An AGC control circuit and voice acquisition device based on audio output

Abstract

Embodiments of the present invention disclose an AGC control circuit and voice acquisition device based on audio output. The AGC control circuit includes a bandpass filter, an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, a primary audio signal amplifier circuit, and a secondary audio signal amplifier circuit. This embodiment of the present invention eliminates the need for complex VAD algorithms and high-cost DSP applications, achieving AGC control of audio output using very low-cost hardware circuits.

Description

Technical Field

[0001] The present invention relates to the field of electronic technology, and in particular to an AGC control circuit based on audio output and a voice acquisition device. Background Art

[0002] Automatic Gain Control (AGC) is an automatic control method that automatically adjusts the gain of an amplifier circuit based on signal strength. AGC is a type of amplitude-limited output that uses an effective combination of linear amplification and compression amplification to adjust the output signal of a voice signal. When the input signal is weak, the linear amplification circuit operates to maintain output signal strength. When the input signal reaches a certain level of strength, the compression amplification circuit activates to reduce the output amplitude. In other words, the AGC function automatically controls the gain by varying the input and output compression ratios. AGC is further divided into input automatic gain control (AGCi) and output automatic gain control (AGCo).

[0003] Existing technologies primarily rely on digital processing, using a DSP to adjust the digital gain of the speech amplitude to achieve AGC (Automatic Gain Control) and ensure the dynamic range of the pickup. While this technology is relatively effective in achieving this function, it requires ensuring that the DSP speed matches the required AGC gain and employing a relatively complex speech VAD detection algorithm. Implementing AGC gain using DSP also results in high costs, significantly limiting the design of cost-effective elevator intercom terminals, for example. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art. The present invention provides an AGC control circuit and a voice acquisition device based on audio output, which can abandon the complex VAD algorithm and high-cost DSP application and realize AGC control of audio output with very low-cost hardware circuits.

[0005] In order to solve the above problems, the present invention proposes an AGC control circuit based on audio output, which includes: a bandpass filter, an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, a first-stage audio signal amplifier circuit, and a second-stage audio signal amplifier circuit, wherein:

[0006] The bandpass filter is used to filter out low-frequency and high-frequency interference signals in the audio signal;

[0007] The audio amplifier circuit is connected to the output end of the band-pass filter and is used to amplify the voltage amplitude of the output signal of the band-pass filter;

[0008] The envelope detection circuit is connected to the output end of the audio amplifier circuit, and is used to extract the amplitude characteristics of the audio signal and output the envelope characteristics of the audio signal amplitude;

[0009] The inverting amplifier circuit is connected to the output end of the envelope detection circuit, and is used to make the output result inversely proportional to the amplitude of the audio signal output by the envelope detection circuit, and output the output result to the audio signal first-level amplification circuit;

[0010] The audio signal primary amplification circuit is connected to the output end of the bandpass filter and the output end of the inverting amplifier circuit, and is used to perform AGC control based on the output result of the inverting amplifier circuit and the output signal of the bandpass filter, and output the AGC result to the audio signal secondary amplification circuit;

[0011] The audio signal secondary amplification circuit performs audio output based on the AGC result.

[0012] The bandpass filter includes a low-pass filter, a high-pass filter, and an operational amplifier, wherein: the low-pass filter is connected to the high-pass filter, and the high-pass filter is connected to the operational amplifier.

[0013] The low-pass filter is a first-order passive low-pass filter, and the high-pass filter is a first-order passive high-pass filter.

[0014] The audio amplifier circuit includes a first transistor, and the audio amplifier circuit amplifies voltage amplitude based on the first transistor.

[0015] The envelope detection circuit is a parallel diode envelope detection circuit, or a series diode peak value detector.

[0016] The series diode peak detector includes a diode and a low-pass filter, and the diode is connected to the low-pass filter.

[0017] The audio signal first-stage amplification circuit includes a second triode, and the audio signal first-stage amplification circuit performs AGC control based on the second triode.

[0018] The second transistor is an NPN transistor, the NPN transistor adopts a common emitter connection method, and the base of the NPN transistor is connected to the output end of the bandpass filter and the output end of the inverting amplifier circuit; or the second transistor is a PNP transistor or a MOS transistor.

[0019] The audio signal secondary amplification circuit is an inverting amplifier.

[0020] Correspondingly, the present invention also proposes a voice acquisition device, and the elevator intercom system adopts the above-mentioned AGC control circuit based on audio output.

[0021] In the embodiment of the present invention, the AGC control circuit adopts an analog circuit solution in its entirety. It achieves the purpose of AGC by utilizing inexpensive and technologically mature transistors and operational amplifier circuits. The input gain of the microphone can be automatically adjusted according to the voice intensity based on the AGC control circuit, ensuring high fidelity of the audio signal and a high dynamic range. The circuit can also implement bandpass filtering, with low overall cost and good performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 1 is a schematic diagram of the structure of an AGC control circuit based on audio output in an embodiment of the present invention;

[0024] Figure 2 is a circuit schematic diagram of an AGC control circuit based on audio output in an embodiment of the present invention;

[0025] Figure 3 Schematic diagram of transistor output characteristics in an embodiment of the present invention;

[0026] Figure 4 1 is a schematic diagram of transistor input characteristics in an embodiment of the present invention;

[0027] Figure 5 Schematic diagram of the relationship between input and transistor gain in an embodiment of the present invention;

[0028] Figure 6 Schematic diagram of the input vs. output relationship in an embodiment of the present invention;

[0029] Figure 7 Schematic diagram of filtering characteristics in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] Figure 1A schematic diagram of the structure of an AGC control circuit based on audio output in an embodiment of the present invention is shown. The AGC control circuit includes: a bandpass filter, an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, a first-level audio signal amplifier circuit, and a second-level audio signal amplifier circuit, wherein: the signal output end of the bandpass filter is connected to the audio amplifier circuit and the first-level audio signal amplifier circuit; the signal input end of the audio amplifier circuit is connected to the bandpass filter, and the signal output end of the audio amplifier circuit is connected to the envelope detection circuit; the signal input end of the envelope detection circuit is connected to the audio amplifier circuit, and the signal output end of the envelope detection circuit is connected to the inverting amplifier circuit; the signal input end of the inverting amplifier circuit is connected to the audio amplifier circuit, and the signal output end of the inverting amplifier circuit is connected to the first-level audio signal amplifier circuit; the signal input end of the first-level audio signal amplifier circuit is connected to the bandpass filter and the inverting amplifier circuit, and the signal output end of the first-level audio signal amplifier circuit is connected to the second-level audio signal amplifier circuit.

[0032] Specifically, the bandpass filter here is used to filter out low-frequency and high-frequency interference signals in the audio signal;

[0033] The audio amplifier circuit is connected to the output end of the band-pass filter and is used to amplify the voltage amplitude of the output signal of the band-pass filter;

[0034] The envelope detection circuit is connected to the output end of the audio amplifier circuit, and is used to extract the amplitude characteristics of the audio signal and output the envelope characteristics of the audio signal amplitude;

[0035] The inverting amplifier circuit is connected to the output end of the envelope detection circuit, and is used to make the output result inversely proportional to the amplitude of the audio signal output by the envelope detection circuit, and output the output result to the audio signal first-level amplification circuit;

[0036] The audio signal primary amplification circuit is connected to the output end of the bandpass filter and the output end of the inverting amplifier circuit, and is used to perform AGC control based on the output result of the inverting amplifier circuit and the output signal of the bandpass filter, and output the AGC result to the audio signal secondary amplification circuit;

[0037] The audio signal secondary amplification circuit performs audio output based on the AGC result.

[0038] based on Figure 1When the audio output-based AGC control circuit shown is applied to a voice acquisition device or an elevator intercom system, the audio signal picked up by the microphone in the voice acquisition device or the elevator intercom system will pass through a bandpass filter. The function and role of the bandpass filter is to filter out low-frequency and high-frequency interference signals, and then divide its output signal into two output paths. One output signal of the bandpass filter undergoes analog processing through an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, and the like, and then provides a bias voltage to the audio signal first-stage amplifier circuit. The principle is as follows: the audio signal is input to the audio amplifier circuit for amplification of the audio signal's voltage amplitude. The envelope detection circuit outputs a waveform that is the envelope signal of the audio signal and is directly proportional to the audio signal's amplitude. The envelope detection circuit outputs the envelope characteristics of the audio signal's amplitude to the inverting amplifier circuit. The inverting amplifier circuit outputs an audio signal that is inversely proportional to the audio signal's amplitude to the audio signal first-stage amplifier circuit. The other audio signal from the bandpass filter serves only as input to the audio signal first-stage amplifier circuit. The bias voltage of the audio signal first-stage amplifier circuit is controlled by the signal output by the inverting amplifier circuit. If the audio signal's amplitude increases, its bias voltage decreases, and the gain of the audio signal first-stage amplifier circuit decreases. AGC control is achieved through this method, and the audio signal is ultimately output through the audio signal second-stage amplifier circuit.

[0039] Specifically, Figure 2 The schematic diagram of the AGC control circuit based on audio output in an embodiment of the present invention is shown. The AGC control circuit includes: a bandpass filter, an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, a first-level audio signal amplifier circuit, a second-level audio signal amplifier circuit, and the like.

[0040] The bandpass filter includes a low-pass filter, a high-pass filter, and an op amp, wherein the low-pass filter is connected to the high-pass filter, which is connected to the op amp. The low-pass filter is a first-order passive low-pass filter, and the high-pass filter is a first-order passive high-pass filter.

[0041] The bandpass filter in the embodiment of the present invention is composed of a first-order passive low-pass filter, a first-order passive high-pass filter, and an operational amplifier. Its frequency response can be expressed as:

[0042]

[0043] Where ω is the input angular frequency, ω0 is the center frequency of the bandpass filter, A0 is the gain of the bandpass filter, A(jω) is the frequency response function of the bandpass filter, j represents the imaginary part, and Q is the quality factor.

[0044] The audio amplifier circuit amplifies the voltage amplitude of the output signal of the bandpass filter and uses the amplified signal as the input of the envelope detection circuit. The audio amplifier circuit includes a first transistor and performs voltage amplitude amplification based on the first transistor.

[0045] The envelope detection circuit may be a parallel diode envelope detection circuit, or may be a series diode peak detection circuit.

[0046] The envelope detection circuit in the embodiment of the present invention utilizes a series-connected diode peak detector, which includes a diode and a low-pass filter, with the diode connected to the low-pass filter. Envelope detection, also known as amplitude detection, primarily extracts the amplitude characteristics of an audio signal (voice signal) and outputs the envelope characteristics of the audio signal (voice signal) amplitude. The envelope detection circuit is composed of a diode and a low-pass filter connected in series. When an input signal is present, the current passing through the diode generates an average voltage in the low-pass filter circuit. This voltage then acts on the diode (called the average voltage negative feedback effect), affecting the current passing through the diode, thereby achieving linear detection.

[0047] The inverting amplifier circuit is connected to the output of the envelope detection circuit, and is configured to output an inversely proportional output to the amplitude of the audio signal output by the envelope detection circuit, and output the output to the audio signal first-stage amplifier circuit. The output voltage of the inverting amplifier circuit is inversely proportional to the output of the envelope detection circuit, and then provides a bias to the base of the audio signal first-stage amplifier. The bias voltage can be expressed as:

[0048]

[0049] Where Vcc is the supply voltage of the inverting operational amplifier, K0 is a constant coefficient, Vin is the input of the pickup, and A(jω) is the frequency response function of the bandpass filter.

[0050] The audio signal primary amplification circuit is connected to the output of the bandpass filter and the output of the inverting amplifier circuit, and is configured to perform AGC control based on the output of the inverting amplifier circuit and the output signal of the bandpass filter, and output the AGC result to the audio signal secondary amplification circuit. The audio signal primary amplification circuit includes a second transistor, and the audio signal primary amplification circuit performs AGC control based on the second transistor.

[0051] The second transistor is an NPN transistor, which is connected in a common-emitter configuration, with its base connected to the output of the bandpass filter and the output of the inverting amplifier circuit; or the second transistor is a PNP transistor or a MOS transistor. The audio signal secondary amplification circuit is an inverting amplifier.

[0052] Correspondingly, the present invention also proposes a voice collection device, which adopts the above-mentioned AGC control circuit based on audio output.

[0053] Here, the audio signal first-stage amplifier circuit is amplified using an NPN transistor as an example, and the NPN transistor adopts a common emitter connection. Figure 3 The following diagram shows the output characteristics of a transistor. A transistor has three operating states: cutoff, amplification, and saturation. In the amplification region, the base current Ib and collector current Ic are linearly related, and Vbe = 0.7V (using a silicon transistor as an example).

[0054] There is a critical state between the cutoff region and the amplification region of the transistor. In this critical state, Vbe < 0.7V, and Vbe is greater than the threshold voltage of the BE pole. This area is the conduction area of ​​the BE pole of the transistor. Figure 4 The figure shows the transistor input characteristic diagram. At this time, the base current Ib is very small and the CE pole presents a large impedance, so the collector current Ic is also very weak. As the Vbe of the transistor increases, Ib also increases, and the current gain of the transistor is It also increases with the increase of Ib, and eventually slowly enters the amplification region. At this time, the transistor gain is a constant value β. The β value is determined by the characteristics of each transistor. After the transistor leaves the factory, it is a constant value under certain conditions.

[0055] Since the transistor has the critical state between the cutoff region and the amplification region, the amplification factor K1 of the transistor in this critical state can be controlled by the base voltage. In the embodiment of the present invention, the Vbias output by the inverting amplifier circuit provides a voltage bias for the base of the common-emitter amplifier circuit (the first-stage audio signal amplifier circuit). As Vin increases, Vbias decreases, and therefore K1 also decreases. Figure 5 The figure shows the relationship between input and transistor gain, which is the measured data using 2N5551. Figure 5 As can be seen in the figure, as the input signal amplitude increases, the transistor gain gradually decreases, eventually reaching a negative gain. At this point, the transistor is in an attenuation state. Therefore, the transistor in the first-stage audio amplification of the present invention can realize the AGC function. The second-stage audio amplification then provides gain for the entire circuit, keeping the output amplitude within a specific range.

[0056] In order to verify the overall effect, input a 1kHz audio signal from the input end of the entire AGC circuit with an amplitude ranging from 0.5 to 14V, and then test the Vout amplitude, such as Figure 6 The input vs output relationship diagram shown in Figure 6As can be seen in the figure, the input audio signal amplitude increases 28 times, but the output only changes by a factor of 2. The output initially rises, then slowly decreases, and finally the output amplitude levels off. The maximum and minimum outputs show that the input-output relationship is not linear. As the input amplitude increases, the input does not continue to increase. This solves the problem of cutoff distortion caused by close-range pickup, achieving the goal of a high dynamic range for speech amplitude.

[0057] In order to explore the response relationship between output and frequency, the input audio signal amplitude is set to 2V, and its frequency is changed. The frequency response can be tested, such as Figure 7 From the filtering characteristic diagram shown, it can be seen that the AGC circuit has a bandpass filtering function.

[0058] Based on the AGC control circuit in the embodiment of the present invention, all analog circuit solutions are adopted. The purpose of AGC is achieved by utilizing inexpensive and technologically mature transistors and operational amplifier circuits. The input gain of the microphone can be automatically adjusted according to the voice intensity based on the AGC control circuit, ensuring high fidelity of the audio signal and a high dynamic range. The circuit can also realize bandpass filtering, with low overall cost and good performance.

[0059] The above is a detailed introduction to the AGC control circuit based on audio output and the elevator intercom system provided in the embodiments of the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the ideas of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. An AGC control circuit based on audio output, characterized in that: The AGC control circuit includes: a bandpass filter, an audio amplifier circuit, an envelope detection circuit, an inverting amplifier circuit, an audio signal first-stage amplifier circuit and an audio signal second-stage amplifier circuit, wherein: The bandpass filter is used to filter out low-frequency and high-frequency interference signals in the audio signal; The audio amplifier circuit is connected to the output end of the band-pass filter and is used to amplify the voltage amplitude of the output signal of the band-pass filter; The envelope detection circuit is connected to the output end of the audio amplifier circuit, and is used to extract the amplitude characteristics of the audio signal and output the envelope characteristics of the audio signal amplitude; The inverting amplifier circuit is connected to the output end of the envelope detection circuit, and is used to make the output result inversely proportional to the amplitude of the audio signal output by the envelope detection circuit, and output the output result to the audio signal first-level amplification circuit; The inverting amplifier circuit is connected to the output end of the envelope detection circuit, and is used to make the output result inversely proportional to the amplitude of the audio signal output by the envelope detection circuit, and output the output result to the audio signal first-stage amplifier circuit. The output voltage of the inverting amplifier circuit is inversely proportional to the output of the envelope detection circuit, and then a bias is provided to the base of the audio signal first-stage amplifier. The bias voltage is expressed as: Where Vcc is the supply voltage of the inverting op amp, K0 is a constant coefficient, Vin is the input of the pickup, and A(jω) is the frequency response function of the bandpass filter; The audio signal primary amplification circuit is connected to the output end of the bandpass filter and the output end of the inverting amplifier circuit, and is used to perform AGC control based on the output result of the inverting amplifier circuit and the output signal of the bandpass filter, and output the AGC result to the audio signal secondary amplification circuit; The audio signal secondary amplification circuit performs audio output based on the AGC result.

2. The AGC control circuit based on audio output according to claim 1, characterized in that: The bandpass filter includes a low-pass filter, a high-pass filter, and an operational amplifier, wherein: the low-pass filter is connected to the high-pass filter, and the high-pass filter is connected to the operational amplifier.

3. The AGC control circuit based on audio output according to claim 2, characterized in that: The low-pass filter is a first-order passive low-pass filter, and the high-pass filter is a first-order passive high-pass filter.

4. The AGC control circuit based on audio output according to claim 1, wherein: The audio amplifier circuit includes a first transistor, and the audio amplifier circuit amplifies voltage amplitude based on the first transistor.

5. The AGC control circuit based on audio output according to claim 1, wherein: The envelope detection circuit is a parallel diode envelope detection circuit, or a series diode peak value detector.

6. The AGC control circuit based on audio output according to claim 5, characterized in that: The series diode peak detector includes a diode and a low-pass filter, and the diode is connected to the low-pass filter.

7. The AGC control circuit based on audio output according to claim 1, wherein: The audio signal first-stage amplification circuit includes a second triode, and the audio signal first-stage amplification circuit performs AGC control based on the second triode.

8. The AGC control circuit based on audio output according to claim 7, characterized in that: The second transistor is an NPN transistor, the NPN transistor adopts a common emitter connection method, and the base of the NPN transistor is connected to the output end of the bandpass filter and the output end of the inverting amplifier circuit; or the second transistor is a PNP transistor or a MOS transistor.

9. The AGC control circuit based on audio output according to any one of claims 1 to 8, characterized in that: The audio signal secondary amplification circuit is an inverting amplifier.

10. A voice collection device, characterized in that: The voice collection device adopts the AGC control circuit based on audio output as described in any one of claims 1 to 9.

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