The Irock 400FM is a short-range stereo FM transmitter designed to let you send the output from a portable MP3 player (or any other audio device) to any standard FM radio. The primary intended use is in the car, since most car stereos have an FM radio, but no external audio input. I use it in the car with my iPod, since it's quicker and easier to plug in the Irock than to burn a CD for the car CD changer. All I have to do is plug the Irock into the headphone jack of the iPod and tune the car stereo to the right FM frequency to receive the signal.

The Irock is a pretty small and simple unit. The 8" plug-in cord ends in a standard 3.5mm stereo headphone plug, and does double duty as the antenna. The only controls are a power switch on the front, and a 4-position slider switch to select transmit frequency on the side. A red LED illuminates in the power switch when the unit is on. It has a soft rubber coating on the sides, but really it would have been more helpful to have this coating on the bottom where it's likely to be resting on a surface in the car.

The FM transmit frequencies available are 88.1, 88.3, 88.5, and 88.7 MHz. If there are already stations on or close to all of those frequencies in your area, you're out of luck with this unit. In theory the Irock can overpower a weak FM station if it is very close to the receiving antenna, but in practice it will cause noise and interference. Some source volume adjustment is needed to get the optimal level, i.e. loud enough to give a strong signal without background hiss, but not overloading the Irock's input to cause distortion on the peaks. There are no indicators, so you have to play it by ear. I found I had to turn the headphone volume level up pretty high to get the best signal with all the music sources I connected to the Irock.

Range is 10-30 feet, as advertised. The best signal is received when the Irock is within 5 feet of the receiving antenna. Beyond that it starts to fade, and antenna position and obstacles become significant. Under best conditions a signal can be received up to 30 feet away. It works fine in my car when positioned on the console at the front, where it has a clear line-of-sight to the radio antenna embedded in the rear window about 6 feet away. It's possible that people with an external radio antenna might have more problems finding a good position for the Irock where there isn't a lot of metal car body in the way. The transmit power is limited by regulation, so the only thing which could be done to improve it a little is to extend the length of the antenna.

The sound quality is adequate at best. It's better than AM radio, but not as good as a good FM station. Cruising along in the car with lots of ambient noise it can sound ok, but closer listening tests tell the story. Even under optimum conditions the extremes of bass and treble are lacking, and the stereo channel separation is minimal. When I directly compared the same digital MP3 tune played locally by my Nomad MP3 player and received from the computer via the Irock and the Nomad's FM tuner, the inferior sound via the Irock was obvious. However, it does sound better on the car FM stereo - perhaps because the car's tuner is better than the Nomad's? Anyhow, most of the time I find that I am just listening to the music without thinking "wow, that sounds inferior", so it can't be that bad.

On the plus side it doesn't suffer from the "pumping" volume effect of many pop FM stations that misuse dynamic limiting to maintain a high average volume without overloading on peaks. But I do get an intermittent repetitive background noise which I haven't been able to pin down, a sort of faint whining which pulses with a period of about 1/2 a second. Could be something in the car, could be an outside source of interference - it's hard to tell.

The Irock 400FM is identical to the earlier Irock 300W (listed separately here on Epinions) except that it includes a 3 volt car cigarette lighter power adapter to save batteries. The 400 can also run on two AAA batteries, but it comes without batteries included to save money.

I have to agree with users of the earlier 300W model - the Irock cannot be recommended as a battery-powered device. It uses expensive and short-lived AAA batteries (always a big negative), performance deteriorates as the batteries drain, and the unit has a faulty design which causes the batteries to be drained accidentally quite easily (probably the problem is the over-sensitive and unprotected power switch, but I'm not entirely convinced that it isn't a more fundamental electrical problem which drains the batteries even when not in use). In any case, this device should only ever be used with its power adapter, and you should never leave batteries in it when not in use.

There are other FM transmitters, including some which are cheaper and have more frequency options, but no others which include the car power adapter with the transmitter for one low price. Others promise longer battery life though. You'll have to decide for yourself if the sound quality is adequate for you.

Recommended: No