radiosgalore said:
Couple of questions here. I'm in the Uk so forgive me for asking daft questions
is HD really HD as in higher quality than FM? From what i know HD radio uses the FM sidebands and have heard that the bitrate can be as low as 32k. Is some fancy codec used to make it sound awesome? What effect does this have on adjacent channel interference for DXers and anoraks? and please tell me it's not true HD radio is on AM. What with night time skywave I'd have thought it would raise havoc on the band at night obliterating some clear channel stations. WJR and WABC were audible when i went to N.E Ohio but being 10 khz apart is that even possible now?
The only daft questions are those you don't ask.
FM HD is better quality than FM analog from the standpoint of signal to noise ratio. If the station devotes full 96k to their HD-1 channel, I cannot hear digital artifacts on a very good system. If they split that between an HD-1 and and HD-2, the situation is different. I can definitely begin to hear artifacts on both - but I have a trained ear and know what to listen for. Someone with an expensive trendy table radio with small speakers will not hear the difference. I have not heard audio split 3 ways with HD-3, but the artifacts can only be worse. The best compromise is probably a single HD-2 channel because the earbud iPod trendy table radio crowd is not used to the quality a good analog signal chain can provide, they have never heard it to know better.
FM IBOC jams first adjacent frequencies, which in my view is unacceptable under any circumstances. Tightly spaced allotments exist in most urban areas, especially on the East and West Coast. Some other method of doing digital that does not jam first adjacents should be found. Especially with the push to go up a decade in power. This would hurt non-commercial and NPR stations the worst, as that portion of the band is where first adjacents appear most often.
In the UK and most of Europe, I understand the channel spacing is 100 kHz instead of the 200 kHz spacing in the US. So when I speak of first adjacents, I am speaking of second adjacents in a 100 kHz scheme. The situation would be dismal for 100 kHz first adjacents. 200 kHz would be completely jammed, and 300 kHz would be degraded seriously. Only 4th adjacents, 400 kHz away, would be clear of interference in Europe.
HD radio is on AM / MW. The sidebands are robust enough they can be heard for 1000 miles in rural areas of the West in the daytime. Long after every other trace of the signal including carrier are gone. At night the IBOC sidebands are extremely robust and cause interference to a local station here in my areas within its protected countours. The system is not very robust - I need advanced AM DX techniques to get reliable decode on AM stations where I am literally in sight of the towers. A power increase has also been proposed for AM sidebands, but the havoc they are producing right now will literally be ten times worse with more power. Many US groups that have stations on adjacent frequencies hundreds of miles apart have had to shut down IBOC power. WJR and WABC are often sited as an example of why the system is unworkable. Similarly the trio of WLW, WOR, and WGN. There are very few reports of AM IBOC working reliably at night. I have never personally gotten the system to work on stations that otherwise have a very strong nighttime signal. I suspect that no matter how robust those sidebands are, they are very susceptible to interference and the slightest hint of analog over them cause loss of lock.
IBOC sidebands obliterate first adjacent AM frequencies with interference considerably louder in volume than the analog channel - a fact that confuses auto scanning radios. Second adjacent AM interference also happens, but the station is still audible if it is reasonably strong - the directional ferrite bars can also null the second adjacent interference if geometries are right. There have been reports of third adjacent jamming by AM IBOC sidebands, but I find that to be untrue - it is a result of poor selectivity on the receiver.
European 9 kHz spacing will slightly worsen the effects of first and second adjacent jamming.
AM HD - when it does decode - does very curious things to music. High frequencies are extrapolated and not sampled, I often use the example of a Jesse McCartney song "Beautiful Soul" which has a high frequency "ding" in the song. Over regular analog and the old C-Quam system, the ding was pure and undistorted. In the HD system, it is muted and the pitch is changed. Very bizarre effect.
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