The NES's GPU generates sync pulses on the video line - one at the end of each line to make the electron gun's aim to return to the left side, and a long one at

The NES's GPU generates sync pulses on the video line - one at the end of each line to make the electron gun's aim to return to the left side, and a long one at the end of each frame for the electron beam path to return to the top left. Circuitry in the TV uses these pulses to keep the picture stable. https://en.wikipedia.org/wiki/Analog_television https://en.wikipedia.org/wiki/Analog_television