AM-HD Undergoes Radical Redesign

Judging from the demonstrations at the National Association of Broadcaster’s convention, the system seems to deliver as promised and offers enormous potential for FM and AM stations….The systems are just emerging from the R&D stages and still need fine-tuning before real-world implementation is possible….Once a recommendation is made, the Federal Communications Commission then has to decide how to implement technical parameters for a DAB system. So the industry is still a few years away from a wholesale DAB implementation – but not that many. (Radio World Editorial, “A Little DAB’ll Do Ya,” May 17, 1995, p. 5.)
With little fanfare and less than two weeks before the National Association of Broadcasters’ Broadcast Engineering Conference in Las Vegas, iBiquity Digital Corporation (the proprietors of HD Radio) announced a radical redesign of its hybrid analog/digital AM-HD waveform.
To be discussed in detail at the conference itself, which on Sunday, April 11 will spend half its sessions focused on HD Radio implementation more generally, iBiquity’s “v4.3 Exciter MPS Framework” makes several notable changes to the AM-HD signal structure.
Most obviously (as seen at right, click for a larger and more detailed image), entire portions of the hybrid analog/digital signal that were sharing bandwidth have been removed. The upper half of the image, which represents iBiquity’s current AM-HD hybrid waveform, shows “secondary” and “tertiary” digital data imposed upon the same bandwidth as occupied by the analog AM signal (that you and I can actually hear). In the real world, this “underlying” digital data causes hiss or white-noise artifacts that are noticeable within an analog AM signal.
newamhdwfTo compensate for this problem, many AM-HD enabled stations actually decreased the bandwidth allocated to the analog portion of their signal in order to improve the quality and/or robustness of the digital one. This has angered more than a few analog AM radio broadcasters and listeners, who do not like the idea of a preexisting, legacy service being degraded for an unproven technology.
To correct this problem, as illustrated in the lower half of the image, iBiquity has simply removed the secondary and tertiary digital data signals from the AM-HD waveform. Under the old regime, a hybrid analog/digital AM-HD radio signal could transmit as much as 48 kilobits per second of digital data; it’s not clear just how much that paltry capacity has been cut by this move.
iBiquity itself seems to be touting cross-purposes of this “improvement.” On the one hand, it claims one of the major goals of this revision was to increase the fidelity of existing analog AM signals. This would suggest that it’s not just listener, but intra-industry pushback from the effects of AM-HD on analog AM radio that have caused iBiquity to sacrifice digital capacity, fidelity, and/or robustness to restore the analog listening experience. On the other hand, iBiquity claims that this new configuration will add new choices on how to tweak the digital audio quality specific to a station’s format, and adds “AM HD Radio data support,” whose details are unknown. It is difficult to see how these particular “advancements” work with what is now essentially one-third of the useable bandwidth reserved for a station’s analog signal.
Ultimately, the most fundamental change is the one not there – an overall net reduction in the spectral footprint of AM-HD radio signals. This makes it clear that iBiquity and its proponents remain unconcerned about the destructive nature of AM-HD signals at night, when skywave interference becomes a real problem. This redesign is about improving the listener experience within the daytime, or groundwave, listening area of AM radio stations.
It, in effect, signals iBiquity and its proponents’ firm intention to gradually phase out the notion of long-range listening on the AM band as we’ve known it, and “localize” the coverage area of all AM radio stations. Apologies to those of you who live in rural areas with no stations of your own, who rely on “distant” stations as a primary means of radio listenership: you’re out of luck. This is no conspiracy – you simply don’t exist anymore:
[T]he fact is that wide-area nighttime skywave service, while a fascinating propagation phenomena and very much a part of radio’s historic past, is largely irrelevant in today’s world. The number of persons deriving radio service via skywave propagation is tiny. Most persons under the age of 40 do not even know that this propagation mode exists, let alone listen to it. The need for a medium wave wide area skywave service in the U.S. has largely passed….Skywave signals will suffer somewhat greater interference then currently is the case and will have their availability decreased as noted in the iBiquity submissions. This is, by far, the most significant compromise attendant to 24-hour AM digital operation. However, the upside of high fidelity — robust digital service in the stations’ groundwave service area — is worth the attendant deterioration of the skywave service component inasmuch as the vast majority of these stations’ listeners and advertisers are within the groundwave service areas. For AM to have a future, it must transition to digital. To transition fully to digital will require this necessary compromise.(Reply Comments of iBiquity investor Greater Media, Inc., MM 99-325, July 30, 2004, p. 4-5.)