FCC Proposes Allowing All-digital AM

Making Waves – Commentary

The Federal Communications Commission last week adopted a Notice of Proposed Rulemaking that recommends giving AM stations the flexibility to voluntarily adopt all-digital broadcasting.

FCC LogoIt hasn’t been easy to watch AM broadcasting slowly wither as stations go mute after low listenership and sales. It seems like every week we hear of another AM station downing its towers and selling its land to housing developments.

FCC Commissioner Ajit Pai has taken a special interest in doing what he can to prop up AM broadcasters, first with his initiative that gave them some access to low-power FM translator channels, with mixed results that, in fact, did nothing for the actual AM spectrum.

Should AM stations be given the right to adopt all-digital modulation?

PROS: 

  • It gives some relief from radio frequency interference (RFI) due to the nature of digital transmission and its ability to disregard noise.
  • Increases fidelity, promoting music formats again. RF spectrum bandwidth is not discussed in the FCC’s NPR, but keep in mind that you can fit more bits in all-digital than in hybrid modes. Assuming the continued use of Xperi HDR compression, music should sound better than analog within the primary coverage area, as demonstrated by existing all-digital WWFD-AM in Frederick, MD. Currently, in the MA3 mode you get 40 kilobits per second of bandwidth in the Core and Enhanced modes, 20 kbps in Core-only mode. Remember that HDR is like AAC-HE, so very data-efficient. The FCC will need to deal with occupied bandwidth because “IBOC” is anything BUT “In-Band On-Channel” because IBOC occupies two 10 kHz channels and that wreaks havoc at night. How about 20 kHz wide daytime, 10 kHz nighttime?
  • Eliminates apparent co-channel interference. AM analog inherently allows the listener to hear background co-channel interference at whatever ratio it’s received, making it distracting to listen to nearly any nighttime AM broadcast since the elimination of the concept of national “clear channels” beginning in the 1960s. Digital radios can demodulate the desired signal as long as the undesired signal is low enough in level. In ATSC TV, you need just over 15 dB D/U ratio.
  • Coverage range is extended somewhat due to the aforementioned ability to overlook noise and co-channel interference.
  • You can pass meta (“RDS”) information about your station’s call letters, format, sponsors, music, or news.

CONS:

  • Old radios are not compatible. Just like old TVs can no longer decipher ATSC without an adaptor. You can buy a little AM transmitter to play your collection of big band songs into your antique Philco catheral radio if you really need to, Jim.
  • You’ll have that nasty dropout effect. Signals in the digital domain are either there or not. You can tweak the modulation to have multiple modes of modulation for carrying a baseline bandwidth as CDMA does, but eventually you’re going to lose the signal “off the cliff.” This is annoying as you drive away from the transmitter, especially in mountainous country where it could come and go and you have fewer signal choices in the first place.
  • Broadcasters will need to upgrade their equipment. Unprofitable AM stations may have to upgrade their transmitters and antenna systems to transmit a signal sufficiently flat through the required bandpass. Stations that have already multiplexed onto a single tower may have an additional challenge in making their filters sufficiently broadband and low in group delay.
  • Broadcasters will be asked to pay for a technology license. The FCC is likely to support keeping Xperi in the driver’s seat here, and stations will pay forever.  Some new equipment will be required.
  • Increases the background noise for remaining analog stations. Although it depends on how many stations remain, how the FCC will treat future adjustments to coverage, and how many broadcasting corporations might rush back into AM.
  • Allows the FCC to continue to ignore the increasing creep of broadband noise from switching power supplies and the likeIt’s one thing to have Part 15 rules, quite another to enforce them.
  • Sources of emergency broadcasting will effectively decrease. Ajit Pai talks about the importance of emergency broadcasting in his defense of allowing AM stations to switch to all-digital, but I would have to ask in response: “How many portable AM HD radios do you have in your house?” Yeah, I didn’t think so. Of course, this will adjust over time.

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