tribalbeeyatch
A photo-optical uses a light souce to provide gain reduction, rather than a VCA, which uses voltage for gain reduction. The main difference is VCA compressors under gain reduction, and even in a soft knee ratio, work full bandwidth, so they squash out the mid and upper end frequencies, and even pump and breath under higher ratios.
VCA devices are really more of a limiter than a true compressor. The compressor was invented as an effect, and not a protection device. Those are called limiters as in "Brick Walling" which prevents signals from passing a predetermined level. For vocals, VCA compressors have a tendency to make them muddy. While they are good for bass guitar, and drums, and as a protection device, Photo-Optical compressors are really good for vocals, acoustics, guitars, etc., as they let the mid and high end pass through, so the end result is a silky smooth warm sound.
The light source from a photo-optical compressor produces a 2nd order harmonic distortion that gives it its very warm tube like tone. This is a very desirable sound sought after by many. Most photo-optical devices rarely go over a 10:1 ratio, as anything over that is really limiting. It is a device that you certianly want in your bag of tricks. It is not the only type of compressor you should use, but they are really good.
I hope this helps you.
Alan Hyatt
