Re: PWM -> Audio



MooseFET wrote:


A small increase in the PWM rate can make quite a large improvement in
the quality of the voice data. If you assume an upper limit on the
bandwidth of the audio of 4KHz, you want to sample at more than twice
the 4KHz.

The Nyquist frequency is the first frequency you can't fully
reepresent and not the last one you can. You want to suppress Nyquist
and above. When Nyquist equals the top of the band, this is
imposible.

PWM is the nonlinear operation, and this makes the things complicated. Although the Nyquist condition applies for the PWM, the nonlinear processing of the original signal is required to restore it acurately. That is not very simple; the brute force way to deal with that is to boost the PWM rate to somewhat x10...x20 of the maximum audio frequency of interest.


The PWM data can have its frequency
responce adjusted to correct for the shape of the filter. This allows
a much simpler filter because the power in the signal decreases
greatly above 1KHz. Your filter could have its 3dB point at 1.5KHz and
the data could be boosted above 1.5KHz.

Furthermore, it is good to apply the preemphasis to the original voice data and design the output PWM filter slope so it would do the deemphasis. This improves the quality quite significantly.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
.

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