If you have the background experience, you can grab the ambient
noise, invert it, and feed it back into an amp 180 out along with
the original, and thus zero out the noise. It's a method often
used to zero out static, switch pops, hums, etc.. I made a lot
of use of it in the electronic organ arena in its heyday. Works
very well with headphones also.

Pop



"zipzit" wrote in message
om...
| Anybody out there work on Audio circuits?
|
| I'm trying to design & build a motorcycle intercom, similar to
the
| Starcom1. see http://www.starcom1.com/
|
| I found some pretty good reference materials, particularly an
old
| newsgroup posting for an airplane intercom. This posting is
| exceptional, with lots of details on why and how.
|
| Do a google groups search for:
| Subject: Intercom Schematic wanted
| From: Graham E Laucht )
| Newsgroups: rec.aviation.homebuilt
| Date: 1997/02/25
|
| The intercom should be able to take multiple inputs (cell
phone, cd
| player and two way radio) and have the ability to mute music
when
| radio is used. The unit should be small/ compact and use low
power.
| We're only driving headphones speakers.
|
| One feature that I found of interest is a automatic volume
adjustment
| scheme. Things can get pretty loud in a motorcycle helmet at
higher
| speeds. I'm looking for a cheap man's speed sensitive volume
system.
| I know the auto industry uses actual vehicle speed input for
radio
| volume adjustment (I'm an Automotive Engineer. with close ties
to
| radio design folks.) I'm thinking the Starcom system uses
helmet
| microphone input to take a measure of increasing ambient sound,
then
| adjusts audio amplifier gain to match.
|
| I've done lots of searching on the web, and the only postings
that I
| can find on the subject are for volume actuated switching. Do
a
| google group search for:
| Subject: Sound Activation
| From: )
| Newsgroups: sci.electronics.components
| Date: 2002-05-19 21:02:20 PST
|
| The hint from that posting is "to amplify the audio, rectify
the
| output of the amplifier, and use the rectified voltage to
charge a
| capacitor. The voltage on the capacitor then turns on a
transistor
| which operates the relay."
|
| I don't want to operate a relay, I want to affect the gain of
dual
| LM386 audio drivers. Currently the plan is to use a stereo
volume
| control on the input signal. The LM386 also has external gain
inputs.
| The gain modification circuit is normally meant to be fixed
with the
| addition of a RC series circuit. No circuit = 20 gain, 1.2k
Ohm +
| 10uF = 50 gain, 10uF alone = 200 gain.
|
| I guess it would be possible to use a comparator, and use the
| comparator outputs to switch in alternate resistors. and go
step
| function on volume increases. Somehow that just doesn't seem
very
| elegant.
|
| Is there anyway to use microphone sound density input to
generate a
| isolated variable resistance output, to work as an automatic
volume
| control for varying ambient noise levels? Are there other ways
to do
| this?
|
| Thanks in advance for any design hints.
| LB
| Detroit, Michigan
|
| Cross posted to:
| sci.electronics.design
| rec.audio.tech
| sci.electronics.basics