----- Original Message -----
From: "Peter L. Peres" <plp@ACTCOM.CO.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, May 24, 2000 12:42 PM
Subject: Re: DTMF decoding with a PIC (sigma-delta)


> Hi Scott, Walter,
>
> Scott, your intro on DFT is excellent ! I have thought of a slightly
> different scheme for the hardware (using a comparator between the C and
> the PIC, comparing with 1/2 Vcc).
>
> I am going to make an application that uses a PIC to communicate to
> another PIC over a phone line, to talk FM to each other. It would be nice
> to make it possible to have the receiver be a modem using perhaps the old
> Bell 202 (?) standard for Rx only (300 or 600 Bps). I do not think that
> there will be enough 'room' in the PIC for a receiver for the same
> standard. I hope to rely on the phone line/central for filtering
> square wave FM beyond my line transformer and a simple RC low-pass <g>. I
> have no room for magic sine waves or other such things.
>
> Now, how does one dial DTMF with a PIC ? Sine wave lookup table, the
> algorythm on your page, Microchip's application note wrt DTMF
> dialing or something else ?
>
> Am I grossly mistaken if I say that one can make a DTMF dial tone set that
> is pretty far from standard wrt. harmonics 9but accurate in frequency and
> amplitude ratio) and still get away with it ? In other words, if I send
> two plain square waves out through two RC lowpasses and sum them then
> might a receiver see what I want it to see or is this too far off the
> standard ? I think that it will see what I want it to see if I supply even
> a minimum of low-pass. Has anyone tried this ?
>
> thanks,
>
>         Peter

DTMF tones were intentionally selected to minimize interference due to
harmonics and intermod products so you may be fine. There is a method to
generate single tones from digital functions using Walsh functions. I'm not
sure how easy it is to use for multiple tones.

Dan