Re: PWM -> Audio

On Dec 6, 8:52 am, Silvar Beitel <silverbee...@xxxxxxxxxxxx> 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.

Yes. This is voice audio < 4 kHz sampled at 8 kHz.
But how much less than 4KHz? For voice 3KHz is usually considered
the highest frequency. 3KHz is too close to 4KHz for a really cheap
filter.

You can stop short of that. 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.

Interesting thought, but computationally not something I want to do in
the microcontroller!

Wimp!

Temp = PrevSample;
PrevSample = NewSample;
NewSample = NewSample - Temp/2;

This gives a 3X boost at Nyquist. The divide is a simple shift to the
right.

For voice data sourced from a server or burned
into ROM, sounds like a good idea.

The latter was my case.


4) Just try to get the PWM frequency as high as the rest of the system
will allow and then drive the earphones/speaker directly, using the
speaker/headphones as part of the filter. Certainly cheap, but what
about emissions, especially out of probably non-shielded headphone
cords?

If you do this never test it to see if it passes FCC.

I can buy stickers cheap :-)

Thanks for your input.

--
Silvar Beitel

.

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