piclist 1998\11\06\102321a >
Thread: sin function
picon face BY : Peter L. Peres email (remove spam text)

On Fri, 6 Nov 1998, Dennis Plunkett wrote:

> Ok so it was for generation, forgive me for skipping over that. As for
> reception, the frequency range of DTMF is 697 to 1633, thus we only have to
> sample at 3266 (Or slightly higher) the 8kHz is only for the full voice
> stuff, in which case it would be connected to a CODEC of some type. This
> CODEC will produce Mu LAW or A LAW compression which wouldh ave to be
> converted to get the 16 bit value (This we would not want to do as it will

Why 16 bit value ? What for ? I was cutting every corner that came my way
just to avoid anything that may generate a carry on 8 bits when added, and
you'd like to consider 16 bits ?

> introduce distortion and take up processing time). Then we have the level
> problem, a DTMF signal must be received with a level range of 20dBm, and
> with high to low frequency tone power of +4 to -8dBm. Also levels of less
> than -35dBm must be rejected. On top of that the decoder must see periods of

And this is ablessing, and this is what I had referred to when mentioning
clipping decimation and a buffer storage. If all this pre-processing is
done correctly then the lower energy frequency can probably be deduced
from the buffer contents after the high frequency component was selected
and removed by substraction in the first pass. Low amplitudes are rejected
by decimation etc.

> breaks of 8 to 12mS as non valid loss (Number still OK) And must ignore a
> signal with a tone devation of +/- 3.5%.

3.5% is easy to decode with a 8kHz sample as I had described. The other
option is, to sync the buffer fill with the higher frequency tone and
analyze the buffer contents to get the lower frequency tone.

> So while it is possible to do a basic decode I don't think that a PIC could
> handle a full decode.

If by 'full decode' you mean, read all the 16 DTMF symbols, I think it can
be done. If you mean, meet all the specs of a phone connectable device,
the answer is: yes, but with some pre-filtering. This is my opinion only.



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