 The ZX81 's built @-@ in RF modulator can output a video picture to either a UHF 625 @-@ line colour or monochrome television ( used in the UK , Australia , and most western European countries ) . France required a slightly modified version of the machine to match the positive video modulation of SECAM sets , while the USA and Canada required a different ULA chip and modulator to cope with their 525 @-@ line VHF ( NTSC ) television systems . Both the ZX81 and its predecessor , the ZX80 , have a significant drawback in the way that they handle visual output . Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display . On the ZX80 , this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place . 
