Pdm Microphone Hardware Characteristics - XMOS xCORE-200 Multi-channel Audio board Design Manual

USB Audio Design Guide
Up to sixteen PDM microphones can be attached to each high channel
count PDM interface ( mic_array_pdm_rx() ).
mic_array_decimate_to_pcm_4ch() , each process up to four channels. For 1-4
channels the library requires two logical cores:
Figure 24:
One to four
channel
count PDM
mic_array_pdm_rx()
interface
for 5-8 channels three logical cores are required, as shown below:
Figure 25:
Five to eight
count PDM
mic_array_pdm_rx()
interface
The left most task, mic_array_pdm_rx() , samples up to 8 microphones and filters
the data to provide up to eight 384 KHz data streams, split in two streams of four
channels. The processing thread decimates the signal to a user chosen sample
rate (one of 48, 24, 16, 12 or 8 KHz).
More channels can be supported by increasing the number of cores dedicated to
the PDM tasks. However, the current PDM mic integration into USB Audio limits
itself to 8.
After the decimation to the output sample-rate various other steps take place
e.g. DC offset elimination, gain correction and compensation etc. Please refer
to lib_mic_array documention for further implementation detail and complete
feature set.

3.13.1 PDM Microphone Hardware Characteristics

The PDM microphones need a clock input and provide the PDM signal on a data
output. All PDM microphones share the same clock signal (buffered on the PCB as
appropriate), and output onto eight data wires that are connected to a single 8-bit
port:
XM0088546.1
Div 8 Div 4
Decimator Decimator
Div 4
Decimator
Div 8
Decimator
Div 4
Decimator
Div N Select
Decimator DC offset
N=2,4,6,8 FIR comp
or 12 Gain comp
mic_array_decimate_to_pcm_4ch()
Div N Select
Decimator DC offset
N=2,4,6,8 FIR comp
or 12 Gain comp
Select
Div N
DC offset
Decimator
FIR comp
N=2,4,6,8
Gain comp
or 12
mic_array_decimate_to_pcm_4ch()
One to four processing tasks,
Overlapping
frames
WIndowing
Bit reverse
Overlapping
frames
WIndowing
Bit reverse
Overlapping
frames
WIndowing
Bit reverse
40/110
Voice
app
Voice
app