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Ignore above, I just RTFAed.

Turns out it's not a separate FM signal as I assumed, but an extra subcarrier (beyond the stereo and RDS signals) in an existing FM signal. This does indeed require electronics skills to generate, though it wouldn't be very hard to add in to a kit transmitter like the mpx-96 we built in my advanced electronics lab.

I did the same, with a hot-rodded MPX-96 from north country in the closet with my Eee 701 music server. Not only do I get crystal-clear music on a couple pairs of retired computer speakers as well as my stereo, I can use a pocket radio, including my Sansa or N800, and soon my N900, not only through my entire flat, but also out to the carpark and dumper.

Control-wise, I use mpd on the server, and I can VNC in to use Gimmix on the server, or use any mpd client directly over the network.

The one thing that bothers me is not the slight, but discernable, loss of separation, or the background noise in quiet sections, but the inability to use the same distribution system for watching TV or movies -- I'm hoping to set up pulseaudio to fix that at some point.

I use the MPX96 FM modulator from North Country Radiohttp://www.northcountryradio.com/Kitpages/mpx 96.htm

Also, the ability to power your transimtter from AC is part of the trick -- it's easy to get them to run quietly from a battery (DC). If you use a wall-wart transformer without careful precautions, you will get a lot of hum in the signal. The kit above has instructions on how to use an AC transformer, but results may vary. Mine works great.

http://www.northcountryradio.com/Kitpages/mpx2000. htm

once upon a time, i begin to write some code to deliver arbitrary content via UDP / LAN broadcast packets. i never finished it. it can be done, but not all wireless routers respect broadcast packets (at least back then, i had to find a switch in the RG1000 to support it), and this would only leave you close to sync'd. nothing will provide sync like FM. obviously, FM doesn't deliver to all your computers (unless you buy USB FM) but it does deliver to radios.

For 64 bucks you can buy a kit for an RF field detector good up to 3gHz and sniff your car once every morning...more often if you have something to hide.
Or am I wrong in assuming that the transmitter would be on all the time or at least periodically and emit a detectable field when powered up?

You might want to look at the specs before you buy one. Check the distortion and channel separation numbers. Is it even stereo? (Many of the cheaper kits are mono. A single electret mic is a clue, here.) Is it PLL controlled or will the output frequency drift all over the place?

I built the MPX96 and I've been happy with it for about two years.

The range is about 200 feet (1/2 mile with a longer antenna - oops!) and the sound quality is "good" - that's somewhere between "fair" and "great".

If you just want a radio station in your home it would seem to make more sense to use a Carrier Current transmitter. What this guy is doing is pretty damn illegal and if you do what he did you can be looking at tens of thousands of dollars in fines!

From what I can tell anyone can setup a Carrier Current station(check for yourself), you can find some pretty cheap kits.

This guy doesn't seem to be all there, he says he has a HAM licence, but is stupidly violating FCC laws. He also says the thing has a BNC type connector but the picture looks like a type RG6 connector.

Try this if you want to build a free-standing FM transmitter from a kit, or this if you want to drop a PCI card into your PC and be on the air instantly.

Yes. If you mean stations that play the music you like, then yes, you will.

Then you can play the MP3s you like and everyone else be damned!