JPS6351603B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention is a noise reduction device for a tape recorder, which detects the output level of a variable filter circuit and sets a DC voltage to control the frequency characteristics of the variable filter circuit. The present invention relates to a detection circuit for a noise reduction device.

[Technical background of the invention]

As is well known, various noise reduction (hereinafter referred to as NR) devices have become widely used in the field of tape recorders, and in cassette tape recorders in particular, it has become possible to sufficiently reduce the hiss noise present in the tape itself. As a result, good sound reproduction is now possible.

Figure 1 shows the basic configuration of the currently most widely used Dolby (trademark) NR device B type 11, which combines an encoder that enables NR recording and a decoder that enables NR playback. . First, when operating as an encoder, connect recording/playback switch S ₁ to the R (record) side,
An audio signal from an audio amplifier (not shown) or the like is supplied to the input terminal IN. After this audio signal is amplified by an inverting amplifier 12, it is supplied to a differential amplifier 14 via a main signal path 13, and a sub signal path 15.
is supplied to the variable filter 16. This variable filter 16 extracts high-frequency components of the input signal, and its output signal is supplied to a variable filter control circuit 18 via a buffer amplifier 17. The variable filter control circuit 18 is composed of an inverting amplifier 19 and a detection circuit 20, and detects the output level of the variable filter 16 by the rectification circuit 21 and integration circuit 22 of the detection circuit 20, Convert to voltage. This DC voltage is then supplied to the control input terminal of the variable filter 16 to change the variable impedance inside the variable filter 16 and control the cutoff frequency to be moved in accordance with the output level of the variable filter 16.

That is, the variable filter 16 passes only low-level high-frequency components of the audio signal on the sub-signal path 15, and the audio signal is supplied to the differential amplifier 14 via the buffer amplifier 17 and limiter circuit 23. Then, the differential amplifier 14 adds the signal on the main signal path 13 and the signal on the sub signal path 15, which has only low-level high-frequency components, and encodes it into an audio signal with enhanced high-frequency components. The signal is supplied from the output terminal OUT to a recording amplifier of a tape recorder (not shown), making NR recording possible.

On the other hand, when operating the B type NR device 11 as a decoder, the recording selector switch _S1 is set to P.
(play) side, and supplies a playback signal from a playback amplifier of a tape recorder (not shown) to the input terminal IN. This reproduced signal is amplified by an inverting amplifier 12 and then supplied to a differential amplifier 14 via a main signal path 13. The output signal of the differential amplifier 14 is supplied to the differential amplifier 14 through the sub-signal path 15 as a signal containing only low-level high-frequency components, as in the case of the encoder described above. The signal on the auxiliary signal path 15 is processed by the differential amplifier 14 so that it has an opposite phase to the signal on the main signal path 15. When the differential amplifier 14 adds both signals, the high-frequency component is subtracted from the reproduced signal. Recorder playback signal is decoded and output terminal
This is what is output to OUT.

FIG. 2 shows a specific configuration of the sub-signal path 15 of the B-type NR device 11.
The input signal of 5 is the capacitor 2 of the variable filter 16.
4, 25 and a resistor 2 that controls the cutoff frequency.
6 and resistor 2 that constitutes a variable impedance circuit.
7. High frequency components are extracted by FET 28 and power supply 29. Then, the signal is attenuated by a time constant controlled by the ratio of the impedance of the resistor 26 and the variable impedance circuit and is output. This low-level high-frequency signal is supplied to a rectifier circuit 21 of a detection circuit 20 via a buffer amplifier 17 and an inverting amplifier 19. This rectifier circuit 21 performs half-wave rectification with a germanium diode 30 and smoothes it with a capacitor 31, converts the input signal into a DC voltage, and converts the input signal into a DC voltage.
Supply to 2. This integrating circuit 22 is a nonlinear integrating circuit composed of a silicon diode 32, a resistor 33, and a capacitor 34. That is, when the input level is low, the silicon diode 32 is turned off, and the charging speed created by the resistor 33 and capacitor 34, that is, the time constant, slows down the response to DC voltage changes, and when the input level is high, the silicon diode 32 turns off. The diode 32 is turned on to speed up the response. The charging voltage of the capacitor 34 is determined by the FET 28 of the variable filter 16.
It is applied to the gate G to control the variable impedance and the amount of attenuation.

The output signal of the variable filter 16 controlled in this manner is supplied to the differential amplifier 14 via the buffer amplifier 17 and the limiter circuit 23, and is added to the signal on the main signal path 13.

Recently, a C-type NR device, which is a development of the B-type NR device 11 and further enhances the NR effect, has begun to be widely used. As is well known, this C-type NR device has two stages of B-type NR devices 11 connected in series, with the first stage serving as a high-level stage for encoding or decoding high-frequency signals, and the latter stage serving as a low-level stage for encoding or decoding mid-high frequency signals. The operating principle is the same as that of the B-type NR device 11, so the explanation thereof will be omitted.

Therefore, by turning off the low level stage of this C type NR device and switching the constants that create the encoding or decoding characteristics of the high level stage, it can be used as a B type NR device 11 very easily.

Currently, this type of NR equipment is integrated into ICs, but
In order to make it possible to switch with the B-type NR device 11 that is already in widespread use, the element that creates the encoding or decoding characteristics of the high-level stage is attached externally to the IC, as described above, and a switching element such as a transistor is used. Each constant is switched by

Figure 3 shows a configuration in which each element is externally attached to the high-level stage detection circuit section of a C-type NR device integrated into an IC. Inside the IC, it is connected to the amplifier output terminal of a variable filter control circuit (not shown). germanium diode 36 of the rectifier circuit 35
is connected to the silicon diode 38 of the integrating circuit 37 together with the IC external terminal a. This silicon diode 38 is connected together with an external terminal b to a variable impedance circuit of a variable filter (not shown). A capacitor 39 is connected to external terminal a of such an IC, a capacitor 40 is connected to external terminal b, and terminals a and b are connected via a resistor 41. In this state, it operates as a B type detection circuit, but
Furthermore, in order to enable switching with the C type detection circuit, switching elements 42 and 43 inside the IC are used.
are connected to capacitor 4 via external terminals c and d, respectively.
4, 45, switching elements 42, 43
By closing the capacitors 44 and 45, the capacitors 44 and 45 are connected in parallel to the capacitors 39 and 40, respectively.

That is, the time constant of the detection circuit is changed by switching the number of capacitors, and the B type and C type NR device detection circuits can be easily switched by opening and closing the switching element.

[Problems with background technology]

However, the above method of changing the time constant by switching the number of capacitors requires a large number of external IC elements, which increases the space taken up by the NR device inside the tape recorder. Also, IC
If switching is performed internally, the number of IC terminals must be increased, which is not preferable.

[Purpose of the invention]

This invention improves the above drawbacks, reduces the number of external elements of the NR device IC, further reduces the number of IC terminals, and provides a detection circuit for a noise reduction device that can switch the time constant inside the IC. It provides:

[Summary of the invention]

That is, the detection circuit of the noise reduction device in this invention uses a two-stage differential amplifier, the first stage is for small signals, the second stage is for large signals, and the current corresponding to the input signal is connected to an external capacitor. The capacitor charging current source and resistor supplied to each differential amplifier in this detection circuit are
The time constant is changed by changing the amount of current flowing to the capacitor by switching a switch inside the IC.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 4 is an equivalent circuit diagram showing the configuration,
Inside the NR device IC, an amplifier output terminal of a variable filter control circuit (not shown) is connected to the base of a buffer transistor _Q1 at a detection circuit input terminal 46 to amplify the low-level high-frequency signal output from the variable filter. The collector of this transistor Q ₁ is connected to the +V _CC power supply, the emitter is connected to the base of the transistor Q ₂ which is the input terminal of the first stage differential amplifier circuit 47, and the second stage differential amplifier circuit 47 is connected via the resistor R ₁ . It is connected to the base of the transistor _Q3 , which is the input terminal of the amplifier circuit 48, and is also connected to the constant current source _I1 that operates the transistor _Q1 .

That is, the offset voltage ΔV (=
I ₁ · R ₁ ), and as long as the emitter output signal of transistor Q ₁ is small, the first stage differential amplifier circuit 47
When the output signal becomes large and exceeds the offset voltage ΔV, the second stage differential amplifier circuit 48 is operated via the resistor _R1 .

In the first stage differential amplifier circuit 47, the emitter of the transistor Q ₄ and the diode D ₁ are connected to the +V _CC power supply.
are connected to form a current mirror circuit. The output end of this current mirror circuit is connected to each collector of transistors Q ₂ and Q ₆ , and the currents flowing through each transistor Q ₂ and Q ₆ are controlled so that they are always equal. This transistor Q ₂ ,
The emitters of Q ₆ are commonly connected and connected to constant current source I ₂ and I ₃ which are connected in parallel in accordance with the operation of switching element S ₁₀ to form a differential circuit. The switching element _S10 switches between the B type and C type NR device, and the constant current sources _I2 , _I3, etc. constitute a first variable constant current source.

Transistor Q ₂ which becomes the output end of the above differential circuit
The collector of is connected to the base of transistor Q ₅ ,
Transistor Q ₅ corresponding to the output signal of the differential circuit
control the collector current of this transistor
Q ₅ protects transistor Q ₅ by connecting diode D ₂ between the emitter and base, the emitter is connected to +V _CC power supply, and the collector is connected to the transistor Q 5.
It is connected to the base of Q ₆ to control the differential circuit, and supplies charging current to the external capacitor C ₁ via the external terminal 49.

The second stage differential amplifier circuit 48 has almost the same configuration as the first stage differential amplifier circuit 47, and includes a transistor Q ₆ and a diode D ₃ forming a current mirror circuit.
The +V _CC power supply is connected to the transistor of the differential circuit by
Supply to Q ₃ and Q ₈ . The emitters of these transistors Q ₃ and Q ₈ are connected to a resistor R ₂ and a switching element S ₁₁
It is connected to the resistor R ₃ connected in parallel corresponding to the operation of , forming a differential circuit, and this resistor R ₂ ,
_R3 constitutes a second variable constant current source.
That is, the resistors R ₂ and R ₃ are of high resistance, and a constant current source is used instead. Further, the switching element _S11 is used to switch between the B type and C type NR device.

The above differential circuit has a transistor _Q3 at the output end.
The output signal of the differential circuit is supplied from the collector of the output transistor _Q7 . This transistor Q ₇
is connected between the base and emitter via a protection diode _D4 , and +V _CC is connected in response to the supplied signal.
A charging current is passed through the capacitor _C1 and outputted to the transistor _Q8 to control the differential circuit. Incidentally, the capacitor _C1 supplies its charging voltage to a variable filter circuit (not shown) via an external terminal 49. Further, a switching element _S12 and a constant current source _I4 connected in parallel to the capacitor _C1 are used to switch the discharge time constant of the capacitor _C1 .

The NR device detection circuit configured as above is
When used as a NR device B type, switching elements S ₁₀ , S ₁₁ , and S ₁₂ inside the IC, which are interlocked as NR device external changeover switches, are set to the OFF state. When the low level high frequency signal of the variable filter output signal is supplied from the amplifier to the transistor _Q1 , the first stage differential amplifier circuit 47 operates until the amplitude of the input signal exceeds the offset voltage ΔV.
When the amplitude of the signal exceeds the offset voltage ΔV, the second stage differential amplifier circuit also begins to function. This function corresponds to that of the silicon diode 38 and resistor 41 shown in FIG.

In the first stage differential amplifier circuit 47, the transistors Q ₂ and Q ₆ of the differential circuit are operated by the constant current source I ₂ , so the maximum output current of the differential circuit is I ₂ /2 due to the current mirror circuit. Therefore, the maximum base current of the output transistor Q ₅ is I ₂ /2, and the current is controlled within a range up to the corresponding collector current to charge the capacitor C ₁ .

On the other hand, the potential at point B in the figure becomes equal to the potential at point A due to the action of the differential circuit, but when the potential at point A decreases, the potential at point B does not decrease due to the charging voltage V _C of capacitor _C1 . . That is, while the potential at point A is decreasing, the output current of the differential circuit remains constant due to the base voltage V _C of the capacitor _Q6 . This operation corresponds to the operation of the rectifier circuit 35 in FIG.

The second stage differential amplifier circuit 48 controls the transistor Q ₇ within the maximum output current range of the differential circuit determined by the resistor R ₂ in the same way as the differential amplifier circuit 47 described above.
The collector current of this transistor Q ₇ is passed through the capacitor C ₁ together with the output of the first stage differential amplifier circuit 47.
Supplies charging current to. Therefore, the charging speed increases rapidly, shortening the rise time, ie, the time constant, of the charging voltage V _C of the capacitor C ₁ . This function corresponds to the operation of the integrating circuit 37 shown in FIG.

That is, the peak voltage V _IN1 as shown in FIG.
When a tone burst wave (≦ΔV) is input, only the first stage differential amplifier circuit 47 operates, and its output voltage V _OUT1 corresponds to the period and amplitude of the tone burst wave as shown in Figure B. The value increases step by step and finally corresponds to the input peak voltage V _IN1
V _OUT1 . The peak voltage since this input is
The time taken to reach V _OUT1 is the time constant t ₁ . Similarly, the peak voltage V _IN2 as shown in FIG. 6A
When a tone burst wave of (≧ΔV) is input, the second stage differential amplifier circuit 48 also starts operating.
As the charging current increases, its output voltage V _OUT2 changes at a rising speed of time constant t ₁ ', as shown in diagram B. Furthermore, when the input signal shown as A in Figs. 5 and 6 ceases to change, each output signal shown in Fig. B discharges with a time constant T ₁ or T ₁ ' determined by the impedance of the next stage variable filter. I will do it.

When the above-mentioned detection circuit is used as a type C NR device, switching elements S ₁₀ , S ₁₁ , and S ₁₂ are turned on. At this time, the constant current source of the first stage differential amplifier circuit 47 becomes I ₂ +I ₃ and the maximum output current becomes I ₂ +I ₃ /2. The speed at which the output voltage changes when the input voltage of the differential amplifier circuit 47 changes is proportional to the current value of the constant current source, so the voltage rise time of the capacitor _C1 , that is, the time constant, is the constant current source _I2. It will be shorter than when it is alone. This also applies to the second stage differential amplifier circuit 48, and the capacitor 4 in FIG.
4,45. C in FIGS. 5 and 6 indicates the waveforms of the output voltages V _OUT1 and V _OUT2 when the tone burst waves indicated by A are input to the detection circuit of the C type NR device, respectively. The rise time constants t ₂ and t ₂ ′ of are
t ₂ <t ₁ , t ₂ ′<t ₁ ′. Also, the discharge time constant T ₂ and
T ₂ ′ is determined by constant current source I ₄ .

Therefore, the number of external terminals and external capacitors can be reduced without impairing the characteristics of the conventional detection circuit.

Figure 7 shows an equivalent circuit of the NR device detection circuit in which the collector current of the output transistor is varied by a constant current source based on the _above embodiment. The emitter current is set based on the offset voltage ΔV generated by the constant current source I ₅ and the resistor R _4. When the amplitude of the input signal is small, the first stage differential amplifier circuit 50 is operated, and when the amplitude exceeds ΔV, the second stage differential amplifier circuit 50 is operated. dynamic amplifier circuit 5
1 is also starting to work.

The first stage differential amplifier circuit 50 is a transistor
Q ₁₀ and diode D ₅ form a current mirror circuit, and the same amount of current is supplied from the +V _CC power supply to the collectors of transistors Q ₁₁ and Q ₁₂ that form the differential circuit. The maximum value of this current amount is determined by the constant current source _I6 connected to the common emitters of the transistors _Q11 and _Q12 . The collector of transistor Q ₁₁ , which is the output end of the differential circuit, is connected to the bases of output transistors Q ₁₃ and Q ₁₄ .
These transistors Q ₁₃ and Q ₁₄ have resistors R ₅ and
It is connected to the +V _CC power supply via R ₆ , and supplies a current corresponding to the output change of the differential circuit to the external capacitor C ₂ connected to the external terminal 52 . The emitter of the above transistor _Q14 is connected to the constant current source _I7 via the switching element _S13 , and the switching element
By turning on _S13 , the transistor
_Q14 is cut off and the charging current is reduced. Note that the resistor _R7 and diode connected between the +V _CC power supply and the base of transistor _Q13
D6 is for temperature compensation of transistors _Q13 and _Q14 . The second stage amplifier circuit 51 has almost the same configuration as the first stage differential amplifier circuit 50, and has a transistor
Q ₁₅ and diode D ₇ form a current mirror circuit, and the same current is supplied from the +V _CC power supply to each transistor Q ₁₆ and Q ₁₇ forming a differential circuit. The maximum value of this supply current is determined by resistor _R8 . The collector of transistor Q ₁₆ , which is the output end of the differential circuit, is connected to the bases of output transistors Q ₁₈ and Q ₁₉ . These transistors Q ₁₈ and Q ₁₉ are each a resistor.
Connect to the +V _CC power supply via R ₉ and R ₁₀ , and connect the capacitor with the output current of the first stage differential amplifier circuit 50.
The charging current is supplied to _C2 . The emitter of the output transistor _Q19 is connected to a constant current source _I8 via a switching element _S14 .
Note that resistor _R11 and diode _D8 are transistors.
For temperature compensation of Q ₁₈ and Q ₁₉ , and also for capacitor
Switching element S ₁₅ connected in parallel to C ₂
and constant current source _I9 is for switching the discharge time constant of capacitor _C2 .

The detection circuit configured as above is NR device B.
When used as a type, the IC internal switching elements S ₁₃ , S ₁₄ , which work with the NR device changeover switch.
Turn off S ₁₅ . Then, the low-level high-frequency signal of the variable filter output signal is transferred from the amplifier to the transistor.
When supplied to Q ₉ , the first stage differential amplifier circuit 50 operates as long as the amplitude of the input signal is smaller than the offset voltage ΔV (=I ₅ · R ₄ ) at point A, and the amplitude of the signal exceeds the offset voltage ΔV. When the voltage exceeds the threshold, the second stage amplifier circuit 51 also comes into operation.

In each of the differential amplifier circuits 50 and 51, the base current of the output transistors Q ₁₃ , Q ₁₄ or Q ₁₈ , Q ₁₉ is adjusted by the respective differential circuits at a rising rate of change, that is, a time constant, which is determined by the amplitude and frequency of the input signal. Raise it step by step. Accordingly, the charging current to capacitor _C2 will change.

In addition, when this detection circuit is used as an NR device C type, switching elements S ₁₃ to S ₁₄ are turned on to reduce the supply current to output transistors Q ₁₄ and Q ₁₉ to reduce the charging current, and S ₁₅ is also turned on. Just turn it on and shorten the discharge time constant.

Therefore, the number of external terminals and external capacitors can be reduced without impairing the characteristics equivalent to those of conventional detection circuits.

〔Effect of the invention〕

As described above, according to the present invention, an IC for NR device
By reducing the number of external elements and the number of IC pins,
In addition, since the time constant can be switched within the IC, manufacturing of the NR device becomes easy and costs can be reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the basic configuration of a B-type Dolby NR device, FIG. 2 is a diagram explaining the sub-signal path of the NR device, and FIG. 3 is a diagram explaining the detection circuit of a conventional NR device. FIG. 4 is a diagram illustrating a detection circuit of an NR device according to an embodiment of the present invention, and FIG.
6 and 7 are diagrams explaining the relationship between the input signal and the output signal of the NR device detection circuit of the above embodiment, and FIG.
The figure is a diagram illustrating another embodiment according to the present invention. 16...Variable filter, 20...Detection circuit, 2
1, 35... Rectifier circuit, 22, 37... Integrating circuit, 46,... Detection circuit input terminal, 47, 50...
First stage differential amplifier circuit, 48, 51... Second stage differential amplifier circuit, 49, 52... External terminal, C ₁ ~
C ₂ ... External capacitor, I ₁ to I ₉ ... Constant current source, S ₁₀
~S ₁₅ ...Switching element.

Claims (1)

[Claims] 1 A means for setting an offset voltage for an input signal, a first differential amplifier circuit that outputs a rectified signal corresponding to the input signal, and a rectified output circuit that outputs a rectified signal only for an input signal having a peak voltage equal to or higher than the offset voltage. a second differential amplifier circuit that changes the output current of each of the first and second differential amplifier circuits, a variable constant current source attached to each differential amplifier circuit that changes the output current of each of the first and second differential amplifier circuits, and a supply of the output current. and a capacitor connected to a terminal connected to the differential amplifier circuit and whose charging and discharging are controlled by the output current of each differential amplifier circuit, and by changing the value of the variable constant current source, the charging time constant of the capacitor is changed. A detection circuit for a noise reduction device characterized by: