2.1.6 Release Time
Most dynamics processing equipment has a knob marked release on the panel. This refers to
release time, and affects the length of time required for the gain to recover to the no-signal
state.
For compressors, the no-signal point is unity gain and this applies to any signal whose level is
below threshold. For expanders the no-signal point is the gain reduction set by the threshold
control under no-signal conditions and this applies to any signal whose level is above
threshold. In this case, the release time control governs how long it takes for the expander to
reduce the gain when the signal disappears.
The release time control allows tailoring the compressor's recovery time to the program
material. Generally, peak limiting is associated with short release times and compression or
leveling associated with longer release times.
The 425's compressor release circuitry has a unique dual-release time feature designed to
make life easier. The dual-release feature makes the release time partially program dependent,
giving you the best of both worlds: fast release for short-duration peaks and a longer release
time for longer-duration peaks. The release time control adjusts the speed of the longer of the
two.
2.1.7 Threshold Setting
The threshold control sets the audio signal level where the compressor/expander/limiter
begins working. In the case of the compressor or limiter, the processor begins working once
the signal has exceeded the threshold level. For the expander, it begins working once the
signal has fallen below the threshold level.
For any of the three processors, the threshold control setting also determines the degree or
amount of gain reduction. Thus, for the compressor, rotating the control counter-clockwise
(towards -40) results in increasing amounts of compression. For the expander, counter-
clockwise rotation raises the level that the signal must exceed to pass through the expander
untouched. This has the effect of "shutting off" the signal once you reach and then pass the
threshold level.
For most compressor applications, moderate amounts of gain reduction are all that is
required, 3-9 dB at the most. If you are using the compressor to minimize level changes of a
wide range of program material (automatic level control), then higher amounts of gain
reduction are needed; the amount of gain reduction corresponds to the range of change that
you can respond to (slower release times are also indicated, too).
2.1.8 Interpreting the Displays
The 425 has many displays; one per processing section. The three displays associated with the
expander, compressor and limiter indicate a parameter called gain reduction. Simply stated,
the gain reduction indication shows how far the gain or amplification was reduced from unity.
Another way of looking at this is: if the gain reduction display says 10 dB of gain reduction,
switching the unit to bypass will result in a 10 dB increase in the output level.
The output display indicates output level, in VU (0 VU = +4 dBm = 1.23V RMS measured at
the balanced outputs). For most applications, just make certain that you never see the CLIP
LED illuminate.
2.2 Using the Sidechain
The sidechain is a patch point in the control circuit of a dynamic range processor, which
provides access to the part of the circuitry that tells the VCA what to do. The 425's sidechain
is routed through a TRS jack located on the rear panel that provides both a send and return
via the same jack. The sidechain connection affects all three processors in the 425.
2-3
Rev 2.0, 10/29/93
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