jackpifm v3

This is a little program that runs on your Raspberry Pi, reads live audio from JACK and broadcasts it over FM through the GPIO pin 7 (GPIO #4). If you add an antenna (20cm of plain wire will do), the range increases from ~10cm to ~100 meters.

The program is able to broadcast both mono and stereo (plus RDS). It has various settings to customize resampling, latency and more, see below.

Quick start

Just get the appropiate tools:

sudo apt-get install jackd2 libjack-jackd2-dev build-essential

And then build!

make

If everything went well, execute with sudo ./jackpifm.

By default, jackpifm emits in 103.3MHz carrier frequency. You can change this with the -f option. You can also pass a JACK port as an argument and jackpifm will connect to it, or you can connect ports manually, you know.

Warning: Don't start this program while sound is being played through the builtin soundcard, or vice versa. This will render both unusable until the next reboot.

Have fun! It's still recommended to read the rest of this README.

Synchronization

With the new version, we use a ringbuffer and a PI controller to acommodate desynchronization between JACK and the GPIO. We do this by changing the rate at which GPIO accepts frames.

After firing jackpifm you'll see some errors; this is normal, the controller is still initailizing. After ~30 seconds, there should be no errors and the pitch should move between a 0.03% range, far low to be perceived or even measured.

Latency

When started, jackpifm will print a bunch of information, including the minimum and maximum latencies, and the targetted (typical) latency. Here "latency" is the time passed between getting a sample in the JACK port, and emitting the FM wave.

The controller will keep the latency as close to the target latency as possible, and it's currently good at it (I see no more than a few milliseconds of deviation in my B+).

However this is not a guarantee, and in some cases the latency can get out of control and reach the maximum or minimum, in which case the controller will be resetted and you'll hear glitches.

Protip: If you hear glitches or get error messages, try increasing -b to improve stability. On the other hand, if you want to force less latency changes, decrease it. See also "Resampling" below.

Resampling

If -r is enabled, jackpifm will resample all sound from JACK into 152kHz before emitting it. This means a bit more load on the CPU and GPIO, and translates into distorsion in FM except when absolute silence is being emitted.

It also means lower latency but higher pitch changes.

It's required if you want to enable Stereo or RDS (see below).

Stereo

If you pass in the --stereo option, jackpifm will open two JACK ports, left and right, and modulate them together. An FM stereo radio should be able to separate both channels back, while on a mono radio you'll hear them mixed (average value) but at 90% of its value.

If you enable --stereo you may pass two ports (left and right) instead of one.

Note: I haven't verified the feature works in this version.

RDS

RDS allows a radio station to embed a little bitstream into the emitted FM. The stream contains data about station name, currently playing program, genre, and notifications.

To make jackpifm embed an already encoded RDS blob in the emission, pass the blob file through the option -R. To emit the example RDS blob, you'd do:

./jackpifm -r -R example.rds

If you want to generate your own blob, you can use rds-utils.

Note: I haven't verified the feature works in this version.

Other options

There are other options not explained here, that allow you to disable the pre-emphasis built-in filter, change the JACK client name, change resampling quality and more. Look at the help message and / or the code.

Emission details

Under the hood, jackpifm communicates with the GPIO controller, and sends commands to output HIGH and LOW voltages at the correct timing to approximate an FM wave.

It's a very rough approximation, and even though it does the job pretty well, keep in mind you're disturbing higher frequencies outside the FM range. Also emitting FM will probably be illegal in your country.

Warning: FM only allows samples at the range [-1, +1]. Any sample exceeding that range will be cropped, and a warning will be output.

When it comes to range:

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.

I still hear a subtle creak every second or so, which I believe to be associated to the GPIO instruction buffer wrapping around and jumping to the start.

If you want to know more about how the emission is done, see the original page.

History

This was originally published here. I took the code and simplified it, rewrote it in C, made it modular and consistent and added JACK support.

Why JACK? JACK makes it easy for applications to share sound in a real-time way. You can even use it over a network too.

Sure, you can always use avconv and netcat to feed PCM to the original program. But if you do, you'll get at least one-second latency, and the emission won't be stable at all.

This corrects some other bugs that arise with a real-time audio source.