Orban Optimod-FM 8300 Operating Manual page 124

3-6
OPERATION

Overshoot Compensation

DSP-derived Stereo Encoder (stereo generator)

Composite Level Control Processor
Input Conditioning: The 8300 operates at 32 kHz sample rate and power-of-two
multiples thereof (up to 512 kHz in the stereo encoder). No commercial A/D con-
verters or sample rate converter chips convert to 32 kHz while maintaining the stan-
dards we demanded for this product. Therefore, to ensure high quality A/D and
sample rate conversion, 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 alleged superiority of
higher sample rates in FM stereo processors, 32 kHz is, in fact, an efficient and excel-
lent-sounding choice for a basic sample rate. 32 kHz allows us to use DSP horse-
power more efficiently, adding features that really improve the sound. By strictly
limiting the output bandwidth to 16 kHz, it also makes it easier to protect the stereo
pilot tone and RDS subcarriers spectrally. Although a 16 kHz bandwidth limitation is
more than is strictly needed to protect the pilot tone, the RDS requires protection
over a substantially wider bandwidth (±2 kHz), and 16 kHz provides this protection.
The 8300's output spectral control is immaculate, ensuring maximum stereo and RDS
coverage. Moreover, the 8300's digital output will pass through any uncompressed
digital STL (including those operating at 32 kHz sample rate) without added over-
shoot and without the need for distortion-producing overshoot compensation
schemes.
A defeatable 30Hz 18 dB/octave highpass filter and a defeatable phase rotator com-
plete the input-conditioning block. These have both been features in Orban FM
processors for many years. Most users will defeat the 30Hz filter and leave the phase
rotator in-circuit, although the choice is always yours.
Stereo Enhancement: The 8300 provides a stereo enhancement algorithm 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 sig-
nal (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).
Gating circuitry detects "mono" material with slight channel or phase imbalances
and suppresses enhancement so this built-in imbalance is not exaggerated. It also
allows you to set a "width limit" to prevent over-enhancement of material with sig-
nificant stereo content, and will always limit the ratio of L–R / L+R to unity or less.
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 material's level falls within a user-settable window
(typically 3 dB), then the release time slows to a user-determined level. It can be
slow enough (0.5 dB/second) to effectively freeze the operation of the AGC. This
ORBAN MODEL 8300