Ambient Noise - Volume Adjustment Intercom Circuit

From: zipzit (zipzit_at_yahoo.com)
Date: 10/04/04 

Date: 3 Oct 2004 18:36:25 -0700

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: Re: Intercom Schematic wanted
     From: Graham E Laucht (graham@ukavid.demon.co.uk)
     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: Re: Sound Activation
     From: ehsjr@bellatlantic.net (ehsjr@bellatlantic.net)
     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

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