First post, by Great Hierophant
- Rank
- l33t
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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