VOGONS


First post, by Great Hierophant

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The VGA Feature connector, found on most older 8/16-bit ISA and 32-bit VLB cards, outputs digital data. It outputs the blanking, synchronization, dot clock and palette addresses, almost everything you need to reconstruct the VGA picture. It is missing one crucial element, however. There are 256 palette registers on a vanilla VGA card. Each register holds an 18-bit RGB color value. That value gets converted into an analog voltage level by the DAC and sent to the video port. You cannot use the feature connector alone to tell you how picture can be reconstructed. You need a separate expansion card to snoop in on the palette writes. But if you could design a card, ISA or PCI, that could obtain the needed signals and combine them with the signals from the feature connector, and convert them into DVI you could obtain pure digital VGA output from just about any card. Pure digital output does not contain any analog noise and should compress far more efficiently than analog output. Moreover, you could convert the 70Hz VGA synch rate into a 60Hz rate for current display friendliness by dropping every 7th frame. Given that VGA games tend not to be pushing high frame rates too often, this is usually just dropping duplicate frames.

Later cards supported the VESA Advanced Feature connector, which included support for 16-bit and 32-bit graphics modes. For these cards, you may not even need to snoop because they may not be outputting palette addresses but actual RGB values.

Of course, no such devices exist, but it would be nice if they did.

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Reply 1 of 4, by Deksor

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Or maybe instead of making an expansion card, just plugging something in the VGA connector that would read each pixel's color and apply it to the numeric/sharp picture. This will probably be more complicated and if your card has too dim/bright colors, this would also be applied to the numeric picture, however this would also free up one expansion slot which might be crucial for some of us.

This makes me think to something else too : it could be a neat feature to be able to mod the voodoo 1/2 to get a very sharp output. Some people would find this useless as the later voodoos have really sharp picture, but I think it could still be a good thing to have if you want to play the games that don't like anything but the voodoo 1 or 2

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Reply 2 of 4, by Jepael

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What a marvellous idea - you are absolutely right. Snoop the palette somehow and get pixel and sync data from feature connector.

If it does not exist, let's build one.

There's just a few issues that might need some more thought.

While you get pixel clock, H/V sync and blank signals from the connector, which do are everything you need to capture pure digital data, there might be slight differences between cards (hardware and software) in the video signal timing. There is no "data enable" signal that would say which pixels contain the image data, unless the blanking signal is used for that. However the blanking signal contains the border area as well, so if you could feed the pixel data through palette lookup to a DVI transmitter, you would have a non-standard custom video signal larger than 640x400 that also contains the borders, and the monitor would try to scale to full-screen. The signal might also be not centered. It could be possible to have some offset adjustments there to clip to 640x400 window based on the hs and vs pulses (that's called a DE generator).

The other issue is the refresh rate. Modern monitors and TVs do not necessarily accept 70Hz signal at all from their digital inputs, so your idea of converting that to 60Hz is valid. But in order to do that, you'd have to receive the 70Hz signal (or any custom resolution and refresh rate combination) into a frame buffer memory and send it out at 60Hz with standard clocks and timings accepted by monitors and TVs, maybe including scaling features to keep aspect ratio, borders or just fill the screen. That's only doable with special video processing circuits or FPGAs.

Reply 3 of 4, by idspispopd

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Deksor wrote:

This makes me think to something else too : it could be a neat feature to be able to mod the voodoo 1/2 to get a very sharp output. Some people would find this useless as the later voodoos have really sharp
picture, but I think it could still be a good thing to have if you want to play the games that don't like anything but the voodoo 1 or 2

Voodoo 1/2 cards don't have a feature connector. I think the SLI connector transmits analog signals.

Reply 4 of 4, by The Serpent Rider

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Yep, Voodoo 2 SLI is analog. At least it's true for mainstream cards. Quantum3D cards could be digital though.

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