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Show HN: An easy way of broadcasting radio around you (looking for feedback)
The project identifies as "Your Raspberry Pi FM network". It uses a fork of the well-known PiFmRds as a backend to stream audio on FM radio. It allows the control of multiple pi units with a central server, that can also be temporarly hosted on google cloud shell or github codespaces.
Luckily the FAQ already mentions why this is a bad idea.

> 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.

> Syncing networked clients to play audio at exactly the same time is a solved problem.

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.

> Syncing networked clients to play audio at exactly the same time is a solved problem.

Say more?

Pi-FM-RDS, from which this project is forked, warns under its Warning and Disclaimer section that "In most countries, transmitting radio waves without a state-issued licence specific to the transmission modalities (frequency, power, bandwidth, etc.) is illegal. Therefore, always connect a shielded transmission line from the RaspberryPi directly to a radio receiver, so as not to emit radio waves. Never use an antenna." They literally warn users not to do what this project tells them to do. I can't be arsed to do the proper math but a casual check suggests that the output power from the Pi could easily exceed the FCCs low power FM transmitter exemption.

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.

The funny thing about doing something like this that is illegal in most countries, is that it actively broadcasts the exact location where you are doing the illegal thing.
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Beside the already mentioned and imo highly valid concerns, no concern regarding the curl some url and pipe to sudo?
On requirements maybe include “antenna or shielded transmission line”
Why is this useful (assuming the issues other people mention are fixed)?