April 9, 2008
Broadcasters Disagree over Increase in Digital FM Power
IBOC signals as currently regulated can't cover same area digitally as their analog counterparts: Despite what the industry has told me for years, a not-very-secret bit of information has come to the fore. The FCC limits IBOC (digital AM/FM as approved in the U.S.) to 1/100th the signal strength of analog. If you have a 100,000-watt analog transmitter, you can broadcast 1,000 watts of digital signal. I have been repeatedly over three years that this provides nearly the same footprint for digital coverage as for analog. Not so. The NAB (National Association of Broadcasters) is split over a proposal to ask the FCC for a 10fold increase in digital power. Some stations are concerned that portable devices with HD Radio built in, when they finally start to appear, will be unable to receive strong enough signals. Critics of the notion, which is still being tested by major networks and NPR, are concerned that the power boost would simply cause interference with adjacent or distant stations' analog signals. Many stations may lack the setup necessarily to power the higher signal, too.October 24, 2007
Pushback on HD Radio in AM: Citadel Turns off Nighttime AM
Criticisms of problems in using HD Radio encodings for AM stations date back to the earliest days of the technology: The FCC approved nighttime broadcasting of HD Radio over AM a few months ago, but there are still relatively few AM stations using HD Radio, and thus problems are only starting to emerge. At night, the so-called D region of the ionosphere changes character and reflects rather than dissipates signals in the AM radio range. AM radio signals already travel thousands of miles further than intended at night, and both hobbyists (DXers) who enjoy tuning in far-off stations, and regular radio listeners could be affected by digital signal skipping.
I've had a mixed opinion on this front, expecting that if there were real problems, then the FCC wouldn't approve nighttime broadcasts, that broadcasters wouldn't deploy such signals, and that lawsuits would be filed. I don't mean to be naive about the money involved, because broadcasters are equally concerned about interference that prevents their own stations from being heard.
Radio World reports that Citadel Broadcasting, which has HD Radio enabled on 16 of its 66 AM stations, decided to turn off the 10 stations that were broadcasting digital signals at night due to reports of interference. The problem occurs when listeners hear "hiss" and stations on nearby channels hear noise.
There have been rumors all along that AM digital encoding is inadequate and will need to be entirely rethought--that is, the underlying encoding replaced with a superior one. This would require replacement of all the HD Radio sets on the market, except those that have upgradability through various means (USB ports and discs that can upgrade firmware) almost certainly, as any solution would likely require new silicon.
We'll see what happens here. It's possible that in production HD Radio won't be able to work at night; that's always been a concern, and now they have to tweak parameters while still maintaining compatibility.
December 13, 2006
Hear That Xylophone!
Many audiophiles dispute HD Radio's quality: I don't want to get into a debate over very fine points, because audiophiles who say that HD Radio is not "CD quality" are absolutely correct. One person has proposed calling it "MP3 quality," which is more accurate, but not correct, either. HD Radio has a far higher dynamic range and frequency response than analog FM, which means that you can hear both deep and high notes better, while also hearing the original sound reproduced with greater fidelity for loudness and softness.
While HD Radio isn't CD quality, partly because it's a compressed format, it does have the range of a CD. It's not MP3 quality, even though the digital radio compression uses a 96 Kbps encoder that's similar for primary FM multicast channels. Why isn't it the same? Because of two factors. First, iBiquity chose a set of optimizations that are designed to make this compression work over the radio. Second, radio stations with any know-how are using preprocessing to optimize their sounds into the compressed format. This allows weaknesses in the compressor to be overcome through clever, real-time choices in the preprocessor. It also means that two stations broadcasting the same song might sound somewhat different.
In this Star-Telegram article about HD Radio, a radio program director suddenly hears the xylophone in a Sly and the Family Stone song--he's been in radio so long, he'd forgotten that these very high, pure notes really exist.
That's my experience, too. When I listen on headphones or in a room to an HD Radio broadcast, it seems spectacular in comparison to analog radio. A CD will always be better, because it's uncompressed sound, so your CD player makes all the choices, not the compression algorithm. But satellite and Internet radio won't sound any different, and may sound worse, especially Internet radio that's using less-than-ideal compressors and have limited bandwidth.