On Mon, 06 Jul 2009 10:39:03 +0100, JohnT wrote:

On Mon, 6 Jul 2009 08:17:31 +0100, "David Looser"
wrote:

"Brian Gaff" wrote in message
.com...
This may or may not be a daft question here, but..
If the system is of restricted bandwidth, would not this fact do the job,
or do you get funny effects with wider bandwidth signals. Cannot say that
feeding nice quality down grotty phone lines sounds any less grotty if
band limited or not.

The nasty bodgit design inc solution is a high value capacitor in series
with the mic and a vvery ngative feedbacked op amp for the amp which will
maybe not be as noise free as it could be though.

Brian


The problem is that John hasn't told us what the application is, so we don't
know what sort of performance he is looking for, nor do we know the reason
for asking for a telephone bandwidth. We don't know what sort of budget he
has either. He says he doesn't have the time to design and build, yet in
fact reserching for a commercial product to perform this could take him far
longer! Ordinary tone controls will not, of course, simulate a telephone
bandwidth with any kind of accuracy. Telephone band filters exist, they have
been used for many years, originally in front of modulators for FDM cable
systems and more recently in front of voice codecs for digital systems, but
these are industrial products not readily available in small quantities.


Thanks for the comments.
What I am trying to do is to optimise a voice signal in the presence
of high background noise levels, so I need a fairly sharp bandpass
filter.
I have of course tried simple RC sections for HPF and LPF but this is
not sufficiently sharp for the purpose.

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.

The Texas Instruments web site knowledge base section offers
interactive filter design pages that will do what you need. I'm sure
that every other op amp designer has similar application notes.

d