3
Usage Notes
How The Compressor Works
The basic concept behind compression is simple: the audio level in the signal path is measured
and this value is then used to control the overall output level by some amount proportionate to
the measured level. There have been a number of different schemes and circuit topologies de-
signed over the years to accomplish this task – it’s well beyond the scope of this user manual to
delve into all of them in detail. The Diamond Compressor uses a simple and relatively non-intru-
sive resistive divider gain reduction system that utilizes an analog optical isolator to map output
signal level to a variable resistance. The opto-isolator (for short) is placed in a feedback loop – it
uses the output level from the gain reduction system to stabilize its own gain reduction system.
This stabilization can seem somewhat counterintuitive, but basically the louder the signal, the
more gain reduction takes place, which lowers the signal level, and reduces the gain reduction,
and so on… until the output level stabilizes at some reduced level. The amount of reduction that
takes place is dependent on the input signal level, the amount that level is above a gain-reduc-
tion threshold, and the steepness of the gain reduction slope once the threshold is met. Basi-
cally, any input signal below the threshold is not compressed, and any signal rising above the
threshold is reduced in level by a gain-reduction slope or ratio. The Diamond Compressor has a
very simple system for providing control over compression. The Comp control is in reality a con-
trol for a voltage amplifier placed in the signal path which performs several duties:
it provides make-up gain to compensate for overall loudness loss due to the resistive
divider compression loss circuit
it provides a mechanism to boost the input signal which in turn has the effect of increas-
ing the amount of signal above the fixed threshold in the signal level measuring circuit
(aka sidechain). The more signal above the threshold, the deeper the attenuation of the
resistive divider, and the more perceived ‘compression’.
The Volume control is simply a final attenuator that the user can use to balance the perceived
loudness between effect on and off. This volume balance between effect on and off is very much
dependent on how hard the player picks – try setting the Volume and Comp controls iteratively
to the desired amount of compression and overall level while playing a chord or run at your
usual picking strength.
Unfortunately, there’s still a bit more to it all… Along with the threshold and compression ra-
tio, the ‘personality’ of a compressor is defined by just how quickly (or not quickly) it reacts
to changes in level. Things can get really confusing here… Each guitar note played has certain
dynamic characteristics. Sound synthesis people often talk about ADSR – Attack, Decay, Sustain
and Release when designing synthesizer sounds or patches, and many of those concepts hold for
how a compressor reacts to a guitar waveform. Basically, when a note is picked, there is a short
Attack and Decay period that covers the initial transient of the picking action. The string then
begins to vibrate in a more steady-state fashion for a certain period of time, constantly decay-
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General