 The ZX81 's primary input / output is delivered via four sockets on the left side of the case . The machine uses an ordinary UHF television set to deliver a monochrome picture via a built @-@ in RF modulator . It can display 24 lines of 32 characters each , and by using the selection of 2 × 2 block character graphics from the machine 's character set offered an effective 64 × 44 pixel graphics mode , also directly addressable via BASIC using the PLOT and UNPLOT commands , leaving 2 lines free at the bottom . Two 3 @.@ 5 mm jacks connect the ZX81 to the EAR ( output ) and MIC ( input ) sockets of an audio cassette recorder , enabling data to be saved or loaded at a rate of 50 bauds . This provides a somewhat temperamental storage medium for the machine , which has no built @-@ in storage capabilities . The ZX81 requires 420 mA of power at 7 – 11 V DC , delivered via a custom 9 V Sinclair DC power supply . 
