In article 4a53c6a2.2006077968@localhost,
Don Pearce wrote:
The just make a multiple pole filter using op-amps. There are plenty
of cook book style recipes to do this. The usual way to maximise
speech intelligibility would be, as you say, a pass band of 300Hz to
3kHz (approx), but between those extremes, you should have a 6dB per
octave rise to accentuate the top end.

I'd go up to approx 4.5Hz. Same as AM transmitters. The 'presence'
frequencies help with clarity.

--
*Young at heart -- slightly older in other places

Dave Plowman London SW
To e-mail, change noise into sound.