N900 FM radio transmitter

The SI4713 chip is used to transmit FM radio in the range 76-108MHz from a digital or analog audio input.

It can also recieve the signal level on the tuned frequency. This enables transmitter application to scan all frequencies, checking for vacant ones, and suggesting empty frequencies to the user for selection as transmission frequencies.

(which is used on the n900?)

https://www.silabs.com/products/audiovideo/fmtransmitters/Pages/Si471213.aspx

The documentation for the driver, along with some information on the chip is at http://www.mjmwired.net/kernel/Documentation/video4linux/si470x.txt