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Radio AM to FM: June 10, 2005

More on digital radio

I received an interesting letter from a reader in response to last week's article on HD Radio. Interesting for two reasons: the subject, and the fact that it was sent anonymously using an anonymous email sending service. I did not know you could do that. But I digress.

"One pro mentioned for IBOC AM/FM is the ability to multicast. But your article did not mention that analog FM already has this feature, called FM SCA," the anonymous reader wrote.

"Current analog FM has all the technological means to provide all the features of IBOC FM (multicast sound and data), but this isn't without problems ... maybe within the next 10 years, I'll read about this topic in your column."

The anonymous reader is right, to a point. Yes, analog FM can provide these features, such as multiple distinct formats on the same frequency, and data services such as RDS -- Radio Data Systems -- that gives information such as song titles, station information, or even storm warnings. But as mentioned, there are problems.

First off, the sound quality of FM-SCA side carriers is dismal. In order for it to not interfere -- much -- with the regular FM broadcast, SCA broadcasts are limited in fidelity, limited in dynamic range, and sent at a lower transmission level than the main signal. I know at least one engineer who absolutely detested SCAs because of the interference they caused to the main carrier, even with those precautions.

Digital multichannel broadcasts, on the other hand, are supposedly of higher quality than analog FM-SCA, and may be split into more distinct broadcasts than FM-SCA's general limit of two.

Secondly, while FM-SCA is a fairly well kept secret -- in the past SCA was often used for subscription services such as Muzak -- multichannel digital is gearing up to be part of the consumer radio receiver standards. So while most people cannot buy FM-SCA radios at their local Radio Shack, if HD Radio catches on, it appears that everyone will be able to hear those extra channels.

And finally, while RDS is cool, it -- like FM-SCA -- needs a really strong antenna. Without a strong signal, RDS often leaves out letters or just refuses to work at all. The presumption is that both RDS and SCA will work better with digital.

Not that I think digital AM and FM is necessarily a good idea. As I mentioned last week, I am cynical. Audio samples I have heard for digital AM and FM leave me very underwhelmed. Further, the multiple signals require a lower bitrate on the main signal, lowering audio quality by introducing digital "artifacts." I have been told that it is not noticeable by the average person, however.

Two quick clarifications to wrap up the topic: HD AM cannot broadcast multiple signals, and I have actually covered the topic before, on December 10, 2004 when I mentioned "even current FM stations can already broadcast multiple audio channels using the sides of each frequency on which they broadcast (SCA broadcasts often leased for commercial or nonprofit use)."


Copyright © 2005 Richard Wagoner and The Copley Press.

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