405 lines from Raspberry Pi 5

[ppppenguin]

Further thought for making the carefully selected input frequency to the modulator. I'm sure Frank knows how to do this, but you can easily make a non-integer frequency divider with an adder and register. Sometimes called a discrete time oscillator. The main disadvantage is that the period of each output cycle will jitter by the input clock period. This may or may not matter for the input to the modulator. Only way to find out is by experiment.

You can use a DTO to make accurate sine waves such as PAL or NTSC subcarrier. The designers of high grade synthesised signal generators have various techniques to get very good spectrum purity. To reduce the jitter problems slightly you can do simple stepwise approximation with 2 or 3 bits and some resistors.