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.

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

In article 4a53c6a2.2006077968@localhost, Don Pearce
wrote:
On Mon, 06 Jul 2009 10:39:03 +0100, JohnT wrote:


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.

That is so. I use cookbooks for such purposes. FWIW I've put some of the
simpler design rules on the web at

http://www.st-andrews.ac.uk/~www_pa/...rt3/page2.html

The snag is that we still don't know the signal levels, impedances, etc,
that would suit John's requirements.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

On Mon, 06 Jul 2009 11:32:27 +0100, Jim Lesurf
wrote:

That is so. I use cookbooks for such purposes. FWIW I've put some of the
simpler design rules on the web at

http://www.st-andrews.ac.uk/~www_pa/...rt3/page2.html

The snag is that we still don't know the signal levels, impedances, etc,
that would suit John's requirements.

Well, given the job it is to do, I doubt that levels are important
unless they are so high they cause overload - easily dealt with. And
for impedances, my normal way is to make the input high and the output
low; you seldom go far wrong that way.

d

On Mon, 06 Jul 2009 11:27:54 GMT, (Don Pearce) wrote:

On Mon, 06 Jul 2009 11:32:27 +0100, Jim Lesurf
wrote:

That is so. I use cookbooks for such purposes. FWIW I've put some of the
simpler design rules on the web at

http://www.st-andrews.ac.uk/~www_pa/...rt3/page2.html

The snag is that we still don't know the signal levels, impedances, etc,
that would suit John's requirements.

Well, given the job it is to do, I doubt that levels are important
unless they are so high they cause overload - easily dealt with. And
for impedances, my normal way is to make the input high and the output
low; you seldom go far wrong that way.


There are plenty of circuits available, and yes I could buy in the
components and make them up. But charging my time to do this will make
the result too expensive for the customer.
It would be cheaper to buy these:
http://www.behringerdownload.de/FBQ8...Info_Rev_A.pdf
which are really too good for the job. But in the absence of anything
simpler, that is what I will do.

JohnT wrote:

There are plenty of circuits available, and yes I could buy in the
components and make them up. But charging my time to do this will make
the result too expensive for the customer.
It would be cheaper to buy these:
http://www.behringerdownload.de/FBQ8...Info_Rev_A.pdf
which are really too good for the job. But in the absence of anything
simpler, that is what I will do.


Suggestion...find out what your customer really wants. For example....

I've recently completed work for a client who wanted some audio networks for
training groups of students in comms protocols. One part of the spec was for
a restricted frequency range. They wanted it to sound like a "radio comms
network". My solution was very very simple and pragmatic, consisting of
nothing more than a judicious choice of feedback and coupling capacitors
among a few basic op amps. I didn't even bother to check the overall
frequency response until my prototypes sounded "right". My client was not
interested in numbers, he just wanted a certain sound quality and I provided
it. It was a very cheap solution in terms of components and design time and
the client was delighted with the results. After the first system was
delivered he came back and ordered two more.

BTW, thanks to those in here who gave me valuable advice when I was clueless
at the start of this work some six months back...you know who you are ;-)

"TonyL"


BTW, thanks to those in here who gave me valuable advice when I was
clueless


** Got some news for you - pal.

YOU are still, 100%, ****ing clueless.




...... Phil

On Tue, 7 Jul 2009 10:45:22 +0100, "TonyL"
wrote:

Suggestion...find out what your customer really wants. For example....


I do know *exactly* what the customer wants, which is to eliminate
low- and high-frequency machinery noise from an operator's microphone.
I have ordered the FBQ8000 which are probably overkill but are
guaranteed to do the job and are much cheaper than I would charge for
my time in building something suitable.
Thanks to everyone for their help.

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.

In article ,
JohnT wrote:
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.

Better way might be to use a noise cancelling mic.

--
*I almost had a psychic girlfriend but she left me before we met *

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