fm-engineer said:
A lot of people are talking about the single sideband option for existing analog FM. The FCC would have to approve the option since having only one L-R would require doubling the level to equal the L+R. I read Frank Foti's paper on this a while back, and I have a tremendous amount of respect for his achievements. I would also like to hear Orban's opinion on the single sideband issue since he was around when the FCC adopted the system we use today. At least we should be permitted to give it a test run since it does not alter any existing receiving equipment.
It is an interesting proposal but it needs more testing before it is ready to be a proposed rulemaking.
My comments below were written for a reader who had already read Frank's article.
The main computational load required to implement this system is the linear-phase FIR lowpass filters needed to realize the Weaver modulator that would generate the SSB component. The L+R would also have to be delayed by the group delay of these filters. If SSB operation is required down to 0.15 Hz (which would be required to preserve the peak level of a highly processed audio waveform), then these FIR filters would be very long (they would be required to have a transition region between passband and stopband of 0.15 Hz) and would probably need to be computed by fast convolution. This implies a long coding delay, which would cause problems if talent needed to monitor off-air through headphones. (Our processors offer an "ultra-low-latency" structure that can be put on-air to address this.)
I have examined the mathematics, and it turns out that one could also do a vestigial sideband realization where the upper sideband was allowed to have frequency components up to a few hundred Hz. This would substantially ease the filter requirements and would also result in less coding delay. It is possible to arrange the VSB such that the peak modulation of the composite baseband signal is still correctly controlled; the sum of the gains of corresponding upper and lower sidebands must be unity. (Conventional DSB FM stereo is a limiting case of this, where mirror-image sidebands of 0.5 gain appear throughout the stereo subchannel spectrum.)
One other important thing to consider is how certain receivers might react to this waveform. I know that Sony in its most advanced receivers uses the quadrature component of the stereo subchannel to estimate channel noise. This noise estimate is used to control a variable-blend-to-mono algorithm in sub-bands. This exploits the fact that in conventional FM stereo, the quadrature component of the stereo subchannel is zero, so material appearing in quadrature can assumed to be due mainly to transmission channel degradation, including multipath and noise. I do not know if any other brands of receivers use this information, but if they do, it's important to know this because the SSB modulation will result in a quadrature component that is equal in magnitude to the in-phase component when demodulated with a conventional double-sideband synchronous detector. If the receiver misinterprets this, it could result in a premature blend to mono.
In addition, I believe that before transmissions using this technique are allowed to proceed, laboratory measurements of co-channel RF protection ratios should be made both under ideal conditions and with multipath. While the results may well favor the SSB technique, it is important to know this with certainty before this proposal is placed before the regulatory authority because this would affect the interference environment in the FM band.
Bob Orban
