 Another hardware quirk produced one of the most distinctive aspects of the ZX81 's screen display – during loading or saving , moving zigzag stripes appear across the screen . The same pin on the ULA is used to handle the video signal and the tape output , producing the stripes as an interference pattern of sorts . The ULA cannot maintain the display during <unk> and LOAD operations , as it has to operate continuously to maintain the correct <unk> rate for data transfers . The interference produces the zigzag stripes . 
