On Thu, 11 Sep 1997, Mike Smith wrote:

> Dids't you mean this to go to PICLIST  it went direct to me.  (seems
> to happen to me when I don't want it to sometimes...)

        Yup, I meant it for the list, so I am sending the whole kit and
kaboodle to the list -- my original, your reply, and my reply to that.

>
> On 10 Sep 97 at 14:04, Pierce  Nichols wrote:
>
> > On Thu, 11 Sep 1997, Mike Smith wrote:
> > >
> > > Somewhat difficult on the average PIC, these FFT's!
> >
> >  Well, there's always the ever-popular look-up table. I also
> > thought of another one that is a little less processor intensive. It
> > is still a raw idea, but bear with me:
>
> Even lookup tables are inefficient on the PIC, with its non-readable
> code forcing the retlw nn and some dancing around to get it.
> I find spi EEPROMS to be more efficient sometimes.
>

        In this case, I don't think it would matter because a PIC is
*light years* faster than DTMF tones, which max out at about 10 tones/sec.
You are right in general of course.

> >
> >  The DTMF tones are on the scale of one kHz -- no problem for the
> > average PIC to sample accurately. You put a filter with its passband
> > straddling the DTMF range right in front of the ADC input. You then
> > count the period between maximum peaks (i.e., when the two tones are
> > in phase) to bring out the beat frequency, which should tell you
> > which tone-pair is on the line, because the tones were selected by
> > bell labs so that their beat frequencies are all different. You
> > stick that into a lookup table and read off the hex digit. As I
> > said, it's off the cuff, but it ought to work. OTH, a DTMF decoder
> > chip is very cheap, and they generally have a 4-bit parallel
> > interface.
> >
>
> Sounds feasible.  Maybe the person who wanted to do it would like to
> try.  Me, I'd use the dedicated chip unless I was making 1000's, and
> they had to cost next to nothing.

        DTMF decoders are cheaper than most PICs, I think, but I am not
sure. Doing it with a PIC is awfully klugey, but it might be a fun thing
to try...

        Pierce