OPTIMOD SURROUND PROCESSOR OPERATION
3-9
Signal Flow
The signal flows through the following main blocks in each of the 8685’s processing
sections (multiband and 2.0). (See page 6-101.)
• Input Conditioning,
including sample rate conversion, defeatable highpass (2.0
processing only) and lowpass filters, and defeatable phase rotation
• Two-Band Gated AGC, with target-zone window gating and silence gating
• Equalization, including high-frequency enhancement
• Multiband Compression in either two or five bands, depending on the proc-
essing structure
• Automatic Loudness Control using Orban’s third-generation CBS Loudness
Controller™ algorithm
• Look-Ahead Limiting
Input Conditioning
A sample rate converter converts the sample rate at the digital input to the 8685’s
internal 48 kHz rate. This 48 kHz rate accommodates a 20 kHz audio bandwidth with
a comfortably wide 4 kHz transition band for the anti-aliasing filter.
We are aware of no bias-controlled double-blind studies that have ever
demonstrated that sample rates higher than 48 kHz are audibly superior
to 48 kHz (or even that there is any audible difference at all). Moreover,
the noise and distortion produced by a given digital filter at 48 kHz is
about 6 dB lower than the N&D produced by a filter having the same
frequency response but operating at 96 kHz. The 8685 uses many digital
filters, both in its equalizer section and for the crossovers in the multi-
band compressor. Hence, we believe that 48 kHz is the ideal rate for the
8685’s audio processing.
A sharp phase-linear lowpass filter, a sweepable 18 dB/octave highpass filter, and a
defeatable phase rotator complete the input-conditioning block. The lowpass filter
can present overshoot due to spectral truncation when the 8685 is driving a link that
cannot pass full 20 kHz audio bandwidth (like a 32 kHz sample rate link). The high-
pass filter is useful for production applications where it is necessary to remove low
frequency rumble from a recording. The phase rotator makes speech more symmet-
rical, reducing its peak-to-average ratio by as much as 6 dB without adding nonlin-
ear distortion. Hence, phase rotation can be very useful for loudness processing of
speech.
Two-Band Gated AGC
The AGC is a two-band device, using Orban’s patented “master/bass” band coupling.
It has an additional important feature: target-zone gating. If the input program ma-
terial’s level falls within a user-settable window (typically 3dB), then the release time
slows to a user-determined level. It can be slow enough (0.5 dB/second) to effec-
tively freeze the operation of the AGC. This prevents the AGC from applying addi-
tional, audible gain control to material that is already well controlled. It also lets you
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