This program implements a decimation of a Pulse Density bit-stream. For example, the bit-stream generated by a digital microphone. The typical use-case of this program is to decimate the PDM stream of a microphone to the usual audio sampling-rates. For example 16kHz for the keyword recognizer of Alexa voice services.

 

A bit-exact demonstration files and executable (BATCH_SRC.BAT) is located at :

http://firmware-developments.com/WEB/P6x/PDMDEC/DEMO/

The documentation is located here

 

The code is ported and benchmarked for ARM Cortex-M4 / M7, and can be executed on any processor using 32-bits arithmetic. The first processing stage is a 4th order CIC decimator with decimation rate of 32 or 64, followed by an half-band decimator and DC-offset canceller using floating point arithmetic.

 

Example with a PDM rate is 2.048MHz with an Over Sampling Ratio (OSR) of 128, the targeted sampling rate is 2.048MHz/128 = 16kHz.  The CIC decimator consumes 9.65 cycles of a Cortex-M7 per PDM-bits, 39 cycles/samples at OSR=2 for the second decimator, 10 cycles for the DC-offset canceller.

 

The computation of 16kHz output sample from a 2.048MHz PDM bit-rate takes 21MHz of CPU load.