Research And Design Of Embedded Audio Signal Processing System

In modern audio signal processing systems,PDM signals and PCM signals play an indispensable role.?-? modulators can convert analog signals into PDM signals,with high-frequency and low-precision.By digital down sampling filters,PDM signals turn into PCM signal.The research and design of ?-? modulators and down-sampling filters has been widely used in MEMS microphones and professional audio decoding chips.This article focuses on the PDM and PCM signals commonly used in the audio field,with the aim of realizing high-order high-precision ?-? modulators and completing PDM-PCM signal conversion.The audio signal processing system designed in this paper mainly includes two major components:a ?-? modulator and a down-sampling filter.The SNDR of the fourth-order one-bit single-loop modulator designed in this paper reaches 105.6dB,which can be used to generate high-frequency and low-precision 1-bit PDM signals.The down-sampling filter designed in this paper realized 64-time decimation,filtered out the noise outside the signal band,successfully converted 1-bit PDM signal into 16-bit PCM signal.Also,FPGA verification of digital down-sampling filter was completed.The main research results of this article are as follows(1)Completed the design of high-order high-precision modulator suitable for audio field.The loop design of the fourth-order modulator is completed through system modeling,parameter selection,and structure selection.Zero-point optimization was introduced to improve the signal-to-noise ratio of the ?-? modulator.MATLAB tools was used to model and simulate SIMULINK of single-loop modulators for each loop filtering form.(2)Improved stability of higher order modulators.Because the stability of the high-order single-loop modulator was poor when the amplitude of the input signal is large,an amplitude reduction unit was introduced to suppress the quantizer overload phenomenon,and the pole position of the loop filter is strictly controlled when the ?-? modulator is designed.The range of the modulator's input signal was enlarged and the stability was improved.Finally,the fourth-order one-bit CRFF ?-? modulator has a wide input range of-0.78V to+0.78V.The SNR of the modulator reached 105.6dB,and the ENOB reached 17.24bit.The modulator successfully converted the analog signal into a PDM signal.It can be used as the first-stage unit of the audio system.(3)The conversion from PDM signal to PCM signal was completed.The CIC filter,compensation filter,and low-pass filter are cascaded to realize the overall filter,which converts high-frequency and low-precision PDM signals into low-frequency and high-precision PCM signals.The CIC filter was used to achieve 16 times decimation.The compensation filter and low-pass filter were each used to achieve 2 times decimation.By using the overall filter designed in this paper,the PDM signal was reduced from the oversampling frequency to near the Nyquist frequency and converted into PCM signal.(4)Improved the disadvantages of CIC filters.On the one hand,because the passband roll-off of the CIC filter was too large,the lifted FIR filter was finally selected to achieve passband attenuation compensation by comparing the lifted FIR filter and the ISOP filter,;On the other hand,the traditional multi-stage CIC filter has low operation speed and large power consumption.By introducing polyphase decomposition technology and Nobel transform,the traditional recursive structure was optimized into an improved non-recursive structure.The task of 16 times decimation was distributed to each sub-filter in order to reduce the power consumption of the CIC filter and improve the operation rate.(5)Built an embedded system for verification.The RTL implementation of the digital down-sampling filter was completed through the hardware description language Verilog HDL,and simulation verification was performed by Modelsim-tools.The FPGA platform was used to verify the hardware design results of the down-sampling filter.In addition,the STM32 single-chip audio system was also built to test ST company's processing technology.The similarities,differences and advantages of different embedded PDM-PCM signal conversion systems were analyzed.