> For instance, many chipsets have an integrated FM receiver that is essentially a high-sensitivity VHF radio. By taking the raw audio output and applying a Software Modem (AFSK/FSK) in the user-space, you bypass the kernel/firmware complexity entirely. You don’t need to sideload a modem driver if you treat the audio jack or the internal FM bus as your physical layer.

This is fascinating. Happy to do the research myself, but do you have any recommended reading/sources to learn more about this?

I'm glad you find it interesting. I developed the theory at university, studying how ASK and FSK modems work. To build this, you’ll need to understand the Shannon-Hartley theorem, band-pass filtering, Fourier transforms, and convolution.

For the practical 'how-to,' I recommend studying GNU Radio and SDR++; they show how to process IQ data or raw audio streams directly, and for sure there are other libraries. On the 'ancestor' side, look at the AX.25 Packet Radio protocol and AFSK (Audio Frequency Shift Keying). These are the same 'softmodem' principles used in FidoNet nodes decades ago.

GSM Arena can help you find phones with integrated FM receivers. You'll notice that many features are market-dependent, meaning: the receiver is often physically present but simply disabled by software.