pifmrds: FM with RDS

Broadcasts an audio file as FM, with RDS data a car radio can display: a station name, a scrolling radiotext and a PI code. It runs rpitx’s port of Pi-FM-RDS. This is the one module that takes its frequency in MHz. Code: pifmrds.go.

Fields

JSON	Go field	Type	Required	Rules
freq	Freq	float64	yes	MHz, 0.1 to 1500, at most one decimal place (107.9 passes, 107.95 fails with ErrFreqPrecision)
audio	Audio	string	yes	Path to an existing audio file
pi	PI	string	no	Exactly 4 hex digits, such as 1234 or FFFF
ps	PS	string	no	Station name, at most 8 bytes, not only spaces
rt	RT	string	no	Radiotext, at most 64 bytes
ppm	PPM	*float64	no	Clock correction in parts per million, any value
controlPipe	ControlPipe	*string	no	Path to an existing named pipe for run-time changes

The audio file goes through libsndfile, so WAV and Ogg/Vorbis work and MP3 does not. A stereo file gives an FM stereo signal. Pi-FM-RDS can read audio from stdin with -audio -, but gorpitx checks that the path exists as a file, so stdin input is not available through this module.

Without ps, Pi-FM-RDS alternates the station name between Pi-FmRds and a counter.

Example

{
  "freq": 107.9,
  "audio": "/home/pi/sound.wav",
  "pi": "1234",
  "ps": "GORPITX",
  "rt": "Hello from Go"
}

Runs:

stdbuf -oL "$GORPITX_PATH/pifmrds" -freq 107.9 -audio /home/pi/sound.wav \
  -pi 1234 -ps GORPITX -rt "Hello from Go"

The frequency is always printed with one decimal place.

Changing PS and RT while it runs

Create a named pipe before the call and pass its path as controlPipe:

mkfifo /tmp/rds_ctl

While the module runs, write one command per line into the pipe:

echo "PS NEWNAME" > /tmp/rds_ctl
echo "RT Now playing something else" > /tmp/rds_ctl
echo "TA ON" > /tmp/rds_ctl

PS and RT change the station name and radiotext, and TA sets the traffic announcement flag. These commands belong to Pi-FM-RDS; gorpitx only passes the pipe path with -ctl.