Talking of AM demodulation, both synchronous and non-synchronous, what
is done is SDRadio is the following :
- Non-synchronous : IMHO the software implementation of the rectifying
process normally done in HW should be avoided, as it is highly
non-linear and produces distortion. Since both I and Q are available,
the AM modulation is just the variation in magnitude of the rotating
versor described by the I/Q couple. So nothing is easier than computing
AF = sqrt(I*I + Q*Q)
where AF is the recovered audio signal. Of course, you will also have
a DC component, which is easily removed with a two-pole high pass IIR
filter.
- Synchronous : SDRadio uses both the USB and LSB demodulators, adding
then their outputs. Additionally, the phase of the carrier, computed as
phi = atan(Q/I)
is used to lock the NCO used for the final downconversion to baseband,
ensuring that no beat tones are generated. The lock range is about 40 Hz.
73 Alberto I2PHD
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