Screen wobble and bending problem on PCXT core in 15Khz crt monitor(video timing?)

[suww37]

I put a lot of effort to get a 15khz screen output on the crt monitor (ex. sony pvm) on the pcxt core, but I failed and I'm currently only using the pcxt core on the lcd monitor. But I desperately hope that the pcxt core runs well on my sony pvm monitor.

I've been raising this problem steadily before pcxt core became public core, but it's still not resolved. I think the 320x200 15khz output of the crt monitor should be normal if the CGA, TANDY mode of the real XT PC is normal. I know that 15Khz output on the crt monitor in MiSTer's other core (minimig core and apple II core) is working well without any screen shaking and bending problems.

I don't know what video timing is the cause of this core, but I think there's something wrong. I would like to know if this problem is unsolvable.

[dmckean]

I've had issues too with 15khz output from this core, I was waiting for the core to mature though before I bugged anyone about it. I guess the core is getting pretty close to that now though.

[FoxbatStargazer]

Isn't this 320x200 more of a 70hz resolution? Have you tried other 70hz resolutions on that PVM?

Many similar cores do include some kind of buffered 60hz mode though, precisely for wider CRT compatibility. But it would probably mean extra black lines above/below at minimum.

[suww37]

Isn't this 320x200 more of a 70hz resolution? Have you tried other 70hz resolutions on that PVM?

Many similar cores do include some kind of buffered 60hz mode though, precisely for wider CRT compatibility. But it would probably mean extra black lines above/below at minimum.

Minimig core and Apple II core have a black bar as you said. Does this mean bufferd 60hz? I wonder if cga and tandy mode only support 70hz, not 60hz. My sony pvm crt monitor only supports 50-60hz. If PCXT core supports buffered 60hz, is it possible for PCXT core to output 15khz crt normally?

[FoxbatStargazer]

I'm wrong, CGA should be near 60hz, and furthermore IBM intended TV compatibility. Makes me wonder how it worked for compatibility on VGA monitors back in the day, when 400p was largely a 70hz thing.

[thorr]

I'm wrong, CGA should be near 60hz, and furthermore IBM intended TV compatibility. Makes me wonder how it worked for compatibility on VGA monitors back in the day, when 400p was largely a 70hz thing.

VGA was compatible with RGBI CGA, but CGA also had composite output. I don't know if VGA CGA modes were at the same refresh rate as CGA RGBI or not. This could be tested on ao486 by running a CGA game and seeing what frequency it uses.
https://youtu.be/niKblgZupOc

[suww37]

I'm wrong, CGA should be near 60hz, and furthermore IBM intended TV compatibility. Makes me wonder how it worked for compatibility on VGA monitors back in the day, when 400p was largely a 70hz thing.

VGA was compatible with RGBI CGA, but CGA also had composite output. I don't know if VGA CGA modes were at the same refresh rate as CGA RGBI or not. This could be tested on ao486 by running a CGA game and seeing what frequency it uses.
https://youtu.be/niKblgZupOc

I don't think the cga that is sub-compatible on the vga card is not 15khz. 15khz is not supported on the ao486, the actual 486 pc vga is also recognized only on the 31khz monitor.

[dmckean]

This is what I found for the timings of rgbi and composite output of a cga card

CGA card generates for an RGBI display:
H-visible:320 Front porch:40 Hsync:80 Back porch:16 V-visible:200 Front porch:24 Vsync:3 Back porch:35 Freq:60Hz Dot clock: 7159091

For a composite display:
H-visible:320 Front porch:56 Hsync:32 Back porch:48 V-visible:200 Front porch:24 Vsync:3 Back porch:35 Freq:60Hz Dot clock: 7159091

This seems to suggest 60hz

[thorr]

15khz is not supported on the ao486

This is actually not correct. ao486 requires use of the scaler and it can scale to anything. What I was referring to about testing CGA in the ao486 core is finding out what the source resolution is that is fed into the scaler. This is what the 486 is natively thinking the video mode is, but the MiSTer then scales this. I created modelines that match the source resolution and refresh rate and scales them 1:1 on VGA monitors so they look and act like a real PC would, except with minor lag due to the scaler. I didn't try a CGA game in ao486 to see what refresh rate the source is. You have to turn on the display of this information in the MiSTer.ini to see it when video modes change. Besides the modelines for VGA monitors, I also provided modelines that will scale anything to be displayed on CRT TV's with ao486 with 1:1 pixel accuracy in 320x200 games. I also wrote a CRT modeline creation guide. I didn't originally publish it on the website and wasn't expecting my name to be attached to it lol. https://github.com/MiSTer-devel/Main_Mi ... uide#ao486 https://github.com/MiSTer-devel/Main_Mi ... TION-GUIDE

The actual 486 pc vga is also recognized only on the 31khz monitor.

This is correct on real hardware. The MiSTer can scale this to work on 15KHz CRT's as explained above.

[Newsdee]

I don't know what it is, but there is something off with the video timing of the core even at 31khz.

I usually run it on an old Eizo 19" VGA monitor, using vga_scaler=1 in MiSTer.ini so the video is forced to 1280x1024.

Quite often the lower parr of the screen becomes shifted (usually after a resolution change), and I have to use the monitor's "auto sync" feature to correct it.

What is really strange is that this does not happen at all on the HDMI output. That tends to lose sync for a second or two then comes back to a stable image (probably it is doing the auto sync on its own).

It doesn't massively bother me, but thought I'd mention it if somebody wants to try to tighten timings.

[thorr]

This is what I found for the timings of rgbi and composite output of a cga card

CGA card generates for an RGBI display:
H-visible:320 Front porch:40 Hsync:80 Back porch:16 V-visible:200 Front porch:24 Vsync:3 Back porch:35 Freq:60Hz Dot clock: 7159091

This seems to suggest 60hz

This is 15.7KHz and 59.92Hz, so it is made for regular 15KHz CRT's. The VGA version of CGA is made for VGA monitors, so it will have different higher frequency modelines. My guess is they will be 70.1Hz. This can be tested and verified on ao486, but I can't do it myself right now.

[thorr]

but thought I'd mention it if somebody wants to try to tighten timings.

If the mode provided by dmckean is correct, then theoretically, the core should be outputting those timings, at least for CGA. I don't know if it is or not. We probably want the composite mode timings.

[Newsdee]

If the mode provided by dmckean is correct, then theoretically, the core should be outputting those timings, at least for CGA. I don't know if it is or not. We probably want the composite mode timings.

I feel that the overall horizontal and vertical setings are right but the pixel clock might be slightly off. So it throws off the display sync.

[akeley]

Like I said before, there is some sort of problem, but it's not universal. The core seems to display OK on at least some 15kHz sets.