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PDM - PCM CONVERSION AND THEIR CHARACTERISTICS

Started by NAranguiz_ 4 months ago2 replieslatest reply 4 months ago109 views
I am working on a university project.

The part of the work that I had to delve into has to do with PDM-PCM conversion from a mems type microphone from the knowles company model ("SPH0641LU4H-1"). (Max frequency clock = 4.8Mhz in ultrasonic mode)

After researching PDM modulation and its origins (sigma-delta modulators), I have found some ways to demodulate a bitstream pdm to pcm, but unfortunately I have many loopholes and structural doubts.

I have seen some articles from certain MCUs, as well as some STM32 videos: "PDM TO PCM CONVERT" to understand, and I have come across some logical stages to achieve said demodulation.

According to what I found, the demodulation logic steps will be generated from 4 steps:

a. Low pass filter

b. Decimation

c. High Pass Filter

d. Gain Stage.

To complete point b, my question is:
1.- Is there a defined topology to achieve this objective?

Based on what I found, this step can be completed by implementing a CIC COMB decimator filter, is this correct? and if so, what would be the correct way to tune the parameters "Decimation Ratio" and the number of stages "k" considering that I need to generate a 16 bit pcm signal?

In short, I need to know the way to demodulate a pdm signal (from the mentioned microphone with a maximum clock frequency equal to 4.8mhz in ultrasonic mode) to pcm format with 16-bit resolution and I don't know if I understood the logic stages or the correct topologies well to use.

I apologize for my level of English; it is not good enough.
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Reply by fharrisJune 2, 2021

Naranguiz,


I have worked on this problem for many years and have come to the conclusion that the sequence you described is not the correct approach to solve the problem. Even though it is the most common approach! I have attached an appropriate paper that explains the reason why. The argument that the CIC is the best filter because it performs filtering without multiplies. Since the input series to the CIC is a single bit data stream, every FIR filter using this stream can be implemented without multiplies. in fact the polyphase filter bank can perform the filtering and down sampling with less energy then required by the very wide accumulators of the CIC integrators operating at the high input sample rate.


Would be glad to interact with anyone who would like to follow my approach.

Best regards,


Many of my ideas are embedded in modern cell phone chips which contain numerous sigma-delta audio signals and have reduced power by replacing the CIC with polyphase filters


fred h  


 iscas_cic_3  

[ - ]
Reply by andrewstanfordjasonJune 2, 2021

I have written a PDM to PCM library in software. The relevant documentation is:https://www.xmos.ai/download/lib_mic_array-[usergu.... It documents the filters used the decimation ratio at each of the three stages and the performance of each stage of filtering and overall.

If it's helpful, I did a presentation at AES a few years back on FIR filtering which demonstrated some of the techniques used to efficiently implement a PDM to PCM converter.

LTI Filters.pdf

Hope that helps