Before going into the schematics, here's essentially what they do: given an input voltage from a guitar pickup, the compressor limits the output voltage below a preset value. It doesn't do that by just clipping any voltage above a certain value (like distortion pedals do) but instead by progressively decreasing the amplification of the incoming signal. The total dynamic range of the sound is "compressed" by limiting high voltage signals while maintaining low voltage signals, which results in a crisper sound with high sustain. Here's a clip of someone using an MXR Dyna Comp pedal, for example, and here's the sound you can get by combining a fuzzbox and a compressor.
The dynamics of the voltage manipulation break down into several aspects: the attack is the time until the compression effect kicks in after an input voltage is detected, the release time is how long it takes for the compression effect to diminish; the threshold is the voltage amplitude higher than which the compression effect takes place; the compression ratio is the amount by which the gain is reduced (this can be set such that any given input amplitude will leave the pedal with a constant output amplitude, giving you loud harmonics); finally you may need to increase the overall output amplitude to make up for the selective damping, with a level control.
This can all be accomplished with a few integrated circuits and passive electronics, which is what I'll be building over the next couple of weeks. To pick up some more intuition about the effect of compressors, though, I've been playing around with the audio manipulation software Audacity. Here's the spectrum of a distorted guitar lick without compression.
|Audio sample, no compression|
Now going into the compression settings:
The basic controls that will be available on the physical pedal are here, too. If I lower the threshold to its minimum value and max out the compression ratio, it applies a significant compression effect to sounds of every amplitude in the clip.
You can see based on the spectrum that the difference between the smallest and largest amplitudes has decreased -- in other words, we've compressed the dynamic range of the sound. That's what it's all about!
Another way to look at the effect is by plotting the absolute value of the output voltage amplitude as a function of the corresponding input:
The degree of compression in this plot is quantified in units of a potentiometer's resistance setting -- 0 Ohms corresponds to no compression effect (the purple line), which is linear since any V(in) gets mapped to itself; as the value of the resistance is increased, every V(in) gets mapped to the same V(out), and in this case the dynamic range of the sound is completely compressed. Note that the frequency of the sound isn't being altered in any of this, just the amplitude -- so you'll still get distinct notes.
As for the electronics, here's the circuit diagram I'll be using:
It uses two op-amp integrated circuits and four current-controled transconductance amplifiers, along with potentiometers to vary the level and compression factor and passive electronics to vary everything else. In my next post I'll describe the circuit in detail and post some design plans!