github.com
> Please note that due of the hardware of the raspberry pi, harmonics ARE generated on 3x, 5x, 7x and 9x the desired frequency. Those harmonics are polluting important services like emergency services a d others.
Please do not use the GPIO of a Raspberry Pi to generate a high frequency to emit radio waves. Putting square waves on the air is always asking for trouble. A simple filter is not a proper solution if the source is so dirty.
For a project as well built of this seems, it seems odd that they would advice people to use such a hacky way. Syncing networked clients to play audio at exactly the same time is a solved problem.
Say more?
WebRTC makes it possible by timing is still limited to NTP who has is far above one sample, you couldn’t possibly get sample accurate playback but you could get it to within a mS or so.
Depending on how far away your sources are that might be fine, for instance two speakers in two rooms where you won’t get significant phase issues this is trivial to do(well trivial is an understatement but you can do it purely with web technologies).
I was going to point out that with the variance in FM demodulation chips, using a pile of FM receivers probably wouldn't get you perfectly synced audio these days at all, even more so if it's going through usb/software/audio stacks.
Then I re-read the Ops comment and this actually seems to be a network of _transmitters_. I'm not sure what problem they're trying to solve, but I can't believe multiple PiFMs is ever the answer.
Commercial DAB radio does use single frequency networks (with tight timings and clever calculated offsets), and I am somewhat curious how analogue FM responds with regard to offset destructive interference, but this isn't that.
Please don't do this. For context, a car FM transmitter is limited to 250nW (in many jurisdictions). A Pi GPIO pin with the right bit of wire is potentially capable of 10mW or more. 40,000 times more powerful and a lot more noisy. One could be causing problems for people surprisingly far away.
The project also cursorily mentions the need to use a bandpass filter to avoid interference but fails to explain the specific device needed or how to wire it up in the instructions.
[EDIT] I debated whether or not to say it but I will: this is just irresponsible. There is no way to legally use this project as is. The user would have to have additional knowledge of RF hardware and regulations to make it compliant.
Again, the project you forked from literally tells users never to attach an antenna; they have enough RF knowledge to know a tiny bit of signal can go a very very long way. You clearly do not.
We live in the time of Cargo, pip and npm. So no. /s