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Sync Signals
Synchronisation is transmitted via negative-going pulses; in a composite video signal these are approximately 0.3V below the 'black' level. The horizontal sync signal is a single short pulse which indicates the start of every line. Two timing intervals are defined - the front porch between the end of displayed video and the start of the sync pulse, and the back porch after the sync pulse and before displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display 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 of 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 mid-way through).