This is so cool! Does anyone know how the data is encoded in the signal? Is it just data for the game that's transmitted (and the logic resides on the ZX Spectrum) or is logic also transmitted?
The format is very simple, basically a square wave with different pulse widths for 1s and 0s [1]. Spectrum software was typically sold on audio cassettes and the radio broadcasts are the exact same format, typically people would record the broadcasts to cassette. The loader program in ROM just copies all the data from tape (or radio) into RAM so yes any game logic would be included in that.
[1] https://en.m.wikipedia.org/wiki/ZX_Spectrum_software