https://github.com/richardghirst/PiBits/blob/master/PiFmDma/PiFmDma.c presents a rewrite using DMA, which uses much less CPU than this version. Please use that version in preference.
(Created by Oliver Mattos and Oskar Weigl. Code is GPL)
Download the code here, and compile:
make clean all
Now connect a 20cm or so plain wire to GPIO 4 to act as an antenna, and tune an FM radio to 100.0Mhz
sudo python
>>> import PiFm
>>> PiFm.play_sound("sound.wav")
Just run the above code in the same folder. The antenna is optional, but range is reduced from ~100 meters to ~10cm without the antenna. The sound file must be 16 bit mono wav format.
In pifm.c, see line 106. The "5a" number here is a hardware specific password -
ignore it (see the datasheet
for specifics). The "0x5000" is the carrier frequency, it should be interpreted
as 5.000 (in hex), and m is the added audio modulation. This chooses how the
500Mhz system clock is divided down to produce the 100Mhz FM carrier. Hence if
you wanted a 99.0Mhz carrier, you would divide by 5.050505 (decimal), which in
hex is 5.0CF. Hence if you changed the number on that line to 0x50CF you should
be able to get a 99.0Mhz FM signal. Most radio receivers want a signal to be a
multiple of 0.1 MHz to work properly.
The details of how it works
Below is some code that was hacked together over a few hours at the Code Club pihack. It uses the hardware on the raspberry pi that is actually meant to generate spread-spectrum clock signals on the GPIO pins to output FM Radio energy. This means that all you need to do to turn the Raspberry-Pi into a (ridiculously powerful) FM Transmitter is to plug in a wire as the antenna (as little as 20cm will do) into GPIO pin 4 and run the code posted below. It transmits on 100.0 MHz. When testing, the signal only started to break up after we went through several conference rooms with heavy walls, at least 50m away, and crouched behind a heavy metal cabinet. The sound quality is ok, but not amazing, as it currently plays some clicks when the CPU gets switched away to do anything else than play the music. The plan was to make a kernel mode driver that would be able to use the DMA controller to offload the CPU and play smooth music without loading the CPU, but we ran out of time. If you're v. smart, you might be able to get stereo going!
The python library calls a C program (provided both precompiled and in source form). The C program maps the Peripheral Bus (0x20000000) in physical memory into virtual address space using /dev/mem and mmap. To do this it needs root access, hence the sudo. Next it sets the clock generator module to enabled and sets it to output on GPIO4 (no other accessible pins can be used). It also sets the frequency to 100.0Mhz (provided from PLLD@500Mhz, divided by 5), which provides a carrier. At this point, radios will stop making a "fuzz" noise, and become silent. Modulation is done by adjusting the frequency using the fractional divider between 100.025Mhz and 99.975Mhz, which makes the audio signal. The fractional divider doesn't have enough resolution to produce more than ~6 bit audio, but since the PI is very fast, we can do oversampling to provide about 9.5 bit audio by using 128 subsamples per real audio sample.
This is a copy of the original documentation and code from http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter, all rights of the original authors reserved.