zipzit wrote:
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
As far as I understood your description, you really need 2 different
automatic volume adjustments, one activated by the voice mike to reduce the
music volume, and another one activated by an environmental mike to increase
the main volume with speed.
The first can be done in a single 10-20dB step and also activate a gate,
that mutes the mike signal when not used. This can indeed be done with a
comparator and a FET-switch. You will need a mixing stage before the power
amp and can use the fets in the summing node. It will be also useful to
filter the voice signal with a bandpass to increase intelligibility and
detection.
The speed compensation is more difficult if you want a stepless operation. I
wouldn't use the feedback point of the power amp as you describe, but do it
as well in the mixing stage by varying the feedback resistor, either a Fet
again or some VCA circuit like the SSM2164. This chip works controlling the
current into a summing node. There are 4 independent VCAs in one package,
which will allow stereo operation. The control signal can be obtained by an
environmental mike, which samples the noise level without getting much of
the voice. Maybe you can even use the same mike as for voice with some 10k
highpass filter to suppress the voice frequencies, but this should be
experimentally veryfied. You also have to linearize the control voltage
because the chip has a dB-linear characteristic.
I have the impression you will need much more help to get this working with
your actual state of knowledge, maybe better to buy the ready made solution?
--
ciao Ban
Bordighera, Italy