Orban 5750 Technical Manual Operation 3-9
manifests itself as a headroom issue. Before applying the system, engineering, programming and management
should be aware of the tradeoffs using SSB/VSB mode vs DSB operation.
In SSB/VSB mode, the bandwidth of the 5750’s composite output signal extends to 38,150 Hz when the 5750’s
composite limiter is not used. When the composite limiter is used, the limiting action will produce energy up to 55
kHz (as it does with normal DSB operation) but this energy will be much lower in level than the energy that would
have been produced by normal DSB operation in the frequency range occupied by the upper sideband.
SSB operation causes irreducible, “laws of physics” composite peak modulation overshoots to occur with certain
combinations of left and right channel signals that are independently peak limited to 100% modulation, which is the
correct limiting technique for conventional double-sideband transmission. The worst-case irreducible SSB overshoot
occurs when the left and right channels contain correlated signals whose phase difference is 90. The 5750’s
Multipath Mitigator, which removes inter-channel phase shifts and converts input audio to “intensity stereo,” is
important to optimum SSB/VSB operation because its action minimizes the amount of modulation overshoot.
Suboptimal system design can cause additional overshoots. To prevent this type of overshoot, the 5750’s SSB/VSB
generator uses constant-delay filters and its frequency response extends to DC (because of the VSB operation below
150 Hz).
To control irreducible overshoots, the SSB generator includes a look-ahead overshoot limiter. To eliminate all
overshoots, this limiter must be used together with the 5750’s Half-Cosine Interpolation composite limiter, which is
located after the look-ahead limiter in the system block diagram.
The group delay of the phase-linear filters needed to create the SSB/VSB waveform and the audio delay in the look-
ahead limiter together add approximately 12 ms to the delay of the stereo encoder. When diversity delay is applied
to the 5750’s composite output, the 5750 adjusts the delay automatically so that it is constant regardless of mode.
SSB stereo encoder mode can be selected from the MODULATION TYPE drop-down in the INPUT/OUTPUT >
COMPOSITE screen. Choose SSB to turn on SSB/VSB operation or STEREO to turn on normal DSB operation. It can
also be controlled via the 5750’s GPI inputs and by PC Remote.
The look-ahead overshoot controller is always active in SSB mode, while the Half Cosine Interpolation Composite
Limiter is controlled by the COMPOSITE LIMIT DRIVE control as usual.
Composite Limiter/Clipper: Orban has traditionally opposed composite clipping because of its tendency to interfere
with the stereo pilot tone and with subcarriers, and because it causes inharmonic aliasing distortion, particularly
between the stereo main and subchannels. Protecting the pilot tone and subcarrier regions is particularly difficult
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 subcarrier protection and without adding the pumping typical of simple gain-control “look-ahead”
solutions.
We succeeded in our effort. The 5750 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
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