Recording setup featuring a dual channel tube VCA compressor
This one might just be my favorite—not to use, necessarily, but to describe. Why? Because it depends on light, or more specifically, light-dependent resistors!
But wait, what’s a resistor? To properly get into this stuff requires talking about the nature of electricity. This would result in a heady discussion that would bore the pants off of both of us, so let’s skip the science and go right to a metaphor commonly used to describe resistance—that of water going through a pipe.
Just picture it: water flows through the pipe, and the pipe carries the water where it needs to go.
So far so good, right?
But what if we put a cap on that pipe, one with a few small holes? Yes, water may only trickle through the holes, but the resistance of the water on the other end of that pipe—the pressure of it—has increased as it builds up. Thus, when a pop-science article on resistors states that “if you turn the volume down, you're actually turning up the resistance”, the metaphor helps us to understand why this the case. We now begin to see the function of resistors in the circuit of a compressor—they help put the squeeze on the signal we need to tame.
But how does this impact the sonic characteristics of optical circuits?
In an optical compressor, the resistors are light-dependent: the audio signal feeds a lighting element (such as an LED), which shines upon a light-sensitive resistor. The resistance of this light sensitive element informs the compression circuit how much and how quickly to attenuate the audio signal.
The wrinkle here is that this interplay between the light source and the resistor, while fast, is not instantaneous. Furthermore, different types of light sources illuminate at different speeds, and a resistor can react differently depending on the material from which it is made. For this reason, an optical compressor’s sonic behavior is highly dependant on the types of materials used in its construction.
But here’s a commonality between them, no matter the make: the attack and release of an optical circuit is (at least most of the time) definitely not linear, often involving a bit of delay before the attack kicks in, and additional delay as the release drops off.
For instance, the harder you hit an optical compressor, the quicker its initial release time can be—but the slope back to a normal, uncompressed sound will not fall in a linear fashion. It will “curve.” So if the circuit gives you 10 dB of gain-reduction, the first five decibels might release much more quickly than the following five.
This bit of behavior is something you can hang your hat on when it comes to most plug-in emulations: the specific timing will certainly change depending on emulation, but the attack and release will act in a way that is a) often slower than many other compressors, and b) more meandering as it starts and stops.
The behavior of these time constants can result in a compression that is quite musical and often smooth. In general, vocals, lead lines, and other elements that need an intangible “rounding out” (not so much a hard squash as a general evening out, or a shapely bolstering) can benefit from optical designs and optical emulations. It’s not as functional for transient shaping, though of course there are exceptions to this rule given the wide variety of types of light sources and resistors available.
To me, optical compression is quite poetic, as it involves the communion of light and sound. It elucidates their wave-like commonalities, getting to the core of how they can influence each other.