Util.hpp

template<unsigned MIC_COUNT>
void mic_array::deinterleave_pdm_samples(uint32_t *samples, unsigned s2_dec_factor)

Deinterleave the channels of a block of PDM data.

PDM samples received on a port are shifted into a 32-bit buffer in such a way that the samples for each microphone channel are all interleaved with one another. The first stage decimator, however, requires these to be separated.

samples must point to a buffer containing (MIC_COUNT*s2_dec_factor) words of PDM data. Because the decimation factor for the first stage decimator is a fixed value of 32, 32 PDM samples from each microphone is enough to produce one output sample (a MIC_COUNT-element vector) from the first stage decimator. 32*s2_dec_factor PDM samples for each of the MIC_COUNT microphone channels is then exactly what is required to produce a single output sample from the second stage decimator.

The PDM data will be deinterleaved in-place.

On input, the format of the buffer to which samples points is assumed to be such that the following function will extract (only) the kth sample for microphone channel n (where k is a time index, not a memory index):

Input Format

unsigned get_sample(uint32_t* samples, 
                    unsigned MIC_COUNT, unsigned s2_dec_factor, 
                    unsigned n, unsigned k)
{
  const end_word = MIC_COUNT * s2_dec_factor - 1; // chronologically first
  const unsigned samp_per_word = 32 / MIC_COUNT;
  const words_from_end = k / samp_per_word;
  const uint32_t word_val = samples[end_word-words_from_end];
  const unsigned bit_offset = (k % end_word) + n;
  return (word_val >> bit_offset) & 1;
}

Here, the words of samples are stored in reverse order (older samples are at higher word indices), and within a word the oldest samples are the least significant bits. The LSb of a word is always microphone channel 0, and the MSb of a word is always microphone channel MIC_COUNT-1.

Upon return, the format of the buffer to which samples points will be such that the following function will extract (only) the kth sample for microphone channel n:

Output Format

unsigned get_sample(uint32_t* samples, 
                    unsigned MIC_COUNT, unsigned s2_dec_factor, 
                    unsigned n, unsigned k)
{
  const unsigned subblock = (s2_dec_factor-1)-(k/32);
  const unsigned word_val = samples[subblock * MIC_COUNT + n];
  return (word_val >> (k%32)) & 1;
}

Here, each word contains samples from only a single channel, with words at higher addresses containing older samples. samples[0] contains the newest samples for microphone channel 0, and samples[MIC_COUNT-1] contains the newest samples for microphone channel MIC_COUNT-1. samples[MIC_COUNT] contains the next-oldest set of samples for channel 0, and so on.

Template Parameters

MIC_COUNT –

Number of channels represented in PDM data.

One of

{1,2,4,8}

Parameters

• samples – Pointer to block of PDM samples.

• s2_dec_factor – Stage2 decimator decimation factor.