buffer of a computer graphics display imitates the dynamics of a cathode-ray display, if it is updated with a new image while the image is being transmitted to the display, the display shows a mishmash of both frames, producing a page tearing artifact partway down the image. Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval, thus ensuring that only whole frames are seen on-screen. Software such as video games and computer-aided design (CAD) packages often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This produces a small penalty in latency because the program has to wait until the video controller has finished transmitting the image to the display before continuing. Triple buffering reduces this latency significantly. 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. Horizontal and vertical hold Analog television receivers and composite monitors often provide manual controls to adjust horizontal and vertical timing. The sweep (or deflection) oscillators were designed to run without a signal from the television station (or VCR, computer, or other composite video source). This provides a blank canvas, similar to today's "CHECK SIGNAL CABLE" messages on monitors: it allows the television receiver to display a raster to confirm the basic operation of the set's most fundamental circuits, and to allow an image to be presented during antenna placement. With sufficient signal strength, the receiver's sync separator circuit would split timebase pulses from the incoming video and use them to reset the horizontal and vertical oscillators at the appropriate time to synchronize with the signal from the station. The free-running oscillation of the horizontal circuit is especially critical, as the horizontal deflection circuits typically power the flyback transformer (which provides acceleration potential for the CRT) as well as the filaments for the high voltage rectifier tube and sometimes the filament(s) of the CRT itself. Without the operation of the horizontal oscillator and output stages, for virtually every analog television receiver since the 1940s, there will be absolutely