VOGONS


First post, by Truffle

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Attached are two photos taken of an Eizo Flexscan 9060S monitor (multisync 15.5 to 38.5Khz) connected to an EGA card (WD Paradise PEGA2A), and a CGA card (TULIP YAMAHA V6363-F). This is running on an 8088 system (Philips P3105).

To my surprise, the EGA image has no prominent scanlines, despite the graphics mode being 320x200, which I would expect to run in 15Kz mode. It looks double-scanned like VGA. The CGA on that very same monitor has the classic 15Khz scanline look.

Can someone explain why the EGA image looks like it's double-scanned?

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Reply 1 of 9, by mkarcher

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Have a look at This post on vcfed: The card seems to support a "24kHz mode", which might very well be double-scanned 200 lines, which can be enabled by a jumper. Your monitor is perfectly able to sync on 400 visible scanlines at 24kHz.

Reply 2 of 9, by Truffle

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mkarcher wrote on 2022-01-22, 10:39:

Have a look at This post on vcfed: The card seems to support a "24kHz mode", which might very well be double-scanned 200 lines, which can be enabled by a jumper. Your monitor is perfectly able to sync on 400 visible scanlines at 24kHz.

24 refers to the clock speed in Mhz, not the horizontal scan rate in Khz. But perhaps you're right that some EGA cards have this ability. This 24Mhz is really close to to the typical 25Mhz VGA pixel frequency.

However, I seem to remember that most EGA cards typically 200 single-scan lines. Maybe I'm wrong about this.

Reply 3 of 9, by Tiido

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That particular card is definitely double scanning, so it must be set that way to do so. Real EGA definitely lacks this ability, but I don't know about clones which can have variety of enhancements.

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Reply 4 of 9, by Benedikt

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I am not familiar with that particular card, but the ATI EGA Wonder 800+ that I have is basically a VGA card minus the RAMDAC.
If yours is a similarly late EGA card that actually uses a VGA chip, the double-scanned 400-line EGA mode is essentially a free bonus.

Reply 5 of 9, by Truffle

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Benedikt wrote on 2022-01-23, 20:42:

I am not familiar with that particular card, but the ATI EGA Wonder 800+ that I have is basically a VGA card minus the RAMDAC.
If yours is a similarly late EGA card that actually uses a VGA chip, the double-scanned 400-line EGA mode is essentially a free bonus.

A bonus, well that depends 😀 I prefer the old school single-scan look.

My current hypothesis is that EGA cards that do not have a 16Mhz clock generator (like the one my CGA card also has), are not capable of creating single-scan video. The EGA cards I know can produce single-scan video all have such a generator. E.g.:
http://www.vgamuseum.info/index.php/cards/item/188-ibm-ega
http://www.vgamuseum.info/index.php/cards/ite … 7-gemini-vc-001
http://www.vgamuseum.info/index.php/cards/ite … enoa-gn006001-b

And let me go out on a limb and state that only single-scan types can be hooked up to a CGA monitor. I haven't been able to do so with my PEGA2A card, despite exhaustively trying all possible switch/jumper settings.

Reply 6 of 9, by Benedikt

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Truffle wrote on 2022-01-24, 13:05:

My current hypothesis is that EGA cards that do not have a 16Mhz clock generator (like the one my CGA card also has), are not capable of creating single-scan video. The EGA cards I know can produce single-scan video all have such a generator.

That sounds fishy to me. IIRC, the 16.257MHz pixel clock is only used for the 640x350 pixel modes while the 14.31818MHz pixel clock for the 640x200 pixel mode may come straight from the ISA bus.

Reply 7 of 9, by Truffle

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Benedikt wrote on 2022-01-24, 13:57:
Truffle wrote on 2022-01-24, 13:05:

My current hypothesis is that EGA cards that do not have a 16Mhz clock generator (like the one my CGA card also has), are not capable of creating single-scan video. The EGA cards I know can produce single-scan video all have such a generator.

That sounds fishy to me. IIRC, the 16.257MHz pixel clock is only used for the 640x350 pixel modes while the 14.31818MHz pixel clock for the 640x200 pixel mode may come straight from the ISA bus.

I stand corrected. I realize now that the 16.257MHz clock on my CGA card is there because it's a hybrid CGA/Hercules card.

Reply 8 of 9, by mkarcher

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Truffle wrote on 2022-01-23, 12:21:

24 refers to the clock speed in Mhz, not the horizontal scan rate in Khz. But perhaps you're right that some EGA cards have this ability. This 24Mhz is really close to to the typical 25Mhz VGA pixel frequency.

OK, I stand corrected. 24kHz horizontal sounded quite low for 400 visible scan lines, so indeed 24 MHz dot clock makes more sense. This is Super-EGA stuff, though.
You can get Super EGA card (with digital output) with up to 800 x 600 pixel resolution, you need a matching monitor, though. Your Eizo monitor is a very good starting point for Super EGA experiments, as it will sync basically every timing ever generated by any kind of Super EGA card.

Truffle wrote on 2022-01-23, 12:21:

However, I seem to remember that most EGA cards typically 200 single-scan lines. Maybe I'm wrong about this.

You are perfectly correct that standard EGA doesn't double-scan 200-line modes. Standard EGA supports two modes: 200 scan lines at 60Hz / 15.6kHz / 14.318MHz ("Mode 1") and 350 scan lines at 60Hz / 21kHz / 16.257MHz ("Mode 2").

Reply 9 of 9, by rmay635703

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It wasn’t just EGA clones
Many early “POS/business color” PCs did tricks to display better text
The NCR 286 systems came with a matching NCR branded 640x400 “CGA Compatible “ screen and video card. While you could display CGA the screen was completely incompatible with non-NCR cards

Back in the day I could only get vibrant 640x400 text,
no idea if it actually supported graphics in double scan which was unfortunate