Video: old VIC2 chroma effect on odd chars

[Michael Huth]

I currently found no option in the video emulation to get the chroma effect a real C64 with old VIC2 has on odd character positions.
(See image from a real machine)
How do I have to configure video to get the same representation in emulation?

Best regards,
Michael

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[PiCiJi]

Do you mean the red touch in bright colors each second column? Can you tell me the monitor type? You mean the new VIC-II doesn’t produce this effect ?

[Michael Huth]

Yes, the red touch every second char. It is visible with the old VIC-II.
For the screenshot I took the capture from a BT878 based capture card, where the C64 is connected by S-Video.

Here is an original resolution screenshot from a new VIC-II PAL-C64 with the same setup:

It shows 288 scanlines, each sampled with 768 pixels. Here I have not rescaled it to 4:3 aspect ratio that the correlation between sample point and pixel does not get lost.
With the new VIC-II you don’t see the red tint effect on each second character. What you see with the new VIC-II is that on the very beginning of each scanline it starts with a grey dot. Of course you can not see that on a regular 1084S as it is too far to the left, however it might be of interest for the emulator if you want to show full borders.

For the red tinted chars, do you also want a photo off a 1084S CRT tube?

[Michael Huth]

Ok I did a few pictures from actual 1084S monitors. First from a 1084S-D2:

If you look closely you can see that on every second char the odd lines (counting vertical from 0) show the purple tint. Like the O, M, D, R in COMMODORE. Whereas the C, M, O, O, E do not show this effect.

I changed the monitor now to a 1084-P with a different tube:

The picture shows mainly the tube difference. The purple char effect is only slightly visible. However, when I do a unsharp closeup the difference can be seen clearly:

So the effect is here as well and it is not related to using a capture card for my initial screenshots.

[PiCiJi]

Thank you for the images. I have never heard of this effect before. If you know any good web sources, i would appreciate it. pal emulation doesn’t mention it. it seems this effect is called something like color banding.

[Michael Huth]

Here is a thread on Forum64 describing the same problem: https://www.forum64.de/index.php?thread/94529-jedes-zweite-zeichen-rosa/&pageNo=1

[PiCiJi]

ok I will look into it

[PiCiJi]

it’s hard to replicate the pattern. basic screen use same chroma for both colors. luma distinguish between light and dark blue. it’s no coincidence that the pattern repeats every 16 pixel, because chroma is transfered phase shifted with a ratio of 16:9 to dot clock. It seems the red color is the inverse color in YUV color wheel. If there is no change in luma the effect isn’t noticeable. Luma change triggers the effect. it seems chroma is tainted by a luma change.

Here is an another interesting thread: https://www.melon64.com/forum/viewtopic.php?f=12&t=10444

Have a look of the screen in the first post of thread. the pattern differs. I don’t have a VIC producing this anomaly. If you have some time, could you genereate a few more screenshots with 1084S-D2.

1. with different chroma on same line (simply change text color one time, two times in next line, three times in next line,… ) for color change, press “Comodore key + digit”, please use most or all colors
2. change pattern of luma change (press “left control and plus” for a half line and normal chars in the rest of line, repeat the pattern more times in next line and so on

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