Orban Optimod-FM 8300 Operating Manual page 126

3-8
OPERATION
achieve cleanest sound, the digital radio branch is unaffected by the bass clipper
embedded in the multiband limiter's crossover.
"Intelligent" Clipping: The 8300 prevents excess clipping distortion by dynami-
cally reducing the drive level to the clippers as required, using an intelligent analysis
of the clipping distortion produced in the final clipper and overshoot compensator.
DSP-derived Stereo Encoder: The 8300's stereo encoder is derived from algo-
rithms first developed for the high-performance Orban 8218 stand-alone encoder.
The 8300's stereo encoder operates at 512 kHz-sample rate to ease the performance
requirements of the D/A converter's reconstruction filter, making it possible to
achieve excellent stereo separation that is stable over time and temperature.
The 8300 has two independent composite outputs, whose levels are both software-
settable. For convenience, two SCA inputs sum into the 8300's analog composite
output amplifier. The second input can be configured to provide a 19 kHz-reference
output for subcarrier generators that need it.
The 8300 does not digitize SCAs.
Composite Limiter: Orban has traditionally opposed composite clipping because
of its tendency to interfere with the stereo pilot tone and with subcarriers, and be-
cause it causes inharmonic aliasing distortion, particularly between the stereo main
and subchannels. Protecting the pilot tone and subcarrier regions is particularly dif-
ficult with a conventional composite clipper because appropriate filters will not only
add overshoot but also compromise stereo separation — filtering causes the single-
channel composite waveform to "lift off the baseline."
Nevertheless, we are aware that many engineers are fond of composite clipping. We
therefore undertook a research project to find a way to peak-control the composite
waveform without significantly compromising separation, pilot protection, or sub-
carrier protection, and without adding the pumping typical of simple gain-control
"look-ahead" solutions.
We succeeded in our effort. The 8300 offers a patented "Half-Cosine Interpolation"
composite limiter that provides excellent spectral protection of the pilot tone and
SCAs (including RDS), while still providing approximately 60 dB of separation when a
single-channel composite waveform is clipped to 3 dB depth. To ensure accurate
peak control, the limiter operates at 512 kHz sample rate.
Like conventional composite clipping, the new algorithm can still cause aliasing dis-
tortion between the stereo main and subchannels. However, this is the inevitable
cost of increasing the power-handling capability beyond 100% modulation above 5
kHz — the characteristic that makes some people like composite clipping. This ex-
ploits the fact that the fundamental frequency in a square wave has a higher peak
level than the square wave itself. However, any process that makes squared-off
waveforms above 5 kHz creates higher harmonics that end up in the stereo sub-
channel region (23-53 kHz). The receiver then decodes these harmonics as if they
were L–R information, and the decoded harmonics appear at new frequencies not
harmonically related to the original frequency that generated them.
ORBAN MODEL 8300