The Technical Stuff
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Perhaps the most important thing about electro-optical compression and limiting is that it is 100%
program-dependent; in other words, both the degree of gain reduction and the compression ratio vary
continuously with the incoming signal, making for a very natural sound. What’s more, unlike other
compressors which allow the user to adjust the attack and release times, both of these parameters
are automatic and are completely determined by the response of the EL panel and photo-cell in the
T4.
The
electroluminescent panel utilized in the T4 discharges most of its light very quickly, resulting in
a fast attack time (which can be as low as 10 milliseconds, depending on the frequency of the
incoming signal), Even more critical to the sound of the LA-610 compressor is its signal-dependent
release time. Short transients are released quickly, while longer, more sustained parts of the sound
are given a much slower release. Like the original T4, the T4 actually releases in two stages: the
initial release generally takes place in about 40–80 milliseconds (which is relatively fast), followed
by a gradual release that can take as much as several seconds. This kind of program-dependent dual
stage quick-then-gradual release results in a warm and natural sound without the “pumping” which
plagues so many other compressor designs.
The amount of time it takes for the photo-cell to recover after the light is removed depends on how
long light had been shining on it and how bright the light was. This causes something called
“memory effect.” As a result, you can actually “train” the T4’s response characteristics by pre-rolling
material for a minute or two, essentially saturating the photo-cell. Similarly, because the amount of
time it takes the luminescent panel to light up determines the attack, you can “prime” the T4 to light
up faster so that the first note's transient doesn't sneak by too aggressively. To do so, simply have
the musician trigger the compressor by playing a note just before recording begins.
Another interesting phenomenon which affects the threshold (and, to a lesser degree, attack time and
release time) of an electro-optical compressor is “panel aging,” something which is more related to
the amount of actual use rather than age in years. The more “aged” the EL panel, the greater the
amount of gain reduction will need to be applied. Panel aging is probably a major reason why the
same model of electro-optical compressor can sound subtly different between units.
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