So I'm developing my own SDR software from scratch (I like to "own" the code). The channel between the RF-to-USB hardware and the software is 192kHz sample-rate IQ audio samples. The degrees of freedom seem to include at least 4 frequencies and a switch. The 4 frequencies are: RF filter frequency, external tuner reference oscillator frequency, software complex multiplication oscillator frequency, software bandpass filter center-frequency (also filter bandwidth). Then select from I, Q, I+Q, I-Q, or abs(I,Q) for audio output. Then resample as necessary to match the audio output API.
So assume I set the 1st frequency (front-end RF filter) to the HF band of interest, the 2nd HF reference oscillator to the middle of some CW HF band. Now say I find a QSO 12.500 kHz up from the HF reference oscillator, and I'd like the resulting Morse Code audio side tone frequency to be 750 Hz.
Where do I set my software oscillator frequency for the complex multiplication and what do I want my software bandpass filter center frequency to be to hear Morse Code with the desired side tone? (Do I have 2 choices? If so, how to choose?) Which final IQ mux output do I select to feed the audio speaker?