JPH0158688B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AGC circuit that stabilizes amplitude by controlling AGC elements.

Conventionally, the purpose was to stabilize the amplitude in audio etc.
The AGC circuit has a configuration in which the rectifier circuit is followed by charging with a relatively short time constant and discharging with a long time constant (this is called fast attack slow release <fast
attack slow release>).
This short time constant is required to be several milliseconds or more when operating at the average value of the audio level, and if it becomes shorter than this, it will start to malfunction in response to high noise for a short period of time.

In the case of a car phone, etc., the noise inside the car is different from that inside the car, and the conditions are more severe than those for indoor use. In other words, the short-term, high-level noise from oncoming cars and car horns is much harsher than indoor noise.
The following phenomenon occurs in the charging time constant of the AGC circuit. For example, if a high-pitched noise such as a horn comes in while a speaker is talking, the AGC control signal will increase at that moment.
After that, the speaker's voice level becomes lower even though it is below the level at which the AGC operates.

To prevent this malfunction, if the charging time constant is lengthened, it will take time for the control signal to rise at the beginning of the speaker's speech, so the loud voice level will gradually shift to the average voice level with AGC applied. It has the disadvantage that the beginning of speech becomes unnatural. Therefore, if you shorten the discharge time constant so that it can quickly return to normal state in the event of a malfunction, the discharge will occur if there is a pause between conversations, and you will have to recharge it every time you start speaking. It has the disadvantage that the beginning of the conversation becomes loud and the conversation becomes unnatural.

The present invention aims to eliminate these drawbacks, and provides an AGC circuit that can prevent malfunctions caused by noise and realize a natural and stable audio level.

AGC according to the present invention to achieve the above object
In an AGC circuit, the circuit has a gain control circuit that controls the gain of an operational amplifier in response to a control signal.
a bandpass filter connected to the output of the AGC circuit for removing noise included in the audio signal from the AGC circuit; and a first time constant connected to the bandpass filter and having a first time constant. a second time constant circuit connected to the bandpass filter and having a second time constant shorter than the first time constant; and a larger one of the output signals of the first and second time constant circuits. and means for supplying a level signal to the gain control circuit as the control signal.

According to the above configuration, even under severe operating conditions with high noise levels for short periods of time, the audio level will not be reduced by the noise, and even if there is a pause between speeches, the beginning of the speech will remain at a high level. The object of the invention is fully achieved, without making the conversation unnatural, and reducing the effects of high noise for a short time, and being able to operate only by audio signals.

The present invention will be described in more detail below based on examples.

FIG. 1 shows a first embodiment of an AGC circuit according to the present invention. In the figure, a bandpass filter 30, a rectifier circuit 40, an amplifier 50, and a double time constant circuit 60 form a control circuit section that controls an AGC element that is a gain control circuit. Also
By controlling the feedback amount of the operational amplifier 20 using the variable gain element 10 as the AGC element, it is possible to vary the gain composed of the variable gain element and the operational amplifiers 10 and 20 for the control output Vcont. Therefore, when the input voltage Vin is added and Vout increases, Vcont also increases, the amount of negative feedback of the amplifier 20 increases, and the gain decreases.
It is configured to keep the Vout level constant.

The operation of this circuit will be explained in order by dividing it into a case where high noise is heard for a short time and then a voice is heard, and a case where a normal conversation is being carried out without high noise.

FIG. 3 is a waveform diagram of each circuit section in the former case. Now, suppose that a waveform as shown in a in the figure is input to the control circuit section under the above conditions. Of the input waveforms,
Since short-term high noise has different frequency components from voices, a bandpass filter (hereinafter abbreviated as BPF) as shown in Figure 1 is used.
30, the noise component is attenuated to the level indicated by the broken line in FIG. 3a. This signal is transmitted to the rectifier circuit 40 in FIG.
After passing through the amplifier 50 and converting it into direct current, it is input to the dual time constant circuit 60. In the same figure 60, the upper circuit has attack (attack) time constants for Ra and Cf, and release (release) time constants for Cf and Ra, which are shorter than those of the lower circuit, so the third As shown in figure b, the first part of the conversation rises quickly. Also, even if high noise is input for a short period of time and the control proceeds excessively, the system will quickly stop. For the circuit below 60 in Figure 1, Ras
Cs is the time constant for attack, and Cs Rr is the time constant for release, and each is longer than that of the circuit above, so it can be used against high noise for a short time as shown in c in Figure 3. They do not respond and respond slowly to long conversations such as conversations. At 60 in Fig. 1, the waveforms of the above points f and s are input to the AGC element △G via diodes Xf and Xs, respectively, and are connected to the higher level one of these two waveforms. One diode turns on and the other diode turns off, so as the Vcont signal,
The waveform input to the AGC element 10 rises in response to high noise for a short time as shown in d in Figure 3, but does not maintain that value and also rises quickly in response to the rising part at the beginning of a conversation. Even under severe operating conditions with high noise levels for short periods of time, the sound level will not be reduced by this noise.
Also, you can avoid starting to speak loudly and sound unnatural.
It will be possible to demonstrate the AGC function.

FIG. 4 shows a case where a normal conversation is being carried out without high noise. Now, suppose that under these conditions a waveform as shown in a in the figure is input to the control circuit section. The signal that has passed through the BPF, rectifier circuit, and amplifier is input to a dual time constant circuit. Thinking in the same way as in Figure 3, in the case of a short time constant, the beginning of the conversation rises quickly, as shown in Figure 4 (b).
When it comes to the pose part, it falls quickly. In the case of a circuit with a long time constant, as shown in c in Fig. 4, it is difficult to start up at the beginning of a conversation, and control is delayed, but the previous value is maintained even when there is a pause. As a result, the waveform input to the AGC element as the Vcont signal, as shown in d in Figure 5, does not have the unnaturalness of the conversation that occurs when the control is delayed and the voice level increases at the beginning of speaking, and there are no pauses. Since the amount of control is maintained even during the conversation, the AGC function can be performed naturally throughout the conversation.

FIG. 2 is a simplified version of the dual time constant circuit in FIG. That is, it responds to short-term high noise with a short time constant τ _f , but does not respond to a long time constant τ _s , so the control amount appearing at point b falls quickly with the fall of the noise.
Furthermore, since a long time constant τ _s responds to something that continues for a relatively long time, such as voice, the control amount at point b is maintained even in a pause state.

As explained above, if the above AGC circuit is used, even under harsh operating conditions such as mobile communication, where short periods of time and high noise levels are used, the voice level will not be reduced by this noise, and the voice level will not be reduced due to the noise. Even if there is a pause, the voice level remains stable without causing the conversation to become unnatural due to the high level at the beginning of the conversation.
A circuit that can perform the AGC function can be provided.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the AGC circuit according to the present invention, and FIG. 2 is a simplified circuit diagram of the dual time constant circuit of the AGC circuit shown in FIG. 1.
FIG. 3 is a waveform diagram showing temporal changes in the control signal when voice is input after high noise is input for a short period of time. FIG. 4 is a waveform diagram showing temporal changes in the control signal when a normal conversation is being carried out without high noise. Vcont...control signal, Ra, Rf, Cf...resistor and capacitor forming short time constant τ _f , Ras,
Rr, Cs...Resistor and capacitor forming a long time constant τ _s , Xf, Xs...Diode for signal selection, Rr...Release resistor, 10...Variable gain element for current input/output, 20, 50...Operation amplifier , 30...
Bandpass filter, 40... rectifier circuit, 60... double time constant circuit.

Claims (1)

[Claims] 1. In an AGC circuit having a gain control circuit that controls the gain of an operational amplifier in response to a control signal,
a bandpass filter connected to the output of the AGC circuit for removing noise included in the audio signal from the AGC circuit; and a first time constant connected to the bandpass filter and having a first time constant. a second time constant circuit connected to the bandpass filter and having a second time constant shorter than the first time constant; and a larger one of the output signals of the first and second time constant circuits. and means for supplying a level signal to the gain control circuit as the control signal.