Dreamcast Dithering through VGA

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  • October 1, 2017 at 6:03 PM #15577
    Wilco
    Participant

    Hi.

    I’m getting a strong dithering and checkerbox pattern across my screen when I use my Dreamcast witg my VGA box through the OSSC. When I plug the VGA box straight into the TV, however, I get a completely clean image. I have tried and tested with the same results on two televisions and two different VGA cables. Has anyone else come across this issue? Could It be that The OSSC doesn’t like the signal from my VGA box.

    Cheers.

    October 1, 2017 at 9:07 PM #15582
    BuckoA51
    Keymaster

    Never noticed anything like that here, and Dreamcast VGA boxes should all be the same. Are you using the recommended settings for Dreamcast VGA as per the wiki?

    October 2, 2017 at 5:08 PM #15607
    Wilco
    Participant

    Yeah, I’m using DTV in 480, but it still looks bad.

    This is a picture of the image straight through into my TV.

    View post on imgur.com

    And this one is when it passes through the OSSC.

    View post on imgur.com

    The image is not only darker in the second picture but the dithering and checkerbox effect is dramatically increased. It’s also apparent when capturing the image via an Elgato HD60.

    Any ideas?

    October 2, 2017 at 5:16 PM #15609
    BuckoA51
    Keymaster

    Hmm, that is strange I wonder if this is what happened to Adam Koralik’s Dreamcast (only other person to complain about poor video quality with OSSC and Dreamcast so far).

    Did you try the other fixes i.e changing H.active to 640 in the advanced timing menu and ajusting H.backporch to centre image?

    October 2, 2017 at 5:38 PM #15611
    Wilco
    Participant

    Yeah I have also tried that but still, nothing makes it better. I saw Adam’s review and wondered whether this may be the same problem. What do you suggest I do? As the Dreamcast was a big reason for me getting this.

    Cheers.

    October 2, 2017 at 5:55 PM #15614
    BuckoA51
    Keymaster

    You say you have tried two different VGA cables, was that the full cable and box or just a cable.

    Admittedly I’m clutching at straws here, really puzzled.

    October 2, 2017 at 6:18 PM #15615
    Wilco
    Participant

    Yeah me too. It was two separate VGA cables both tested with the same VGA box. It’s super weird as everybody else has raved about the Dreamcast image being incredible. I hope I’m not missing something super obvious. Would it be worth sending the unit back for testing?

    October 2, 2017 at 9:37 PM #15617
    paulb_nl
    Participant

    This appears to be a case of dithering being revealed by the clearer picture of the OSSC. The Dreamcast connected directly to your TV looks much blurrier than the OSSC.

    If you search on Google for ‘Dreamcast dithering’ you will find lots of results with people noticing the dithering even on RGB scart.

    October 3, 2017 at 11:09 AM #15638
    BuckoA51
    Keymaster

    I can spot the dithering on some screens on my Dreamcast now I’m looking for it. It definitely only affects certain screens though, for instance there’s no dithering on the polygons on Crazy Taxi

    October 4, 2017 at 2:09 AM #15660
    BubbaMc
    Participant

    Agreed with paulb_nl, if you look closely it seems the dithering is present when hooked directly to the TV, to OSSC just reveals this.

    December 16, 2017 at 10:51 AM #18025
    Tanbao
    Participant

    Hi, I am using this thread, because I have the same problem. My Dreamcast is connected though VGA with the OSSC and it gives me some square lines all over the LCD. If it’s hooked up with the TV only, the picture is okay, just a bit blurry, but clean.

    I tried it with the DC Wiki settings from junkerhq, but it didn’t solve the problem. Also, I don’t have any other cables to test with, only the VGA ones.

    Picture 1
    Picture 2

    Any ideas?

    December 16, 2017 at 1:10 PM #18030
    SirRockALot
    Participant

    I’m 99% sure that the DC is supposed to have terrible dithering. Mine looks exactly like yours (both through the OSSC and directly on my TV), has dithering patterns in solid colors like the GDemu background etc. Even visible through SCART RGB. When I bought this up on other DC discussion forums, people were all like, duh, that’s like complaining an N64 is blurry, that’s what it looks like.

    I guess it’s simply your TV’s AD conversion / processing blurring out the dithering patterns?

    December 16, 2017 at 2:30 PM #18031
    Tanbao
    Participant

    I don’t know if it’s my TV’s fault or just the cable. It’s a simple VGA cable from ebay. If iz’s connected with my TV only, it’s fine.

    Maybe a VGA -> Scart cable could solve the problem. Or ones from retrogamingcables…

    VGA Cable

    December 16, 2017 at 3:13 PM #18034
    Teddy Rogers
    Participant

    The dithering is normal on the Dreamcast and most visible and pronounced when displayed through VGA. A lot of people complain when they get a VGA cable because there are a lot of vertical lines on screen. Those vertical lines are due to the dithering and the screen mode being squished or stretched because of incorrect DTV timings. A perfect image should look something similar to this…

    http://i67.tinypic.com/14kgr3o.jpg

    Ted.

    December 16, 2017 at 3:40 PM #18036
    Tanbao
    Participant

    And how to get rid of the problem? Another TV or cable? 🙁

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