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Optimod-FM 5500 Supports SSB

Manufacturer releases free software update

Orban released free software to allow its Optimod-FM 5500 stereo encoder to operate in an experimental compatible single sideband/vestigial sideband mode, the company stated.

The National Radio Systems Committee is studying this issue, as RW has reported and as Orban summarized, “to determine if it is adequately compatible with current FM radios and if it decreases multipath distortion compared to conventional double-sideband transmissions.”

Orban states: “The 5500’s subchannel modulator acts a pure SSB generator for L–R material in the frequency range of 150 Hz to 17 kHz and as a vestigial sideband generator below 150 Hz. The software supports CSSB operation in both 5500 operating modes: stand-alone stereo encoder and audio processor plus stereo encoder. CSSB mode produces better than 55 dB of left/right channel separation, 30-15,000 Hz, as measured through a Belar FMSA-1 stereo monitor.”

It continued, “CSSB stereo encoding 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 peak limiting technique for conventional double sideband modulation. To control such overshoots, the 5500’s CSSB generator combines a composite-domain look-ahead overshoot limiter with Orban’s patented Half-Cosine Interpolation composite limiter.”

It said users who are required to comply with the ITU-R BS.412 multiplex power limits should note that CSSB operation puts 3 dB more power into the stereo subchannel than conventional DSB operation does. “The 5500’s built-in ITU-R MPX Power Controller automatically compensates for this increase.”

Download the software (FTP).


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