PICTURE IMPROVEMENT
The RF video signal from the ZX80 deviates from the standards defined for the PAL and NTSC formats with regard to both timing and signal waveform. It fails to include a back porch signal in the video output which is required to define the voltage level that represents black (the black reference level) in the video signal. This failure was replicated in the first two versions of the ZX81's ULA but corrected in the third and final version. To understand the importance of the backporch signal, consider the line synchronisation waveforms output by the ZX80 and final ZX81 ULA for a pure white scan line:
 |
 |
|
| ZX80 | ZX81 ULA 2C210E |
The lack of a back porch in the video signal output 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 ZX80 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.
The ZX80 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 80 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 ZX80 and from the Chroma SCART connection is shown below. Click the buttons to the left to select the photo to display.
|
|
||||||
| RF Connection |