• Please review our updated Terms and Rules here

What is the proper aspect ratio adjustment of the IBM 5153 CGA monitor?

Trixter

Veteran Member
Joined
Aug 31, 2006
Messages
7,478
Location
Chicagoland, Illinois, USA
For decades, I was under the impression that the viewable area of an IBM 5153 CGA monitor (and, possibly, all CGA monitors) was supposed to look like it was evenly spaced from all sides of the bezel, such that the overscan border was of an equal thickness on all sides. My own hardware has been configured like that for decades, and looks like this:

fX0hdy7.jpg


Further reinforcing my impression of this was the fact that DeluxePaint from the 1980s has built-in aspect ratio correction, which you could turn on and off as needed. I used it to create the following aspect ratio test picture, which displays a perfect circle/square with the monitor adjusted as shown directly above. That test picture looks like this:

vOYrWf3.jpg


(The wording in the left circle is incorrect, please ignore the wording for now)

However, it was recently brought to my attention that the designers may have intended the display aspect ratio to be 1.37, which is closer to NTSC than I would have expected. To match this, it means the correct adjustment of the viewable area would have very thin overscan borders on the top and bottom compared to the sides, like this:

V3r1DqW.jpg


(Again, note how there is almost no overscan on the bottom.)

Which is correct? I've checked the 5153 tech manual as well as the SAMS Computerfacts document and there is no mention of what the proper adjustment of the viewable area is supposed to be. I thought I could answer this question by grabbing samples of the 5153 from youtube videos, images, and old magazines in my collection, but after two hours I could only gather a decent sample size of 22 images (where the viewable area/overscan border were very clearly visible) and the results were inconclusive.

What did IBM's designers intend? What is the correct adjustment for viewable area and overscan on an IBM 5153 CGA monitor?

(I'm hoping this post brings some of the old monitor repair people out of the woodwork, as it's possible the only people who might know are those who directly serviced these monitors...)
 
I don't know the exact answer, but the situation is additionally complicated for Tandy 1000 users, due to Tandy's 225-pixel-high text modes (which nicely eliminates the visual claustraphobia of having the bottom of descenders touch the top of ascenders on the line below them) vs. the standard 200-pixel-high graphics (and optionally text) modes. I usually adjust an RGB monitor so that the 225-line mode is approximately what you show in the dotted line photo, with about half a text row height's worth of overscan at the top and bottom. That way, it still fits on the screen, while the 200-line mode appears about the same as your first photo, with an even amount of overscan on all sides... as seen in this quick video:

 
Trixter,

I am not sure it matters. My reasoning: It is so simple to screw up the adjustment that there is no way IBM expected anyone to ever maintain some pre-conceived ratio. I've basically always set it so that the lettering looks good to my eye. If a screen mode after that looked bad I reached to the back made an adjustment. If this was some sort of critical ratio then the knobs to adjust things would be much harder to access IMHO. Of course as I said this is all conjecture on my part so if you do find a magical ratio do tell ;). Then of course we would have to go hunting for said magical ratio for the 5151, 5154, and 5157....
 
I think it might, actually, since the user width control is not user serviceable. Here's the relevant portion of the 5160 Guide to Operations: "Turn the Vertical Size control clockwise until the black areas just disappear. If one of the black areas disappears before the other, continue to turn the control until the second black area is gone." That supports my original calibration theory represented in the first picture I posted (and is, maybe not coincidentally, the exact method I used to adjust the monitor before I took the picture).

This is highly encouraging, but not definitive. I'm open to more opinions or specs.

On page 192 of that document, there are some "these are bad" sample images. Unfortunately, none of them are applicable to determining the proper ratio except that the "too narrow" is obviously not right and the "too shrunken" is obviously not right, so the correct result is somewhere in the middle.

I wish there was a way to track down the actual engineers. They are unknown, and most of the people (Don Estredge, William C. Lowe, etc.) who were close to the project are deceased.
 
Last edited:
You'd think the proper width setting would be so that programs which shift the display to the left or right (such as MODE ,L or MODE ,R does) would not run off the side of the screen? I've come across some old games which automatically shift the display to the right, because old TVs with lots of overscan tend to cut off the left edge of the display at its normal centering.
 
I am inclined to support the NTSC-aspect ratio with minimal vertical borders. My reasoning is that when IBM designed the CGA card, they did not design the 5153 at the same time. When CGA was released, the 5153 did not follow for two years and was very pricey when it was first released. Many graphics artists and programmers had to use the NTSC aspect ratio because all they had was a TV or a composite monitor to use. I'm not sure if there were many non-IBM CGA monitors prior to the 5153 or how obtainable they would have been.

Given the relatively small size of these screens, borders were seen as a necessary artifact but one that should be minimized. Most users probably would have preferred minimal borders to show the active display area as large as possible.

If you draw a circle in IBM PC BASIC, does it look perfectly circular with an NTSC aspect ratio?
 
CIRCLE is aiming at 4:3...

http://www.retroarchive.org/dos/docs/basic_ref_1.pdf#page=150

aspect affects the ratio of the x-radius to the y-radius. The default for aspect is 5/6 in medium resolution and 5/12 in high resolution. These values give a visual circle assuming the standard screen aspect ratio of 4/3.

And, that manual is dated May 1982, and has low-intensity yellow as brown, not dark yellow. Now to find a first-edition BASIC reference manual...
 
Last edited:
Given the relatively small size of these screens, borders were seen as a necessary artifact but one that should be minimized. Most users probably would have preferred minimal borders to show the active display area as large as possible.

This is not applicable to the Commodore 64, which has always had an equal-size border. Most purists consider it bad calibration to reduce or eliminate a C64 border.

If you draw a circle in IBM PC BASIC, does it look perfectly circular with an NTSC aspect ratio?

That is an excellent suggestion! I just did it, and the circles are only perfectly circular on my (calibrated, broadcast) NTSC monitor. However, that only proves bhtooefr's point, that they are only meant to be circular on a 4:3 monitor. It does not prove that the 5153 was meant to have a perfectly 4:3 viewable area.

Staying as objective as possible, I think it is a valid argument that the designers of the PC only ever meant for there to be one aspect ratio, and that they would want the RGB monitor to be a crisper version of an NTSC monitor or TV. Extending this argument further, if they DID mean for the aspect ratio of the 5153 to be changed, there probably would have been more noise about it in PC Tech magazine or programming journals or something.

Still looking for more proof or arguments... There is one strong argument on either side, so I'm afraid this is still up in the air...
 
I remember from when I did CRT repairs that we had to achieve convergence within a viewable area. The 5153 takes a 370mm diagonal tube which is 14.5". Part of it is hidden behind the bezel so considered a 14" display. Viewable area
is considered to be 13.2" (???). A 4:3 surface ratio correlates to a 3/4/5 right triangle. Doing the math places 11.2" width and 8.4" height of the 14" glass and 10.56" x 7.9" viewable area so roughly 1/4" recessed from all sides of bezel. 640x480 resolution
is a true 4:3 but cga is 640x400 or 320x200 which is an 17% reduction in height which reduces height by roughly another 1/2" top and bottom. Of course, where the raster starts until the actual video signal starts/stops will fudge the borders too.

PS - I came out of the woodwork but my math is still rusty ... :)
 
Thanks for the info, although I'm afraid you might be thinking of a different monitor, since the 5153 is considered a 12" diagonal monitor, not 14"...

This may help get the info we're looking for: What was the rule of thumb when adjusting the vertical and horizontal size pots? On CGA monitors, was the viewable area after adjustment meant to be a perfect 4:3?
 
I found an IBM announcement from 1983 that calls it a 13". I was going by the CRT# in the Sam's which is 370RKB2-TC14. Typically, 370 was the diagonal measurement in millimeters. I remember there was controversy how the measurement was advertised
whether it be viewing area, phosphor area or size of the glass. I do also remember some tube part#'s didn't follow the measurement either. As far as horizontal size, the control is a coil denoted as factory adjustment, so never touched by a service tech. Looking at schematic, that would also detune the flyback so would affect high voltage. I bet it was epoxied also. Since the width is fixed, that only leaves putting up a test pattern to get a centered, round circle at the optimal resolution. Whether it ends up 4:3 only a tape measure could tell.

PS - to make the measurement would require measuring the lit part of the raster, not the video
 
Since the width is fixed, that only leaves putting up a test pattern to get a centered, round circle at the optimal resolution.

I think this will give us the answer we're looking for: What was the generator of the test pattern? What did you hook up to a 5153 to generate the circle?
 
Last edited:
This is not applicable to the Commodore 64, which has always had an equal-size border. Most purists consider it bad calibration to reduce or eliminate a C64 border.

A correction about the C64 my friend. The borders on the sides of an NTSC C64 are thicker than on the top and bottom. I have an NTSC C64 and I took a screenshot showing its borders :

20160320_174902.jpg

I believe perhaps the PAL C64 may have equal size borders due to the extra blanking induced by the PAL 625-line standard.

Now, compared to the display of the IBM CGA on the same TV, :

20160320_175519.jpg

you can see that even though 200 lines are still used, the horizontal size of the display is longer. Both systems use a multiple of 160 pixels, 320 in the first picture and 640 in the second. Commodore's video is in no danger of being covered by the monitor's bezel, but IBM's is just a hair too much to the left.

I would note that many composite monitors did not have vertical and horizontal size adjustments, at least not intended to be accessible by an ordinary user. The Commodore 1701 and 1702 Monitors have a vertical hold and a horizontal position control. The Apple II AppleColor Composite Monitor does have a vertical hold and a vertical size control but a horizontal hold control.

I must also note that my TV is a modern one and shifts the picture upward from dead center in order to leave more room for closed captions generated on the bottom of the screen.

Also, even though my CGA monitor is only a Tandy CM-5, it is advertised as a 14" tube/13" viewable display, just as the 5153. I measured at least a fair 12.5" with a measuring tape.
 
Last edited:
Also, I wouldn't be surprised if the BASIC example is Microsoft expecting 4:3... except when properly adjusted, it wouldn't be 4:3 due to viewable area on NTSC anyway.

The only composite monitor that I'm aware of IBM selling - and, after the 5153's launch, so it counts here - is the 5155's internal display, which has no adjustability of height or width. Ignoring borders, it's 6 25/32" wide, 4 31/32" tall - slightly off of 4:3.

Active area with EDIT.COM set to display white on white is 6 3/8" wide, 4 3/8" tall - that's about a 16:11 aspect ratio, not 4:3 (16:12) or 16:10.

In SCREEN 2, a CIRCLE (319,99),238 (the largest drawable CIRCLE in SCREEN 2) is 4 7/16" x 4 5/16", indicating an error of about 2.9% in the circle. That's nowhere near the error that (16/11)/(16/12) presents, though - brightness could be affecting width.

To be honest, I'm wondering if IBM simply didn't care about getting the aspect ratio that precise.
 
I think we have probably put more thought into the discussion than IBM did back then.

That's the conclusion reenigne and I came to about a week ago when we had this discussion over email. I wasn't fully satisfied with the outcome of that conversation, which is why I started this topic. (Which would be finished by now, were it not for the 5160 service manual instructions.)

I'm very interested to learn what retrogear used as a "circle generator" when servicing 5153 monitors.
 
The other question is what the correct adjustment for the 4863 is, and that would tell the rest of the story, I think.

5153 seems to be 16:10, 5155 seems to be 16:11, 4863 might be 16:12?
 
Just to clarify, our shop was not an authorized IBM service center so we didn't have any "official" generator. Just used software to produce the necessary patterns.
I don't even recall what it was. We had a generator for NTSC composite patterns, though.
 
Back
Top