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Charles is looking for some help with a project, read the challenge and post up in the comments if you have ideas! "When you listen to music or speech in a noisy environment you have to constantly, continuously adjust the volume control: boost the gain when the sound level is low, reduce the gain when the level is high. For example trying to listen to a movie in an airplane, or to a program in a car. What's needed is a gadget that automatically changes the gain of the amplifier, as a function of the average sound level, to reduce the dynamic range of the sound source -- an audio compressor."
In the old days of AM radio, the engineer did this job so as to broadcast the highest average power level and have his station noticed. They called it "riding the gain." There have been stereo amps that had this feature, but they are rare.
THE SPECIFIC GADGET DESIRED: A small package that I can use between the airplane audio jack and my headset. It might need to have a small amount of amplification to make up for signal losses in the compressor circuit. Monaural would be fine: combine the two channels from the plane and deliver monaural output to the headset.
Controls: Volume control is optional. But it must have something to control the degree of compression -- from relatively flat dynamic range to full range. The gadget would also be usable for a car radio: hook it up between the volume control pot and the amplifier. All I need is a tested circuit diagram, I can construct it myself. (I'm a vacuum tube, analog circuit person who never made the transition to digital circuitry.)
http://machines.hyperreal.org/categories/DIY/info/compressor.
would be a good start point.
Personally, whenever I need an automatic level control, I rummage up a thrift shop cassette recorder or two and yank the circuitry out of them.
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I don't have a circuit for this, but it's an op-amp problem, not a digital one. Here's a sketch. Feed a tiny microphone picking up ambient sound into an pre-amp (op-amp), then a rectifier (diode), and then into an integrator with an artificially-leaky capacitor (op-amp, capacitor, resistor). This output corresponds to the most recent ambient average environmental sound. Use this signal to control a pair (binaural) of variable gain amplifiers (2 each op-amp, FET). Volume control can be added by scaling the signal to the FET gates (potentiometer). It could fit into an mint box.
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