pixel clock (pclock) problem

[Nikoh]

Hi guys, i have a problem setting up a modeline... I'm using lrmc to generate my modelines; the problem is that when i try to "run" a low res like 320x240 (Modeline "416x288x50.08" 8.000000 416 432 472 512 288 293 296 312 -HSync -VSync or "384x288x50.08" 7.375000 384 392 424 472 288 293 296 312 -HSync -VSync) on my CRT PAL TV i have a black screen; but if i set horizontal res bigger like 640 or 720 (Modeline "720x288x50.08" 13.875000 720 744 808 888 288 293 296 312 -HSync -VSync) it work... it seems a pclock problem (too slow) for my video card, is it possible?

[buttersoft]

I've never heard of anyone circumventing the pixel clock limits. Calamity was certainly never able to, which makes me think it's a hardware limit, or a BIOS one that may be there for a reason.

The normal workaround is to use a double-wide mode like 640x240p and integer scale 2x horizontal. Or use a super resolution and use a higher integer multiple.

[Nikoh]

I've never heard of anyone circumventing the pixel clock limits. Calamity was certainly never able to, which makes me think it's a hardware limit, or a BIOS one that may be there for a reason.

The normal workaround is to use a double-wide mode like 640x240p and integer scale 2x horizontal. Or use a super resolution and use a higher integer multiple.

yes i know, but is there an explanation? Does anyone know the pclock values below which you cannot go and why?

[buttersoft]

No real idea why, i'd guess someone made a design choice based on what the market was thought to want.

The Radeon HD 2000 & 3000 series are about 8MHz, and i think that limit comes back to AMD cards after the last analog video port are dropped, but even newer cards have an even higher limit. Not sure about Nvidia as i've never run up against it.

You might try Toastyx's pixel clock patcher. I think there are versions for both Nvidia and AMD. I'm not sure if that can remove the lower limits or not, it's not really intended for that purpose.

[Nikoh]

Ok, many thank
Last thing... do you know which is the most AMD powerful card with a lower pclock and analog VGA port? (Radeon R7 240 2gb?)

[Nikoh]

I've never heard of anyone circumventing the pixel clock limits. Calamity was certainly never able to, which makes me think it's a hardware limit, or a BIOS one that may be there for a reason.

The normal workaround is to use a double-wide mode like 640x240p and integer scale 2x horizontal. Or use a super resolution and use a higher integer multiple.

Ok, since Blanking = Front Porch + Sync Width + Back Porch, i could increase Blanking and decrease h-disp to have same h-total but low xres right?
Someone tried it?

[buttersoft]

Ok, since Blanking = Front Porch + Sync Width + Back Porch, i could increase Blanking and decrease h-disp to have same h-total but low xres right?
Someone tried it?

Yes, that can be done, and it's a good way to get from about 7.7 to 8MHz, or for small jumps. But you can only pack a modeline with so much blanking before your CRT won't let you expand the H-size anymore - if it has that adjustment to begin with. So you'd end up with a narrow image in the middle of the screen. It's better to double the horizontal res and set your emulators to match that.

I can never remember if the R9 380 or 380X is the last AMD card with analog video. I think the latter. You need to use a passive DVI-to-VGA adapter, but those are a dime a dozen.

I did notice in another thread you were using linux, and i don't know much about linux. The stuff i'm saying is generally relevant to Windows, though things like the dotclock limit aren't going to change.