Can DC outupt custom VGA resolution/refresh rate?

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dark night
Posts: 55

Can DC outupt custom VGA resolution/refresh rate?

Post#1 » Thu Mar 30, 2023 6:17 am

This happened by chance last night, when I was faffing about with boot discs and such.

So, I have the DC plugged to crt VGA monitor, and that is always 640x480@60Hz, right?
Also, my VGA monitor identifies it correctly with on screen message.

I happened to insert PAL Enforcer and the thing booted and displayed a picture!
It's the same picture the disc usually shows but it is placed on top and squashed to about half.
But now, monitor does not identify it as 640x480, actually it says is "New Mode".
But what exactly is this New mode?
If PAL Enforcer is working and changed the refresh rate to 50Hz, is it 640x480@50Hz ?
If not, could it be 640x576@50Hz?

At first I thought this is strange but maybe it can be interesting, perhaps for Amiga emulation purposes?

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Anthony817
Shark Patrol
Posts: 4009

Re: Can DC outupt custom VGA resolution/refresh rate?

Post#2 » Thu Mar 30, 2023 5:40 pm

Yeah sounds like possibly using the 576 PAL resolution.
Image

TapamN
letterbomb
Posts: 149

Re: Can DC outupt custom VGA resolution/refresh rate?

Post#3 » Thu Mar 30, 2023 6:16 pm

I've done testing with modifying the video timing on the DC. Just yesterday, I was testing a new CRT I got to try out 120 hz at 640x240 (adds true scanlines) and 320x480 (my other CRT maxxed out at around 102 Hz). I can change video timing on the VMU screen while the monitor displays a test image.

It is possible to use the PAL timings in VGA, and get a 640x576 at 50hz (and 640x600 is also possible). (I've thought about using that to enable a PAL VGA mode for Gens4All.) There could be multiple reasons why the image was squashed.

The pixel clock is fixed at 27 MHz, which severally limits what can be done. The only "standard" alternate resolution the DC supports at full detail is 640x400x70hz. When the DC runs at 320x240x60hz (or 640x240 or 320x480) it actually has the same timing, but doubles each pixel. It's also possible to add borders, like using 640x480x60hz timing and displaying a 640x448 frame buffer or something.

It's not possible to increase the horizontal pixel density/resolution without running into problems on most monitors (but it extending the framebuffer into the border a bit works fine). By reducing the horizontal and/or vertical resolution, you can increase the framerate (e.g. 640x400x70hz or 512x384x88hz). Increasing vertical without reducing horizontal results in a lower framerate (640x768x43hz, monitor thinks it's 1024x768x43hz. This only works on my smaller CRT). It's possible to increase the vertical resolution without sacrificing framerate, but you have to reduce the horizontal resolution (424x768x60hz, monitor thinks it's 1024x768x60hz).

The 3D hardware doesn't fully support rendering at a resolution higher than 1280x480, so high resolution modes are more complicated to use with the 3D hardware (but not impossible).

cloofoofoo
Dreamcast Swirl
Posts: 534

Re: Can DC outupt custom VGA resolution/refresh rate?

Post#4 » Sun Apr 02, 2023 12:41 am

TapamN wrote:I've done testing with modifying the video timing on the DC. Just yesterday, I was testing a new CRT I got to try out 120 hz at 640x240 (adds true scanlines) and 320x480 (my other CRT maxxed out at around 102 Hz). I can change video timing on the VMU screen while the monitor displays a test image.

It is possible to use the PAL timings in VGA, and get a 640x576 at 50hz (and 640x600 is also possible). (I've thought about using that to enable a PAL VGA mode for Gens4All.) There could be multiple reasons why the image was squashed.

The pixel clock is fixed at 27 MHz, which severally limits what can be done. The only "standard" alternate resolution the DC supports at full detail is 640x400x70hz. When the DC runs at 320x240x60hz (or 640x240 or 320x480) it actually has the same timing, but doubles each pixel. It's also possible to add borders, like using 640x480x60hz timing and displaying a 640x448 frame buffer or something.

It's not possible to increase the horizontal pixel density/resolution without running into problems on most monitors (but it extending the framebuffer into the border a bit works fine). By reducing the horizontal and/or vertical resolution, you can increase the framerate (e.g. 640x400x70hz or 512x384x88hz). Increasing vertical without reducing horizontal results in a lower framerate (640x768x43hz, monitor thinks it's 1024x768x43hz. This only works on my smaller CRT). It's possible to increase the vertical resolution without sacrificing framerate, but you have to reduce the horizontal resolution (424x768x60hz, monitor thinks it's 1024x768x60hz).

The 3D hardware doesn't fully support rendering at a resolution higher than 1280x480, so high resolution modes are more complicated to use with the 3D hardware (but not impossible).


So it's not possible to be like GSM mode selector on ps2 during 3d function/ game and line fills / stretches to 720p ?

Makes me wonder also if the dc could do that what ps2 homebrew did of rendering native 720p as a proof of concept

TapamN
letterbomb
Posts: 149

Re: Can DC outupt custom VGA resolution/refresh rate?

Post#5 » Sun Apr 02, 2023 6:47 pm

You can't do that kind of resolution switching on the DC.

I have had the DC render a 1280x960 image (and 1280x720) downsampled to 640x480, but that uses so much fillrate you'd have to run at 30 FPS. 1280x720 kinda works for simple scenes.I rendered the top and bottom halves of the screen separately to get around the TA's size limitation.

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