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.