OPTIMOD SURROUND PROCESSOR OPERATION
3-51
creating a sense of increasing loudness although peaks are not actually increasing.
At the gain reduction values set by the 2B
MASTER BREAKPOINT and 2B BASS
BREAKPOINT controls, the release rate for these bands becomes constant and density
does not increase with additional amounts of gain reduction.
2B Release Shape
selects a LINear or EXPonential release shape.
Linear causes the Two-Band compressor to release at a constant number of dB per
second above the 2B
BREAKPOINT setting and proportionally to the amount of gain
reduction otherwise.
Despite its name,
EXPONENTIAL actually offers a reverse-exponential characteristic: It
causes the release to commence slowly and then speed up as it progresses. The
E
XPONENTIAL shape allows you to create the open sound of a slow release time with
program material whose level is well controlled while permitting the processing to
quickly correct excessively low input levels. We recommend using Exponential for
general-purpose sound-for-picture programming. For program material dominated
by music, L
INEAR may be a better choice because Exponential may create unnatural
side effects. (If the 2B Release control is set between about 0.5 and 2 dB/second, an
Exponential release shape should cause no problems even with music.)
Note: The B
REAKPOINT controls do nothing when EXPONENTIAL release is chosen.
2B Gate
(“2B Gate Threshold”) threshold control determines the lowest input level
that will be recognized as program material by OPTIMOD 8685; lower levels are con-
sidered to be noise or background sounds and will cause the AGC or two-band com-
pressor to gate, effectively freezing gain to prevent noise breathing.
There are two independent gating circuits in the 8685 Two-Band structure. The first
affects the AGC and the second affects the two-band compressor. Each has its own
threshold control.
The two-band gain reduction will eventually recover to 0 dB (when the
RELEASE
SHAPE is set to LINEAR) or to the setting of the 2B DRIVE control (when the RELEASE
SHAPE is set to EXP). However, recovery is slow enough to be imperceptible. This
avoids OPTIMOD 8685’s getting stuck with a large amount of gain reduction on a
long, low-level musical passage immediately following a loud passage.
In EXP release mode, the two-band gate’s gated gain reduction is the
same as the setting of the 2B
DRIVE CONTROL, which is similar to the be-
havior of the AGC gate. This is because E
XP release is mainly useful in
sound-for-picture processing and is used in the TV
2B GEN PURPOSE and
TV
2B GEN PURP+LC presets. In these presets, the AGC is defeated so that
all gain riding can occur in the two-band compressor. See AGC Idle Gain
on page 3-44 for a more complete discussion.
It is common to set the 2B GAT E control between approximately –35 dB and –25 dB.
Higher values are useful in sound-for-picture processing to prevent background
sounds and underscoring from being pumped up, while lower settings are more
common with musical programming.
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