Question about modded VGA cable

Portables, case replacements, mods etc, all in here!
Post Reply
FriendofSonic
Posts: 6
Joined: Mon Jun 09, 2014 3:48 am

Question about modded VGA cable

Post by FriendofSonic » Wed Apr 04, 2018 7:37 pm

Hey there,

I came into possession of a modded component for VGA cable. When I hooked it up to a flat panel TV display with VGA in, I noticed vertical lines only when it displayed in 480p. I found several older posts from here and elsewhere stating that this is to be expected from flat panel TVs using VGA. Would the issue disappear if using a proper PC monitor (LCD or CRT, or only CRT)? I've heard it will disappear and I've also heard specifically CRTs won't exhibit the issue.

Sorry to retread on what is probably known info, but everything I found was older info. I'm mainly making sure that the cable wasn't improperly modded-- if it's purely a signal issue then that will make me feel relieved. I'm mainly checking to make sure it's safe to sell since the horizontal lines are no good for my current setup. Also apologies if this is the wrong forum.
pstrick
Posts: 16
Joined: Tue Jun 14, 2016 2:31 am

Re: Question about modded VGA cable

Post by pstrick » Thu Apr 05, 2018 12:34 am

I have a Japanese D terminal cable that was modified to VGA.
It works fine on my LCD tv. (a polaroid flm 3232)

Can you post a picture of the lines?
FriendofSonic
Posts: 6
Joined: Mon Jun 09, 2014 3:48 am

Re: Question about modded VGA cable

Post by FriendofSonic » Thu Apr 05, 2018 4:01 am

Image

I did keep digging after this post and found a post from Unseen: https://www.gc-forever.com/forums/viewt ... &start=499
A closer examination of the video signal from the board using an oscilloscope revealed the cuplrit: There is a small interference from the 27MHz pixel clock on the three RGB lines. This does not matter if the display samples the video data with the same frequency because it is just a small, constant offset at each sample point. However, if the display incorrectly interprets the signal as 640x480 instead of 720x480, the phase of the sampling points and the 27MHz interference shifts with every pixel, resulting in vertical stripe artifacts.
Soo... my display sees it as 640x480 when I went and checked. Can you tell me if your TV is correctly displaying as 720x480?
FriendofSonic
Posts: 6
Joined: Mon Jun 09, 2014 3:48 am

Re: Question about modded VGA cable

Post by FriendofSonic » Wed Apr 11, 2018 2:55 am

So, CRT displays this perfectly. I'm going to guess it's because all the prior flat panels displayed the image as 640x480 and that caused the artifacts. Leaving this here in case anyone from the future reads this topic.
pstrick
Posts: 16
Joined: Tue Jun 14, 2016 2:31 am

Re: Question about modded VGA cable

Post by pstrick » Fri Apr 13, 2018 11:18 am

I don't really know how to check the resolution on the TV.

I can try to test it out on another TV, but I'm not going to have time for that for the next couple months.
User avatar
Aleron Ives
Posts: 72
Joined: Thu Oct 13, 2016 3:56 am
Location: California

Re: Question about modded VGA cable

Post by Aleron Ives » Fri Apr 13, 2018 12:46 pm

This is certainly a problem for Dreamcast VGA output. Some TVs do not accept 720x480 over VGA, as it's not an expected monitor resolution, but the DC outputs 640x480 of active image within a 720x480 frame, so you can only use DC VGA directly if you're lucky enough to have a TV with VGA input that also likes the DC's wonky resolution. The fact that you got proper output on a CRT suggests you have HDTVs that don't fully support 720x480 over VGA, either, although the usual result is that you get no signal whatsoever, rather than a faulty image. Does the GC also output 640x480 in a 720x480 frame, or is it actually producing 704x480 of active image within the frame? I don't think "NTSC VGA" is a thing, so that would explain why your TV doesn't like it, if so...
Post Reply