Optical compressors

suspiciousminds

I've just built an optical compressor which has two controls - vol/sustain.
I've used a 1M ldr and a 5mm red led.
Now the circuit seems to work ok - sounds nice but could someone please explain whats the purpose of the ldr/led combination in these circuits?
The Led seems to be "pick responsive" and lights up bright when i play.
I've also housed the ldr/led facing each other  in a small tube but what does this actually do in the circuit?

UnclePsychosis

Optical comps use the combination of a light source and an LDR to achieve the gain control - the light source changes brightness as you play louder or softer and then the LDR reacts to that brightness. The resistance of the LDR is then used to alter the level of the output signal.

The way (speed/knee/ramp/whatever) that the LDR reacts to changes in brightness is what gives optical compressors their sound, really. 

suspiciousminds

Ahh, thx
so in theory if i play harder the signal/vol increases then?

UnclePsychosis

suspiciousminds said:

Ahh, thx
so in theory if i play harder the signal/vol increases then?

Erm... its a compressor?  

As you play harder the input to the compressor will increase and the LED will glow brighter. 

What the output signal volume does is complicated: bear in mind that a compressor makes quiet sounds louder and loud sounds quieter.  

Ovnilabs has a really great overview of compressors (how the work, what they do, what the controls mean, what the difference is between optical and FET...)

http://www.ovnilab.com/faq.shtml