Graphics capability in the original IBM-PC

[Andrettigto]

...I don't see the line of reasoning that says that it was the long persistence phosphor that was the monochrome display's key advantage...

I don't know about it being the key advantage, but it was one advantage over CGA for TEXT. I only mentioned the monitors persistence as one of the reasons IBM's monochrome display lookes so good. Static images were easier on your eyes over a long period of time.

Yes, its character set was also far better than CGA, but knowing all this many users still wouldn't let go of APA graphics.

...Using a CGA on a really good monitor was worse than using a mediocre one...

So you're trying to say that a poor dot pitch will help smooth out the low res. font? Any pixel bleed's a bad thing resulting in poor focus and eye strain. If you were to set-up a side by side comparison, I'm sure you would feel different about that.

 

[mbbrutman]

Yes Mike, but the field rate is the same for both MDA and CGA, so I don't see the line of reasoning that says that it was the long persistence phosphor that was the monochrome display's key advantage. 60Hz is pretty much well inside the human persistence of vision, anyway.

One could, of course, run the MDA (and the CGA) in interlaced mode and the long-persistence phosphor would come in handy, but interlaced displays have their own problems (e.g. "fuzzy" appearance).

I don't think that anybody said it was the 'key advantage'. That is your addition. Andretti said "Sure, MDA characters looked better formed on the L-O-N-G persistence monochrome monitor." You said that "The persistence had nothing to do with it, though IBM did use a long-decay phosphor on the 5151." And I said "The persistence of the phosphor does matter - it reduces the perceived flicker."

The phosphor makes a difference - that is all we said. You were the first to infer 'key', and then knock that down .. kind of like a straw man argument.

60Hz is very noticeable, which is why high end CRT monitors used 72Hz or better refresh rates. The IBM 5151 refreshes at 50Hz, so the choice of phosphor is very important for reducing flicker.

 

[tezza]

Gee guys, I didn’t mean to start an argument.

Setting aside the technical stuff , I still believe the following…

To obtain a graphics-capable IBM-PC a business needed to buy a more expensive monitor and a card (CGA) which had poor font resolution. I take the point that in 1981 the technology wasn’t really up to good colour text at the time. However, monochrome graphics are likely to have been possible at the time MGA was released (or at least very soon afterwards) and not to incorporate them in MCA (or at least make them available via a third card option a little later) was an oversight by IBM. Possibly they underestimated the need for mono graphics for business, or they felt the CGA was up to it?

Anyway, the gap was soon plugged by the likes of the Hercules card.

Tez

 

[paul]

I have a few Tecmar Graphics Master boards on hand which are jumperable to drive either CGA or MDA-type, or composite NTSC monitors. It has a proprietary Tecmar mode that (as best as I understand) interlaces the CGA's standard non-interlaced 60 Hz refresh to get 640 x 480. AutoCAD 10 has a driver for this but it is nearly unusable as horizontal lines flash at 30 Hz and are quite painful to look at.

EDIT: Detailed image here.

 

[Chuck(G)]

IThe phosphor makes a difference - that is all we said. You were the first to infer 'key', and then knock that down .. kind of like a straw man argument.

60Hz is very noticeable, which is why high end CRT monitors used 72Hz or better refresh rates. The IBM 5151 refreshes at 50Hz, so the choice of phosphor is very important for reducing flicker.

My point is that "noticeable" and "objectionable" are very subjective. Film shot for movie theater showing is done at 24 frames per second. While the flicker is noticeable, within a few minutes, the brain ignores it. So you sit in your seat happily watching all 12 reels of The Ten Commandments for three hours and no one ever commented on the flicker.

It wasn't the flicker that made CGA objectionable for text use; it was the lousy resolution that necessitated badly formed characters.

I agree here, that "flicker" is a red herring.

 

[Chuck(G)]

I have a few Tecmar Graphics Master boards on hand which are jumperable to drive either CGA or MDA-type, or composite NTSC monitors. It has a proprietary Tecmar mode that (as best as I understand) interlaces the CGA's standard non-interlaced 60 Hz refresh to get 640 x 480. AutoCAD 10 has a driver for this but it is nearly unusable as horizontal lines flash at 30 Hz and are quite painful to look at.

Many earlier VGA (and some EGA) adapters can be programmed for interlace mode, as well as the original MDA and CGA (see the 6845 datasheet). When there's no motion on the screen, the effect can be dreadful. On the other hand, broadcast NTSC television did just fine with it, probably because the image is moving most of the time.

 

[Fallo]

60Hz is very noticeable, which is why high end CRT monitors used 72Hz or better refresh rates.

Not only that, but the flicker becomes more noticeable the bigger the monitor is.

The IBM 5151 refreshes at 50Hz, so the choice of phosphor is very important for reducing flicker.

Which is why you still get a flicker-free display even if you turn the MDA's interlace on (thereby reducing the frame rate to 25 fps).

I have a few Tecmar Graphics Master boards on hand which are jumperable to drive either CGA or MDA-type, or composite NTSC monitors. It has a proprietary Tecmar mode that (as best as I understand) interlaces the CGA's standard non-interlaced 60 Hz refresh to get 640 x 480. AutoCAD 10 has a driver for this but it is nearly unusable as horizontal lines flash at 30 Hz and are quite painful to look at.

All CGA cards can be put in interlaced mode (it's a feature of the 6845), but as IBM never supported this, the normal 8x8 text is way too small to be useable (even the MDA's 9x14 text is too small). You can't use it in graphics mode either, as there isn't enough video memory on standard CGA. It should work on the PCjr/Tandy, though.

What kind of interlace does this Tecmar card use? Is it type 1 (the method used by broadcast TV where both fields repeat the same picture) or type 2 (each field has a different picture)?

 

[Trixter]

All CGA cards can be put in interlaced mode (it's a feature of the 6845), but as IBM never supported this, the normal 8x8 text is way too small to be useable (even the MDA's 9x14 text is too small). You can't use it in graphics mode either, as there isn't enough video memory on standard CGA. It should work on the PCjr/Tandy, though.

You *can*, but the image repeats on the second half of the screen ;-)

I tested interlaced mode on CGA and it didn't seem to work properly on either composite out or RGB. Video result here: http://www.oldskool.org/pc/cgacomp (starts at 3m08s). Go to archive.org to get the full-quality video since the web version is only 30fps with blended fields and doesn't properly represent what interlaced video on the screen looks like.

In interlaced mode, both fields appear in the same physical place, making it somewhat useless.

 

[Marrr]

I have a few Tecmar Graphics Master boards on hand which are jumperable to drive either CGA or MDA-type, or composite NTSC monitors. It has a proprietary Tecmar mode that (as best as I understand) interlaces the CGA's standard non-interlaced 60 Hz refresh to get 640 x 480. AutoCAD 10 has a driver for this but it is nearly unusable as horizontal lines flash at 30 Hz and are quite painful to look at.

Very interesting... I've been intrigued about Tecmar Graphics Master for ages, since I saw the list of its modes in some ancient version of PC Paintbrush:

Tecmar Graphics Master;320x200 16 color
Tecmar Graphics Master;640x200 16 color
Tecmar Graphics Master;640x400 16 color
Tecmar Graphics Master;640x400 2 color
Tecmar Graphics Master;720x200 4 color
Tecmar Graphics Master;720x400 4 color
Tecmar Graphics Master;720x352 2 mono shades
Tecmar Graphics Master;720x352 3 mono shades
Tecmar Graphics Master;720x704 2 mono shades
Tecmar Graphics Master;720x704 3 mono shades

This may be the only card ever which supports 3-mono-shades graphical modes!
Can you please tell something more about this card?
I guess all those 720x352 modes use MDA-type monitors, and so do 720x704 modes, only with interlace?
Also, what's that third, lowermost connector on that card's bracket?

 

[Fallo]

You *can*, but the image repeats on the second half of the screen

I used a BASIC program that fills the screen with As. Interlace mode 1 (what broadcast TV uses) produces normal-sized text, but the picture shakes up and down. Mode 2 produces tiny, almost illegible text. It's also very flickery. I also didn't get any repeating picture, just a screen full of As. BASIC's Ok prompt and function key bar only appeared in the top half of the screen.

I tested interlaced mode on CGA and it didn't seem to work properly on either composite out or RGB

I've only tried it on my TV. Your video shows it on RGB though, and not composite.

 

[paul]

...since I saw the list of its modes in some ancient version of PC Paintbrush:
This may be the only card ever which supports 3-mono-shades graphical modes!
Can you please tell something more about this card?
I guess all those 720x352 modes use MDA-type monitors, and so do 720x704 modes, only with interlace?
Also, what's that third, lowermost connector on that card's bracket?

I have the original Tecmar docs and disk .. it came bundled with PC Paintbrush. I had never considered if it can also interlace the 5151 monochrome monitor, but those modes seem to suggest it can! The lowest "connector" is a switch, for mono or color.
At work (late 1980's) we used the Tecmar in industrial PCB production machinery that we designed which was run by 2 5170 AT's. The user interface was an Electrohome CGA monitor with a touchscreen installed. The interlacing worked acceptably on the touch icons as they were "blocks" of color.

I did locate an interesting article in InfoWorld 20 Oct 1986 (Special Report Color Graphics Boards). See Google books. The Tecmar is mentioned along with another card I'm familiar with - the BNW Model 15. We used these as a secondary display for AutoCAD on XT's and AT's at 1024 x 1024 x 60Hz (interlaced) at 16 colors driving a Mitsubishi 19" high-persistance color monitor. The CGA-style "digital" output was converted to RGB analog with a cheesy network of diodes that the CAD VAR had kludged together.

The image was very dim and looked primitive in today's terms, but our CAD drafters put up with it for years.

 

[Trixter]

I've only tried it on my TV. Your video shows it on RGB though, and not composite.

Doesn't matter; it is broken by design. Check Andrew's post for today: http://www.reenigne.org/blog/cga-why-the-80-column-text-mode-requires-the-border-color-to-be-set/

"...the output vertical sync pulse is generated in a similar way to the output horizontal sync pulse, only it’s 3 lines instead of 4 LCLKs. It always starts at the beginning of an output hsync pulse, so a field can’t start halfway through a scanline."

 

[Fallo]

Doesn't matter; it is broken by design

I'd like to see someone try the interlace mode on a PCjr or Tandy, since you should be able to use it in graphics mode there.

Check Andrew's post for today: http://www.reenigne.org/blog/cga-why-the-80-column-text-mode-requires-the-border-color-to-be-set/

Setting the border to color 6 produces the normal 16 colors, while other values cause the colors to become tinted. White or gray (colors 7 and 15) produce a black and white picture.