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OPERATION ORBAN Model 8400
Gating circuitry detects “mono” material with slight channel or phase imbalances and
suppresses enhancement so this built-in imbalance is not exaggerated. It also allows you
to set a “width limit” to prevent over-enhancement of material with significant stereo
content, and will always limit the ratio of L–R/L+R to unity or less.
The second stereo enhancement algorithm is based on the popular “Max” technique. This
passes the L–R signal through a delay line and adds this “decorrelated” signal to the un-
enhanced L–R signal. Gating circuitry similar to that used in the “222-style” algorithm
prevents over-enhancement and undesired enhancement on slightly unbalanced mono
material.
Two-Band Gated AGC: One of the 8200’s secrets was that its AGC is a two-band de-
vice, using Orban’s patented “master/bass” band coupling. The 8200’s LCD showed only
the AGC “master” band gain reduction.
In the 8400, we bring out all of the two-band AGC controls so the user can adjust them,
including thresholds, ratio, attack times, release times, and master/bass coupling. We also
add an important feature: target-zone gating. If the input program material’s level falls
within a user-settable window (typically 3 dB), then the release time slows to a user-
determined level. It can be slow enough (0.5 dB/second) to effectively freeze the opera-
tion of the AGC. This prevents the AGC from applying additional, audible gain control to
material that is already well controlled. It also lets you run the AGC with fast release
times without adding excessive density to material that is already dense.
Another user-requested feature is the ability to operate the AGC in left/right or sum-and-
difference modes. The user can preset the maximum amount of gain difference permitted
between the sum and difference channels. Sum-and-difference can provide a different
style of stereo enhancement than the 8400’s purpose-built stereo enhancers, which we
nevertheless prefer for this application because of their more sophisticated gating and
decreased tendency to add multipath distortion.
In sum-and-difference mode, you can operate the sum and difference channels with dif-
ferent thresholds. If you set the sum threshold higher than the difference threshold, you
can force the AGC to reduce separation automatically on program material with exces-
sive separation, like old Beatles records.
For the first time, the AGC contains a compression ratio control that allows you to vary
to ratio between 2:1 and essentially ∞:1. Lower ratios can make gain riding subtler on
critical formats like classical and jazz.
Finally, the AGC now has its own silence-gating detector whose threshold can be set in-
dependently of the silence gating applied to the multiband compressor.
Equalization: The 8400 improves on the 8200’s steep-slope bass shelving equalizer and
adds three bands of fully parametric bell-shaped EQ.
In the 8400, you can set the slope of the bass shelving EQ to 6, 12, or 18 dB/octave and
adjust the shelving frequency. This significantly adds to its versatility.
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