JPS6215847Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an audio circuit that changes the operation of AGC according to an input signal to obtain frequency characteristics according to the input source.

In general, video tape recorders, for example, have the disadvantage that reproduction frequency characteristics deteriorate when high-density recording is performed, and it is possible to improve the deterioration by reducing the bias value of the magnetic head, but the input amplifier section of the audio circuit It is known that the distortion rate of the maximum level at which the AGC operates, that is, the reference output level, changes.

By the way, the AGC in the input amplification section operates with fast attack/quick recovery for pulsed audio signals that require clarity, such as conversations, and relatively smooth sources such as music. For this purpose, it is desirable that the AGC in the input amplifier section operates with fast attack/slow recovery. The input amplifying section of an audio circuit such as a conventional video tape recorder is configured as shown in Fig. 1. In the figure, Aa is an input terminal Ia, and a first input resistor R1 is connected to an input amplifier for conversation or music. The amplifier receives an input signal based on the input source from the input terminal, and outputs an output signal from the output terminal Oa. D1 is the first diode for rectification whose anode is connected to the output terminal of the amplifier Aa, and C
1 is a first smoothing capacitor C provided between the card of the first rectifying diode D1 and the ground point.
2, R2 is a second smoothing capacitor and a second current limiting resistor connected in parallel to the first capacitor C1, and R3 is connected in parallel to the series circuit of the second smoothing capacitor C2 and the second current limiting resistor R2. The third bias resistor Q1 provided is an NPN type first AGC transistor, whose collector is connected to the input terminal of the amplifier Aa, whose base is connected to the cathode of the first rectifier diode D1, and whose emitter is grounded. has been done.

The first transistor Q1 for the switch, the first diode D1 for rectification, the second resistor for current limiting, the third resistor R2, R3 for bias, the first transistor for smoothing, and the second resistor for smoothing.
Capacitors C1 and C2 operate as AGC for amplifier Aa, and the reference input is input to amplifier Aa via input terminal Ia and first input resistor R1, and output terminal Oa.
The output waveform of is T0 from time t0 to time t1 in Figure 2.
As shown in the figure, when the waveform has a constant amplitude, a pulse wave signal that is 20 dB larger than the reference input is input during the short T1 time from t1 to t2, for example, by hitting or dropping the microphone. Then, due to the first capacitor C1 and the third resistor R3, the output waveform of the output terminal Oa reaches the maximum amplitude at time t1, as shown at time T1 in the figure, and then the amplitude reaches a predetermined level with a fast attack. The reference input is then input to the amplifier again at time T2 from time t2 to time t3.
When inputting to Aa, as shown at T2 time in the same figure,
The level temporarily drops slightly from the reference input, and since the second capacitor C2 is hardly charged by the pulse wave signal at this time, the level returns to a level slightly higher than the reference input by quick recovery. And t3
For example, the relatively long T3 time from time to time t4.
When a continuous high-level source signal, that is, a continuous signal 20 dB higher than the reference input, is input, the amplitude reaches the maximum at time t3, and then the amplitude reaches the specified level with a relatively fast attack, as shown at time T3 in the same figure. , and from time t4, the reference input is applied to the amplification section again.
When inputting to Aa, the level drops below the reference input at time t4, and then the large-capacity second smoothing capacitor C2 is sufficiently charged at time T3, resulting in slow recovery as shown after time t4 in the same figure. begins to return to the specified level.

In other words, the audio circuit shown in Figure 1 provides quick recovery for pulsed input signals, such as those generated when a microphone is tapped or dropped, and provides quick recovery for continuous high-level input signals. It will be a slow recovery. Therefore, when a source signal such as a music source containing an input pulse signal with a level sufficiently higher than the level of the reference input is input to the amplifier Aa, for example, as shown in FIG. 3a, the output signal of the amplifier Aa As shown in figure b, the level difference _L0 between the reference output level based on the reference input level and the output pulse signal level based on the input pulse signal is the same as the reference input level and input pulse in figure a. It is smaller than the difference L _i with the signal level.

This is because the second capacitor C2 has a large capacity, so when the pulse component is input, the second capacitor C2 is not sufficiently charged and slow recovery does not occur as described above. , for input signals based on input sources such as music sources, due to their pulse components,
The AGC operating level changes, and the dynamic range, frequency characteristics, and distortion rate deteriorate.

With the above points in mind, this invention, for example,
For input signals that emphasize dynamic range and frequency characteristics, such as music sources, the AGC always operates with slow recovery. This will be explained in detail with reference to the drawings below.

In these figures, Ab is an amplifier to which an input signal is input via an input terminal Ib and a fourth input resistor R4, and an output signal from an output terminal is output via an output terminal Ob. D2 is a second rectifying diode whose cathode is connected to the output terminal of the amplifier Ab and whose anode is grounded; D3 is a third rectifying diode whose anode is connected to the cathode of the second rectifying diode D2; A rectifying section consisting of third diodes D2 and D3, and an amplifier
Rectify the Ab output signal. C3 is a third smoothing capacitor having one end connected to the cathode of the third rectifying diode D3 and the other end grounded, and is set to have a relatively small capacitance, forming a quick recovery capacitor. R4' is the fourth bias resistor connected in parallel to the third smoothing capacitor C3;
is a smoothing section consisting of a third smoothing capacitor C3 and a fourth bias resistor R4', which smoothes the output of the rectifying section 1. D4 and D5 are a fourth backflow prevention diode and a fifth backflow prevention diode connected in series,
The anode of the fourth backflow prevention diode D4 is connected to one end of the third smoothing capacitor C3, and the cathode of the fourth backflow prevention diode D4 and the anode of the fifth backflow prevention diode D5 are connected. Q2 is an NPN type second transistor for switching forming a switching element, the collector is connected to the power supply terminal +B, and the base is the fifth transistor for input.
A fourth diode D4 for preventing backflow via a resistor R5.
connected to the anode of the C4 is a fourth smoothing capacitor having one end connected to the emitter of the second switching transistor Q2 and the other end grounded, and is set to have a relatively large capacity, forming a slow recovery capacitor. 3 is a slow recovery time constant section consisting of a second switching transistor Q2 and a fourth smoothing capacitor C4, and S1 has one end connected to the emitter of the second transistor Q2.
The AGC time constant changeover switch D6 is a sixth backflow prevention diode whose anode is connected to the other end of the capacity changeover switch S1, and whose cathode is connected to the cathode of the fifth backflow prevention diode D5. Q3 is the third transistor for NPN type AGC, and its collector is connected to the input terminal of amplifier Ab.
The base is connected to the cathode of the sixth diode D6, and the emitter is grounded.

Q4 is a fourth output transistor of NPN type, and a fifth output capacitor C5 is connected to the collector. R6 is the fourth output transistor Q4
The sixth resistor for biasing has one end grounded to the emitter of Q4 and the other end grounded, and L1 is the first resistor for resonance whose one end is connected to the emitter of the fourth output transistor Q4.
The coil C6 is a sixth resonance capacitor with one end connected to the other end of the first resonance coil L1 and the other end grounded, and S2 is an equalizer changeover switch whose one end is connected to the other end of the first resonance coil L1. and
Interlocks with AGC time constant changeover switch S1. C
7 is a seventh resonance capacitor with one end connected to the other end of the equalizer switching switch S2 and the other end grounded; 4 is a fourth output transistor Q4; a first resonance coil L1; an equalizer switching switch S2; and a bias capacitor 7. This is an output amplification section including six resistors R6, a fifth output capacitor, a sixth resonance capacitor, and a seventh resonance capacitor C5, C6, and C7, and the output signal of the amplifier Ab is input thereto. L2 is a second coil for the magnetic head, one end of which is connected to the other end of the fifth output capacitor C5 via the input terminal Ic. 5 is a bias circuit whose output end is connected to the input terminal Ic, S3 is a bias changeover switch whose one end is connected to the bias circuit 5 and the other end is grounded, an AGC time constant changeover switch S1, and an equalizer changeover switch S2.
Linked with.

Note that components other than the output amplifying section 4, the bias circuit 5, and the bias switching switch S3 constitute the input amplifying section, including the first resonance coil L1, the equalizer switching switch S2, the sixth bias resistor R6, and the sixth resonant coil L1.
The capacitor C6 and the seventh resonance capacitor C7 constitute an equalizer circuit.

Furthermore, when inputting input signals such as music sources,
AGC time constant changeover switch S1 is connected, and when an input signal such as a conversation source is input, equalizer changeover switch S2 and bias changeover switch S3 are connected.

Next, the operation of the above embodiment will be explained.

First, when inputting an input signal based on an input source such as a conversation source, the output signal of amplifier Ab is rectified by rectifier 1, the output of rectifier 1 is smoothed by smoother 2, and The signal is outputted to the base of the third switching transistor Q3 via the five diodes D4 and D5, and the level of the input signal to the amplifier Ab changes. In other words, AGC operates.

Then, the third switching transistor Q3 is turned on and off with a time constant determined by the fourth bias resistor R4 of the smoothing section 2 and the third smoothing capacitor C3.
Since the capacitance of the third smoothing capacitor C3 is relatively small, the AGC operation is fast attack/quick recovery.

Furthermore, in conjunction with the disconnected AGC time constant switch S1, the equalizer switch S2,
A bias changeover switch S3 is connected to supply a sufficient bias current to the magnetic head coil L2 and to make the resonant frequency of the equalizer circuit relatively small.

Next, when an input signal based on an input source such as a music source is input, the output signal of the amplifier Ab is rectified by the rectifier 1, and the output of the rectifier 1 is converted to the smoother 2.
It is smoothed by the diodes D4 and D5 for backflow prevention, and is output to the base of the third transistor Q3 for the switch. The fourth smoothing transistor is charged with power from the power supply terminal +B to the base of the three transistors Q3.
Capacitor C4 is connected.

Therefore, a reference input is input to amplifier Ab until time t1', and from time t1' to time t2', that is, a pulse wave signal with a higher level than the time reference input at T1' is input.
From time t2′ to time t3′, that is, time T2′, the reference input is input again, and from time t3′ to time t4′, that is, time T3′, a pulse wave signal with a higher level than the reference input is input, and from time t4′ onwards. When the reference input is input again to operate the AGC, the output waveform at the output terminal Ob will have a constant amplitude at a predetermined level due to the reference input until time t1', as shown in Figure 5, and a pulse will be generated at time t1'. After the amplitude changes to the maximum due to the wave signal, it decreases to the specified level at time T1′ until the reference input is input again, and then reaches the specified level at time t2′ due to the reference input input at time T2′ from time t2′. After reaching the level below, it gradually begins to return to the predetermined level with slow recovery, and then changes to the maximum amplitude larger than the predetermined level again at t3′ due to the pulse wave signal input at time T3′ from time t3′. Thereafter, it changes to the predetermined level again, and after falling below the predetermined level at t4' due to the reference input from time t4', it gradually begins to return to the predetermined level due to slow recovery.

That is, the fourth smoothing capacitor C4, which is charged via the second transistor Q2 for the switch, is connected to the base of the transistor Q3 in parallel with the third smoothing capacitor C3 of the smoothing section 1, and based on the operation of the transistor Q2, , fourth capacitor C
4 is quickly charged with the power supply voltage, the AGC operates with a slow recovery of, for example, 2 to 3 minutes even in response to pulsed input signals. For this reason, the level of the output signal of amplifier Ab based on the music source etc. becomes low on average, and
In the time axis spectrum of the sound pressure based on the output signal of , the amplitude envelope is approximately located within the amplitude envelope of the sound pressure due to the reference output within the operating range of the AGC.

Therefore, as shown in FIG. 6a, when an output signal based on a music source containing pulse wave-like peak components is input to amplifier Ab, amplifier Ab
As shown in Figure b, the output signal has sufficient slow recovery even for pulse wave-like peak components, so the AGC operating level does not change, and the output signal level is equal to the average output signal level based on the peak component. Therefore, the difference between the level of the output signal and the
Even when the distortion factor of the output signal of amplifier Ab is at a high level, it becomes a low factor.

Then, in conjunction with the AGC time constant changeover switch S1, the bias changeover switch S2 and the equalizer changeover switch S3 are turned off, and by changing the resonance frequency of the equalizer circuit and the amount of bias current of the bias circuit 5, the input of conversation sources, etc. The bias amount is changed compared to when a signal is input, and the resonant frequency of the equalizer circuit is increased to expand the frequency characteristics and dynamic range.

That is, for example, when applied to a video tape recorder, the bias current of the bias circuit 5 is
When changing from 160μA to 140μA and 120μA,
At 5KHz and 7KHz of the output signal, as shown by the solid line Ia in Figure 7a, at 160μA, 7KHz is lower than 5KHz.
The difference in amplification amount is 3dB lower for 140μA and 120μA.
As shown in the dot-dashed lines Ib and Ic in Figure A, which show the frequency characteristics of the amplification amount of μA,
For example, at 120 μA, the reduction can be reduced by 1 dB at 7 KHz compared to 5 KHz. In this case, if the amplification amount of 333Hz is OdB based on the time of 333Hz, the amplification amount is 7KHz compared to 333Hz.
In the case of , it increases by 6dB and the frequency response is improved by 6dB. Therefore, as shown by the solid line a in Figure 7b, when a bias current of 160μA is applied, the maximum amount of emphasis at 7KHz is 120μA.
When a bias current of μA is applied, it can be set to reach a maximum at 7 KHz or more, as shown by the solid line b in FIG.

Therefore, during recording, by switching the equalizer changeover switch S2 and the bias changeover switch S3,
By changing the resonant frequency of the equalizer circuit and the amount of bias current of the bias circuit 5, when an input signal such as a conversation source is input, the bias current is set to 160 μA as shown by the solid line a in FIG. The resonance frequency is lowered to narrow the frequency band, and conversely, when an input signal such as a music source is input, the bias current is set to 120 μA as shown by the broken line b in the same figure.
It is possible to expand the frequency band by increasing the resonant frequency of the equalizer circuit, and it is possible to obtain recording frequency characteristics according to each input source.

As described above, the audio circuit of this invention comprises an input amplifier section for amplifying an input signal, a rectifier section for rectifying an output signal of the amplifier section, a smoothing section for smoothing and outputting the rectified output of the rectifier section using a quick recovery capacitor, a slow recovery time constant section consisting of a switching element operated by the output of the smoothing section and a slow recovery capacitor charged by the power supply voltage via the switching element, and an AGC transistor for changing the level of the input signal according to the input level to the base, the output of the smoothing section being input to the base via a backflow prevention diode. and a time constant changeover switch which is provided between the base of the transistor and the slow recovery capacitor and which selectively connects, by switching, the slow recovery capacitor to the base of the transistor via a diode for preventing reverse current. The audio circuit also includes an output amplifier which equalizes and amplifies the output signal of the input amplifier and supplies it to a magnetic head, and a bias circuit which supplies a bias signal to the magnetic head, the output amplifier having an equalizer changeover switch and a bias changeover switch which change over in conjunction with the time constant changeover switch.

Therefore, the AGC operation is varied according to the content of the input signal based on the switching of the time constant changeover switch, and is particularly suitable for input signals that emphasize frequency characteristics, dynamic range, and distortion rate, such as input signals of music sources. AGC always with slow recovery
Based on the switching of the equalizer switching switch and bias switching switch, which are switched in conjunction with the time constant switching switch,
Recording can be performed with equalizer characteristics and bias characteristics that correspond to AGC operation, and recording can be performed with recording frequency characteristics that are optimal for the input source.

[Brief explanation of the drawing]

FIG. 1 is a wiring diagram of a part of a conventional audio circuit, FIG. 2 is an explanatory diagram of waveforms in FIG. 1, and FIGS. 3a and b are waveform diagrams of input and output signals in FIG. 1.
4 to 6 show one embodiment of the audio circuit of this invention, FIG. 4 is a wiring diagram, FIG. 5 is a waveform explanatory diagram, and FIGS. 6a and 6 are waveform diagrams of input signals and output signals. , 7 and 8 are frequency characteristic diagrams of a video tape recorder to which the audio circuit of this invention is applied. 1... Rectifier section, 2... Smoothing section, 3... Time constant section for slow recovery, 4... Output amplification section, 5...
Bias circuit, Q2, Q3...second and third transistors, S1...time constant changeover switch, S2...
Equalizer changeover switch, S3...Bias changeover switch, C3, C4...Third and fourth capacitors, D4, D5, D6...Fourth, fifth and sixth diodes.

Claims (1)

[Claims for Utility Model Registration] An input amplification section that amplifies an input signal, a rectification section that rectifies the output signal of the amplification section, and a quick recovery capacitor to smooth the rectified output of the rectification section and output it. a switching element operated by the output of the smoothing part; and a slow recovery time constant part comprising a slow recovery capacitor charged by the power supply voltage via the switching element; The output of the AGC is input to the base via a backflow prevention diode, and the level of the input signal is changed according to the input level of the base.
a transistor provided between the base of the transistor and the slow recovery capacitor, and selectively connecting the slow recovery capacitor to the base of the transistor via a backflow prevention diode by switching. and a time constant changeover switch for supplying a bias signal to the magnetic head. The audio circuit is equipped with an equalizer changeover switch and a bias changeover switch that operate in conjunction with the constant changeover switch.