Orban OPTIMOD 8400 Operating Manual page 115

OPTIMOD-FM
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Multiband Compression with embedded HF clipping and additional HF limiter
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"Intelligent" Clipping with distortion control, distortion cancellation, and anti-
aliasing
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Overshoot Compensation
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DSP-derived Stereo Encoder (generator)
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Composite Level Control Processor
Each of these blocks is significantly improved by comparison to its predecessor (if any)
in Orban's 8200.
Input Conditioning: The 8400 operates at 32 kHz sample rate and power-of-two multi-
ples thereof (up to 512 kHz in the stereo encoder). No commercial A/D converters or
sample rate converter chips convert to 32 kHz while maintaining the standards we de-
manded for this product. Therefore, to ensure high quality A/D and sample rate conver-
sion, we operate both the SRC and A/D chips at 64 kHz-output sample rate and then
downsample to 32 kHz in DSP. By designing and implementing our own downsampler,
we can ensure full frequency response to 15 kHz with very low spurious images.
Despite myths circulating in the marketplace regarding the supposed superi-
ority of higher sample rates in FM stereo processors, be assured that 32 kHz
is far preferable to 48 kHz as a basic sample rate for these devices. 32 kHz
allows us to use DSP horsepower at least 1.5x more efficiently than we
could at 48 kHz, adding features that really improve the sound. It also makes
it easier to protect the stereo pilot tone and RDS subcarriers spectrally by
strictly limiting our output bandwidth to 16 kHz. Although a 16 kHz band-
width limitation is more than is strictly needed to protect the pilot tone, the
RDS requires protection over a substantially wider bandwidth, and 16 kHz
provides this protection.
Like the 8200, the 8400's output spectral control is immaculate, ensuring
maximum stereo and RDS coverage. Moreover, the 8400's digital output
will pass through any uncompressed digital STL without added overshoot
and without the need for distortion-producing overshoot compensation
schemes.
A defeatable 30 Hz 18 dB/octave highpass filter and a defeatable phase rotator complete
the input-conditioning block. These have both been features in Orban FM processors for
many years. Most users will defeat the 30 Hz filter and leave the phase rotator in-circuit,
although the choice is always yours.
Stereo Enhancement: The 8400 provides two different stereo enhancement algorithms.
The first is based on Orban's patented analog 222 Stereo Enhancer, which increases the
energy in the stereo difference signal (L–R) whenever a transient is detected in the stereo
sum signal (L+R). By operating only on transients, the 222 increases width, brightness,
and punch without unnaturally increasing reverb (which is usually predominantly in the
L–R channel).
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OPERATION