Why Analog SSTV has many incompatible modes (Scottie S1/S2, Martin M1/M2, Robot 36, PD120 and so on) that differ in line timing, number of lines and color encoding. Before the picture lines begin, the sender transmits a short digital header of audio tones called the Vertical Interval Signaling code, a 7-bit number plus a parity bit, that names the mode. Receiving software reads the VIS code and automatically switches to the right decoder, so the picture comes out with correct geometry and color instead of skewed or off-color.