Yiz....

A decent zero crossing detector can be made with one or two op-amps. You
are not looking for modulation on the signal (AM) but simply capturing time
from one crossing to the next....to get period duration.

With two tones present, it would seem that one should separate the tones by
band first and then process them.

You might look at some systems that do DTMF decoding and adapt from there.
Perhaps start with the literature with app notes on analog PLLs.

Phil.

On Monday, June 26, 2000 4:28 AM, Yiz Garnoff
[SMTP:yizgarnoff@OPTUSHOME.COM.AU] wrote:
> When the subject of FFT comes up many people reply suggesting the use of
> zero crossing's to detect tones like DTMF etc, where DSP might otherwise
> be used.
>
> I have read a lot about DSP lately, but did not read about the zero
> crossing method.  How is it used?  What are the imputs?  How do you
> process them to detect tones etc?  I can't see how to apply DSP to this,
> since DSP uses ADC values generally.
>
> Also, any good web links on this theory?
>
> TIA.