Technique Aims to Reduce Multipath
SSBSC on FM was a hot topic at last year’s NAB Show. Single-sideband
suppressed carrier generation of FM stereo instead of the conventional DSBSC
has shown promise in a number of markets as an effective means of reducing
multipath distortion. Omnia, Orban and Wheatstone all offer the SSB option in
their top models of FM audio processors. However field results have not been
universally judged as providing meaningful improvement for many stations.
|The first image is a
graph of data extracted from the paper ‘The Psychoacoustics of Multichannel
Audio’ by J. Robert Stuart. Jeff Keith commented, ‘I found this paper online
about two years ago and was glad to see that his data pretty much matched what
I discovered back in 1998.’ The second shows data gathered by Keith from 62
male and female test subjects in the summer of 1998.
Jeff Keith, senior
product design engineer at Wheatstone
Corp., offers an alternative solution to the challenge of multipath
reduction in his planned April 9 presentation at the Broadcast Engineering
Keith had evaluated and added SSBSC as a feature
in the Wheatstone AirAura FM audio processor. “Field experience and feedback from customers
using SSB-equipped processors from both Wheatstone and our competitors has been
mixed,” he says. “A few claim it to be a miracle, a few claim it to be a
detriment and the rest seemingly notice no difference between conventional
DSBSC and SSBSC.”
Although Wheatstone still offers SSBSC as an
option, Keith decided there was an entirely different FM processor design
approach that provides more consistent multipath mitigation.
“The issue of minor receiver compatibilities aside,
what stations really want in a multipath solution is something that is
beneficial to all listeners and is compatible with all stereo receivers
regardless of stereo decoder design. Combining intelligent stereo separation
management with conventional DSBSC modulation achieves that goal.”
Keith built and marketed
the SMO-900 Stereo Enhancer when stereo wars were heating up in the late 1980s.
In 1998, as
chief engineer at WMJI(FM) in Cleveland, he designed and built a processing
device to preserve mono loudness when songs with ping-pong stereo were aired.
“Having had success with the SMO-900, I knew that
excessive stereo enhancement reduced mono loudness. However it had not occurred
to me until WMJI that the early ping-pong stereo recordings were in fact the
same thing as ‘excessive stereo separation,’ a problem that I knew quite well
how to solve.
“The Mono Compatibility Controller
that I built for WMJI was designed to dynamically adjust stereo separation on
the fly according to the program material being fed to it. Its sole purpose was
to even out loudness on mono radios, nothing more,” he said.
He discovered that the “normal” stereo
sound field could be reduced significantly without listeners noticing it.
take very long for the station’s staff to discover two new effects of the processor
being in line. First, there was no noticeable change in stereo separation even
though it was aggressively reducing separation on quite a lot of program
material. Second was the effect on our perceived multipath — it was far less
noticeable in ‘bad’ multipath areas and was virtually gone in others. This was
a complete surprise to all of us and something that I never expected or even
Wheatstone has included the algorithm from Keith’s
original Mono Compatibility Controller in the stereo generators of their FM
processor models from the AP-2000 forward. “Over time, and so as not to give
away exactly what it does or how it works to competitors, we’ve hidden its
function under several different control names and have never explained in our
documentation exactly what’s going on inside it.”
Keith cites feedback from many
hundreds of customers over the past few years that reinforced what he found at
WMJI. “Intelligent stereo separation management apparently works much better at
reducing the audible effects of multipath-induced stereo blending than any
other method except perhaps transmitting in mono,” he concludes. Further, “No
special permission is needed from the FCC, and it is 100 percent compatible
with every FM stereo receiver that was ever made.”
Tom McGinley is
technical adviser and a longtime contributor to Radio World.