JPH0130207B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an audio noise suppression circuit that reduces noise due to crosstalk from adjacent tracks in a magnetic recording/reproducing apparatus that records a frequency modulated (FM modulated) audio signal and a video signal in a superimposed manner. Traditionally, luminance signals are frequency modulated (FM modulation)
A magnetic recording/reproducing device (hereinafter referred to as
It's called VTR. ) Method 1 of recording audio signals in
One known method is to superimpose an FM-modulated audio signal and the video signal and record them on the same track of a magnetic tape using a rotating head (hereinafter referred to as the audio FM superimposition method). Incidentally, the improvement in recording density in recent years has been remarkable, achieving a recording density 17 times higher than that of VTRs from about 10 years ago. As high-density recording technology progressed, efforts were made to make it more compact by making the cassette smaller and the diameter of the rotating cylinder smaller.
VCRs are beginning to be developed. In order to reduce the size and weight of these small VTRs and to reduce the running speed of the magnetic tape, the conventional audio signal recording and reproducing method using a fixed head has been designed to reduce wow and flutter characteristics, playback S/S/
It is becoming difficult to obtain sufficient performance in terms of N, reproduction frequency band, etc., and there is an increasing need to adopt new audio recording and reproduction methods such as the above-mentioned audio FM superimposition method. The features of the audio FM superimposition method are: (1) It is less susceptible to time axis fluctuations due to uneven tape running speed, so it has good wow and flutter characteristics. (2) The playback frequency band does not depend on the tape running speed, and a wide band is possible. etc. Let us now consider the recording frequency spectrum of a VTR that records and reproduces the above-mentioned audio signal using the audio FM superimposition method. The center frequency of the audio signal carrier must be determined so as to minimize its influence on the luminance and chromaticity signals. Also, in small VTRs, especially those with small rotating cylinder diameters, the relative speed between the tape and the head is low, so the recording frequency band is narrow.
The center frequency of the luminance signal carrier cannot be set too high. Therefore, the center frequency of the audio signal carrier wave must be set as low as possible below the FM modulated luminance signal. An example of the frequency spectrum during recording of the video signal and FM audio signal of FIGS. 1 and 2 is shown.
FIG. 1 shows an example in which an FM modulated audio signal A ₁ is placed between an FM modulated luminance signal Y ₁ and a frequency-converted chromaticity signal C ₁ .
FIG. 2 is an example in which the FM modulated audio signal A ₂ is placed below the frequency-converted chromaticity signal C ₁ . However, a major problem with audio FM superimposition is the need to make the head width wider than the video track width in order to perform so-called variable speed playback, which is playing back at a tape speed different from the tape speed during recording, and to obtain tracking margin. If the signal of an adjacent video track is also reproduced due to tracking deviation, etc., noise will occur in the reproduced audio signal due to the influence of the FM audio signal of the adjacent video track (hereinafter referred to as adjacent interference). be. In particular, when attempting to increase the recording density, the video track width becomes narrower, and adjacent disturbances such as tracking deviations become very annoying and become a problem. FIG. 3 shows video tracks T1 and T2 formed on the magnetic tape 21 and a video head H.
It is a top view which shows the position typically. Here, the noise D(t) caused by the above adjacent interference is
is the first FM audio signal obtained from the video track T1 that the video head H is trying to trace (the signal obtained from the part A in FIG. The level of the FM audio signal (referred to as FM audio signal) is a,
Second FM obtained from adjacent video track T2
Audio signal (signal obtained from part B in Figure 3,
Hereinafter, this is referred to as the interfering FM audio signal. ) level b
If the difference frequency between the desired FM audio signal and the interfering FM audio signal is △ω, then it is expressed as D(t)∝b/a△ω(cos△ωt) (1). Here t represents time. That is, the adjacent interference noise D(t) is output as a sine wave of the difference frequency △ω (bead frequency) between the desired FM audio signal and the interfering FM audio signal, and its amplitude is equal to that of the interfering FM audio signal.
Amplitude ratio b/ of FM audio signal and desired FM audio signal
It is considered that it is proportional to a and the difference frequency Δω. Therefore, as an effective method for reducing the adjacent damage prevention in the VTR described above, during recording, the amplitude is compressed according to the dynamic range of the audio signal or a specific band component of the audio signal, and the effective amount of frequency deviation is Increase the interfering FM audio signal and the desired FM
After making the instantaneous difference frequency of the carrier wave with the audio signal almost outside the audible frequency band, it is FM modulated, frequency-superimposed with the video signal, and azimuthally recorded on a magnetic tape. During playback, it is conceivable to perform FM demodulation and then expand the compressed amplitude to restore it to its original value. This method uses the azimuth loss caused by azimuth recording to reduce the level ratio b/a between the interfering FM audio signal and the desired FM audio signal shown in equation (1) to some extent, although it is not sufficient due to the low FM audio carrier frequency. By effectively increasing the amount of frequency deviation of the audio signal, the difference frequency component △ω in equation (1) moves to the high range and outside the audible band, and is inversely transformed during playback. This, combined with the effect of reducing the noise level, suppresses adjacent interference noise to a practically sufficient level. Moreover, this method also has the following features: One, the video track width can be further narrowed by the adjacent interference low-frequency range, allowing for high-density recording, two, noise other than adjacent interference noise can also be reduced, and three, the frequency deviation of the actual audio signal can be reduced. Compared to the method of simply increasing the amount of frequency deviation, because the frequency band required for recording can be small, and the frequency band used when recording the above-mentioned FM modulated audio signal can be small. Therefore, since the recording wavelength of the luminance signal can be made longer, the diameter of the rotating cylinder can be made smaller, and the device can be made more compact. However, the effect of suppressing the interfering FM audio signal level by the azimuth loss used in the method for reducing adjacent interfering noise varies greatly depending on the recording track width, video head width, etc. for each VTR system. For example, as shown in Table 1, the FM audio carrier frequency _p = 1.3 MHz, the azimuth angle = 17 degrees, and the relative velocity V _h = 4.1 m/S,
The recording track width T _p and video head width T _w are each
T _p = 18.7 μm, T _w = 28.0 μm system and T _p =
With a system of 9.3 μm and T _w = 14 μm, the level ratio between the desired FM audio signal and the interfering FM audio signal (hereinafter abbreviated as D/U) is approximately
22dB, and about 17dB for the system, which is a big difference. Here, the recording density of the system is twice that of the system, and if the system is set to a so-called standard mode (hereinafter abbreviated as SP mode), the system will be placed in a so-called long-time mode (hereinafter abbreviated as LP mode). Equivalent to. Therefore, compared to the SP mode, the LP mode generates more adjacent interference noise, which is very unpleasant to the ear, by an amount corresponding to the difference in D/U.

[Table] Therefore, in LP mode, the variation characteristics of the dynamic range that uses the method of reducing adjacent interference noise, that is, the compression/expansion ratio, must be made larger than in SP mode. However, if the compression/expansion ratio is increased unnecessarily, the S/N ratio is improved, but the so-called "breathing" phenomenon in which the noise level fluctuates depending on the signal level, and the distortion rate deteriorates, which is not good for audibility. Therefore, from the perspective of sound quality, it is necessary to define an optimal compression/expansion ratio according to D/U. An object of the present invention is to reduce the adjacent interference in the audio FM superimposition method to a practically sufficient level without deteriorating the sound quality and without hindering the realization of high recording density and miniaturization of the mechanism and circuit. This provides a suppression circuit. The present invention provides a magnetic head device for reproducing frequency modulated audio signals recorded on a magnetic tape as an audio recording locus inclined at a predetermined angle with respect to the longitudinal direction of the magnetic tape, and A demodulation circuit that demodulates the frequency, an expansion circuit that expands the frequency-demodulated audio signal, and a de-emphasis circuit that de-emphasizes the expanded audio signal are provided, and two reproduction signals that are simultaneously reproduced from two adjacent recording trajectories are provided. The audio signal before frequency modulation is compressed in advance according to a predetermined compression characteristic so that the difference frequency of the instantaneous frequency has a frequency deviation exceeding the audible frequency. Each characteristic is set to have an elongation characteristic. Also,
According to a preferred embodiment of the present invention, the level ratio (D/U) of the desired FM audio signal and the interfering FM audio signal
By setting the compression/expansion ratio of the amplitude of the audio signal to be recorded to an optimal value according to the Reduces adjacent interference to a practically sufficient level,
Moreover, it is possible to simultaneously achieve miniaturization of the mechanical system and the circuit system. As mentioned above, Nozomi FM audio signal and interfering FM
In order to reduce adjacent interference noise, the compression/expansion ratio of the amplitude of the audio signal must be changed depending on the level ratio (D/U) with the audio signal. Here, the larger the compression/expansion ratio is, the more noise such as adjacent interference noise is suppressed and the S/N is improved, but the so-called "breathing" phenomenon in which the noise level changes depending on the signal level, distortion rate deterioration, etc. become more noticeable. This results in auditory discomfort. Therefore, it is necessary to define an optimal compression/expansion ratio according to the D/U, which does not cause deterioration in sound quality such as breath-breathing phenomenon and can reduce adjacent interference noise to a practically sufficient level. FIG. 4 shows experimental values showing how much D/U value the improvement effect due to compression/expansion corresponds to when the compression/expansion ratio is changed according to each D/U value. Here, the compression/expansion ratio is approximately in the range of 1:2 to 1:1. The reason for this is that when the compression/expansion ratio greatly exceeds 1:2, the so-called "breathing" phenomenon in which the above-mentioned noise changes depending on the signal level and distortion rate deterioration become significant, resulting in a significant deterioration of sound quality. Furthermore, if the D/U of the system is approximately 15 dB or less, it is difficult to reduce the adjacent interference noise to an audibly acceptable range using the adjacent interference noise reduction method described above.
It is said to be about 15dB or more. Applying a compression/expansion ratio of 1:2 to a system with a D/U of 17 dB results in an adjacent interference noise level equivalent to that of a system with a D/U of 26 dB, and applying a compression/expansion ratio of 2:3 results in an adjacent interference noise level equivalent to that of a system with a D/U of 21 dB. This indicates the level of interference noise. Additionally, experiments have shown that if D/U is about 22 dB or more, the level of adjacent interference noise is within the audibly permissible limit. Note that if high-quality audio such as stereo audio is required, a D/U of 26 dB or more is desirable. Therefore, once the D/U value required for sound quality is determined, the optimum compression/expansion ratio can be found from FIG. In other words, first find the value corresponding to the D/U of the system on the horizontal axis of Figure 4, then find the value equivalent to the D/U required for sound quality on the vertical axis of Figure 4, and find the intersection point of these. The value of the compression/expansion ratio straight line near to is approximately the optimum compression/expansion ratio to be sought.
For example, if the system D/U is 17 dB and the required D/U is 26 dB, there is a 1:2 compression/expansion ratio straight line near the intersection of these from Figure 4, so this 1:2 is optimal. This is the compression/expansion ratio. In addition,
In this example, a ratio of 1:2 was adopted, but 1:1.6,
A nearby value such as 1:2.4 may be used. DESCRIPTION OF THE PREFERRED EMBODIMENTS The speech noise suppression circuit of the present invention will be explained below with reference to embodiments shown in the drawings. Figure 5 shows a rotating head shape that can change recording density, or recording time, in two ways.
1 is a circuit configuration diagram showing an embodiment in which the present invention is applied to an audio recording circuit that records audio signals using an audio FM superimposition method in a VTR. FIG. 6 is a circuit diagram showing an embodiment of the audio reproduction circuit of the VTR to which the present invention is applied. Here, the specifications of the VTR whose recording density can be changed to two are the same as those listed in Table 1 above, and the practically required D/U is
Suppose it is 26dB. Therefore, D/U is about 22dB
Recording density equivalent to the system (SP mode)
According to Figure 4, the compression/expansion ratio is 2:3, and the D/U is approximately
When the recording density corresponds to a 17 dB system (LP mode), the compression/expansion ratio may be set to 1:2 as shown in FIG. In Fig. 5, the audio signal input from input terminal 1 passes through pre-emphasis circuit 2, and then passes through switch 3 to 2/3 compression circuit 6 with a compression ratio of 1.5 in SP mode, and to 2/3 compression circuit 6 with a compression ratio of 1.5 in LP mode. 3 to the 1/2 compression circuit 4 with a compression ratio of 2. Here, the switch 3 is controlled by an SP mode/LP mode switching signal inputted from an input terminal 28. In SP mode, the dynamic range of the pre-emphasized audio signal is reduced to 2/3 by the 2/3 compression circuit 6.
3, and in the LP mode, the dynamic range of the audio signal, which is also pre-emphasized by the 1/2 compression circuit 4, is compressed to 1/2. Here, the gains of the 2/3 compression circuit 6 and the 1/2 compression circuit 4 are controlled by the output signals of the detectors 5 and 7, each of which receives a pre-emphasized audio signal as input. The output signal of the 2/3 compression circuit 6 or the 1/2 compression circuit 4 is inputted to the FM modulator 9 through the switch 8 and subjected to FM modulation. Here, the switch 8 is the SP mode input from the input terminal 28, and the LP mode is input from the input terminal 28.
Controlled by mode switching signal. After removing unnecessary band components from the FM modulator output in a low-pass filter (hereinafter referred to as LPF) 10, it is added to the video signal input from the input terminal 30 in an adder 29.
Azimuth recording is performed on the magnetic tape 12 by the magnetic head 11. Next, in the audio reproduction circuit shown in FIG.
is input. The BPF 13 extracts only the FM audio signal from the reproduced signal. Further, the signal reproduced by the magnetic head 11 is also outputted from an output terminal 25 to a video signal reproduction circuit (not shown). extracted
The FM audio signal is demodulated into an audio signal by the FM demodulator 14. The demodulated audio signal is passed through LPF15
After FM carrier wave leakage and the like are removed in the hold circuit 16, noise caused by head switching is processed by holding the previous value. Here, the hold circuit 16 performs a previous value holding operation for a certain period of time using a control signal synchronized with the head switching signal inputted from the input terminal 26. The output signal of the hold circuit 16 is passed through a switch 17 to a 3/2 expansion circuit 18 with an expansion ratio of 1.5 in SP mode, and to a 2 times expansion circuit 2 with an expansion ratio of 2 in LP mode.
Input to 0. Here, the switch 17 is controlled by an SP mode/LP mode switching signal inputted from an input terminal 27. The hold circuit output signal is
The dynamic range is returned to its original state by the 3/2x expansion circuit 18 in the SP mode, and by the 2x expansion circuit 20 in the LP mode. Here, the 3/2-fold expansion circuit 18 and the 2-fold expansion circuit 20 are gain-controlled by the output signals of the detection circuits 19 and 31 which input the hold circuit output signal, and are based on the dynamic range of the demodulated audio signal. Extends to a range of The expanded signal passes through the switch 22 to the de-emphasis circuit 2.
After being de-emphasized at 3, the output terminal 24
It is output from Here, the switch 22 is interlocked with the switch 17. The reproduced audio signal expanded by the expansion circuits 18 and 20 undergoes the same expansion operation with respect to its noise level, and has a low noise level, so that it is output as an audio signal with suppressed adjacent interference noise. That is, for example, if the FM modulation 9 operates so that a frequency deviation of ±100 KHz occurs when the audio input signal is 0 dB, and the pre-emphasized audio input signal is -20 dB, then this -20 dB audio input signal If the signal is not compressed by the compression circuits 4 and 6 and is FM-modulated by the FM modulator 9, a frequency shift of ±10 KHz (=±100×10−20/20 KHz) occurs. When this FM modulation signal is recorded as adjacent video tracks T1 and T2 and simultaneously played back by video head H, video track T
The difference frequency between the instantaneous frequencies of the two reproduced signals read from T1 and T2 is in the range of 0 to 20 KHz, and all adjacent interference noise is within the audible frequency band of 20 KHz or less. However, if the –20dB audio input signal is 1/
2 Compression circuit 4 (in LP mode) compresses the signal to -10 dB and FM modulates it, resulting in a frequency deviation of ±31.5 KHz. (difference frequency) will be distributed in the range from 0 to 63 KHz, and most of the adjacent interference noise can be at frequencies outside the audible frequency band of 20 KHz or higher. In other words, the adjacent interference noise distributed from 0 to 20KHz is spread in frequency to the adjacent interference noise distributed from 0 to 63KHz, so the noise energy within the audible frequency band is reduced and the adjacent interference noise is almost eliminated. becomes undetectable. In this case, if the difference frequency changes sinusoidally, approximately 80% of the total period will be equal to or higher than the audible frequency. Expressing this in general terms, the input signal is
For the FM modulator 9 whose frequency deviation is ±θKHz at 0dB, the upper limit input level at which all frequency deviations are within ±10KHz (difference frequency is 20KHz or less) is 20log10/θdB. , when this upper limit input level is compressed to 10log10/θdB by the 1/2 compression circuit 4 and input to the FM modulator 9, the frequency deviation is within ±θ×10 ^1/2log10/ 〓KHz (that is, the difference frequency Then, it becomes 2×θ×10 ^1/2log10/ 〓KHz or less). Therefore, even if the input signal level is 20log10/θdB, where all the difference frequencies are within the audible frequency band when they are not compressed, when they are compressed and input to the FM modulator 9, the signals from adjacent video tracks T1 and T2 are simultaneously transmitted. A frequency shift occurs in the FM modulation signal such that the difference frequency between the instantaneous frequencies of the two reproduced signals becomes an audible frequency of 20 KHz or more, and adjacent interference is reduced. Needless to say, this output audio signal has been compressed and expanded at the optimal compression/expansion ratio, so there is little deterioration in sound quality such as the so-called breathing phenomenon in which the noise level changes depending on the signal level, and distortion rate deterioration. be. In the embodiments shown in FIGS. 5 and 6, a switch circuit is used to change the compression/expansion ratio for each of two recording densities, but the configuration becomes a little complicated. Therefore, it is also possible to simplify the two compression/expansion ratios in the embodiment to only one, which is 1:2 in the LP mode. In this case, the reproduced audio signal in the SP mode is improved in terms of S/N, but the above-mentioned breathing phenomenon and distortion rate are slightly degraded. However, these amounts of deterioration are within a sufficiently tolerable range. Figures 7 and 8 show the compression/expansion ratio of the above VTR.
An example of the circuit configuration of the audio recording and reproducing circuit when simplified to only 1:2 in LP mode is shown. Here, circuits in FIGS. 7 and 8 that are the same as those in FIGS. 5 and 6 are given the same numbers. The operation in Figure 7 is in SP mode and LP mode.
In either mode, an audio signal input from an input terminal 1 passes through a pre-emphasis circuit 2, and then a 1/2 compression circuit 4 compresses the dynamic range of the pre-emphasized audio signal to 1/2. Here, the gain of the 1/2 compression circuit is controlled by the output signal of the detector 5 which inputs the pre-emphasized audio signal, and the dynamic range is compressed by 1/2. The output signal of the 1/2 compression circuit 4 is FM modulated by the FM modulator 9. FM modulator output is LPF1
After removing unnecessary band components at 0, adder 29
The signal is added to the video signal input from the input terminal 30, and is azimuthally recorded on the magnetic tape 12 by the magnetic head 11. In the circuit shown in FIG. 8, the signal reproduced by the magnetic head 11 from the magnetic tape 12 is input to the BPF 13, and only the FM audio signal is extracted. Further, the signal reproduced by the magnetic head 11 is also outputted from an output terminal 25 to a video signal reproduction circuit (not shown).
The extracted FM audio signal is demodulated into an audio signal by the FM demodulator 14. The demodulated audio signal is
After the leakage of the FM carrier wave is removed by the LPF 15, the noise caused by head switching is processed by the hold circuit 16 by holding the previous value. The hold circuit 16 operates based on a control signal input from an input terminal 26. The output signal of the hold circuit 16 has its dynamic range doubled by a doubling expansion circuit 20, and is returned to its original range. Here, the gain of the double expansion circuit 20 is controlled by the output signal of the detector 31. The output signal of the double expansion circuit 20 passes through the de-emphasis circuit 23 and is then outputted from the output terminal 24. Further, in the recording circuit of the embodiment, the dynamic range of the pre-emphasized audio signal is compressed so that overmodulation is less likely to occur. Further, although compression and expansion are performed according to the amplitude of the entire band of the audio signal, the amplitude of a specific band component of the audio signal may be used. As explained above, by using the present invention, as shown below: 1. The optimum compression/expansion ratio can be easily defined according to the D/U of each system. 2 The compression/expansion ratio is determined for each system, so
While minimizing the so-called breathing phenomenon in which the noise level changes depending on the signal level and distortion rate deterioration, adjacent interference noise can be reduced to a practically sufficient level with a simple circuit. 3. Noise other than adjacent interference noise can be reduced at the same time. 4. Since the required frequency bandwidth is small, the diameter of the rotating cylinder may be small. It has many features such as these, and is highly effective in downsizing VTRs and reducing adjacent interference noise in audio FM superimposition.

[Brief explanation of drawings]

Figures 1 and 2 are frequency spectrum diagrams showing examples of signal frequency spectra in the audio FM superimposition method, Figure 3 is a plan view of a magnetic tape to explain adjacent interference, and Figure 4 is a D/U of the system. Characteristic diagrams, Figures 5 and 6, showing the relationship between and the apparent D/U of the system improved by the compression/expansion effect.
7 and 8 are circuit diagrams showing an embodiment of an audio recording/reproducing circuit using the present invention. 4, 6... compression circuit, 18, 20... expansion circuit,
5, 7, 19, 31...detector, 3, 8, 17, 2
2...Switch.

Claims (1)

[Scope of Claims] 1. A magnetic head device that reproduces a frequency-modulated audio signal recorded on a magnetic tape as an audio recording trajectory inclined at a predetermined angle with respect to the longitudinal direction of the magnetic tape, and a frequency-modulated audio signal reproduced by the magnetic head device. It consists of a demodulation circuit that demodulates the frequency of the audio signal, an expansion circuit that expands the frequency-demodulated audio signal, and a de-emphasis circuit that de-emphasizes the expanded audio signal. The audio signal before frequency modulation is compressed in advance according to a predetermined compression characteristic so that the difference frequency of the instantaneous frequency of the reproduced signal has a frequency deviation exceeding the audible frequency, and the expansion circuit is compressed according to this compression characteristic. A speech noise suppression circuit characterized by having reverse expansion characteristics. 2. The speech noise suppression circuit according to claim 1, wherein the ratio of the compression and expansion characteristics is 2. 3. The speech noise suppression circuit according to claim 1, wherein the ratio of the compression and expansion characteristics is 1.5. 4. A magnetic head device for reproducing a frequency modulated audio signal recorded on a magnetic tape as an audio recording trajectory inclined at a predetermined angle with respect to the longitudinal direction of the magnetic tape, and a frequency demodulating device for frequency demodulating the frequency modulated audio signal reproduced by the magnetic head device. A voice noise comprising a demodulation circuit, an expansion circuit that expands a frequency demodulated audio signal, and a de-emphasis circuit that de-emphasizes the expanded audio signal, and the expansion circuit has an expansion characteristic ratio of 2. suppression circuit.