How do compressors work?

[Alpha579]

Yeh, how do compressors work?. I was looking at the compressors at tonpad, and i cant make any sense of them. Whats happening here?
http://www.tonepad.com/getFile.asp?id=9
And are there any really simple compressors?
Thanx
Alex

[Rodgre]

The simplest compressor works by taking the input signal and converting it to a voltage that can drive some sort of light source, be it an LED or a lightbulb, or half an opto-isolator (in the case of the CLM6000, an LED, which is attached to a light-dependant resistor).

This light source faces a light dependant resistor and the louder your input signal, the brighter the light, the brighter the light, the more resistance is created with the light-dependant resistor, and if this resistor is put in the path of the audio signal, it attenuates the signal, thus compressing it.

Threshold is determined by how high the input signal needs to be in order to trigger the light source enough to affect the LDR.

VCA-based compressors work via extracting a voltage from the input signal that is able to control a voltage controlled amplifier. It might be similar in concept to the rudimentary optical compressor, but it's a little more involved, using the change in voltage to create a downward change in gain.

I'm way too tired to make much more sense than this. Sorry.

Roger

[Peter Snowberg]

To add a little to Roger's description...

The stage where the audio is converted to a control signal involves first rectifying it, with either a half or a full wave rectifier, and then using that resulting voltage to charge a cap. A resistor is usually used in parallel with the cap to discharge it (providing the release parameter).

The idea is that as you play, you can charge the cap faster than the resistor will discharge it. As the voltage in the cap increases, so does the attenuation. As the attenuation goes up, the signal that keeps charging the cap falls and everything reaches a happy balance.

If you want a compressor that's a little easier to decipher, try looking at an Orange Squeezer. The OS uses a half-wave rectifier which makes it a bit simpler.

As far as the dynacomp schematic... everything to the left of IC1 is just buffering and bias. IC1 does the actual volume control, and the circuit built around Q2, Q3, Q4, and Q5 works as both an output buffer (Q2) and a full wave rectifier. The 10uF cap between Q3 and Q4 is the cap that accumulates the control voltage (fed to pin 5). The dynacomp actually uses the cap in reverse from some other compressors. It gets charged by the 150K resistor, and discharged by the rectified audio signal.

Take care,
-Peter

[StephenGiles]

Try this for size:
http://www.tonmeister.ca/main/textbook/electroacoustics/02.html