Sign in with
Sign up | Sign in
Your question

Ambient Noise - Volume Adjustment Intercom Circuit

Last response: in Home Audio
Share
Anonymous
October 3, 2004 10:36:25 PM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

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
Anonymous
October 3, 2004 11:26:53 PM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

"zipzit" wrote ...
> Anybody out there work on Audio circuits?

Most likely.

> 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.

You might find my circuit, description, and construction project of
some interest... http://www.rcrowley.com/ComClone/default.htm

> 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.

The same ambient sound-level adjustment scheme is used in
many PA/paging systems in public spaces (airports, etc.)

The three basic building-block circuits you need are:
1. Microphone preamp
2. Audio signal rectification (and integrating)
3. VCA (voltage-controlled amplifier)

There should be many examples of these circuits out on the
internet. The mic preamp can likely be a single op-amp, and
you could likely use the other half of a dual op-amp for the
active rectification/integration circuit (similar to those used
for audio level metering). And there should be several good
application notes by the vendors of VCA chips.
October 4, 2004 7:20:54 AM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

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: 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

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
Related resources
Can't find your answer ? Ask !
Anonymous
October 4, 2004 3:08:41 PM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

I read in sci.electronics.design that zipzit <zipzit@yahoo.com> wrote
(in <39a4cdf7.0410031736.579161f3@posting.google.com>) about 'Ambient
Noise - Volume Adjustment Intercom Circuit', on Sun, 3 Oct 2004:

>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?

Not an isolated one, easily, but a grounded one, quite simply. You can
sue a bipolar or JFET as a variable resistor to form the grounded arm of
a potential divider at the input of an op-amp. The op-amp output feeds a
diode rectifier and the d.c. output of this feeds the base or gate of
the control device. You could use the LM386 instead of an op-amp, I
think.

There are lots of examples of this sort of automatic gain control
circuit on the net. To make it into an ambient noise compensator, you
need to use the rectified, amplified ambient noise from a microphone to
*oppose* a bias voltage or current on a control device that is keeping
the gain low in the absence of the noise-related signal.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
October 4, 2004 7:34:48 PM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

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" <zipzit@yahoo.com> wrote in message
news:39a4cdf7.0410031736.579161f3@posting.google.com...
| 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
Anonymous
October 5, 2004 12:34:18 AM

Archived from groups: sci.electronics.design,rec.audio.tech,sci.electronics.basics (More info?)

zipzit@yahoo.com (zipzit) wrote in message news:<39a4cdf7.0410031736.579161f3@posting.google.com>...
> 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/
>
<snipped>
>
> 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

------------------------

LB,

As John Woodgate (I think it was) said, you could use an FET as a
voltage-controlled resistance, to control the gain of an opamp, or a
similarly-controlled amplifier.

I did something that's somewhat similar to what you are wanting to do:

To sense and convert the signal (in your case the mic output) that
should affect the gain, I used an opamp circuit similar to a "peak
detector" or an "envelope follower". There are many examples of both
of those, on the web, especially in some of the opamp application
notes at IC manufacturers' websites. I can email the schematic of the
one that I used, to you, if you like. That circuit should produce a
quasi-DC voltage that is proportional to the mic output's amplitude.
You can then put that through another opamp circuit, to buffer, and
possibly change the level of, and/or the sign of, that voltage. You
could also do other stuff to it, as needed, before using the voltage,
or a current produced with it, to drive some variable-resistance
device that controls an amplifier's gain.

In your case, you may need to do a little more conditioning of the
signal, at some stage, or find some way for the circuit to tell the
difference between the times when there is only ambient noise and the
times when there is a voice or other input, and some way for it to
"hold" the gain-setting voltage that's derived from the ambient noise
level, when there is a voice or other desired stronger input present.

Instead of using an FET as the controllable gain-setting resistance, I
used a simple analog optical isolator, a VTL5C2 "Vactrol", which is a
current-controlled-resistance device and is actually just an LED
encapsulated with a photocell. So there are four leads: two for the
LED (and your control current) and two for the photocell, across which
a "pure" resistance is presented (at low frequencies, at least; never
looked at higher than audio).

In the case of the VTL5C2 model, the resistance across the photocell's
two leads varies from about 2 megohms, with 0 mA through the LED, down
to about 200 Ohms, with 40 mA through the LED. (The VTL5C2 is
available from bgmicro.com, for $0.50 for qty 1.) Check out the intro
and appnotes links for them, at:

http://optoelectronics.perkinelmer.com/catalog/Category...

In my circuit, the final control voltage output from an opamp circuit
was connected to the vactrol's LED's positive lead through a resistor
of a few hundred ohms (with the LED's neg lead connected to ground),
in order to produce the proper current range for my application.

Note, also, that the vactrol's resistance does not vary linearly with
current. In my case, that didn't matter too much, since it was inside
of a feedback control loop and just needed to get to the correct
resistance within about a second or so. But it probably won't matter
for your AGC application, either.

Good luck. I hope this helps.

Regards,

Tom Gootee

http://www.fullnet.com/u/tomg

-----------------------------
August 28, 2011 8:17:40 PM

Quote:

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


Maybe you can solve this problem by having an iPhone and a special app made by me: http://AutoVolumeApp.com video: on Youtube

Having GPS + Phone + iPod Music + Auto Volume Control running in the background is a good choice I think. Let me know if this can help you.

AutoVolume is a cool iPhone app that will constantly measure the outside noise level with the iPhone's built in microphone and then gracefully adjust the music volume depending on the noise outside and your personal settings.

Use it while traveling on noisy transport that is making stops, walking on loud streets. It is also able to instantly lower the music volume when noise is heard (limiter). Making it useful in office or during any kind of activity that requires your sudden attention. More over it responds when you start talking.

Best regards
!