What signals SSTV receiving software to begin a new picture line?

In analog SSTV, synchronization is handled by transmitting specific audio tone frequencies. At the end of each scanned line of the image, the transmitting station sends a very short horizontal sync pulse at a specific frequency, typically 1200 Hz. This distinct tone frequency signals the receiving software that the current line has ended and it is time to move to the beginning of the next picture line. Because SSTV relies on a frequency-modulated subcarrier, using a specific tone frequency for sync pulses is highly reliable over HF bands. Relying on elapsed time or amplitude changes would cause the image to skew or tear due to propagation delays and fading.