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

[Scali]

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

The PAL C64 borders are much larger than what I see here at least.
In your case, even part of the main screen is lost in the top border (could be a poorly adjusted TV as well).
On PAL you had very large borders, and many demos abused these for border-sprite tricks. Most of those wouldn't even be visible on a configuration such as yours.

 

[Scali]

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.

Yes, I think I feel the same about this.
Instead of 'meant to have', what if we say 'supposed to have'?
Namely, CGA was released in 1981. The 5153 was released in 1983.
Until the 5153 was available, the de-facto display for CGA was NTSC. And if that results in 4:3, you would say that is de-facto the correct aspect ratio for CGA.
Which makes me think that the 5153 is supposed to have a 4:3 aspect ratio as well, in order to remain compatible with existing CGA content.

Perhaps there is a reason why IBM chose a CRT and a bezel that does not naturally lead to a 4:3 ratio. In which case I would think 'cost' is that reason.
As others have said, we shouldn't over-think this issue. Back then, a lot of things were done to keep costs down and to keep designs simple. That was more important than having perfectly calibrated systems (another argument can be the change in CGA colours from old to new, which was not officially documented afaik, so was probably not considered an important change. They probably considered both sets of colours to be 'in the ballpark', and left it at that).

 

[bhtooefr]

Well, except it wouldn't be 4:3 - it'd be slightly off due to not using all of the available lines.

 

[geoffm3]

The PAL C64 borders are much larger than what I see here at least.
In your case, even part of the main screen is lost in the top border (could be a poorly adjusted TV as well).
On PAL you had very large borders, and many demos abused these for border-sprite tricks. Most of those wouldn't even be visible on a configuration such as yours.

I wonder if that's because the VIC-II still supports the same resolutions regardless of PAL or NTSC. Therefore they'd have to add additional overscan lines to accommodate for the extra scanlines top and bottom.

 

[Trixter]

If the 5160 operations manual didn't have the "equal overscan" bit in the documentation, I would have said 4:3, same as NTSC, done. Most logical analysis points to that. But those calibration/setup instructions are actual IBM documentation, so I owe it to the community to try to track down someone involved with the design of the monitor to be 100% sure. I've done some research this past week and come up empty, so I reached out to someone who might have connections as a hail mary pass. I'll wait a week before declaring that a dead end.

 

[vwestlife]

Do we really need to overthink this?

1. Adjust the vertical height as per IBM documentation.
2. Use BASICA to draw a circle in 320x200 graphics mode (SCREEN 1).
3. Adjust the horizontal width until the circle is perfectly circular.
Done!

 

[offensive_Jerk]

Funny, I was just reading the manual to Ashton-Tate's SignMaster program.

To correct the aspect ratio the program wants to know dimensions of the monitor.

YOU THEN WILL BE ASKED THE FOLLOWING QUESTIONS CONCERNING YOUR MONITOR:

MEASURE THE RECTANGLE ON THE SCREEN (NOT THE SCREEN ITSELF) FROM LEFT TO RIGHT AND ENTER IT IN DECIMAL UNITS (E.G. "9"). DO THE SAME FOR THE HEIGHT OF THE RECTANGLE AND ENTER IT IN THE SAME MANNER (E.G. "7.5").

The picture shows a rectangle border around the screen. It must be leaving out the overscan area.

Then in the picture it says:

ASPECT RATIO: THE PROGRAM MUST KNOW THE DIMENSIONS OF YOUR DISPLAY IN ORDER TO PRODUCE CIRCLES THAT ARE ROUND. HOW WIDE IS THE FRAME ON THIS SCREEN, IN INCHES (DECIMAL UNITS)?

 

[Trixter]

Do we really need to overthink this?

I know it seems pedantic, but it would be nice to know what was intended. Wouldn't it be nice to know, exactly, what IBM CGA graphics are supposed to look like on the definitive CGA monitor?

3. Adjust the horizontal width until the circle is perfectly circular.

I think you're kidding, but in case you're not: The 5153 has no user-serviceable horizontal width control. Come to think of it, I don't know how many of my other CGA monitors do either. And as was pointed out earlier, the 5155 monitor has no horizontal adjustment on the circuit board so it's not possible even if you try to service it.

Also, your answer suggests you are deeming 4:3/NTSC the correct ratio based on the BASICA CIRCLE. All that proves is that NTSC monitors were meant to be 4:3, not the 5153 and other CGA monitors.

To correct the aspect ratio the program wants to know dimensions of the monitor.

That's an interesting data point -- It indirectly suggests that nobody followed a standard and that it was up to each program to query the user.

 

[bhtooefr]

Another interesting data point is in the HMR: https://www.lo-tech.co.uk/downloads...0088-Repair_Information-PC_and_XT.pdf#page=56

Corroboration of the adjustment procedure in the GTO manual, as well as the same procedure for "Mode 1" (read: CGA modes) on the EGA monitor, yet a different procedure (of equalizing the borders) for Mode 2 (read: EGA modes).

And, I didn't realize that there weren't /any/ adjustments on the 5155, but the HMR doesn't refer to any - the only adjustments that the 5155 HMR lists are to drive speed, the motherboard color trimmer, and the 5153 adjustments.

 

[VileR]

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.

What about Andy Saenz? Dr. David Bradley mentions him as "responsible for the video card": http://www.vintagecomputing.com/index.php/archives/790/the-ibm-smiley-character-turns-30
There are all those other little CGA quirks that'd be nice to have explained straight from the horse's mouth... I thought it'd make for some nice blog fodder, but never got very far in tracking the guy down.

 

[Eudimorphodon]

I know it seems pedantic, but it would be nice to know what was intended. Wouldn't it be nice to know, exactly, what IBM CGA graphics are supposed to look like on the definitive CGA monitor?

As noted in the web page in the OP the difference between 1.37 and 1.33 is 3%; on a 11" diagonal (viewable) display the difference if the monitor were *perfectly* adjusted to the two ratios would be an 8.8"x6.6" rectangle for 1.33 vs. an 8.9"x6.5" rectangle for 1.37. I'm not entirely sure that little difference would be readily visible to the human eye.

(Since the width isn't adjustable on the 5153 if you take the 8.8" number as a fixed width the height of the display will be 6.42" for 1.37 instead of 6.62". On one hand that's almost a quarter of an inch, but that also means that if both displays were perfectly centered the 1.33 will be just one tenth of an inch closer to the top and the bottom of the frame. Not sure I'd notice that unless I had the two right next to each other *and* had a color border.)

 

[offensive_Jerk]

What about Andy Saenz? Dr. David Bradley mentions him as "responsible for the video card": http://www.vintagecomputing.com/index.php/archives/790/the-ibm-smiley-character-turns-30
There are all those other little CGA quirks that'd be nice to have explained straight from the horse's mouth... I thought it'd make for some nice blog fodder, but never got very far in tracking the guy down.

I remember reading an interview with the guy who came up with the smiley face in code page 37. Not sure if it's the same guy or not, but he was open to talk about points that other people wouldn't give a second thought .

 

[vwestlife]

I know it seems pedantic, but it would be nice to know what was intended. Wouldn't it be nice to know, exactly, what IBM CGA graphics are supposed to look like on the definitive CGA monitor?

If you own a 5153 in good condition and a ruler, why not measure the aspect ratio for yourself? The calibration may have drifted somewhat over the years, but probably not any more than the original tolerance of the factory adjustments.

Or get one of these:

 

[VileR]

I remember reading an interview with the guy who came up with the smiley face in code page 37. Not sure if it's the same guy or not, but he was open to talk about points that other people wouldn't give a second thought .

erm, that would probably be the interview I linked to. That's Bradley, but he does namedrop Andy Saenz as being the video card guy (CGA? MDA? both?), so maybe he (or Benj Edwards) would have a lead on Saenz?

 

[Eudimorphodon]

Corroboration of the adjustment procedure in the GTO manual, as well as the same procedure for "Mode 1" (read: CGA modes) on the EGA monitor, yet a different procedure (of equalizing the borders) for Mode 2 (read: EGA modes).

My memory is getting foggy on this point, but wouldn't the need for the separate procedure for "Mode 2" be dictated by the EGA 350 line modes not having a border? (IE, EGA basically has almost no overscan so the "crank up the brightness and center the whole scan block" is the correct tactic.)

In any case, if the official adjustment procedure for CGA is "turn the knob until both the black bars go away" it sort of implies they weren't really aiming for mathematical precision.

 

[Trixter]

Corroboration of the adjustment procedure in the GTO manual, as well as the same procedure for "Mode 1" (read: CGA modes) on the EGA monitor, yet a different procedure (of equalizing the borders) for Mode 2 (read: EGA modes).

Wow, the instructions are different for all three scenarios:
CGA monitor: "Turn the Vertical Size control clockwise until the black areas just disappear." (emphasis mine)
EGA monitor, mode 1: "Turn the Vertical Size control clockwise until both black areasdisappear."
EGA monitor, mode 2: "Adjust the Mode 2 Vertical Size control until the black areasat the top and bottom are approximately the same size as theblack areas at the sides."

Frustrating! #1 and #3 support "even border from all sides of the bezel", and #2 is just indeterminate.

If you own a 5153 in good condition and a ruler, why not measure the aspect ratio for yourself? The calibration may have drifted somewhat over the years, but probably not any more than the original tolerance of the factory adjustments.

I did, reflected in the first image in the first post of this thread. It is visibly not identical to 4:3 NTSC if calibrated according to IBM documentation. The question this thread is trying to answer is: Is that normal for the 5153, or was the 5153 always supposed to be 4:3 just like a composite monitor, and the IBM docs are wrong?

Or get one of these:

...which is clearly mis-marked when it comes to CGA! CGA wasn't a 67Hz, 35KHz horizontal monitor... This must be a device for calibrating arcade monitors.

In any case, if the official adjustment procedure for CGA is "turn the knob until both the black bars go away" it sort of implies they weren't really aiming for mathematical precision.

I don't care about precision, I care about which setting is correct. It's possible to adjust a 5153 to match an NTSC monitor by nearly eliminating the top and bottom overscan areas, but is that correct? Especially when there are many photos in old magazines, including an IBM press release photo I dug up, that show a different calibration?

Research continues...

 

[Trixter]

he does namedrop Andy Saenz as being the video card guy

I've reached out to Saenz; hopefully he will respond.

 

[retrogear]

As far as the BK Precision generator, when switch is set to PC and CGA you get 15Khz CGA. The frequency spec column is for switch set to MAC.
Dang, Mouser has it for $320. Must be a market for it yet?

 

[bhtooefr]

As an aside, I decided to take a different approach, and look through old PC Magazines to get an idea of what monitors IBM recommended before the 5153.

https://books.google.com/books?id=w_OhaFDePS4C&lpg=PP1&pg=RA1-PA68#v=onepage&q&f=false

It's looking like there were two color monitors that IBM and ComputerLand stores sold: the Amdek Color I (a composite monitor), and the Amdek Color II (a RGB monitor that later got an upgrade to RGBI specifically for the IBM PC). So, even if those monitors weren't the design target for CGA, they were the first semi-official CGA monitors, so to say.

 

[Shadow Lord]

...which is clearly mis-marked when it comes to CGA! CGA wasn't a 67Hz, 35KHz horizontal monitor... This must be a device for calibrating arcade monitors.

Well according to the specs:

Specifications
Resolutions:
Type          Resolution        Horizontal          Polarity      Vertical       Polarity
                                          (kHz)                                  (Hz)
MDA          720 x 350         18.4                     +            50                  -
CGA          640 x 200         15.8                     +            60                  +
EGA          640 x 350         21.8                     +            60                   -
VGA          640 x 480         31.5                     -             60                   -
SVGA        800 x 600         35.2                    +             56                  +
8514A       1024 x 384       35.5                    +             87                  +

Crappy table work is all me