How does SDR resolution translate to audio resolution when using an FM demodulation algorithm?
For example: using a 8-bit/2.048 Mhz IQ stream, what is the maximum audio sample rate and bit depth that can be generated from a +/-75kHz mono WFM signal with 20kHz audio?
These are the steps of a basic FM conversion algorithm. My question is, what effect does each step have on the bit depth of the result of the step:
Step 1 - Decimate down to the relevant bandwidth (150khz). Doing so reduces the sample rate but increases the bit depth.
Step 2 - Take the arctan of the resulting IQ samples. The combination of the I and Q parts further increases the bit depth of the current data.
Step 3 - Use a differentiator to get the final audio result. I am not sure how this affects the resolution, but since no data is combined, I suspect there would be no change.
Step 4 - Further decimate the audio to desired sample rate (48khz presumably). Would result in further increase in bit depth, but not by much.
In each step, combinations of data result in more data possibilities, eg:
- Average 2 and 3 to get 2.5
- Arctan(I/Q) provides N^2 possibilities, though not all are unique
Wikipedia has an explanation of the effect of oversampling on bit depth. But I am not sure about the arctan and differentiator processes.
Thank you for your expertise