Universal Audio LA-610 Channel Strip - Manual (Page 16)

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photo-cell resistance can not go completely to zero and hence there will always be some
signal present.
The T4 electro-optical device is the heart of the compressor and its gain reduction
characteristics. Its unique characteristics affect the overall sound and character of the
LA-610.
In addition to the compression curve, the combination of the EL panel and the photo-cell
determine the attack and release characteristics of the LA-610. This is one of the most
important contributors to the sound of the LA-610. Unlike other compressors which
allow the user to adjust these parameters, the attack and release of the LA-610 are
completely determined by the T4.
There are several important characteristics of the T4 which play crucial roles in the
sound of the LA-610. The first is the attack. The LA-2A was the first electro-optical
compressor to use an electro-luminescent panel for the light source. Previous attempts at
electro-optical compression employed either neon or incandescent lights. Both of these
took time to light up, and this delay resulted in slow attacks. The electro-luminescent
panel resulted in a faster attack than exhibited by other contemporary devices.
The next important aspect is that of the release of the compressor. This is determined
almost entirely by the characteristics of the photo-cell. The LA-610 the uses the same
cadmium-sulfide photo-cells. The first important aspect of the cell is its “two-stage
decay”. After the light is removed from the cell, it releases quickly (40-80 milliseconds)
to approximately half of its off resistance. The remainder of its release can take place
over as much as several seconds.
The next aspect is the “memory” of the cell. This results in two important aspects of the
character of the LA-610. The amount of time it takes for the cell to recover after the light
is removed depends on how long light had been shining on it and how bright the light. In
the case of the LA-610 this results in behavior where the release time is slower if the unit
has either been in compression for a while, or the amount of compression is large. This
signal dependent release characteristic is critical to the sound of the unit.
The amount of compression, as well as the compression threshold, is controlled by the
“Peak Reduction” potentiometer. This potentiometer controls the gain of the side-chain
circuit. The greater the gain of this circuit, the lower the threshold and the greater the
amount of compression will be. Many modern limiters and compressors allow for the
direct adjustment of the threshold. Other units such as the 1176LN use a fixed threshold
and provide an input level control, which adjusts the signal level before it is applied to
the compression circuit. In contrast, the LA-610, while also having a fixed threshold,