Re: PWM -> Audio

On Dec 6, 2:27 pm, Vladimir Vassilevsky <antispam_bo...@xxxxxxxxxxx>
wrote:
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.

How do you conclude it must be non-linear? I can think of ways it can
be but it seems that it needn't be. Even if it is non-linear, since
it is before the filter, you can correct the data to make it produce
the linear result.


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.

In the extreme case, you can apply preemphasis and then do a one bit
output to a comparitor and still understand what is being said.


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

.

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