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I've installed IRIX on SGI Octane and have screen looking like this:

Screenshot showing fringing

(I mean the stripes on all elements of the screen, which doesn't seem as a result of CRT monitor - the screen looks normally on login screen)

Is there some way to fix this with Xserver settings?

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    That looks like fringing, where the number of pixels x&y doesn't match the CRT's dot-pitch. The screen mode may be different on a login screen.
    – Chenmunka ♦
    May 30, 2017 at 10:42
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    Just some nasty dithered pseudocolor mode?
    – rackandboneman
    May 30, 2017 at 11:24
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    If you have anything near the display that contains strong permanent magnets (e.g. modern loudspeakers) see if moving them about changes the distortion. Then try degaussing the CRT. Otherwise, it's possible a mechanical shock has messed up he internal alignment of the tube components - in which case there is probably not much you can do about it except replace the tube, but it probably won't get any worse over time.
    – alephzero
    May 30, 2017 at 17:50
  • Also, try swapping the video cable to the CRT if you can get a replacement.
    – alephzero
    May 30, 2017 at 17:52
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    @alephzero, magnetic fields produce magnetic-field-shaped patterns on the screen, not the crisp diagonal stripes seen here. This looks more like a settings issue than a physical-hardware issue.
    – Mark
    May 30, 2017 at 18:32

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The pattern, combined with the horizontal shadow effect visible in places, suggests to me that some EMI is getting into the video signal from some nearby clock signals. It's also possible that the video cable has the wrong impedance relative to the high-frequency interfaces at either end, which may possibly be caused by age.

These effects might not have been visible on a lower-resolution, higher-contrast login screen for a variety of reasons.

If a new video cable doesn't fix the problem, you may need to look for faults (such as a missing shield connection) inside the machine and/or the display, near the video ports.

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