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All digital AM IBOC testing: Ten pounds of dung in a five pound bag, still

Alternate title: Why Medium Wave frequencies do not work for high speed data transmission

Much ink has already been spilled on the merits of HD Radio or lack thereof.  The latest NAB sponsored project is the all digital mode test of AM HD Radio.  As of this writing, four stations have volunteered to conduct the all digital (no analog) test on their facilities.  I predict no great performance improvement in AM HD Radio system will be demonstrated from these tests.

Medium wave is the radio frequencies between 300-3,000 KHz, which is where the Standard Broadcast or AM broadcasting service in the US exists. There are several reasons why trying to make a 20 KHz wide linear digital signal work in those frequencies posses some technical problems:

  • Electrical noise is very prevalent below 1,000 KHz
  • Ratio of bandwidth to available frequency is low, therefore low data rates must be used
  • Narrow bandwidth of existing antenna systems, particularly directional antennas and phasors
  • Existing uses of the frequencies in question

Electrical noise plagues existing Analog AM broadcasting because of the frequencies being used and the type of modulation.  Noise floors in urban areas can be very high, preventing any reception.  Noise generators include things like high tension power lines, street lights, loose electrical connections, cracked insulators, appliances, etc.  Add to that intentional RF generators such as broadband over power line and other power line communications systems and an RF noise cacophony exists.  Digital modulation schemes are only immune to this noise as long as the noise floor is low enough to establish an acceptable signal to noise ratio.  If the noise floor is too high, data errors cause dropouts and eventually the receiver will mute or blend back to analog.  Thus, HD Radio tendency to jump between a higher quality digital audio source and a lower quality (made lower by the presence of IBOC signal) analog audio source, much to the annoyance of listeners.  If the analog signal were not there, it would be silent.  In short, there is no magic bullet when it comes to RF noise in the AM broadcast band.

The bandwidth available in the AM broadcast band is drastically less than in the FM broadcast band.  AM channels are not more than 20 KHz wide (actually 10 KHz without adjacent channel interference), versus FM channels which are 200 KHz wide.  This is due in part because AM is the senior service, founded when frequencies above 1,000 KHz were considered junk.  It was necessary to use less frequency spectrum because there was less usable spectrum to be had.  If AM stations were allowed the same bandwidth as FM station, there would only be room for eight stations in the entire AM broadcast band (520-1,710 KHz in US).  These existing narrow channels can only carry so much data; 24 kbps to about 40 kbps without causing too much interference to adjacent channels.  Any person can tell you, listening to a 40 kpbs .mp3 is not high fidelity or even a pleasant experience.  And yes, iBiquity’s own AM HD Radio CODEC is better than .mp3, but not that much better.

Because the wavelength of Medium Wave AM broadcasting (or medium frequency) is very long compared that of FM, antenna systems tend to be large and inefficient.  The AM Broadcast Band wave lengths run from 577 meters (~1,900 feet) at 530 KHz to 175 meters (~575 feet) at 1710 KHz.  Very tall structures are needed to radiate this signal effectively and the RF energy needs to be distributed evenly about the center frequency through the entire 20 KHz bandwidth, which is difficult to accomplish.  This is due to the physical change in size of an AM radiator over the required bandwidth span of 20 KHz.  The becomes more pronounced at the low end of the band, where the required bandwidth becomes a larger portion of the available RF frequency.    COFDM requires a linear response for its entire bandwidth, otherwise errors will be induced on an already lossy transmission system.  Many existing phasors and antenna systems, particularly directional antenna systems are narrow banded and will not pass the IBOC signal without significant modification, which is expensive.

An example of this, the tower base impedance measurement, WTMN 1430 KHz, Gainesville, Florida:

 

As clearly demonstrated by the graph, the base impedance of this tower is not symmetrical about the carrier frequency of 1430 KHz.  This is a somewhat extreme case, but most AM tower exhibit some asymmetry of sideband energy.  The problem can be fixed, but it will be expensive and cash strapped AM radio stations can ill afford that kind of investment particularly of an experimental nature.

In the US, there is an overcrowding issue on the AM band, thanks mostly to the FCC’s issuance of too many licenses for the frequencies available.  That is water under the bridge, but it does create an interference prone band.  Digital radio signals are not, somehow magically immune to co-channel and adjacent channel broadcast interference.  Similar to the situation with electrical noise, interference from other broadcast stations will induce errors on the received COFDM signal, thus causing drop outs.

Thus, it does not matter whether the system is tested in Hybrid digital/analog mode, or all digital mode, it will not be an improvement over plain analog AM broadcasting.

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5 comments to All digital AM IBOC testing: Ten pounds of dung in a five pound bag, still

  • Paul Vincent Zecchino

    Thank you for bravely publishing this clear, succinct description of the state-sanctioned Jamming Scheme many of us politely call iBLOC.

    Moral cretins educated beyond their abilities concocted this ‘carny shill’ for the express purpose of ‘culling the herd of unnecessary AM stations’.

    Unnecessary? By whose standard? That of the free market? The station owners? Or some sleazy gaggle of BigKorpseorate connivers?

    No matter. For all their lies, hype, and blather, astute citizens long ago examined iBLOC’s supposed virtues and rejected it.

    The fact its promoters continue trying to inflict it strongly suggests yet another ‘public private partnership’ scheme to ‘fundamentally transform’ radio into a closed, pay-to-play, controlled system of limited access to stale propaganda.

    Ten pounds of dog doot in a five pound bag perfectly describes this ruse by which digital garbage is slopped all over formerly pristine spectrum.

    Paul Vincent Zecchino
    Manasota Key, Florida
    07 July, 2012

  • “Could EXB Band Be Your New Home?”

    “The group says most AMs should move to the new band, where they would operate as FMs on channels of 100 kHz width, enjoy more parity with current FM stations in terms of audio fidelity and gain the ability to go all-digital. AMs could transition to 100 channels and operate in the all-digital mode. In this way, AMs ‘can solve the current digital problems they are experiencing, especially at night’, the group states. But while most would move, the existing band could, under their plan, also remain populated with clear-channel stations that would enjoy more elbow room. Under the proposal, filed with the FCC in its diversity proceeding (Docket 07-294), the old AM band would be ‘re-packed.'”

    http://radioworld.com/article/could-exb-band-be-your-new-home/22038

    This is all an attempt to hijack the AM band by iBiquity investors (BIG Group Radio). “See FCC, IBLOCK doesn’t work on the crowded AM band, so the smaller stations should move to the expanded FM band, or buy time on FM-HD side channels.” Remember, Carolyn Beasley had that secret meeting with the FCC recently over the AM band. It’s nice that the smaller AM stations have a word in all this.

  • Chris R

    The problem is that article is way out of date. The FCC changes directions faster then a garbage can in a hurricane. They are trying to force TV back down on to lower channels. The top channel would be 31. There was a way to let a station use only as much of a 6MHz channel as needed but the FCC rejected the idea. Now, they are looking at this again but it is too late, the receiver base out there won’t work.
    The point I am making is that Washington only looks at what brings in the most short term cash. Without making any long term plans, we will continue to drift around in the ocean until we sink.

  • Paul Thurst

    Paul-I wouldn’t necessarily call the pre-IBOC AM band pristine, one would have to go back to the 1950’s to find the Medium Wave band in such condition. However, the rest of your comment is dead on.

    Greg-The FCC has already began moving full power TV stations back into the Channel 5/6 frequency spectrum. Unfortunately, there will be no expanded FM broadcast band in the former VHF TV channels. The UHF spectrum is too valuable for broadband data networks and wireless companies like Verizon will pay top dollar for it. How much would AM broadcasters pay to move into an expanded FM band? Not that much, I’d hazard. It all comes down to money.

    Chris R-You are correct.

  • All of the technical talk is all well and good. AM was in trouble when they started limiting the receivers to 2.5khz. Then broadcasters were limited to 10khz – to minimize splatter. The noise floor has increased incredibly over the past 30 years-rendering lower power AM stations useless to most. The real issue is content. With most AM markets having 2 or 3 viable stations, FM has the distinct advantage. Here we can hear L.A., San Diego and Tijuana. We’re still limited to 3 maybe 4 usable AM signals (content and signal combined). FM has something on most every frequency. AM can’t be fixed at this rate. Sorry.

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