Video quality revisit

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moose
Posts: 12
Joined: Wed Feb 19, 2020 2:07 pm

Video quality revisit

Post by moose »

Hi,

A few months ago I posted about my dragon behaving poorly with a cheap composite to hdmi converter. I eventually switched out the converter for a different one after testing the machine with a TV that had proper composite in, moved on to sorting out other bits of the machine. The new adapter was by no means perfect (flickering occasionally, green periodically looking washed out), but I decided I could live with it.

Since then I've done some more work on the machine, recapped it, got a CocoSDC and adapted a joystick to work with it. The latter means I've been trying more games and running the machine in graphics modes more often. This has now shown up that in 4 colour Green, Yellow, Blue, Red mode the blue and red seem to be nearly indistinguishable. This also seem to be the case when connecting it to the decent TV, which also shows a lot of 'shimmer' when there are bands of red and green. Fiddling with RV1 allows some tuning of picture stability but doesn't seem to help with the colour issue. This is a later rev board without RV2.

I'm not sure where to concentrate further troubleshooting efforts, and I only have limited tools at my disposal (multimeter primarily). Sugegstions welcome.

Stu
sorchard
Posts: 530
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: Video quality revisit

Post by sorchard »

The colour subcarrier crystal is usually just to the left of the power connector, and just under the crystal there is a load cap C7. On some boards this is a 22pF trimmer and can be adjusted to fine tune the subcarrier frequency. If it's a long way off then the display device might struggle to lock on to the frequency. You will also notice that this affects the shimmering dot crawl effect and can be tuned to make it less objectionable. I've added the trimmer to a couple of my machines in an attempt to improve things, though unfortunately tiny adjustments have a large effect, and it drifts with time and temperature so it doesn't stay perfect for long.

Single pixels of red and blue are not easy to distinguish on red/blue backgrounds. That is just the nature of a PAL composite signal: The colour bandwidth is not up to the task. The luminance component has more bandwidth, making light/dark transitions much more well defined than colour transitions.
Stew
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