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Blastfrog

Question about XGA

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If I were to use a modern(ish) video card with analogue VGA output, using a source port like Chocolate Doom at an output resolution of 320x240 (NOT 320x200), would the analogue signal actually output at exactly 240 lines, or would it scandouble it to 480? I take it that if it actually does output 240 lines, that the monitor itself does the scan doubling, right?

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I think it might output at 640x480, but there won't really be any noticable difference.

Also note that using 320x240 directly with chocolate doom tends to make the display look like a blurry mess (640x480 is significantly better). You should basically just make a tradeoff between CPU speed and what your monitor/video card can do. 320x200 native is the most ideal, no scaling whatsoever so it's super silky fast, but it's also only really decent looking on old CRT monitors. LCDs have native resolutions so you're better running at native resolution (let Chocolate Doom do the scaling) or close to it.

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chungy said:

Also note that using 320x240 directly with chocolate doom tends to make the display look like a blurry mess (640x480 is significantly better). You should basically just make a tradeoff between CPU speed and what your monitor/video card can do. 320x200 native is the most ideal, no scaling whatsoever so it's super silky fast, but it's also only really decent looking on old CRT monitors. LCDs have native resolutions so you're better running at native resolution (let Chocolate Doom do the scaling) or close to it.

Well, what I'm trying to do is build a scan converter from VGA to NTSC RGB to use with an Amiga 1084 monitor, and use Chocolate Doom at 320x240 but NOT have it scale it, so there would be 20 lines of black space on the top and bottom of the image. I was asking this to see if the source signal could actually be what I needed, or if it was absolutely gonna be 480 even if I didn't want it to be, so I could know in advance if I need to have the scan converter skip every second line or not.

Note that I haven't actually built this thing yet, and am not terribly experienced in building electronics. I was thinking of trying to use this circuit as a reference to build the reverse of it:

http://elm-chan.org/works/sc/report.html

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Sodaholic said:

NTSC RGB


There's no such beast as "NTSC RGB". NTSC encodes luminance and chrominance plus sync pulses in a single signal, which then you have to demodulate in order to get all components back.

There can be, however, a RGB signal with refresh rates (H & V) matching those of NTSC (or PAL), usable to drive a TV with a RGB input or a multisync RGB monitor that can sync to them, and has a correctly wired lead.

What are you trying to drive? A normal TV or a monitor with just a composite input? There's no need to build your own converter for that: practically all VGA cards made after 1997 have a built-in TV output (often S-Video or even component video with some cards), so you don't need to jump through hoops. At most you'll need a cheapo passive S-Video to Composite video adaptor (which is as simple as mixing the two Y/C signals with a capacitor).

Trying to drive a RGB monitor which can't sync to VGA refresh rates? You can do with a much simpler converter and an utility to set the VGA refresh rate (horizontal) lower than normal directly on the computer. Very popular in the emulation scene.

What the guy in the link you posted built is practically a real-time composite s-video signal digitizer, which then gets projected into a VGA signal without having a PC do the work (every TV/Video digitizer card is able to do that).

The "opposite" of that would be a VGA-to-SVideo converter, which you probably already have on your PC...

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Maes said:

There's no such beast as "NTSC RGB". NTSC encodes luminance and chrominance plus sync pulses in a single signal, which then you have to demodulate in order to get all components back.

Ah, okay. What I meant was 60hz 240p RGB that's TV compatible. I'm using a Commodore 1084s monitor, which is basically an RGB TV that can run in both 50 and 60 hz.

Maes said:

There's no need to build your own converter for that: practically all VGA cards made after 1997 have a built-in TV output (often S-Video or even component video with some cards), so you don't need to jump through hoops. At most you'll need a cheapo passive S-Video to Composite video adaptor (which is as simple as mixing the two Y/C signals with a capacitor).

I know, I've dealt with those before and they all suck. I need 1:1 video output in 240p, but all it does is 480i, and even if you use a 480 line resolution, it will scale it down to what I think is 448 lines with some overdraw. Point is, it's not flexible enough for my needs, and the output is shite anyway.

Maes said:

You can do with a much simpler converter and an utility to set the VGA refresh rate (horizontal) lower than normal directly on the computer. Very popular in the emulation scene.

Exactly what I want to do, but I need a way to get the card to output 240 lines, instead of anything more.

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Maes said:

There's no such beast as "NTSC RGB". NTSC encodes luminance and chrominance plus sync pulses in a single signal, which then you have to demodulate in order to get all components back.

Hence its nickname, "Never The Same Color".

SECAM was better anyway.

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Sodaholic said:

Exactly what I want to do, but I need a way to get the card to output 240 lines, instead of anything more.


The problem is that VGA has a minimum fixed horizontal scan rate of 31 KHz. At 70 Hz vertical scan rate, which is the usual scan rate, that turns out to be more than double of what's needed to actually display e.g. 200 lines at 70 Hz. The same scan rate is used also for "high res" VGA modes and text mode (400, 480 lines), so there's no escaping that. The scan rate is fully used in all modes, but it will just contain duplicated/doubly-scanned lines in lower res modes.

You essentially need to convert the 31 KHz signal to 15 KHz...again, your best bet is to use a video driver designed to drive "normal" TV-like RGB monitors and change it at the source, before even starting soldering any signal-level adaptor electronics. The 1084s itself can only handle 15 KHz, so it really needs this adaptation so it doesn't blow up.

http://forum.arcadecontrols.com/index.php?topic=66402.0

Trying to do it all by hardware is not a beginner's project.

Gez said:

Hence its nickname, "Never The Same Color".

SECAM was better anyway.


Uhmm...my mistake, I was actualy referring to composite video, which is staunchly the same no matter what TV system you use (other than 50/60 Hz refresh rate), and has its own set of problems (crawling dot etc.).

The TV systems PAL, NTSC, SECAM etc. add ANOTHER level of encoding on top of that, some of which are better than others :-p

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