We Ponder an All-Digital AM Future

May 6, 2014

At the 2014 NAB Show''s Broadcast Engineering Conference, there were many papers presented discussing digital AM and IBOC.

No doubt as an engineer you have experienced the “buzz saw” on the AM band from adjacent facilities, particularly at night, or been charged with running your station in digital. The current situation, in many instances, has resulted in a sharp metallic distaste in the mouths of engineers and management. Indeed numerous stations, some even in my own backyard, originally quite keen on IBOC, have subsequently turned off the digital component, and returned to analog-only operations. Although I am definitely a free-market kind of guy, the reality is human nature sometimes requires interventional prodding to get us to travel the path of a little greater resistance to a better endpoint.

Our friends on the television side of the industry underwent a similar transition. While many stations came to the table kicking and screaming for a myriad of reasons, the result has been beneficial. Program material looks and sounds better, there have been subjective gains in coverage, and multiple program streams can be shoved down the pipe. The same can be said, minus the better-looking video of course, for a transition to all digital AM. That is where we should be heading.

AM HD Radio hybrid waveform spectrum (5kHz audio configuration)

It is important to remember that the current situation, a hybrid mode of analog and digital, was intended only to be a stopgap between the good old days, and a bright future. The MA1 mode under which we are currently operating is akin to erecting a gigantic brick wall on the dial. Not only is there the conventional analog signal occupying its theoretical bandwidth of 10kHz, but also the three additional OFDM subcarriers. The primary and secondary subcarriers respectively lie roughly from 10-15kHz and 5-10kHz either side of the carrier. The tertiary subcarrier roughly occupies the real estate from the carrier frequency out to 5kHz either side. Usually the secondary and tertiary subcarriers are suppressed in MA1. This reduces the bandwidth used, but since the primary subcarriers are distantly located from the main carrier, the “IBUZ” problem remains.

Contrast that with the endgame, which in full bandwidth mode places the primary subcarriers within the 10kHz block centered on the carrier frequency, and the secondary and tertiary subcarriers 5-10kHz above and 5-10kHz below the carrier frequency respectively. In the reduced bandwidth configuration, the secondary and tertiary subcarriers are once again suppressed, and the digital signal occupies the usual 10kHz AM channel. The end result is that in either digital condition, the overall bandwidth is reduced, and the waveform is less susceptible to adjacent-channel interference.

AM all-digital waveform spectrum

With the full-bandwidth mode, the secondary and tertiary subcarriers become available for additional program streams. Nyquist, however, pops up again, and since these subcarriers are narrower, they will pass less information. Therefore their use is really only suited for highly compressed material such as weather or traffic reports, information services, and the like. Still, this provides additional outlets for programming, and opportunities for alternate revenue streams.

While all of this is encouraging, the nagging issue of receiver penetration remains. At the 2013 Engineering Conference, it was stated that estimates have the number of extant conventional AM receivers pegged at one billion. The conversion to all digital will most likely relegate these to the E-waste heap, or to non-functioning museum pieces. Oh, I suppose a converter could be made; however, it does not seem likely that the economics would be there for such a device. Furthermore, unless the desire is to keep one of Mr. Kent''s masterpieces functional for nostalgic purposes, the addition of a converter box would seem to destroy the portability that radio offers. Finally, how would such a device interface with a conventional radio? Devices are of course available that take an audio signal, and convert it to a very low-level FM signal for reception by a receiver in close proximity. But the scheme of taking a digital AM signal and converting it to FM to be receivable by a conventional FM radio seems to fall somewhere between a light chuckle and absurdity. This is where the Commission should eventually establish a sunset date for analog operations.

Before reaching that sunset date, however, there would be a fair amount of work to be done. First would be upgrades to transmitter plants. Some of these upgrades would necessarily take the form of replacement transmitters. A transmitter is definitely one of the biggest single-ticket items a station has. No doubt in some instances this may cause some financial hardship. Also in the upgrade column would be necessary changes to antenna systems. In general, the non-directional guys have it fairly easy. For many of them, a change in the phase shift of the ATU may be all that is required. However, in the case of directional antennas, there may be some significant work necessary.

AM HD Radio all-digital waveform spectrum (reduced digital bandwidth configuration)

Back in the day when the tube transmitter would load into a wet noodle, impedance bandwidth of the common point was less of a concern. Sure, the tubes may glow a little red on occasion, but RF was being made, and the information loss inherent in the modulation scheme combined with speaker quality meant you sounded no worse than your neighbor, despite the grimness of your load. But with exotic things like MER and SNR to consider, the load needs to approach the terrain of Eastern Illinois ... flat. Many arrays may require appropriate modifications, which could cause the cost of a retrofit to spiral up further.

However, items that are still being investigated include power levels and coverage. The field tests conducted so far, including those reported in 2013 and 2014, are encouraging to say the least. Not only are there significant increases in the recorded off-air audio quality, but also the coverage seems to be well matched, or better than the corresponding analog coverage. Since only one facility was tested at a time, there may be some variation in observed behavior when everybody on the dial goes fully digital. Indeed one of the papers this year raised this very issue and notes that power levels need to be potentially revisited in concert with the ongoing propagation tests.

While the transition to full digital remains elusive at best, there is no doubt that things look to be moving that direction. Momentum seems to be building for an eventual transition to all-digital. While FM translators for AM stations have no doubt been a boon and most helpful, a transition to all-digital AM will highlight why AM spectrum is valuable, and worth preserving and utilizing. Perhaps this transition will usher in a new Golden Age for radio. If it does, I for one hope it is accompanied by a restoration of some of the cultural refinement of that era. That of course, is a topic of discussion for another day.