The RF video signal from the ZX81 deviates from the standards defined for the PAL and NTSC formats with regard to both timing and signal waveform. Three versions of the ZX81 ULA were released, with the final version incorporating a back porch signal in the video output that improved TV compatibility by defining the voltage level that represented black (the black reference level). To understand the importance of this, consider the line synchronisation waveforms output by the different ZX81 ULA versions for a pure white scan line:
|ULA 2C158E / 2C184E||ULA 2C210E|
The lack of a back porch in the video signal output by the earlier 2C158E and 2C184E ULAs causes a modern TV to interpret the left hand border signal as the black reference level. In effect the TV has been told that when it sees white from the ZX81 it is to display this as black. This tends to result in a black picture with a 'ghostly' image showing white outlines at the transitions between black and white pixels. Sinclair rectified this problem with the release of ULA 2C210E. Modern TV sets are therefore far more likely to display a usable picture from a ZX81 fitted with this newer ULA.
The ZX81 video signal is modulated to become an RF signal and then the TV must demodulate it back into the original video signal. These processes introduce electrical noise and result in reduced picture clarity. The RGB SCART connection from the Chroma 81 interface yields a clearer picture because it avoids the modulation/demodulation stage. The difference in picture quality I obtained from the RF connection for the various ULA types and from the Chroma SCART connection is shown below. Click the buttons to the left to select the photo to display.
|RF Connection (ULA 2C158E)|
Note that the original 2C158E ULA had a tendency to slant the first row of the display to the right after the ZX81 had heated up for a few seconds. The Chroma 81 interface monitors the expansion bus from the ZX81 and generates an identical picture to that being produced by the ZX81. It will therefore also slant the first row of the display. Fortunately, the later versions of the ULA did not suffer from this problem.