Horizontal resolution and CGA monitors question

[TheManuel]

I have read about the challenges of setting up vertical screen size on CGA arcade monitors to account for the different vertical resolutions.
How about horizontal resolutions?
Do all games fill up the entire screen horizontally (provided no black bars are included as part of the video) regardless of the games original horizontal pixel count or is this another adjustment that needs to be made from one game to the other?

Thanks.

[qrz]

game's video horiz freq must be within the sync lock range of the monitor in question....

i.e. feeding 31khz+  to a standard 15khz monitor will result in multiple horiz images- if it kinda locks at all.

qrz

[TheManuel]

My poor understanding of this is that the types of frequencies you quote, 31 and 15kHz are horizontal scan rate frequencies are the product of vertical resolution x horizontal refresh rate.  Nowhere in this calculation is horizontal resolution mentioned.  This is why I ask if whether the original horizontal resolution of a game has any bearing on the size of the picture in a CGA monitor.

[MonMotha]

There are two things to consider here: horizontal active video time and the number of pixels crammed into that time.  The two are completely independent, but are limited by the analog bandwidth of the monitor.

The width of the picture is determined by the active line time.  Each line is surrounded on both sides of horizontal sync by blanking intervals sometimes known as porches.  During these intervals, as well as during horizontal sync, black is sent to the monitor.  In between these intervals (but not during vertical blanking or sync), active video is sent to the monitor.  A game which has short porches will be "wider" than a game with long porches since its active line time is larger (total line time being the same, which is the case if you take both games to be running the same vertical resolution at the same vertical refresh rate).

The number of pixels per line is determined by how fast the video output changes state.  The hardware limitations on the game board end are memory (have to have enough memory to store all the pixels and scan through them) and the update rate of the video DAC.  The limitation on the monitor end is the analog input bandwidth of the monitor.  You can calculate the "pixel rate" or "dot clock" as hsync_rate * total line pixels (which would include blanked pixels).  In order for the monitor to display the picture with significant blurring, the analog input bandwidth must be at least this rate, and preferably at least 2x this rate.  For a feel of numbers here, a typical dot clock for a standard res output might be 7.68MHz (512 total pixels at 15kHz).  You can usually find a specification of analog input bandwidth in the databook for a monitor.

Typical analog input bandwidths for standard res monitors might be in the range of 10-15MHz.  Multisyncs go significantly higher to support higher resolution modes.  IIRC, the KT-2914 (Betson Multisync) is about 55MHz, which is just less about 1.5x the dot clock of most SVGA outputs (the highest resolution that monitor supports, and it blurs a little).

The gist of it is that the monitor doesn't care how many horizontal pixels you feed it.  I've used a Matrox G100 to output 1440x480 interlaced video before, which is compatible with standard res.  The horizontal is way higher than you'd expect, but it brings the dot clock into a range the PLL on the video card can lock onto, and I just stretch everything 2x horizontally, which means everything looks the same in the end.  The monitor has no clue what I'm doing this.

[TheManuel]

Hmm! 

Pretty technical stuff.

1. So I take it by your last statement that the image will not need to be re-sized horizontally when switching between games of different horizontal resolution?

2. Those settings you used in your video card, was that for testing purposes or did you do it to improve the look of something, i.e., desktop, games, etc.?

Thanks for the detailed explanation.

[MonMotha]

Unfortunately, the blanking interval times are not standardized on arcade games, so if you accurately emulate the video (or run real hardware) you will often need to tweak the horizontal size to get it perfect, though I've found that it's often possible to find a setting that is rather acceptable for everything.

As for the weird mode I was using, I was actually testing a RGB to NTSC board I had designed, wasn't using it for anything MAME related (I actually don't run MAME, just real hardware).  The G100 can handle 15kHz horizontal scan rates, but the dot clock can't go down low enough to do 720x480x60i (DVD NTSC res), so I doubled the horizontal res to make the hardware happy.

[TheManuel]

So horizontal screen size still requires some level of compromise but not quite as much as vertical size, is this correct?
For instance, going from a game with 224 vertical resolution to one with 256 will require some decent adjustment or you will loose 32 lines vertically which is pretty significant. 
Is the effect not quite as pronounced with horizontal resolution differences?

The reason I ask all this is because I always see a lot of discussion about adjusting the monitor for vertical size but never horizontal.

Thanks.

[MonMotha]

The actual active resolution in pixels alone does not determine actual image size.  Horizontal active resolution in pixels, in particuarly, has nothing to do with image width.  Remember that the monitor has no clue what resolution you're feeding it horizontally (it can only know the number of total lines).  Only together with other aspects of timing is the actual image size (compared to black) determined.  There's just not quite so much room for difference in the horizontal image size as there is vertical, so it tends to not vary quite as much.

My experience has been that to get everything "perfect" (i.e. imaged area just at the edge of the imagable area on the tube), both settings need to be adjusted for every game, but finding a suitable compromise for all my games has been much easier for horiztonal geometry than vertical.

[TheManuel]

Fantastic!
Thank you for all your help.

[essence25]

Does this mean if running low res games on a large display and use the controls to stretch the image to fit the screens this would also create much more visible scan lines that you need to move away from to see a nice picture correct?

[wpcmame]

Does this mean if running low res games on a large display and use the controls to stretch the image to fit the screens this would also create much more visible scan lines that you need to move away from to see a nice picture correct?

First, you do want to see the scanlines. It is the black lines between them you want as small as possible.

Arcade monitors have a low dot-pitch. This means that the displayed image will not be very sharp. The scanlines "bleed" and can almost fill the distance between them. You can also reduce the black parts by increasing the beam intensity.

[essence25]

Well I should had mentioned that I was mostly refering to 15Khz low res games or consoles like SNES. The scanlines on those are very visible. I do like scalines myself but these are pretty big when image is stretched to all sides via the monitor OSD. I guess the 27" Monitor is pretty big and since it is so sharp and clear they dont bleed much and you are able to see the gap between the scanlines.

Now how do I increase beam intensity? Does it involve adjusting pots? Also higher res 15khz and 24khz and 640xXXX 31khz games look awesome.

10x