OPTIMOD SURROUND PROCESSOR OPERATION
3-13
pling to completely uncoupled operation. The coupling control determines the
maximum amount of gain difference permitted between the left and right
channels in a given band and therefore the amount of stereo image shift permit-
ted in each frequency band.
Although the processing architecture is dual-mono, you cannot adjust setup con-
trols independently on the left and right channels. For most sound-for-picture
applications, this is not a significant constraint.
LFE Processing
The LFE channel has its own dedicated compressor with ATTACK TIME and THRESHOLD
controls. Its other settings (such as R
ATI O and BREAKPOINT) track the settings of the B1
(5-band mode) or Bass Band (two-band mode) compressor. To constrain build-up of
LFE energy, the B1>LFE
COUPLING control clamps the maximum gain of the LFE
channel with respect to the band 1 compressor in five-band mode and the Bass band
in two-band mode. For example, when this control is set to 3 dB (the factory default)
and the B1 compressor exhibits 12 dB of gain reduction, the LFE channel can never
have less than 9 dB of gain reduction even if the LFE compressor would have pro-
duced less than 9 dB of gain reduction if uncoupled.
Low-IM Look-Ahead Limiter
The 8685’s peak limiter prevents overshoots by examining a few milliseconds of the
unprocessed sound before it is limited. This way the limiter can anticipate peaks that
are coming up.
Dialnorm and Limiting: Unlike some familiar compressors and limiters (whose
gain reduction is adjusted via threshold controls), the 8685 limiter’s threshold is fixed
with respect to the input of the 8685’s digital output level control so the limiter’s
drive level solely determines the gain reduction. Two cascaded gain controls set this
drive level. One is MB
LIMIT DRIVE; the second is a “hidden” control whose gain is set
by the 8685’s active D
IALNORM value. In the transmitted Dolby Digital bitstream, set-
ting D
IALNORM to a less negative value automatically turns down the home receiver’s
volume control, so the 8685’s output level must be turned up by the same amount to
maintain a constant loudness at the receiver. Because it is placed before the 8685’s
look-ahead limiter, the 8685’s hidden D
IALNORM gain control achieves this while al-
lowing the 8685’s look-ahead limiter to prevent digital clipping in the downstream
transmission chain regardless of the 8685’s D
IALNORM setting. This arrangement al-
lows the user to set the correct loudness at the 8685’s output solely by adjusting the
8685’s active D
IALNORM value—it is unnecessary to adjust any other controls within a
preset, so all presets (including User Presets) automatically adapt to the 8685’s cur-
rent D
IALNORM value.
Look-Ahead Limiting and Low Bitrate Codecs: It is important to minimize audi-
ble peak-limiter-induced distortion when one is driving a low bitrate codec because
one does not want to waste precious bits encoding the distortion. Look-ahead limit-
ing can achieve this goal; hard clipping cannot.
One can model any peak limiter as a multiplier that multiplies its input
signal by a gain control signal. This is a form of amplitude modulation.
Amplitude modulation produces sidebands around the “carrier” signal.
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