NL8105775A - LINKAGE FOR MODIFICATION OF THE DYNAMICS OF THE AUDIO AND OTHER SIGNALS. - Google Patents

Description

Circuit for modification of. the dynamics scale of audio and other signals.

The invention generally relates to a circuit that changes the dynamics magnitude of audio and other signals, namely compression members that compress the dynamics magnitude, and expansion means that expand the dynamics magnitude. in particular, the invention relates to a compression and an expander whose sensitivity to unwanted signal control has been reduced, this reduction is hereafter referred to as "modulation control" for reasons which will be noted hereinafter.

Compression members and complementary expansion members are often used together (a compander system) to effect noise reduction: the signal is compressed before transmission or recording and expanded from the transmission channel after reception or playback. However, compression means can be used per se to reduce the dynamics size, for example to adapt to the capacity of a transmission channel, without expansion following, namely when the compressed signal is sufficient for the end target. In addition, compression means are used by themselves in certain products, especially audio products, which are intended to transmit or record compressed broadcasts or pre-recorded signals only. Expansions are used separately in certain products, in particular audio products, which are intended solely for the reception or playback of already compressed broadcasts or pre-recorded signals. In certain products, especially audio signal recording and playback products, a single device is often configured for switching operation as a compression means for recording signals and as an expansion means for playing compressed signals. broadcasts or pre-recorded signals.

8105775 - 2 -

The degree of compression or expansion can be expressed in dB. For example, 10 dB compression means that an input dynamics size of ö | iB is compressed to an output size of (K - 10) dB. In a noise reduction 5 system, it is said to have 10 dB compression followed by 10 dB

complementary expansion, yields 10 dB noise reduction.

The invention particularly relates to a circuit for modifying the dynamics magnitude of an input signal which circuit has a bi-linear character (in which "linear" in this context means constant gain), which typically consists of: 1) a linear part at low level up to a threshold value, 2) a non-linear part (with changing gain) at an intermediate level above the threshold value and up to an end point, which provides a predetermined maximum compression ratio or expansion ratio, and 3) a high level linear part with a gain different from the low level part gain.

The characteristic is called a bi-linear characteristic because there are two parts with at least almost constant gain.

In practice, the threshold value and the end point are not always well-defined "points". The two transition areas where the intermediate level portion merges into the low level portion and the high level portion, respectively, may vary in shape from a smooth curve to a steep curve, depending on the steering characteristics of the compression member and the expansion member.

Attention is drawn to the fact that a circuit with a bi-linear characteristic is different from a circuit belonging to one of two classes, namely: 35 (a) a logarithmic or non-linear circuit 8105775 * * - 3 - circuit with either a fixed or a changing slope and without a linear part, that is, the gain changes over the entire dynamics range; (¾) a circuit with a characteristic having 2 two or more parts of which only one part is linear ("unilinear").

The invention also applies to uni-linear circuits, as will be explained below.

A circuit with a bi-linear characteristic has special advantages and is widely used. The threshold can be set above the input noise level or the transmission channel noise level to exclude the possibility of noise control of the circuit. The high level portion with substantially constant gain avoids nonlinear handling of strong signals that would otherwise introduce distortion.

Two well known types of bi-linear circuits are referred to here as shearband and fixed band (or split band or multiband) chains.

20 Shearband chains evoke the specified desired characteristic for the case of high-frequency audio e-compression or expansion by applying high-frequency support (for compression) or suppression (for expansion) by means of a high-pass filter with a variable bottom angle frequency . As the signal level in the high frequency band increases, the filter corner frequency shifts upwards to narrow the supported or suppressed band and exclude the useful signal from the support or suppression. Examples of such chains are found in U.S. Patent Nos. Re 28,126, 3,757,225, 1,072,911, 3,931,190, and in Japanese Patent Application 55529/71. Such chains can also be configured to operate at a low frequency, in which case the low-frequency support or suppression is obtained by a low-pass filter with a variable top angle frequency.

8103775 V * - k -

In fixed band chains, the frequency spectrum is split into a number of bands by means of associated band-pass filters and the compression or expansion in each band is effected by an amplification controller (either an auto-responsive diode-type limiter or a controlled limiter) in the case of a compressor having some form of inverse or complementary expansion device circuitry. Examples of such circuits are found in U.S. Pat. Nos. 3,881,6,719 and 3,903.1 * 85 and in

Journal of the Audio Engineering Society, vol. 15, No. 1, October 1967, pp. 383-388. These fixed band chains provide independent operation in the different frequency bands.

It is known to construct bi-linear compression and expansion members of both the shear band and the fixed band type using only a single signal path. However, it is generally preferred to construct such devices by providing a main signal circuit which is linear in dynamics magnitude with a combining circuit in the main circuit, and a second circuit which derives its input signal from the input or output signal. of the second chain and whose output is connected to the combining chain. The second chain includes a limiter (self-acting or controlled), and the second chain's limited signal amplifies the main chain signal in the combining circuit for the case of compression but presses the main chain signal for the case of expansion.

The bounded second road signal is smaller than the main road signal in the upper part of the input dynamics range. Preferably, the main chain and the second chain are separately identifiable signal paths and this is also the easiest. More than one second chain is usually provided in the case of fixed band devices. A bi-linear device with a main chain and second chains is often referred to as a double-way device.

8105775 ► * - 5 -

Such known double-way compression and expansion means are especially advantageous because they can achieve the desired type of transfer characteristic in an accurate manner without problems of strong signal distortion. The low level portion with practically constant gain is accomplished by giving the second path a threshold above the noise level: below this threshold, the second path is linear. The intermediate level portion is established by the region over which the limiting action of the second path becomes partially effective and the high level portion with at least substantially constant gain occurs after the limiter has become fully effective so that the second path signal ceases to increase and becomes negligible compared to the main road signal.

At the highest part of the input dynamics range, the output signal of the circuit is effectively only the signal passed through the linear major path, ie, which is linear with respect to the dynamics range.

Examples of these known circuits are found in U.S. Pat. Nos. 3,86,719, 3,903, 85, and Re 28, 26. There are also known analog circuits that achieve similar results but in which the second road has properties opposite to limiter properties and in which the output of the second road suppresses the main road signal for compression and amplifies the main road signal for expansion (U.S. Pat. Nos. 3,828. 280 and 3-875-537).

The invention can be applied to any of these known bi-linear circuits in order to obtain the associated advantages. The invention is not limited to bi-linear circuits, but can also be used to improve the operation of the aforementioned un-linear circuits. As discussed below, the invention can also be applied to logarithmic circuits provided that a deviation from a logarithmic transfer function can be allowed. However, the preferred embodiments relate to bi-linear circuits and, except as stated with so many advantages, this description only concerns bi-linear circuits.

As mentioned earlier, it is not essential to achieve the desired shape of the bi-linear characteristic using the aforementioned "double veg" techniques.

There are alternatives that explore with single swipes, such as described in U.S. Pat. Nos. 3,757, 3,967 * 219, .072.91, and 3,909,733, and Japanese Patent Application No. 555.29 / 71, to name a few examples. Although these alternative circuits may not normally provide such good results as double veg circuits or may be less convenient and therefore less inexpensive, they may generally provide similar results. Accordingly, the invention also applies to these known circuits.

The invention also relates to known compression and expansion means in which series-connected (eg multi-stage) bi-linear chains are used.

Such circuits are described in Belgian patent specification 889.28.

In compression and expansion devices, in particular frequency-selective or multi-band devices, it is clear that strong signals in one frequency range do not unduly adversely affect the behavior of signals in another frequency range. . Filtration and equalization used in the various circuits has been the usual mode of handling this problem, as much in logarithmic devices as in specialized devices such as the unlinear and bi-linear circuits described.

In these known circuits, the DC control signal controlling the variable gain / attenuation device (for example, a variable gain device such as a voltage controlled amplifier (VCA) or a variolosser, such as an 8105775-7 # FET attenuator), or else a variable filter, formed from the linear additive combination of the passband signals and the stopband signals reaching the control circuit. The invention effectively changes this simple combination in a level dependent manner so as to optimize the performance of the compressor or expander with respect to passband versus stopband signals. Non-linear operations are performed including rectification of the signals in different parts of the spectrum, and analyzes are made of the relative and / or absolute amplitude values. The final control can be formed by choosing one of the signals, by choosing two or more of the signals, or by performing non-linear operations, such as limiting at least one of the signals.

In the broadest sense, the invention provides a circuit for modifying the dynamics magnitude of an input signal, which comprises a frequency-selective circuit that determines a frequency pass band within which the dynamics magnification modification takes place, and a dynamic Mic Modifier for effecting a progressive dynamics modification of signal components in this passband or for effecting a progressive shift of the frequency passband thereby modifying the dynamics magnitude, whereby the dynamic action of the dyna-25 mics modifier responds to the level of practically speaking the linear additive combination of the bandpass signal components and the stopband signal components within the circuit so that for high level input signals, the dynamics of the dynamics modifier is reduced to a lesser extent respond to stopband signal components. In other words, at a low level of the input signal, the circuit functions at least substantially like a conventional compressor or expander. However, at a high level of the input signal, the operation of the compressor or expander is modified by the modulation control circuit according to the 8105775

V V

- 8 - invention.

A side effect is that the transfer characteristic from input level to output level of the device is modified by a certain frequency or combination of frequencies. The overall effect is immaterial and may even go unnoticed at the predominant frequency in bi-linear systems. However, in logarithmic systems, the effect of modulation control is mainly active in the portion of the high-level dynamics magnitude, which is deviated from a purely logarithmic characteristic. This may or may not be important in a particular application.

The invention stems from the observation that, ideally, in compressors and in expanders, the compression or expansion responds only to the levels of signals within desired frequency pass bands and not to the levels of signals at other frequencies, the latter of which can be said to be in the stop bands. For example, in an ideal circuit, compression or expansion should not be affected by the levels of signals outside the passband of the fixed band or the passband of the sliding band (whether or not in its rest position). In the case of a shift band chain in accordance with the invention, the amount of the variable band frequency shift does not exceed what is necessary to ensure that a dominant controlling signal is not amplified (in the case of compression) above a reference level.

As applied to bi-linear chains, in particular such chains in the double-way embodiment, the invention further benefits from an inherent feature of such chains: at a high level of the input signal, the main road signal is considerably stronger than the signals or the signal in the other way or roads. As a result, manipulations of the strong signal on the other road or side road are at least virtually inaudible and, with the exception of phase shifts, they are practically not measurable (negligible 8105775 * * -9 level changes). This property of bi-linear chains is most easily understood in the context of a double-way chain. However, the principle also applies in single-path bi-linear chains in which two or more signal-5 components are present on the same path, rather than in separately designated paths.

The invention takes advantage of the above data findings regarding the properties of bi-linear circuits. When compared with bi-linear compressors and expanders known in the art, the invention provides additional manipulation of the signal (modulation control) in the region where the input signal has a high level, where the overall response of the compressor or expander is linear.

The comparatively weak noise component of the signal is manipulated in this additional manner only at high signal levels, thereby ensuring that all effects relevant to the signal channel are overshadowed by the strong main signal component.

In dual-way bi-linear circuits, an effect of the invention is to modify the side-way transfer characteristic such that the side-way characteristic itself becomes bi-linear instead of smoothing at a high level of the input signal. or return to zero.

This is a consequence of the proportional aspect of the modulation control. That is, at a high entry level, the level in the side road does not drop below a chosen proportional part of the level in the main road (for example, a quarter or a tenth). This is acceptable because the signal in the side road remains considerably weaker than the signal on the main road at a high level of the input signal and because the stop band is usually shifted considerably in phase with respect to the main road signal channel.

For the same reasons, the invention can be implemented in uni-linear chains that have a linear response at a high signal level.

8105775 β w - 10 -

From another point of view, the object of the invention is to increase the level of stopband signal components in the output signal of the device "at a high signal level, but not so much as to cause problems with the recording or transmission channel since they are still relatively weak. Increasing the level of such stopband signal components is not in itself a particularly advantageous matter, but is necessary in order to obtain an enhancement of the dynamic operation as well as a further reduction of noise in the passband. the level of stopband signal in the output signal of the device at a high signal level is achieved by lowering the level of the stopband signal components in the control signal channel at a high signal level 15, or else by arranging things so that the control signal is generated as if the stopband signal components in the si signal used to obtain the control signal at a high signal level had a lower level (for example, by filtering and limiting in the control circuits 20 or by a frequency-dependent suppression facility for the control signal).

Another advantage of the invention is that in listening tests "pump effects" of single-ended compressors and expanders are significantly reduced if not completely removed. Thus, in addition to its application in complementary noise suppression systems, the invention is particularly useful for use in stand-alone compressors and expanders (ie, compressors used to compress signals that are not subsequently expanded, and expanders used to expand signals that had not been compressed before).

It is in the background of the invention that although various practical embodiments of noise suppression circuits have been successful, in use such circuits deviate to some extent from the ideal because the problem of stopband signals occurring too much occurs. control compression and expansion. The effect of such shortcomings appears on several interconnected screws: 1) a reduction in the noise suppression effect in a portion of the passband of the noise suppression system; 2) noise modulation effects (eg the level of a signal at one frequency modulates the noise level in another part of the holiday spectrum); 3) signal modulation effects (for example, the level of the signal at one frequency modulates the level of a signal at another frequency); U) cross modulation effects (for example, interfering modulation products arising from one of the last modulation effects mentioned above or from both).

The extent to which these shortcomings are discernible depends on the type of circuitry used in the noise suppression system, on the recording and playback equipment, on the recording / playback channel or medium and on the nature of the signal material. In many cases, the shortcomings are at least virtually imperceptible except with the aid of test equipment. It is nevertheless desirable to address these 25 shortcomings. Since the aforementioned shortcomings of the known compressors, expanders and noise suppression systems are associated with modulation effects, either of signals or noise, the invention described herein for suppressing such shortcomings is referred to as modulation control.

The severity of these modulation effects largely depends on the uniformity of the transmission channel between the compressor and the expander. For example, in magnetic tape recording and playback systems, there is a frequency response phenomenon known as "head sheds" 8103775 fc0-12 ("head bumps"). Even in professional systems, especially those operating at j6 cm / s, the playback response below 100 Hz is not uniform due to the relationship between the signal wavelength on the tape and the size of the playback head 5 of the same order of magnitude to be. If the compressor / expander system is sensitive to signals in the head-shock region, such signals may control the expander in a non-complementary manner during playback such that signals or noise at a higher frequency, for example up to 3 kHz, are modulated by the signals in the range of 100 Hz or lower.

Prior art fixed band circuits (single band and multiband) have utilized various filtering techniques to minimize control of compression and expansion by unwanted signals. According to these techniques, narrow filters (for example with steep sides) are placed in the signal path or in the control circuit (of the limiter).

However, the use of signal path-20 filters steeper than 6 dB / octave (for example, single pole filters) in multiband compressors and expanders causes amplitude and phase effects such that when the total signal spectrum is combined again, amplitude and are phase errors. This problem is exacerbated when filters 25 steeper than 12 dB / octave are used. However, a filter steepness of only or 12 dB / octave cannot adequately protect against all unwanted signals. In the examples of multiband (fixed band) bi-linear circuits in U.S. Pat. No. 3-8U6.719 and in Journal of 30 the Audio Engineering Society, vol. 15, No. U, October 1967, pp. 383-388, filters are used with a steepness of 12 dB / octave in the signal path of three of the four fixed bands. Only by using a complex filter characteristic in the frequency band adjacent to the steep filters can a flat overall frequency response be obtained. Obviously, such a solution is not generally applicable.

In the logarithmic multiband (fixed band) compressor / expander circuit described in Rundfunk techn. Mitteilungen, 22 (1978), No. 2, pp. 63-7, the input signal is divided into four bands by single-pole filters. However, the control chains for each band use steep 18 dB / foot flashes. Also, a steep control chain filter (12 dB / octave) is used in a single fixed band compressor / expander chain sold under the trade name "dbx II". However, the use of steep control chain filters results in an undue amplification of high level signals outside the passband of the control chain filter as no large amplitude signals are present within this passband, resulting in a possible overload of the transmission channel unless Steep cut-off filters are also used in the signal channel.

An art known in the art which is referred to as spectral beveling is described in Belgian patent 889.-27, in Audio, May 20, 1981, pages 20-26 and in disclosure J-6 and pre-pressures presented at the November 1981 meeting of the Audio Engineering Society in Hev York, 1ί. Y .. Spectral beveling also relates to the suppression of modulation effects resulting from the non-complementary nature of the compressor and expander due to transmission channel errors. According to the spectral bevel technique, at least in the compressor at a frequency clearly within the normal passband of the system and within the flat response region of the transmission channel, steep filtering is provided. Although the spectral bevel succeeds in reducing the disturbing signal modulation effects caused by channel irregularities, it does not address the problem of an undue frequency shift in shear band systems or of an undue attenuation in fixed band- systems.

8105775

V V

- · | ί + -

Thus, the invention aims to minimize control of expansion and compression by unwanted signals without the associated side effects and / or complexity of the prior art.

Although measurable modulation effects are not totally suppressed by the invention, the effects of the invention in audio applications are complemented by psychoacoustic masking effects such that the noted effects for most listeners and most of the music material be inaudible. That is, only the modulation of a signal (or signals) sufficiently frequency-separated from the modulating signal is noticed by human hearing. Such modulation is reduced to a minimum by the invention. Although the modulation of a signal (or of signals) by another signal that is close in frequency is less likely to be affected or improved by the invention, such phenomena are not likely to be perceived by hearing. as a result of two related effects: a) a weak signal that is close to a strong signal in terms of frequency is masked by the strong signal such that the weak signal is inaudible, or b) if the nearby signal is heard. Before the compression or level is increased by the compressor so that it becomes audible there is a psychoacoustic tolerance of modulation effects due to the small frequency distance.

Thus, human hearing is unable to distinguish modulation effects of signals at nearby frequencies, and thus the invention need not be fully effective for such signals.

The operating conditions for the invention are determined by a fixed band or shear band compressor or expansion circuit incorporating a variable circuit 8105775 - 15 - which is usually controlled by a DC control signal operating primarily in the lower part of the total dynamics -size. According to the invention, in the upper part of the dynamics magnitude modulation controllers are used to prevent the operation of the variable chain member from being greater than is necessary to provide the nominally desired suppression of dominant signals, or to have frequencies in the pass band or in the stop band. In practice, controlling the operation of the variable chain member usually means acting on the controller controlling the chain member.

The modulation control may take the form of active or passive control signal limiting devices operating at a high signal level, or of means utilizing circuits 15 which detect the presence of the high level signals and generate signals which increase the control signal level counteract. Such a control signal limitation can take place in one or more frequency-selective control signal channels: if more than one means are present for selecting or combining the control signals such that the variable chain element is provided with an optimal control signal. When using a high level signal detection circuit or a modulation control generator, it can operate in various ways that will measure signal levels in at least the upper portion of the dynamic size. For example, the modulation control signal can be derived from the input signal or the output signal of the compressor or expander. The modulation control signal effectively provides a reference for the DC control signal 30 applied to the variable circuit element (VCA or voltage controlled filter). The reference signal is combined with opposite phase to the DC control signal (for example with opposite sign or the like to counteract) which control signal is mainly generated in response to stopband signal components to yield an 8103775-16 limit as to "how large the control signal to the variable chain element may become responsive to signals in the stopband, ie outside the passband of the fixed band or the slider band. In practice, this boundary can be made relatively "hard" or relatively "soft" That is, progressive increases in the control signal may abort relatively suddenly or otherwise be allowed to continue at a reduced speed.

The modulation control signal can also be derived from the variable chain (VCA or variable filter) by measuring the voltage or current components of the variable chain and, if necessary, equalization to generate a signal useful in providing a limit about how large the control signal to the variable circuit can become in response to signals in the stopband.

In terms of results, the invention, when applied to either fixed band or shear band devices, provides substantial immunity to signals outside the pass band of the fixed band or shear band. In the case of sliding band devices, the invention provides an additional but related advantage, namely that the sliding band shifts only as far in response to a dominant signal as is necessary to bring the gain at the signal frequency to substantially the unit, at least for levels at or above a reference level. The reference level is located at or near the top surface of the dynamic operating range of the device, for example within about 6 - 20 dB of the maximum allowable level. Prior art sliding band circuits are prone to excessive shifting such that the angular frequency of the variable filter is pushed further than is necessary with high level signals, which not only causes potential modulation effects but also results in loss of the effect of noise reduction in part of the spectrum.

When applied to two-way circuits with 8105775 - 1T - sliding tape, the invention in the simplest embodiment ensures the rectification and smoothing of the input signal or output signal and the combination of the obtained DC voltage reference signal with the control signal that is presented to the variable filter. . The level of the reference signal can be adjusted for a desired proportionality limit on a dominant second road signal relative to the corresponding component in the main road signal. For example, the modulation control circuit can be performed to operate so that, in, say, the top 20 dB of the dynamics magnitude, the limiter provides only the attenuation needed to keep the dominant signal component in the second away in a relatively constant ratio to this component in the main road signal (for example, 15 dB lower).

When applied to fixed-band two-way switches, the invention in its simplest embodiment, as in the sliding band version, rectifies and smoothes the input or output signal to generate a modulation control signal which responds mainly to the signal component and with high level of input signal. However, in the case of fixed band circuits, a steep filter is used in the passband control circuit to provide a passband control signal. In addition, a stopband control circuit is used to provide a stopband control signal. The modulation control signal provides a reference for the stopband control signal (i.e., it counteracts this at high signal levels). The stopband control signal held against the reference is compared to the passband control signal and the two are generally combined to favor the greater through a maximum signal selection circuit which then controls the VCA. The overall effect of the chain is to provide the required attenuation (total compression rule) in the passband, while avoiding control of the passband attenuation by large signal components in the stopband, and while allowing 8103775 - 18 - excessive amplification of high level signals in the stop band as they appear at the output of the overall compressor is avoided.

In these and other embodiments in which a reference blanking signal is generated, the signal can be derived from the input signal or the output signal because at a high signal level where the invention operates, the input level and the output level are substantially equal.

In certain embodiments, the modulation control signal may be subject to filtering or equalizing before the rectification. Such equalization interacts with the fixed or variable filtering or equalization used in the signal chains and in the control chains, to provide total modulation control that is most effective in suppressing control by signal components in the stopband , while at the same time interfering as little as possible with the control by signal components in the passband.

Other embodiments of the invention are described below. For example, the amplified AC voltage output of the sliding band variable filter can be divided into two or more bandpass channels, each channel exposed to selected threshold thresholds, and rectified and combined to provide a control signal. By selecting suitable thresholds, the DC control circuit of the sliding band compressor or expander has a frequency dependent maximum output signal characteristic which acts to minimize the control of the compressor or expander by signals outside the sliding pass band.

In a variant that uses only a single control chain channel, a low-frequency gain chain is provided in the control amplifier. This is followed by an amplitude limiter and then a high frequency boost circuit. The resulting AC voltage signal is then rectified and smoothed to form the control signal.

The invention will be described in detail below by way of example with reference to the drawing, in which: Figure 1 is an example of a set of curves showing the complementary bi-linear compression and expansion properties.

Fig. 2 is a circuit diagram of a sliding belt compressor known from the prior art.

Fig. 3 is a circuit diagram of a sliding belt expander known from the prior art.

Fig. is a circuit diagram of a modification of the schemes of Fig. 2 and Fig. 3.

FIG. 5 is a block diagram of a sliding belt compressor known from the prior art.

Fig. 6 is a set of probing tone curves showing the sliding band action of the circuit of FIG. 2 and FIG. B.

FIG. T through FIG. 10 are a series of sounding tone curves illustrating the effects of modulation control according to the invention performed in a sliding band compressor.

Fig. 11 is a block diagram of a preferred embodiment of the invention performed in a sliding belt compressor.

Fig. 12 through 15 are block diagrams of other embodiments of the invention made in sliding belt compressors.

FIG. 16 and FIG. 17 are block diagrams of a prior art fixed belt compressor and expander.

Fig. 18 through 20 are response curves illustrating the effects of modulation control 35 according to the invention performed in a sliding belt compressor.

8105775 - 20 -

Fig. 21 is a block diagram of a preferred embodiment of the invention performed in a fixed belt compressor.

Fig. 22 is a block diagram of another embodiment of the invention performed in a fixed band compressor.

In Fig. 1 exemplary bi-linear complementary compression and expansion transfer characteristics (at a given frequency) are shown which indicate (for the compression characteristic) the low level part of a practically constant gain, the threshold, the part where the dynamic action takes place, the end point and the high level part with practically constant gain.

Details of a particular two-way slider-15 band bi-linear circuit are given in Figures 2, 3 and b. The slide belt embodiments of the invention have been described with reference to this circuit, although the invention is not limited to use in such circuits.

Fig. 2, FIG. 3 and FIG. 4 are the same as FIG. 4, FIG. 5, and FIG. 10, respectively, from U.S. Patent 28.U26, and further details of these circuits, their operation, and the underlying theory are set forth therein . Fig. 5 is a block diagram of FIG. 2 (with or without the modification of FIG. U). The following description of FIG. 2, FIG. 3 and FIG.

25 ^ is largely derived from the American patent specification Re 28Λ26.

In particular, the circuit in FIG. 2 is designed to be included in the recording channel of a home tape recorder requiring two such circuits for a stereo recording device. The input signal is applied at the terminal 10 to an emitter follower stage 12 which provides a low impedance signal. This signal is first presented via a straight through main path formed by a resistor 14, to an output terminal 16 and secondly via 8105775-21 - a second path, the last element of which is a resistor 18 which is also connected to the output terminal 16 is connected. Resistors 14 and 18 add the main road and second road outputs to fulfill the required compression rule.

The second path consists of a fixed filter 20, a variable cut-off filter 22 containing a FET 2k (these form the filter / limiter), and an amplifier 26 whose output is connected to a double diode limiter 10 28 and at the resistor 18. The non-linear limiter suppresses overshoot of the output signal with suddenly increasing input signals. The amplifier 26 secondly raises the signal to a level such that the knee in the limiter or overshoot suppressor characteristic 28 containing silicon diodes is effective at the appropriate signal level under transient conditions. The effective threshold of the overshoot suppressor is slightly above that of the previous filter / limiter. Resistors 1k and 18 are sized so that the desired compensating amount of attenuation is then obtained for the signal in the second path.

The output of amplifier 26 is also connected to an amplifier 30 whose output signal is rectified by a germanium diode 31 and integrated by a smoothing filter 32 to provide the control voltage 25 for the FET 2h.

Two simple RC flashes are used, although equivalent LC or LCR filters could be used. The fixed filter 20 produces a cut-off frequency of 1700 Hz (now 1500 Hz) below which compression takes place at a reduced rate. The filter 22 consists of a series capacitor 31 and a parallel resistor 36, followed by a series resistor 38 and the FET 2h whose supply-drain path is connected as a parallel resistor. In the state of rest with a signal zero at the control electrode of the FET 2k, the FET is pinched off and shows a practically infinitely large 8 1 0 ü 7 7 5

% V

Impedance: the presence of the resistor 38 in this case can be left out of consideration. The cutoff frequency of the filter 22 is thus 800 Hz (now 750 Hz), which, as will be noted, is considerably below the cutoff frequency of the filter 5 filter.

When the signal at the control electrode increases enough to lower the resistance of the FET to less than say 1 kohm, the resistor 38 effectively parallels the resistor 36 and increases the cutoff frequency, 10 which reduces the transmission band of makes the filter much narrower.

Obviously, the increase in the cutoff frequency is progressive.

The use of a FET is convenient because, within a suitably limited range of signal amplitudes, such a device operates practically as a linear resistor (for signals of both characters), the value of which is determined by the control voltage on the control electrode.

Resistor 36 and FET are returned to an adjustable runner k6 in a sputter divider containing a temperature compensating germanium diode h-8. The runner k6 makes it possible to adjust the compression threshold of the filter 22.

Amplifier 26 includes complementary transistors that provide high input impedance and low output impedance. Since the amplifier drives the diode limiter 28, a finite output impedance is required which is provided by a coupling resistor 50. The diodes 28 are, as already noted, silicon diodes and have a sharp knee around 1/2 volt.

The signal on the limiter and thus on the resistor 18 can be short-circuited to earth by means of a switch 53 when it is necessary to switch off the compressor.

Amplifier 30 is an HPH transistor with an emitter-time constant network 52 that provides 8105775-23 amplification at high frequencies. Powerful high frequencies (such as a cymbal beat) will therefore lead to a rapid narrowing of the band in which compression takes place, thus avoiding signal distortion.

The amplifier is connected to the smoothing filter 32 via a rectifying diode 31. The filter includes a series resistor 5 ^ and a parallel capacitor $ 6. The spring position 5k is connected in parallel to a silicon diode 58 which allows a fast charging of the capacitor 56 for a fast response coupled with a good smoothing under stationary conditions. The voltage on the capacitor 56 is applied directly to the control electrode of the FET 2b.

A complete circuit diagram of the complementary expander is shown in Figure 3, but a full description thereof is not necessary since the circuit is practically identical to that in Figure 2 and therefore the component values are for the most part not shown in Figure 3. indicated.

The differences between FIG. 2 and FIG. 3 are the following: In FIG. 3, the second path derives its input signal from the output terminal 16a, the amplifier 26a is an inverting amplifier, and the signals combined by the resistors 1U and 18 are applied to the input (base) of the 25 emitter-follower 12 whose output (emitter) is connected. to the terminal 16a. To ensure a low drive impedance, the input terminal 10a is connected to the resistor 14 through an emitter follower 60. Appropriate steps must be taken to prevent a preset 30 from occurring in the expander.

The amplifier 26a is inverted by deriving the output signal from the emitter instead of from the collector of the second (PUP) transistor. This change involves the displacement of the resistor 62 35 with value 10 kohm (fig. 2) from the collector to the emitter 8105775

• X

- 2k - (fig. 3) which automatically provides a suitable output impedance for driving the limiter. The spring position 50 is therefore omitted in Fig. 3.

It should be noted that it is important in tuning a complete noise reduction system to have signal levels of the same emitters of transistors 12 in as much of the compressor as the expander. Measuring terminals M on these emitters are shown in the drawing.

FIG. k shows a preferred embodiment of a circuit to substitute for the chain between points A, B and C in Fig. 2 and Fig. 3. When the FET 2b is pinched, the second RC network 22 is inoperative and determines the first RC network 20 in that case the response of the second veg. The improved circuit combines the phase benefits of having only a single RC section in the quiescent state with the 12 dB per octave attenuation properties of a TV section RC filter in the presence of signal.

In the preferred embodiment utilizing MPF 10U type FETs, spring position 36a with 39 kobm is required to provide a finite supply impedance to act in the FES. In this way, the compression ratio at all frequencies and all levels is kept at a maximum of about 2. The spring position 36a provides the same compression ratio limiting function in the improved circuit as the spring position 36 in the chain of Fig. 2 or Fig. 3. In addition, this spring position provides a low frequency veg for the signal.

Certain details of the circuit in Figures 2, 3 and 3 have been developed over time and more modern embodiments of the circuit have already been published and are well known in the art. For the sake of convenience, reference is made here to the specific circuitry in U.S. Patent Re 28. 26.

FIG. 5 is a block diagram showing the major elements of the compressor in FIGS. 2 and U. The combining chain 15 represents the combining spring positions Ik and 18 in Figures 2 and 3.

The variable band offset of the sliding band 5 device can be studied in FIG. 6, each figure showing a true recording of a solder tone response obtained from the circuit of FIG. 2 including the circuit of FIG. H. The variable band distance is given by turning off the eompressor frequency response by means of a low level solder tone (of which the level is below the compressor threshold) in the presence of a high level signal: the test tone is detected at the compressor output using a tracking filter. The high level signal allows the compressor circuit 15 to cool, the graph showing the effect on the filter sweep frequency.

In a shear band device according to the invention, the amplitude of the strong or dominant signal that causes the shear band forking should not cause undue shifting, nor should the presence of other high level signals outside the bandwidth of the shearing band be undue shifting cause. Overdue means a displacement of the variable filter's crossover frequency that goes beyond what is necessary to provide a shear band compressor characteristic that avoids chasing the dominant signals above a reference level. The absolute value of the reference level is chosen by the devastator of the system, but is usually 10 dB below the normally used highest level values.

FIG. 7 shows another set of true records of solder tone curves for the case of a slider band compressor circuit according to the same design as that in FIG. 2 (with the modification of FIG. 4), but with a low level gain of 8 dB and a quench frequency of the filter 35 in the amount of 800 Hz. The solder tone level is - 10 dB, 8105775 - 26 - below the compressor threshold. The curves are recorded for a 100 Hz signal at - 20, - 10, 0, + 10 and + 20 dB, where 0 dB is the reference level, and a curve is drawn in case the 100 Hz signal is missing. The 5 recording curves for - 10, 0, + 10 and + 20 dB all start at approximately 200 Hz. This is also the case for Fig. 8. In Fig. 9 and Fig. 10 curves are also drawn in case the signal is missing.

Referring again to FIG. 7, ideally there should be no shift in response to a 100 Hz signal because it is clearly outside the circuit's pass band at its lowest (quench) frequency. Nevertheless, as the level of the signal of 100 Hz increases, the band shifts upwards. The curves for - 10, 0, 15 + 10 and + 20 dB need not slide further than the curve for - 20 dB to avoid any significant boost of the 100 Hz signal. The unnecessary shift has two effects: a) a significant noise reduction effect is 'lost' (during playback) because there is no rushing at frequencies where it may otherwise occur, and b) as the amplitude of the 100 Hz signal varies, modulate signals at higher frequencies as the shift band varies under its control, resulting in a possible incorrect repair of the signal by the expander if the recording or transmission channel has an irregular frequency response in the vicinity of 100 Hz.

Fig. 8 shows a set of true records of solder tone curves for the same circuit, but adding the modulation control circuit as described below.

At least virtually no shift occurs for the same levels of the 100 Hz signal as in the situation of FIG.

7. The sliding band compressor is practically made immune to strong signals outside its passband. The sliding band response is practically the same as its response below the threshold value 35 in the absence of dominant signals.

8105775 - 27 -

The effect of modulation control for sliding-belt compressors is further explained by Figures 9 and 10, which also show real recordings of solder curves recorded with the same circuit and the same level of solder tone as in Figures 7 and 8. In in this case, the effect of a dominant signal at 800 Hz, a frequency within the desired active region of the circuit, has been proposed. Ideally, a shift is necessary to go so far that the 800 Hz signal is not rushed above the 10 dB reference level. Thus, in the response of FIG. 9 without modulation control, the shift caused by the 800 Hz signal at the levels of. - 10, 0, + 10 and + 20 dB uneven. Fig. 10 shows the response of the modulation driven circuit: the shift at and above 0 dB 15 is significantly reduced. The effect is progressively less for low signal levels, but it is noticeable to some extent at the - 10 dB signal level.

Fig. 11 generally shows a preferred embodiment of the modulation control according to the invention, implemented in a two-way bi-linear sliding belt device. The reference numbers have been kept as much as possible as in Fig. 5 for the same and for functionally equivalent elements. The solder tone response curves of FIGS. 7 to 10 are recorded with a sliding tape device as shown generally in FIG. 11 with the modulation control sub-circuit elements in the dotted line block 100 taken from the circuit for the response curve without modulation control. For the sake of explanation, the circuit of FIG. 11 is substantially the same in detail as that of FIGS. 2 and k. The circuit can be modified as described above without affecting the fundamental operation of the modulation control sub-chain.

As shown in FIG. 11, the modulation control sub-circuit derives a DC control signal from the input signal of the circuit (or optionally from 8103775-28-).

output signal of the combining circuit 15), by means of an amplifier 30 'a rectifier 31' and a smoothing circuit 32a '. A potentiometer 102 is shown to indicate that the signal from smoothing circuit 32a 'has adjustable gain. In practice, the amplification is usually preset in the design. A combining circuit 32 subtracts the signal supplied by the sub-circuit 100 from the main control signal supplied by the amplifier 30, the rectifier 31 and the smoothing circuit 32a.

The smoothing circuit in Fig. 11 is divided into two stages in order to save on the price of the components.

Thus, blocks 32a and 32a 'may be identical and each consist of only a single RC filter section, and block 32b further smoothing the combined control signal may consist of an additional RC filter section.

The signals are rectified (by rectifier s 31 and 31 ') before they are combined by circuit 33 to avoid the ambiguity with the sign that would occur if AC voltage signals were combined and then rectified (ie with AC voltage signals two possible stable states would be).

The arrangement of the embodiment of Figure 11 thus provides a reference level for stabilizing the DC control signal, a reference level that dynamically changes with the input signal level, shifting or transferring part of the dynamic operation of the variable filter to a level region determined by the reference level. The device functions to maintain the maximum amplitude of dominant signals in the noise suppression bypass in a constant relationship to the input signal at high signal levels. The relative level from the modulation control sub-circuit 100 is selected to minimize the sliding action in response to signals outside the sliding band 35 pass band.

8105775 - 29 -

Although the embodiment of FIG. 11 functions effectively when the input signal for the modulation control sub-circuit 100 is derived from the wide band input (or output) signal, other circuits that include a measurement of signal levels at the upper end of the dynamics. size, possibly. For example, modulation control effects are obtained even to some extent if the input signal of the sub-circuit 100 is taken from the output signal of the band-pass filter 20. Ideally, 10 in amplifiers 30 and 30 'utilize equalization to optimize the overall modulation control effects ( control by passband components as opposed to stopband components), taking into account the combined frequency response effects of the filters 20, 22 and the equalization applied in the control amplifier 26.

When the invention has been implemented in serial coupled devices, such as described in Belgian patent 889. ^ 28, a single modulation control sub-circuit can be used to supply a reference signal to each stage. Such a circuit advantageously derives its input signal from the output signal of the last compressor stage when the series-connected stages are arranged in the preferred order such that the first stage has a threshold at the highest level. By deriving the reference signal from the output signal, the low level stage or stages receive the modulation control effect at a lower signal level and thereby enhance the modulation control effect.

As mentioned above, it is also possible to obtain modulation control of sliding band chains by means other than by deriving a control signal reference from the input signal (or output signal). One or more control signals can be derived from the output of the variable filter and limited so as to obtain results comparable to those obtained with the blanking device as shown in FIG. 11: the essential 8103775-30 result is the same namely, desensitizing the dynamic modification operation of the circuit for high level signals "within the stop hand. FIGS. 12, 13 and are directed to such embodiments in which limiting is used.

In the embodiment of FIG. 12, the control signal generating means (blocks 30, 31 and 33 in FIG. 5) is divided into three paths by amplifiers 30, 116 and 121), filters 110, 118 and 126, respectively, a 10 high-frequency road, a medium-frequency road and a low-frequency road. Each road contains a limiter (112, 120, 128) that has a preset threshold. The limiters can be oppositely connected diodes, such as the diodes 28 in FIG. 2. For a high-frequency audio compressor with a performance generally shown in FIGS. 7 to 10, the filter frequencies may be are as follows: filter 126, 200 Hz low pass; filter 118, 200 to 800 Hz band pass; and filter 110, 800 Hz high pass. The output of each limiter is rectified by rectifiers 11U, 122 and 130, combined (or selected at maximum value) and applied to the smoothing network 32. Otherwise, the limiting functions can be obtained after rectification. In operation, the low-frequency and medium-frequency bandlimitors are set to minimize the effect on shift by signals outside the pass band. Little or no limitation may be required in the high-frequency path and the control effected by this path can be enhanced by providing the amplifier 30 with a high-frequency boost, as represented by block 52.

Fig. 13 shows another split path control circuit 30. In this example, two roads are used, a high-frequency road and a low-frequency road. The high-frequency path is essentially the same as in the embodiment of Fig. 12, except that the limiter 112 is omitted.

In the low-frequency path, an amplifier 132 is included which has a high-frequency damping network 13 ^. The output of 8105775 - 31 - the amplifier - is applied to a low-pass filter 136 and to a limiter 138. The limiter threshold - is set together with the different filter and amplifier filter properties to achieve the greatest insensitivity to shear band control. obtain stopband signals. The signals in the two paths are rectified by rectifiers 11 if and lifO, respectively, and combined at the input to the smoothing circuit 32.

Fig. 14 shows a simplified embodiment of the embodiment in Fig. 13. The high-pass filter 110, the low-pass filter 136 and the amplifier / high-frequency attenuation network 13 are omitted. The high-frequency preamplification network 52 of amplifier 30 is different from that in network 52, in that the high-frequency boost at a higher frequency becomes effective. Thus, only the wide band path containing amplifier 132 carries low-frequency signals (along with high-frequency signals). The threshold of the limiter 138 is set in conjunction with the high frequency drive properties of the network 52 'to minimize the effect on shift by stopband signals.

20 'FIG. 15 shows an embodiment with a one-way control circuit containing a frequency dependent amplifier 1 Ui with a low-frequency boost network 1 ^ 2, followed by a limiter 1¼ and an amplifier 146 with a high-frequency boost network 1 if-8. In operation, the low-frequency portion of the spectrum that tends to cause unwanted shift is first driven and then limited.

The limiter 1UU is preferably syllabic with its own feedback amplifier, rectifier, smoothing circuit and controlled gain element (such as blocks 276, 280, 282 and 30 270 in Fig. 17). The amplifier 1k6, which is provided with a high-frequency boost network 1U8, restores any high-frequency emphasis that may be required. The output of amplifier 1U6 is then rectified and smoothed by blocks 114 and 32, respectively. In this one-way control circuit, the high level stopband signal components 8103775 ► <- 32 - are significantly attenuated at the point 114 rectification.

For convenience and simplicity, the sliding belt embodiments have been described in connection with a particular design of the sliding belt compressor. The invention is equally applicable to expanders without any change in the second-way noise reduction control circuits shown in the embodiments of Figures 11 to 14. In noise suppression systems using compressors and expanders, it is preferred that the modulation control invention be applied to both devices to ensure complementarity. The invention is equally applicable to low-frequency sliding band chains in which the compression and expansion action is intended to occur in the low-frequency range.

FIG. 16 shows a block diagram of a fixed band bi-linear compressor and expander design in two-way design. The fundamental aspects of the system are described in U.S. Pat. Nos. 3,846,719 and 3,903,485 and in Journal of the Audio Engineering Society, Vol.

20 15, no. 4, October 1967 »pp. 383-388.

In the known embodiment of Figure 16, the second-way networks 250 provide four bands. Bands 1, 3, and 4 have conventional 12 dB / octave input flashes: a low-pass filter 252 at 80 Hz at the input of band 1, a high-pass filter 254 at 3 kHz at the input of band 3, and a high-pass filter 256 at 9 kHz at the entrance of band 4.

Each of the filters is followed by an isolating stage 258 with an emitter follower. A band 2 has a frequency response that is complementary to that of bands 1 and 3. Such a response is obtained by (in the adder 260) the output signals of the emitter followers 258 in bands 1 and 3 on to count and subtract this sum from the total input signal (in a subtractor 262). The output of the emitter follower 258 in each of the bands and the output of the subtractor 262 are applied to the respective limiters 26k and 26k1. The limiters 26k and 26k1 are identical except that the limiters 26k 'in bands 1 and 2 have a time constant twice that of bands 3 and U. The outputs of bands 1 to 5 are combined with ^ with the signal on the main road in a combination means 266. The output of the compressor is presented to a noise input channel to be sent to the complementary expander in which the output of the identical second-way mains networks is subtracted from the input signal 10 to thus obtaining the complementary expansion characteristic.

Fig. 17 shows further details of the limiters 26k and 26k1. Each includes an FET attenuator 270 that operates in response to a control signal. The attenuator output is amplified by a signal amplifier 272 whose gain is adjusted to obtain the desired low-level signal gain. The output signals of all bands are combined with the main signal in such a way that a low-level output signal from the compressor is delivered that is uniformly 10 dB higher than the input signal up to about 5 kHz, above which the increase in level gradually increases to 15 dB at 15 kHz.

The FET attenuator is controlled by a control signal auxiliary circuit which provides an e-compression threshold of U0 dB below the peak operating level. The control auxiliary circuit includes a control signal amplifier 276 which is followed by a phase splitter 278 which drives a phase-phase rectifier 280. The DC voltage obtained is applied to a smoothing power 282 whose output signal is the control signal. Network 282 includes a first RC integrator, an emitter follower, and an RC end integrator that interact with diodes such that both the first integrator and the end integrator have a nonlinear characteristic provided by the diodes. Rapid, large changes in the signal amplitude 8105775 - 3U - are passed quickly, while small changes are propagated slowly. This dynamic smoothing action provides optimum results with respect to modulation effects, low-frequency distortion and distortion components generated by the control signal. The chain achieves fast recovery and low signal distortion.

Fig. 18 shows actual recording on a sheet writer of a response graph below the compression threshold of a fixed band compressor with a low level gain of 8 dB and a pass band filter frequency of 800 Hz high pass. Within the active frequency range of the device (determined by the angular frequency of 800 Hz), a boost is obtained up to levels of about -10 dB (relative to a 0 dB reference level).

FIG. 19 shows the effect on compression when a high level signal (+ 10 dB) is present at 100 Hz, which frequency is well below the angular frequency of 800 Hz of the filter. The strong 100 Hz signal in the stop band effectively blocks the compressor and prevents any compression. 20 sie within the passband. The result is that the desired noise suppression in the passband is omitted. In addition, if the 100 Hz signal is intermittent, the compression in the passband will be effected or lost with the controlling 100 Hz signal, which will cause noise modulation and / or signal modulation.

Fig. 20 shows the effect of adding a modulation control auxiliary circuit as described below to a fixed band circuit. Compression is restored in the pass band area even with the occurrence of the strong (+ 10 30 dB) signal at 100 Hz. The modulation control auxiliary circuit effectively desensitizes the fixed band circuit to the strong stop band signal.

Fig. 21 shows in. generally the preferred embodiment of the invention as applied to one belt of a fixed belt bi-linear two-way compressor of the 8105775-35 type as is described in connection with Fig. 16. Two additions have been made to the described circuit to obtain modulation control A modulation control auxiliary circuit 198, similar to that in the sliding belt embodiment 5 of Fig. 11, is provided and includes a straightener 208 'and a first smoothing stage 210a'. fed from the compressor output signal. Elements 208, 208 'and 210a, 210a' may be identical (but sheet separate). The level of modulation control signal from smoothing circuit 210a * is adjusted by an attenuator 212 or by any other suitable and is combined by the opposite sign circuit 214 with the dust band DC voltage control signal from the smoothing circuit 210a. output from an FCA 204 and an amplifier 206 applied to a filter 216 which preferably has the same angular frequency as the filter 202, although this is not essential: the comparative graphs in Figures 19 and 20 were made with a simple 6 dB / octave low pass filter 216 with an angular frequency of 3 kHz. After 20 minutes, the filter 216 should ideally have a relatively steep cutoff characteristic, for example 12dB or 18dB per octave (with a two- or three-pole filter, respectively) with approximately the same cutoff frequency as the filter. 202. The output of the filter 216 is rectified and smoothed by blocks 218 and 220 to form the passband control signal. The smoothing provided by blocks 210a, 210a 'and 210 "may be a preliminary filter stage followed by further smoothing in the circuit 210b. The output of the passband filter channel is presented to a maximum selector 222 which at its other input receives the output from the modulator 214, the modulation-controlled stopband control signal In its simplest form, the maximum selector comprises two diodes which transmit the greater of the two input signals: in more sophisticated circuits, the used operational amplifiers 8105775 - 36 - to eliminate diode voltage drops and increase accuracy.

In operation, the signals in the stopband are subject to the operation of the auxiliary circuit 198 if there are no dominant signals within the passband where the compression action is desired. Thus, although a strong signal, such as that of + 10 dB at 100 Hz, generates a large control signal by blocks 208 and 210a (and 210b), that control signal is counteracted by the modulation control auxiliary signal so that the gain of the VCA 20k is not pushed down to cause loss of compression in the passband. On the other hand, if a signal of 100 Hz occurs in the level range of -20 dB, the counteracting action is considerably reduced, and the stopband control signal in that case controls the operation of the compressor correctly each time the signal conditions are such that the passband control signal compressor does not control. If strong signals are present within the passband in the active region, the output of the narrow filter channel, the passband control circuit will control the maximum selector and allow the YCA to respond accordingly.

The level of the modulation control auxiliary circuit relative to the input signal or the output signal is adjusted so as to obtain a dynamic reference signal (relative to the input signal) of sufficient level to result in a significant insensitivity of the compressor operation to strong signals from the passband.

The comments made with regard to equalizing control and modulation control amplifiers in sliding band chain applications also apply to fixed band embodiments. Thus, filter equalizers 22h and 226 may optionally be inserted into the respective paths to the rectifiers 208 'and 208. However, the possibilities of advantageously operating one frequency-dependent characteristic 8105775-37 against another are in the case of the fixed hand versions less than with the sliding belt: this is the reason why an extra control chain is required in the fixed belt case (three chains versus two).

It is also possible to obtain modulation control of fixed band chains by means other than deriving a control signal reference from the input signal (or output signal) of the compressor or expander. One or more control signals can be derived from the output of the controllable element (attenuator or VCA) and can be limited in order to achieve results corresponding to those obtained by means of the counteracting embodiment according to Fig. 21. Figs. 22 is directed to such limiting embodiments.

In the embodiment of FIG. 22, the control signal generating means (blocks 276, 278, 280, and 282 in FIG. 17) is split into two paths, one with an amplifier 228, a steep cutoff filter (as in the embodiment of FIG. 21) and a rectifier 218, and the other with an amplifier 230, 20 a limiter 232 and a rectifier 218 '. The threshold of the limiter 232 which may consist of, for example, oppositely connected diodes) is chosen such that the limiting action begins at a relatively high level, namely at about the same level where the output signal from the combination member 25 becomes dominant in the embodiment. according to fig. 21.

The output signals of the rectifiers 218 and 218 'can be combined and applied to a smoothing circuit 210, the output signal of which is supplied as the control signal to the VCA 2Qk or the rectifier output signals of which can be applied to (or serve as) a maximum selector chain (such as block 222 in Fig. 21) and its output is applied to a smoothing network 210.

In operation, the embodiment according to Fig. 22 functions in a similar manner to the embodiment according to Fig. 21.

8105775

Claims (10)

1. A circuit for modifying the dynamics magnitude of an input signal, comprising a frequency selective circuit defining a frequency pass band within which the dynamics magnification modification takes place, and a dynamic modifier to provide a progressive dynamic modification of signal components in that pass band or to effect a progressive shift of the frequency pass band modifying the dynamics magnitude, with the dynamic action of the dynamic modifier responsive to increasing levels of the linear additive combination of the passband signal components and the stopband signal components within the circuit, characterized in that for high level input signals, the dynamic operation of the dynamic modifier becomes less responsive to steam band signal components. Circuit according to claim 1, characterized in that the dynamic action modifier 20 comprises a variable gain or variable damping member, or a variable filter whose variable dynamic action is controlled by a control circuit. 3. A circuit as claimed in claim 2, characterized in that the control circuit comprises a means responsive to stopband signal components to counteract the respective progressive dynamic modification or progressive shift at high level input signals. Circuitry according to claim 3, characterized in that the counteracting means comprises a means for a non-linear processing of the stopband signal component and. Circuit according to claim 3, characterized in that the counteracting means comprises a sub-circuit for generating a counteracting reference signal 35 as a component of the signals to which the control circuit 8105775 - 39 - responds, the reference signal providing information at least "at high level input signals such that the effect of stopband signal components in the control circuit at a high level of the input signal is reduced. 6. Circuitry according to claim 3, characterized in that the control circuit comprises at least one limiter and frequency-selective circuit member such that the limiter or limiters preferably act on signals in the stopband, so that the effect of stopband signal components in the control chain at a high level of the input signals is reduced. A circuit according to any one of the preceding claims, characterized in that the circuit has a bi-linear characteristic composed of a low-level portion with a practically constant gain up to a threshold, an intermediate level portion above the threshold with changing gain providing a maximum compression ratio or expansion ratio, and a high level portion with a practically constant gain that differs from the gain of the low level portion, or else a uni-linear characteristic including the portion has a changing gain at low level. Circuit according to any one of the preceding claims, characterized in that the circuit is bi-linear and is a two-way circuit with a main road that is linear with respect to the dynamics magnitude, a combining circuit in the main road and a second road whose input is connected to the input or the output of the main road and whose output is connected to the combining circuit, the second road 30 provides a signal that at least in an upper part of the frequency band chases or counteracts the main road signal by means of of the combining circuit, but limited so that in the upper portion of the input dynamics magnitude, the second road signal is smaller than the main road signal. Circuit according to claim 8, 8105775 - Uo - characterized in that also. the second way has a bi-linear characteristic such that at high input signal level, the second way signal level is not less than a selected fraction of the level of the signal in the main road. The circuit of claim 2, for audio signals, wherein the dynamic operation modifier comprises a variable filter which filter provides a boost or cut in a high-frequency region or a low-frequency region of the signal band and responds to dominant signals in order to cause the filter corner frequency to shift in the direction that narrows the hunted or truncated region, and the control circuit comprising a rectifier, a smoother and an amplifier, which provide a control signal derived from the filter output signal, namely, a controlled impedance means of the filter to effect shifting of the filter angle frequency, characterized in that the control circuit comprises a sub-chain which provides a reference signal for counteracting the control signal and which is derived from the input or output of 20 the circuit in which the reference signal varies dynamically in level with the level of the input signal or the output signal, the sub-circuit comprising a means for adjusting the gain of the reference signal such that the effects of the stopband signal components in the control signal at high input signal levels are reduced. 11. A circuit according to claim 2 for audio signals, wherein the dynamic operation modifier comprises a variable filter which filters a boost or cut in a high frequency region of the signal band or in a low frequency region of the signal band and responds to dominant signals in order to shifting the filter corner frequency in the direction that narrows the hunted or truncated region, and the control circuit comprising a rectifying, smoothing and amplifying means providing a control signal derived from the filter output signal to a controlled 8105775-In-impedance means of the filter to effect shifting of the filter angle frequency, characterized in that the control circuit comprises at least one limiter and frequency-selective circuit member such that the limiter or limiter 5 preferably acts on signals in the stop band and so that effects of the stopband signal component n are reduced at high input signal levels. 12. The circuit of claim 2 for audio signals, wherein the dynamic operating means comprises a variable gain or damping means which provides dynamic modification of signals in the passband, and responsive to dominant signals causing progressive dynamic modification, and wherein the control circuit comprises a means supplying a control signal to a control input 15 of the variable gain or damping member to effect progressive dynamic modification, characterized in that the control circuit comprises a first sub-chain containing at least one sharp filter having a frequency characteristic similar to that of the frequency-selective circuit, as well as a rectifying, smoothing and amplifying means for supplying a first signal, and a second sub-circuit deriving a second signal from the output signal of the amplification - or damping member and which have a rectifying, flattening e n includes an amplifying member and derives a third signal 25 from the input or output signal of the circuit and includes a second rectifying, smoothing and amplifying member, the signal forming a reference signal for counteracting the second signal in order to fourth signal, the reference signal 30 dynamically varies in level with the level of the input or output signal, the second sub-circuit includes means for adjusting the gain of the reference signal such that the effects of stopband signal components in the third signal at a high level of the input signal is reduced, and a means for presenting the largest 8105775-k2 signal of the first and the fourth signal as the control signal at the control input of the variable gain member or cushioning. The circuit of claim 2 for 5 audio signals, wherein the dynamic operation means comprises a variable gain or attenuation means, the latter means providing a dynamic modification of the signals in the passband and responding to dominant signals to provide a progressive dynamic modification and wherein the control circuit comprises a means which provides a control signal derived from the output signal of the variable gain or attenuation means, namely a control input of the latter means to effect the progressive dynamic modification, with the characterized in that the control circuit comprises a first sub-chain with at least one sharp filter with frequency properties similar to that of the frequency-selective chain, furthermore a rectifying, smoothing and reinforcing member, and a second sub-chain with at least one limiter and a frequency selective circuit 20 means such that the beg preferably operate on signals in the stopband such that the effect of the stopband signal components is reduced at a high level of the input signal. 8105775