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.
(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