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4.5. FAR-FIELD VOICE PROCESSING
4.5.1. PDM MICROPHONE INTERFACE
The PDM microphone interface converts Pulse Density Modulation (PDM) audio input from the
microphones to Pulse Code Modulation (PCM) format allowing further processing. The PDM
microphone interface consists of the physical pins connecting to the two microphones and a series of
filters resulting in a 16kHz PCM, two-channel output stream suitable for far-field voice processing.
Please refer to the datasheet for the physical and electrical details of the PDM pins.
The processing consists of four filter stages:
-
Decimate by 8 FIR filter to 384kHz
-
Decimate by 4 FIR filter to 96kHz
-
Decimate by 6 FIR filter to 16kHz
-
DC Blocking, single-pole IIR filter
The PDM microphone interface uses 32-bit internal processing to provide very low distortion with a
specification exceeding -110dB THD+N with a 140dB dynamic range.
The frequency response of the FIR filter has a stopband attenuation of at least 70dB with a passband
ripple of less than 0.9dB and a passband of 6.8kHz. The total group delay from pin to the XVF3510
audio pipeline input is 1.125 milliseconds.
A DC blocking filter is placed at the end of the PDM microphone interface pipeline and is tuned to
have a 5Hz -6dB point and removes any DC offset present in the PDM input.
The output from the PDM microphone interface may optionally be shifted or attenuated providing a
'power of two' gain control. Saturation may be applied in the case that the gain is greater than one. By
default, the gain block shift is set to zero (a gain of 2
normal use.
The PDM interface control parameters are shown below:
Table 4-13 Microphone commands
COMMAND
SET_MIC_SHIFT_SATURATE
GET_MIC_SHIFT_SATURATE
XM-014232-PC
Figure 4-10 PDM microphone input processing stages
TYPE VALUE
arg1 <shift value
(left shift)>
uint32
arg2 <saturate,
enable if !=0>
uint32
0
= 1) and this is the recommended setting for
DESCRIPTION
Write the gain (power of 2) on the raw mic
signals before entering the audio
pipeline.
Read the gain (power of 2) on the raw
mic and Saturate Enable signals before
entering the audio pipeline.
NOTE
S
51
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