The earliest gain control is analog circuit detection control, but the analog circuit design is relatively cumbersome and it is difficult to achieve a wide range of gain control. Therefore, with the development of digital signal processing devices (DSP), the use of DSP for gain control has become the mainstream. At first, the general method of digital device processing is that large signals reduce gain, and small signals are not processed. There are also ways to amplify small signals, but because of the concern that too much gain adjustment without signal input will increase the background noise, so the gain adjustment range is not large and the ideal control effect cannot be achieved. In addition, the input signal is basically detected, that is, feedforward control, and the output signal is not detected, so that when the gain is large when the input is lost, the output will be limited, affecting the listening effect. And the cost of DSP scheme is relatively high. This solution uses a low-cost microcontroller as the processing core, and completes automatic gain control through a simple gain control algorithm.
1 System hardware design
As shown in Figure 1, the entire system is designed with the collection and processing of audio signals at its core. The audio signal input and output terminals on both sides of the audio control chip PGA2311 are transferred to the A / D port of the MSP430F149 after being level shifted by the amplifier TL084 for sampling (the signal input / output terminals are tested to solve the situation when there is no signal at the input terminal The question of whether the gain is adjusted, and at the same time to avoid excessive gain resulting in output limiting). The sampled data is processed by a software algorithm to obtain a gain value, which is configured to PGA2311 through a level shifter 74HC245. The buttons and digital tube complete the setting and display of the output level threshold range.
1.1 Main control circuit
The main control chip MSP430F149 is a 16-bit, 48 8-bit parallel I / O port, with a reduced instruction set, ultra-low power consumption (only 0.1μA in power-saving mode), its addressing space is 64 KB RAM is 2KB, which brings great convenience to system development. It contains a 12-bit A / D converter ADC12, sample-and-hold and analog multiplexer. ADC12 has the characteristics of high speed and universality, and can perform A / D conversion on 8 external analog sources and 4 internal reference power sources (including internal temperature sensor source). ADC12 also provides a variety of sampling trigger methods, conversion clock cycle, conversion mode selection.
PGA2311 is a two-channel, programmable gain amplifier that interacts with the MSP430F149 via the SPI bus, and its gain range is + 31.5 ～ -95.5 dB.
In Figure 1, MSP430F149 is a 3.3 V power supply, and PGA231l is a CMOS device with a ± 5 V power supply. Therefore, it is necessary to pay attention to I / O logic level matching. Level shifter 74HC245 is used to achieve level matching when driving PGA2311.
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