"JohnT"
"David Looser"


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.


** Very likely, that is a 100% wrong assertion.

Obtaining good voice intelligibility from a microphone used in high noise
environments is a problem that was SOLVED about 70 years ago. One simply
buys or creates a "noise cancelling" microphone.

http://en.wikipedia.org/wiki/Noise-canceling_microphone

Two mics in one really, one picks up the voice plus nosie the other the
noise only.

Subtraction produces a result that is largely ambient noise free.



..... Phil

Have you looked into some of the add on devices used by ham radio folk to
tailor the sound either in the outgoing or incoming audio sections? Sounds
like your needs are more akin to this group of users than prof audio ones.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"JohnT" wrote in message
...
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.

"Brian Goof"

Have you looked into some of the add on devices used by ham radio folk to
tailor the sound either in the outgoing or incoming audio sections?


** None of which are of ANY use when the noise originates at the mic
diaphregm.

In such cases, two way radio users employ "noise cancelling" mics.

Dickhead.


..... Phil

Yeah, I know, , many years ago when I could see made one out of an old 741
op amp. Crap signal to noise, but you could reall tailor the responce very
well with cheap components, and end up with a healthy output.

As you say, its intriguing why pre grottification might be needed!
Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"David Looser" wrote in message
...
"Brian Gaff" wrote in message
om...
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.

David.