Software interface: digital signal processing

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Software interface: digital signal processing

The data coming from the microphone is sent to the decimation process, which consists of

two parts: a decimation filter converting 1-bit PDM data to PCM data, followed by two

individually configurable IIR filters (low pass and high pass). The reconstructed audio is in

16-bit pulse-code modulation (PCM) format. After the conversion, it produces raw data that

can be handled depending on the application implementation (stored as wave/compressed

data in a mass storage media, transferred to an external audio codec DAC through I2S

peripheral...).

Figure 2.

Digital signal processing

4.1

PDM digital filtering and decimation

The PDM signal from the microphone is filtered and decimated in order to obtain a sound

signal at the required frequency and resolution.

The frequency of the PDM data output from the microphone (which is the clock input to the

microphone) must be a multiple of the final audio output needed from the system. For

example, to perform a decimation of 80, for the output rate of 30 kHz, we need to provide a

clock frequency 2.4MHz to the microphone.

The output of the filter pipeline is a 16-bit value, we consider [-32768, 32767] as the output

range for a unitary gain (0 dB).

4.2

Digital signal conditioning

The digital audio signal resulting from the previous filter pipeline is further processed for

proper signal conditioning.

The first stage is a high pass filter designed mainly to remove the signal DC offset. It has

been implemented via an IIR filter with a cut-off frequency below the audible frequency

range in order to preserve signal quality.

The second stage is a low pass filter implemented using an IIR filter.

Both filters can be enabled/disabled and configured (cut-off frequencies) by using the filter

initialization function.Gain can be controlled by an external integer variable (MicGain) as

shown in the following equation: G = MicGain/64.

MS19893V1

PDM digital filtering

and decimation

Digital signal

conditioning

PDM in

Audio out