https://www.youtube.com/watch?v=1XhudGdTAm8
I got the code from here
https://github.com/ChristopheJacquet/PiFmRds
if you do “try this at home” (or anywhere else) – be well aware that although it is impressive the radio signal generated by this method (using some very clever maths and DSP that I hardly understand) is very rough and full of harmonics – it makes use of a “feature” of PWM signals that can often be a nuisance (like LED displays that are pretty but make unwanted noise on audio and radio equipment…)
Transmitting these can upset a lot of other important radio communications on other frequencies, including aviation and emergency services comms and even some mobile telephones. That can get you in trouble with your country’s Communications Ministry!
In many nations (even where mini FM transmitters are permitted below a certain power) they also generally do not approve of RDS data being transmitted as well in case it confuses or distracts people driving motor vehicles.
In my country (UK) even licensed broadcasters must only transmit static channel IDs (unlike elsewhere in mainland Europe) – there is more leeway with regard to RT (radiotext) that does not automatically appear on most older receivers.
I used the very shortest length of wire that would give a reliable signal to receivers a few metres away and placed the RPi low down in an area where the signal would be blocked by various other metal items connected solidly to earth; and even then only ran this for a few minutes as a demo of how RDS works (otherwise I just use a legal mini FM TX without RDS)
