How does the horizontal sync signal work?
The brightness information comes to the television as a steady stream. While the television knows that this information should control the brightness of adjacent spots on the screen, from left to right, it needs to be told when each horizontal line begins and when each vertical sweep begins. It knows that a new line is coming when the brightness information contains a “blacker-than-black” level. This level seems to say that the electron gun should not only stop sending electrons at the screen, it should send less than no electrons at the screen! Actually, this level is an instruction to the television’s electronics, telling the television to bring its electron beam back to the left side of the screen to begin a new horizontal line. A long “blacker-than-black” level is an instruction to the television to begin a new vertical scan down the screen.