What is this “HD radio” stuff they’ve been touting as of late (esp. the car dealers)? Do I need a special tuner? What is the main difference from conventional radio - more stations? Is it worth it, or is satellite radio the better way to go? - Jinx 
HD Radio is the US’s standard for digital terrestrial radio. It is designed to be transmitted at the same time and on the same frequency as the radio station’s analogue signal. You need a special radio to receive it.
HD Radio is used nowhere else in the world; even Canada and Mexico currently use the international standard DAB for digital radio (though this may change; Canada’s rollout of DAB has stalled, and apparently the CRTC has allowed some stations to test the US system). I now believe that DAB and HD Radio (and terrestrial radio in general) are being overtaken by satellite radio on one hand, and audio streams across wireless internet connections on the other hand, and won’t have nearly as easy a time as they might have had five years ago.
HD allows an FM station to broadcast in CD quality and AM to broadcast in FM quality. The signal can carry two or three broadcasts on the same signal. Interference is filtered. You need to buy a special receiver.
You still have to put with endless commercials, and I would never trade my Sirius for one.
I don’t think it’s a serious threat to satellite, but watch for wireless internet coming in five years or so in new cars. That IMO will put a dent in satellite radio.
On preview, what Sunspace said.
(Disclaimer: None of this is firsthand experience, just stuff I learned while considering an HD Radio purchase)
It’s a form of digital radio that’s sent over the standard AM/FM frequencies.
Yes, you need a special tuner, but some newer cars might come with one.
I think there is some unique content available, but some of the stations are just simulcast local AM/FM stations (meaning you hear the same station, but it’s digital – the sound quality is better and there might be less interference).
Compared to satellite radio: You don’t have to pay a monthly subscription fee, but you don’t get the full variety of content and there may eventually be commercials. You only get local coverage, not the anywhere-with-of-a-view-of-the-sky reception that you get with satellite.
Links… HD Radio on Wikipedia for background info, official site for information in marketingspeek and a station listing for your area.
HD radio gives stations the ability to split the bandwidth into two channels and broadcast different content on each channel. So, for example, your local NPR station may use one channel to broadcast classical music and the other would broadcast news and information programming.
The sound quality of split channels is about what standard FM sounds like; there is no improvement in quality. In fact, I’ve heard some HD radio stations that actually sound worse than analogue because of the location of their transmitter relative to my location. To be fair, I live in a fairly hilly region where I can’t always get good signals for some stations. I just don’t see the need to actually spend a few hundred bucks in the hopes of listening to one or two of the few stations that broadcast in HD, and where at least one has a dodgy signal.
Robin
What I find deceptive about the HD ads is they say you can receive “hundreds of additional stations”. But that’s counting all of the HD signals nationwide. You will only be able to hear the small number of stations that happen to be nearby.
What remains unspoken about digital broadcast signals is the ability to multi-cast. The license holder can use the bandwidth to transmit a single high-def signal, or multiple regular-def signals, which they can then sell time on and increase (or try to) their revenue stream.