Signal-chain overview

The path from antenna to loudspeaker, with the second FFT enabled (the general case). Stages 2–5 are the wideband DSP; stages 6–9 the narrowband DSP.

1. Input fills timf1 with raw complex (I/Q) or real samples, one or two RF channels, from an A/D device, the network, or a file.

2. First FFT transforms timf1 into the wideband complex spectrum fft1 and the main spectrum/waterfall.

3. Strong/weak split + back-transform routes each FFT bin to a strong-signal slot or a weak-signal + noise slot (per the liminfo[] classification), and back-transforms both to the time function timf2. See First FFT and the strong/weak channel split.

4. Noise blanker operates on the weak-signal part of timf2, where impulse noise is exposed. See Noise blankers.

5. Second FFT recombines the two timf2 parts into one time function and transforms it at high resolution for AFC and spur removal.

6. First mixer + AFC tunes the selected signal to 0 Hz and decimates it, implemented as a limited back FFT rather than an NCO + FIR. Output is timf3.

7. Third FFT + second mixer apply the baseband filter, optionally combine the two RF channels adaptively (Diversity and adaptive polarization), and produce the complex baseband.

8. Baseband processing does AGC detection, AM/FM demodulation, coherent-CW carrier extraction, and the baseband spectrum/waterfall.

9. Output applies the BFO, AGC gain and squelch, resamples to the exact D/A rate, and writes audio. See Mixers, filters, baseband and output.

When the second FFT is disabled, AFC uses first-FFT transforms directly and the blanker is less effective.