FM transmitters work by passing our sounds as a radio signal over a definite frequency. Our car radio also picks up the same signal when it is tuned correctly and plays it through the speakers in the car. They are usually run at very low FM frequencies, where no other signal will be competing with our sound, although there are some which can run at any frequency .
Quality varies by transmitter also. A wireless transmitter, such as an iTrip, runs away the iPod's battery also. Battery power is not as strong as AC or DC power, and as such it would not give out a strong signal. This leads out to a low quality sound, as well as draining off the iPod battery. If this is for an iPod, then I absolutely recommend a Monster FM transmitter. They are usually the best, with no comparison.
The purpose of FM transmitter is to allow you to hear to any external audio device played through a car stereo, or any other radio with an FM band. An FM -Frequency Modulated transmitter, which is also called RF-Radio Frequency modulator or FM modulator, is a after market source that you connects the audio from the earphone jack of a portable audio source, such as an iPod, a Zune, or an MP3 enhanced cell phone, and also an MP3 player, a CD player or a satellite radio system, into the FM transmitter. The sound from the portable audio device is then broadcasted through the FM transmitter as an FM radio station, and is picked upon our car radio as an FM station and played through the FM band through the speakers out.
Lets look at how the FM transmitter works in a stage by stage description. The first stage is that the digital to analog converter that takes in the digital quality audio output from say, your audoi device, which would be connected to a FM transmitter by using a cable from the device's earphone jack to the FM transmitter's left and right RCA type connectors, and then it gets converted into analog audio within the FM transmitter. In the second stage, the now converted analog audio signal is again converted into an FM signal by modulating the audio using FM modulation. This FM modulated signal is then laid on to a RF transmitter signal from the FM transmitter on a define frequency as an FM radio station signal in the final stage. You can then tune your car's radio to the definite FM station frequency that the FM transmitter is about to transmit on and hear to the audio in your car on the FM band as an FM station.