Why Analog SSTV is an FM/audio-subcarrier mode: everything, including timing, is encoded as tone frequency. Brightness runs from 1500 Hz (black) to 2300 Hz (white), and a 1200 Hz tone burst below the video band serves as the horizontal sync pulse that tells the decoder to start the next scan line. Because 1200 Hz never appears as picture data, the software can reliably detect it and stay in step with the transmitter.
Watch out Elapsed time alone is tempting since each line has a fixed duration, but drift between the sending and receiving sound cards would slant the picture without periodic sync tones; amplitude carries no information at all in SSTV, since the mode is frequency-based.
SSTV speaks in pitch, not volume: 1200 Hz = new line, 1500-2300 Hz = black to white.