> >My solution to TT decoding was to precede each bank
> >of 567's (two tone ranges) with an appropriate low or
> >high pass op-amp filter. The 567's would then actually
> >'work' whereas before operation could not in any way
> >be guaranteed ...

If memory serves me, on 567's data sheet they present the DTMF decoding
solution without this preamplifier.

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