Orban OPTIMOD 8400 - Advanced AGC Controls

OPTIMOD-FM OPERATION
3-33
Advanced AGC Controls
The following AGC controls are found only in the Advanced Modify screen.
AGC Max Delta GR
determines the maximum gain difference permitted between the
two channels of the AGC. Set it to “0” for perfect stereo coupling.
This control works the same regardless of whether the AGC operates in
left/right or sum/difference
Matrix modes, in both cases controlling the
maximum gain difference between the “channels.” Depending on the
Matrix
mode setting, the “channels” will handle left and right signals or will handle
sum and difference signals. When the AGC operates in sum/difference
Ma-
trix
mode, this control determines the maximum amount of width change in
the stereo soundfield.
Window Size determines the size of the “target zone” window in the AGC. If the input
level falls within this target zone, the AGC release time is set to the number specified by
the
Window Release control. This is usually much slower than the normal AGC release,
and essentially freezes the AGC gain. This prevents the AGC from building up density in
material whose level is already well controlled. If the level goes outside the window, then
the AGC switches to the release rate specified by
Master AGC Release, so the AGC can
still correct large gain variations quickly.
The normal setting for the Window Size is 3 dB.
Window Release (see Window Size above.)
AGC Matrix
allows you to operate the AGC in left/right mode or in sum/difference
mode. Usually you will operate in left/right mode. However, sum/difference mode can
give a type of stereo enhancement that is different from the enhancement modes offered
in the 8400’s built-in stereo enhancer. This will only work if you allow the two channels
of the AGC to have different gains. To do this, set the
AGC MaxDeltGR control greater
than zero.
It is unwise to set this control beyond 3 dB. Multipath distortion could in-
crease because the amount of L–R energy builds up excessively. We prefer
using the 8400’s stereo enhancer because its built-in gating circuits prevent
over-enhancement.
AGC Ratio determines the compression ratio of the AGC. The compression ratio is the
ratio between the change in input level and the resulting change in output level, both
measured in units of dB.
Previous Orban AGCs had compression ratios very close to ∞:1, which produces the
most consistent and uniform sound. However, the 8400 compressor can reduce this ratio
to as low as 2:1. This can add a sense of dynamic range and is mostly useful for subtle
formats like classical and jazz.
This control reduces the available range of AGC gain reduction because it
acts by attenuating the gain control signal produced by the AGC sidechain.