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Topic: device that converts microphone level into dc voltage
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Randy Stankey
Film God
Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99
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posted 04-16-2017 11:42 AM
Do you want automatic volume control for your lobby so that the music will get louder when the atmosphere gets noisy?
BTW: "Gain" and "Volume" refer to different things:
"Gain" is the input/output ratio of an audio signal as it passes through an amplifier or some piece of equipment.
"Volume" is simply the perceived loudness of an audio signal from a listener's point of view.
I don't think you want gain control. I think you want "loudness control." Am I right? (Am I understanding you correctly so far?)
Simply hooking a microphone to a loudness control in order to control volume is likely to cause problems.
If there is a sudden change in the noise level of the area, the sound level will suddenly change. For instance, if somebody shouts, the volume will get louder, too. The volume level of the music could suddenly become ear-splitting loud! If a truck drives by, if an airplane flies over, if there is thunder, the volume level could be jumping up and down all day.
If the microphone "hears its own sound" (so to speak) you could also start a feedback loop where the volume level seems to get louder or softer for no apparent reason.
I think we could design a device which listens to the sound level of a room and "decides" whether to raise or lower the volume of the sound system. I have a general idea how that is done but I am not sure how to design such a circuit.
Maybe there is already some such device on the market? I don't know.
To be honest, I wouldn't do that kind of thing. I would rather put a remote control on the sound system so that you can adjust the level from a more convenient place instead of "running upstairs."
Depending on the equipment being used and the location, it could be as simple as running a set of wires and attaching a potentiometer. (Volume control) I bet a smart person could design a way to do it with your iPhone.
I wonder if there is already an app for that! ![[Wink]](wink.gif)
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David Buckley
Jedi Master Film Handler
Posts: 525
From: Oxford, N. Canterbury, New Zealand
Registered: Aug 2004
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posted 04-16-2017 03:21 PM
What you want is a Symetrix 571 SPL computer, which is a tool designed to do exactly what you want. No longer available new, but are readily available on eBay. The manual (with schematics) is still on Symetrix's website.
I have one of these on the foyer of our little community theatre (not a movie house!) for the BGM speaker; as the foyer racket level goes from silent to many people talking loudly the BGM level just follows the room noise.
This is a discontinued product; This device, like several other audio black (or perhaps, rack) boxes (anyone remember room combiners?) have now been replaced by the configurable DSP audio platforms, like Soundwebs, HAL etc, which have this function available as a drag-and-drop module. However the scale of investment required for a DSP platofrm is much greater, even on eBay, and unless you need the myriad functions that these boxes can perform, a 571 will be the lower cost option.
One of the biggest user of these devices used to be McDonalds...
Edited to add: forgot to mention: the idea of a simple AGC scheme by having a noise-to-voltage converter and using that to control the mixer is doomed to failure. The reason is the measurement microphone is picking up both crowd noise, and BGM noise, which gives a composite level. And what you have is a positive feedback loop, so as the noise level goes up, the BGM volume goes up. So the overall volume is now measured as "higher", so more BGM is needed. So the level is now measured as "even higher", so more BGM is needed. Rinse and repeat until BGM is full on and can go no higher.
The SPL computer measures the RMS energy of the music, and subtracts it from the RMS energy of the room, thus giving the actual crowd noise level, and that is then averaged to set the BGM level.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 04-16-2017 05:45 PM
So close. If you are going to provide an RDL solution, one closer to the mark is the ST-GCA3 http://www.rdlnet.com/product.php?page=233#!prettyPhoto (Gain Controlled Amplifier). it is an AGC with configurable ballistics (to avoid sudden jumps up/down in level). Mind you this isn't a solution but possibly a building block.
Honestly, making what you want isn't all that hard, but doing what you want with it is just the beginning of what you are trying to do. That is, take a mic preamp, run its output through a bridged rectifier with a suitable capacitor and you can generate a DC voltage that is a representation of the level of the input. Then, using comparators (e.g. op amps) you can set thresholds of when you want the BGM to go up/down and also set up a hysteresis to avoid rapid jumping around.
If I was trying to do what you are thinking of, I'd probably to the drag-n-drop DSP type of box. I suspect that one could adapt an AEC (Acoustic Echo Cancellation) module into doing what you want. It essentially has the bits and pieces. It normally has two inputs (near end and far end...or in your terms, music and noise), it adds the far end inverted back into the signal so that as it is modulating, it is cancelled out of the signal...which is what you want to happen to the BGM when it comes to altering the level so what should come out of the AEC module is just the noise, in your case. But even without an AEC module (some make than an expensive option) you still can have your BGM signal add in to the AGC control inverted to cancel it's contribution out...just measure it with an empty lobby. So long as your amplifier gain is fixed, it should track your music. Then see what your DSP has for an AGC module. A lot of them have side-chain inputs where the side chain is what triggers what happens to the processing of the signal.
This is what I thought of about as fast as I was typing...it isn't a solution but an idea of what you can do. Then there is good old experimentation.
I just checked, both Rane and Q-Sys have forms of ambient noise compensation. I haven't investigated beyond to see that your solution is probably already figured out.
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