 By alternating between two screens on every frame interrupt (50 Hz), it is theoretically possible to simulate a doubling of the vertical display resolution from 192 to 384 lines.The Timex clones and ZX Spectrum 128K implement a 'shadow' screen area which can be switched into place through the use of a single OUT command, and this is often utilised to rapidly switch between two images for this purpose (although this can also be achieved with a standard block copy, albeit not over the entire screen).When viewed on a CRT television screen (the standard at the time), the flicker is less noticeable than on a modern monitor.