Mention noise interference to radio reception, and most people’s thoughts immediately jump to the AM band. But noise is also an issue for FM reception, the subject of Glynn Walden’s NAB Show presentation, “Noise Contributions to Recovered Analog FM Reception.”
Walden, now a consultant for Entercom, says FM noise is a different animal.
‘‘When we listen to AM radio, it’s easy to distinguish between noise and a weak signal. With FM, the limiter in the receiver masks most of the noise, and what’s left might sound like just slightly more background hiss.’’ Although this noise is largely indistinguishable, it nevertheless becomes a limiting factor in FM reception.
NOT WELL DOCUMENTED
While noise in the AM band has been researched and discussed, much less is really known about how it works on FM.
‘‘When I first became interested in this topic,’’ says Walden, ‘‘I worked for iBiquity, now Xperi. I did extensive research on existing noise levels in the FM band, and discovered that neither the FCC nor the radio consulting community knew what the noise level was.
“What we do know is that the total noise is the sum of atmospheric noise, the co-channel interference within some finite distance from the station and the noise figure of the FM receiver.’’
Walden adds that when evaluating received signal-to-noise ratios in car radios, he generally ignores the noise floor of the receiver, as it is too low to be a contributing factor.
We also know that, comparatively speaking, the FM band is increasingly noisy. Walden notes that scientists at iBiquity’s predecessor USA Digital Radio observed that noise floor in the FM band is about 10 dB higher than it is on the lower adjacent band or the upper adjacent aviation band. That observation led them to explore why this is so.”
“A lot of the noise, particularly in urban areas, is manmade,’’ says Walden, ‘‘and there is an additional component that is simply a result of the number of FM co-channel signals adding asynchronously in a matter that resembles Gaussian noise.’’
The session will focus on the effects of IBOC transmissions, at –10, –14 and –20 dBc on the received signal-to-noise ratio at the –60 dBu contour. Field measurements that Walden made were compared with FCC F 50:50 contours and Longley Rice matrixed predicted contours. The actual measured signal levels, at the point identified as the FCC F 50:50 60 dBu contour, were used to calculate the signal-to-noise ratios with IBOC power levels at –20, –14 and –10 dBc.
HD Radio has been a large part of Walden’s career for the past 25 years. As the senior VP of engineering for CBS Radio, he was responsible for numerous technology rollouts, including the CBS conversion to HD. He helped found USA Digital Radio in 1991; that consortium of broadcasters was responsible for developing in-band, on-channel technology. Before that, he was VP of engineering for Westinghouse Broadcasting’s Group W Radio, where he was involved with numerous improvements to AM and FM radio, including digital broadcasting.
Over the years, he continued to work on IBOC technology, and currently holds four patents involving digital radio. For those efforts, he received the Westinghouse Signature of Excellence. He was presented with a Lifetime Achievement Award at the 2004 NAB convention for his contributions to the industry and its transition to digital radio.
Walden’s session “Noise Contributions to Recovered Analog FM Reception” will be presented on Sunday April 8 at 2:10 p.m. in N255.