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Ambient Noise - Volume Adjustment Intercom Circuit

Discussion in 'Electronic Basics' started by zipzit, Oct 4, 2004.

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  1. zipzit

    zipzit Guest

    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 ()
    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: ()
    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
     
  2. Most likely.
    You might find my circuit, description, and construction project of
    some interest... http://www.rcrowley.com/ComClone/default.htm
    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.
     
  3. Ban

    Ban Guest

    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?
     
  4. 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.
     
  5. Pop

    Pop Guest

    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



    | 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 ()
    | 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: ()
    | 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
     
  6. ------------------------

    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.aspx?CategoryName=VTL5C+Series

    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

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