MMSSTV demodulators
The original MMSSTV documentation describes three methods for demodulating the SSTV picture tones:
Zero-cross detector
Converts tone timing into picture-frequency information.
PLL
Uses a phase-locked loop to track picture-tone frequency.
Hilbert transform
Uses quadrature signal processing before frequency detection.
Signal path
MMSSTV separates received audio into several processing paths. Dedicated filters detect VIS symbols and synchronization, while another path demodulates the 1500–2300 Hz picture information.
Spectrum display
MMSSTV provides three spectrum-display choices:
OFF
No spectrum calculation.
FFT
Conventional frequency-domain spectrum display.
FM demodulator
Fast display suited to short synchronization pulses.
The original documentation notes that the FM-demodulator display has a fast response but does not represent signal level in the same way as the FFT display.
Automatic Frequency Control
AFC is enabled from the AFC button in the DSP pane. The original default is ON.
When AFC is locked to the received SSTV signal, MMSSTV changes the synchronization-frequency marker to indicate the lock state.
LMS filters
Later MMSSTV versions provide several LMS filter modes selectable from the DSP pane: