on the receiver screen. A sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. Horizontal synchronization The horizontal sync pulse, separates the scan lines. The horizontal sync signal is a single short pulse that indicates the start of every line. The rest of the scan line follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or vertical synchronization pulse. The format of the horizontal sync pulse varies. In the 525-line NTSC system it is a 4.85 μs pulse at 0 V. In the 625-line PAL system the pulse is 4.7 μs at 0 V. This is lower than the amplitude of any video signal (blacker than black) so it can be detected by the level-sensitive sync separator circuit of the receiver. Vertical synchronization Vertical synchronization (also called vertical sync or VSync) separates the video fields. In PAL and NTSC, the vertical sync pulse occurs within the vertical blanking interval. The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line. The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or midway through). The format of such a signal in 525-line NTSC is: pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines) long-sync pulses (5 pulses) post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines) Each pre- or post- equalizing pulse consists in half a scan line of black signal: 2 μs at 0 V, followed by 30 μs at 0.3 V. Each long sync pulse consists of an equalizing pulse with timings inverted: 30 μs at 0 V, followed by 2 μs at 0.3 V. In video production and computer graphics, changes to the image are often kept in step with the vertical synchronization pulse to avoid visible discontinuity of the image. Since the frame