Opposed to VCA, Variable-Mu or FET based approaches, opto-electrical compression takes advantage of using a light-sensitive resistor and a small light emitter (a LED or electroluminescent panel) to obtain a gain reduction voltage in the sidechain path. This technique is well-known to add some smoother gain riding characteristics to the signal because of the specific attack and release response which comes from the inertia and inherent memory effect of the photoresistor element.
Typical classic opto-electrical circuit designs do have a significant and highly program dependent impact on the processed sound and this comes mainly from three factors:
- The adaptive release time characteristic which gets faster at higher compression activity and can be fairly long when leaving the compressors duty cycle.
- The specific compression transfer curve which features soft-knee characteristics by nature and limits the dynamic range.
- The frequency dependency and non-linearities which impacts the actual behaviour of things like the compression transfer curve.
In addition, opto elements do have an inherent lag time in their attack response which is typically not fast enough to catch short transients but adds up to an overall smooth gain riding impression.
Light and shadow
Opto compression really shines on overall adaptive and smooth gain riding purposes such as for vocals and solo instrument performances, whereas faced with rather complex program material it can easily tend to sound rather quirky. This might be the reason why it is typically not used that often on the mix-bus or if so then just with few amounts of gain reduction.
In digital implementations where the opto element effects are modelled in an explicit fashion this kind of weirdness can be avoided and both, frequency dependency and non-linearities can be applied in a way to better support full program material.