AirPort Express

Doc Searls asks of Apple's new announcement of an AirPort Express: "Why go wireless-to-wired?" It strikes me as a question from somebody who has had a very different experience using FM Transmitters than I have. I have an iTrip that allows one to transmit an iPod's output across the FM spectrum. It's not bad. But it is not good either. It's a constant game of cat and mouse to find a station to use. I've had significantly better luck, and quality from the wired solutions I've had. Further, this way it doesn't take a tuner. According to the documentation you can plug it into any powered speakers. So for that second room a set of decent computer speakers just became a nice way to listen to music.

Now for the feature I wish it had. I'd like to send different songs at the same time to different places. Send the pop music into the den where the kids are gathered while having the news on the kitchen speakers.



Dumb, underpowered Part 15-compliant FM transmitters

There are two problems with iTrip, iRock and so many other little FM transmitters.

First, they're underpowered; but not because their transmitters are weak, though they are to some degree. It's because an efficient FM antenna is about 30 inches long, give or take. That's a quarter-wave antenna, and about what you get with most cars on the receive side. It's also what you get with a WalkMan, which uses the headphone cord for an antenna (also on the receive side). If you want your iTrip or iRock or iFMtransmitter of any kind to radiate more efficiently, get a phono cord extender with a male jack at one end and a female at the other. It'll give you a lot more range. Never mind if it's more than 30 inches. It'll still be better than the nothing that you get with the basic unit.

Second, they're not agile. That is, they have no intelligence about what channel to use. Without much trouble they could be receivers as well as transmitters, and look for empty channels to use. Also, again with very little trouble (technically), they could add 87.9 to the list of channels they use. There is only one station in the whole country using 87.9, and it's a little 10-watter at a high school in Mountain View, CA. It goes about 3 miles, tops. There are a few pirates out there, and Channel 6 TV audio is close by (at 87.75); but in most of the country it's wide open. Nearly all digital receivers have it on the roster, and all analog (dial) tuners can get it too. At the very least, the transmitters could allow selecting any channel from 87.9 to 107.7. I can see leaving out the upper half of the dial, since receivers can send out blanking signals 10.x MHz removed from the tuned signal, which conceivably might cause interference with cockpit communications in airplanes (the reason why walkmen might be more dangerous to commercial planes than cell phones). The aviation band starts at 118MHz, just around 10MHz up from the FM band.

Radiation (or interference) limitations are set by Part 15 of the FCC rules. They say this: "The field strength of any emissions within the (FM) band shall not exceed 250 microvolts/meter at 3 meters..." That's about what you'll get from an FM station when you're reaching the edge of its noise-free coverage area. Compliance is possible with more strength than these devices are putting out, in my opinion, although I can understand manufacturers' conservative intentions.

In any case, there's no reason a device like Apple's new one can't make both a wired and an unwired (FM) connection to an audio system.


Good Point

Doc makes a really good point here. I've experienced first hand the difference the right sized antenna makes in using VHF radios in the public service sector. It points out an assumption I made when I read Doc's post which is I was assuming it would be a built in antenna. If Apple put an antenna jack on the AirPort that would allow for an external antenna (ala the AirPort Extreme Base Station) it would be really cool. I especially like the idea of including BOTH. Maybe somebody (though they might need Apple's help) could figure out how to use the USB port to power a small external transmitter that either gets its signal from the USB port or uses the headphone jack to do it.