405 lines from Raspberry Pi 5

[`FRANK.C]

I have always bit banged the second channel. If I had wanted to use just one I would probably have used a analogue switch. To switch the data line to the modulator I wanted to program.
Where I would normally use 2K2 pull up resistors on the I2c lines. If using the analogue switch instead of the 2K2 pull up resistors I would put a 4K3 pull up resistor on each connection to the switch.

The output of the modulator is quite square. I don't know if that would have any effect on mixing signals.
Photo below of the CH1 audio carrier.

Frank