Ambient Noise - Volume Adjustment Intercom Circuit

Anybody out there work on Audio circuits?

I'm trying to design & build a motorcycle intercom, similar to the Starcom1? see

formatting link

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 ( snipped-for-privacy@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: snipped-for-privacy@bellatlantic.net ( snipped-for-privacy@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

Reply to
zipzit
Loading thread data ...

Most likely.

You might find my circuit, description, and construction project of some interest...

formatting link

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.

Reply to
Richard Crowley

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
Reply to
Ban

I read in sci.electronics.design that zipzit wrote (in ) about 'Ambient Noise - Volume Adjustment Intercom Circuit', on Sun, 3 Oct 2004:

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.
 Click to see the full signature
Reply to
John Woodgate

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

Reply to
Pop

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

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:

formatting link

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

formatting link

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

Reply to
Thomas P. Gootee

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.