"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

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.

On Mon, 06 Jul 2009 12:04:57 +0100, "Dave Plowman (News)"
wrote:

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.

This is true, but I am assuming he doesn't have access to the source,
and is trying to make the best of what he is receiving.

Somewhere in a junk box I have a pair of throat mics that used to do
service in a Lancaster bomber. They are quite uncanny - you could use
them next to a revving F1 car, and hear nothing but the voice. The big
problem is that they don't catch any kind of sibilant, so you have to
be careful what words you say if you want to be understood.

d

"Don Pearce Congenital IDIOT"
"Dave Plowman"

Better way might be to use a noise cancelling mic.

This is true, but I am assuming he doesn't have access to the source,
and is trying to make the best of what he is receiving.


** Funny that the OP's first post starts out like this:

" I need a small, simple preamp to tailor the output of a standard
electret mic, to limit the bandwidth to approximately a telephone.. "

He has access to the source signal and mic.

Dickhead.



.... Phil

On Mon, 06 Jul 2009 11:58:22 GMT, (Don Pearce) wrote:

Better way might be to use a noise cancelling mic.

This is true, but I am assuming he doesn't have access to the source,
and is trying to make the best of what he is receiving.

Absolutely correct. The microphone is built in to a piece of
engineering quipment and for 'elf 'n safety reasons I cannot get
access to it - only to a remote connection point.

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.

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

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