Absolute Control Of Peak Modulation - Orban OPTIMOD-FM 8500S Operating Manual

OPTIMOD-FM DIGITAL

Absolute Control of Peak Modulation

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The 8500S provides universal transmitter protection and audio processing
for FM broadcast. It can be configured to interface ideally with any commonly
found transmission system in the world, analog or digital.
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The 8500S provides pre-emphasis limiting for the internationally used pre-
emphasis curves of 50μs and 75μs.
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The 8500S achieves extremely tight peak control at all its outputs—analog,
AES3 (for both the analog FM and HD channels), and composite baseband.
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The stereo encoder has two outputs with independent level controls, each ca-
pable of driving 75Ω in parallel with 47,000pF, (100ft / 30m of coaxial cable).
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By integrating the stereo encoder with the audio processing, the 8500S elimi-
nates the overshoot problems that waste valuable modulation in traditional ex-
ternal encoders.
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The 8500S prevents aliasing distortion in subsequent stereo encoders or trans-
mission links by providing bandwidth limiting and overshoot compensated
15 kHz low-pass filters ahead of the 8500S's audio outputs and stereo encoder.
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The 8500S has an internal, DSP-based stereo encoder (with a patented
"half-cosine interpolation" composite limiter operating at 512 kHz sample
rate) to generate the pilot tone stereo baseband signal and control its peak
level. The composite limiter is a unique, "you can only do this in DSP" process
that beats composite clippers by preserving stereo imaging while fully pro-
tecting the stereo pilot tone, RDS/RBDS, and subcarriers.
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The stereo encoder's stereo subchannel modulator can operate in normal double
sideband mode and in an experimental compatible single sideband mode
that is offered to enable users to compare and assess the two modes. See SSB
Stereo Encoder Operation on page 3-10.
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The Digital Radio processing chain offers an ITU-R BS.1770-3 Loudness Meter
and Safety Limiter for use in countries that enforce a BS.1770 loudness limit on
digital radio broadcasts.
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The 8500S/HD implements "true peak" control in its HD processing chain by
oversampling the HD peak limiter's sidechain at 256 kHz. This allows the 8500S
to prevent clipping in a playback device's analog signal path by predict-
ing and controlling the analog peak level following the playback device's recon-
struction filter to an accuracy of better than 0.5 dB. For typical program mate-
rial, accuracy is 0.2 dB
Without true peak control, analog clipping can occur even if all peak values of
the digital samples are below 0 dBFS. This phenomenon has also been termed
1-3
INTRODUCTION