has to handle high power. Sync separator Image synchronization is achieved by transmitting negative-going pulses; in a composite video signal of 1-volt amplitude, these are approximately 0.3 V 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 the displayed video and the start of the sync pulse, and the back porch after the sync pulse and before the 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 midway through). In the television receiver, a sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen. Counting sync pulses, a video line selector picks a selected line from a TV signal, used for teletext, on-screen displays, station identification logos as well as in the industry when cameras were used as a sensor. Timebase circuits In an analog receiver with a CRT display sync pulses are fed to horizontal and vertical timebase circuits (commonly called "sweep circuits" in the United States), each consisting of an oscillator and an amplifier. These generate modified sawtooth and parabola current waveforms to scan the electron beam in a linear way. The waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface. The oscillators are designed to free-run at frequencies very close to the field and line rates, but the sync pulses cause them to reset at the beginning of each scan line or field, resulting in