JMPXRDS 
An FM MPX signal generator on top of Jack Audio Connection Kit with support for:
- Typical DSB modulator
- Multiple SSB modulators for the stereo subchannel (L-R)
- RDS Encoder with support for all basic fields
- Built-in RTP server for sending the generated signal on a remote site
The generator output has a fixed sampling rate of 192000, it outputs data to a local unix socket (for the GUI to do FFT analysis and/or other uses e.g. netcat/sox), and through a built-in RTP server. Note that RTP server sends the signal FLAC-encoded to reduce bandwidth. A client for the RTP server is also available as well as a GNU Radio - based receiver for debugging. The signal can be used to drive an FM exciter via a normal sound card. We 've tried this with RPi + HiFi Berry and Odroid + HiFi Shield.
The optional GUI is based on GTK3+ and can be used to fully control the generator, alternatively a set of command line tools are also available for headless systems. The generator itself is a standalone app and uses shared memory to communicate with the GUI/tools.
It's currently used in production on 3 radio stations in Crete (UoC Radio 96.7, MatzoRe 89.1, Best 94.7). In order to operate this properly you should also use some audio processing, we recommend Calf Studio Gear. The default settings together with a properly configured compressor/limiter will get you within the allowed deviation constraints (75KHz). You should also make sure your exciter and amplifier are properly set up.
An overview of JMPXRDS is included in a presentation at FOSDEM 2018.