What signals SSTV receiving software to begin a new picture line?
Specific tone frequencies✓Bonne réponse
Elapsed time
Specific tone amplitudes
A two-tone signal
Explication
Each SSTV line begins with a sync tone at a frequency deliberately placed below the range used for picture brightness, and the receiving software starts a new line when it detects that specific frequency. Timing alone would drift, and amplitude carries no information in SSTV.