Some computer
monitors, especially some older
'graphics' displays of
high resolution for their time, are sync on green. In order to properly display
video received over an
analog connection, a conventional
CRT monitor needs four streams of information:
- Red signal strength
- Green signal strength
- Blue signal strength
- Vertical Retrace (synchronization) signal
Typically, on these high-end monitors, there would be four coaxial cable connections; one locking connector and cable for each color signal, plus an additional cable for the sync signal. These would be abbreviated R,G,B and S; If color-coded, the sync cable was typically black or brown.
However, some monitors only had three connections; or, similarly, some video cards only put out three signals. In this case, they would use the timing of one of the three color signals to synchronize the retrace interval; typically, the green signal was used (although some monitors/video cards could manually select which signal to use). Thus, only three connectors were needed, similar to Sun's 13W3 standard. This was described as 'sync on green.'
One primary advantage of having four connections is that it allowed each isolated cable to carry a single signal, which minimized interference. Interference in the signals would show up as blurred or shifted pixels on the monitor. Sync on green wasn't as good as full RGBS, but it minimized the interference to one signal, which was still much better than a conventional multiline connector such as the 15-pin VGA standard. These cables, while improving over time, typically use much narrower wires in close proximity, meaning it's harder to achieve really high resolutions without some interference. The use of switchboxes doesn't help either, although some are less intrusive than others.