PDM MIC with SSC Clock

[mccreacasey]

Is it possible to run any of the ST PDM mic sensors with a spread spectrum clock input? What implication does that have on audio quality? For example, is it possible to run any of the sensors with a clock input of 3.075MHz PDMCLK with 0.5% center spread frequency deviation at a modulation rate of 33kHz?

Best Regards,

Casey

[Eleon BORLINI]

Hi @mccreacasey​ Casey,

The PDM mic output is a 1-bit and, in your case, 3.075MHz signal.

If you correctly sample (i.e. synchronize the data reading on the rising/falling edge of the clock) the stream at DSP level, you will be able to correctly decode the signal.

Please consider however that most of the time the audio format are a standard, i.e. 16kHz, 44.1kHz, 48kHz, 96kHz, since in this way you can easily import them in the most common audio editors/analyzers (e.g. Audacity).

-Eleon

[mccreacasey]

Hello Eleon,

My question is not about reception of the 1-bit stream at the DSP level. My question is what is the consequence of clocking the mic in this way for the audio quality? Will this lead to a degradation of the THD? Increase of noise floor? Other?

I would suspect that if the modulation rate of the spread spectrum was >> frequency than the audio sampling rate, then it would not have significant impact on the audio quality, But in this case, lets say we go for a standard audio sampling rate of 48kHz; this is actually higher frequency than the SSC modulation rate of 33kHz. So will this translate to THD? Does ST have any experience in clocking a PDM mic with a spread spectrum clock?

Best Regards,

Casey