A correspondent sent this link along about the latest developments in HD radio, along with a scathing commentary about what this latest insanity portends:
From http://www.radio-info.com/newsletter/html/tri-11302011.html this morning (11/30):
Improving HD Radio reception is the goal of an FCC Public Notice on “Asymmetric sideband operations”, and now we’ve got the comment dates. Comments are due by December 19, with reply comments due January 3. This is the story TRI told you about on November 2 – “’Asymmetric’ may not sound sexy, but it might be one key to improving coverage for HD Radio FMs.’ Basically, the Commission says “a significant number of FM stations are precluded from taking advantage of the full 10 dB digital power increase permitted by the order, due to the presence of a nearby station on one but not both of the first first-adjacent channels.” If stations could run an “asymmetrical” signal – stronger on one side – they could raise digital power.
So the Federal Cookie Company is moving ahead with this idiotic idea. It won’t improve “HD” coverage significantly, but it will increase interference to adjacents, at least on one side of the analog channel.
The “HD” signals are not sidebands in the literal sense of the word. They are two independently generated digital signals, one occupying the closer half of each first-adjacent channel. And they are not synchronized. That accounts for both for the excessive time delay imposed on the analog signal, which is necessary to keep it synchronized with the digital output at the receiver, and for the “HD” system’s relative (not absolute) advantage in the face of multiplex under some (not all) conditions. (The two “HD” signals seldom suffer identical interference, and an “HD” receiver delays the two side-channel signal, picking and choosing whichever parts of each signal seem most intact to reconstruct an undamaged digital stream.)
Of course, that won’t work if one of the two digital side-channel signals is too weak to use!
So where could this increase the coverage range for “HD” FM? Only on fixed (not mobile) receivers with (presumably) outdoor antennas. Indoor antennaswould be subject to the effects of people — or pets? — getting too close and interferreing with marginal signals. So who has that? How about translators?
I can see no practical purpose (and I use the word “practical” loosely!) for asymmetrical “HD” except to enlarge the area where a network of analog translators could be used to make it possible for a signal from an HD-2 or HD-3 subchannel to reach a real audience.