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3-23
OPTIMOD
OPERATION
AGC REL (AGC Release) control determines how fast the AGC compressor releases. It
is ordinarily operated in the slow end of its range to allow the AGC to do gentle gain
riding. The two-band compressor does the hard work to increase program density (if de-
sired). See
(Two-Band Release), below, for a further discussion of release time.
2B REL
AGC B CPL (AGC Bass Coupling) determines the amount of bass coupling in the two-
band AGC. Because the AGC is generally operated slowly, one usually sets this control
at 80% or higher to prevent the AGC from significantly changing the frequency balance
of the program material. "Automatic equalization" is ordinarily done in the two-band
compressor section following the AGC. For a further discussion see BASS CPL on page
3-24.
GATE THR (Gate Threshold) control determines the lowest input level that will be
recognized as program by OPTIMOD-DAB; lower levels are considered to be noise or
background sounds, and cause the compressor to gate, effectively freezing its gain.
The two-band gain reduction will eventually recover to 0dB and the AGC gain reduction
will eventually recover to –10dB even when the compressor gate is gated. However, re-
covery is slow enough to be imperceptible. This avoids OPTIMOD-DAB's getting stuck
with a large amount of gain reduction on a long, low-level musical passage immediately
following a loud passage.
It is common to set the
control to –40. Higher settings are primarily useful
GATE THR
for radio drama, outside sports broadcasts, and other non-musical programming which
contain ambiance, low-level crowd noise, and the like. Slightly higher settings may in-
crease the musicality of the compression by slowing down recovery on moderate-level to
low-level musical passages. When such passages cause the gate to cycle on and off, re-
covery time will be slowed down by the ratio of the "on time" to the "off time." This
effectively slows down the release time as the input gets quieter and quieter, thus pre-
serving musical values in material with wide dynamic range (classical music, for exam-
ple).
2B DRIVE control adjusts signal level going into the two-band compressor, and there-
fore controls the density of output audio by determining the amount of gain reduction in
the two-band compressor. The resulting sound texture can be open and transparent, solid
and dense, or somewhere in between. The range is 0-25dB.
Regardless of the release time setting, we feel that the optimal amount of gain reduction
in the two-band compressor for popular music and talk formats is 10-15dB. If less gain
reduction is used, loudness can be lost. For classical formats, operating with 0-10dB of
gain reduction (with the gain riding
set to
) maintains a sense of dynamic range
AGC
off
while still controlling levels effectively. Because OPTIMOD-DAB's density gently in-
creases between 0 and 10dB of compression, 10dB of compression sounds very natural,
even on classical music.
2B REL (Two-Band Release) control determines how fast the two-band compressor
releases (and therefore how quickly loudness increases) when the level of the program
material decreases. It can be adjusted from 0.5dB/second (slow) to 20dB/second (fast).
Settings toward 20dB/second result in a more consistently loud output, while settings
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