This refers to a wire link on the ZX81 PCB. This changes the timing used by the video generation system so that it works on a 60Hz EIA 525/60 system.sirmorris wrote:The unit can be switched to NTSC using a jumper on the board. I can't tell you what it is off the top of my head but I'm sure someone will be along any second to fill in the details
Which composite mod did you do. Some give better results than others depending on the TV.
Older CRT type TV's are generally more tolerant of the non-standard Zeddy video signal.
Some TV's do not like the DC voltage on the video signal that the modified Zeddy produces. In this case add a 100uF 16V electrolytic capacitor in series between the composite video output from the ZX81 and the lead to the TV.
Also note that the ZX81 does not generate a PAL signal, as it does not produce colour!
PAL is a colour encoding system. The signal that the ZX81 produces is a monochrome 625-line 50Hz video signal defined as CCIR 625/50. The UK broadcast system is known as "CCIR system I" (letter I NOT number 1). This is an interlaced video signal with a frame rate of 25Hz, a field rate of 50Hz, a horizontal frequency/line rate of 15625Hz (or 15.625kHz) and a video band-width of 5.5 MHz. When colour information is combined to this, it is often called PAL I.
If you want to know more about the 525/60 link, shout out