RE: DAC glitch (was: Recommended specs for new system)

["gary" <gsellani@xxxxxxxxxxxxx>]

Video DAC glitching is common. Video DACs are a combination of binary
weighted and thermometer coded dacs. Use an example of 2 bits of binary
weighted and 6 bits of thermometer code. This means the thermometer code dac
consists of 63 individual current sources weighted at 1LSB. As the gradient
is ramped, one additional LSB is turned on at a time. To go from 63 to 64,
all 63 individual LSB weighted DACs are turned off and the 64 LSB weighted
current source is turned on. This is where you get a glitch. Another glitch
happens at 128.

To see these glitches, you need a high bandwidth monitor. For whatever
reasons, most small monitors (17 inches and under) don't have the bandwidth
to clearly define the glitch. The BNC inputs on my Nokia 445xPro have a
bandwidth around 280Mhz, which really shows the glitches.

I haven't owned a Matrox card since the G200, and it glitches like the rest
of the cards on the market. For reasons I don't need to repeat, I'll never
buy a piece of hardware from Matrox again. In fact, if given a free Matrox
card, I'd think twice about installing it.

Make sure you do your gradient test in a primary color. Timing skews between
DACs can mask the glitches. If you can find the old Nokia Ntest, it has a
gradient test done in RGB.

> -----Original Message-----
> From: owner-scan@leben.com [mailto:owner-scan@leben.com]On Behalf Of
> Paul D. DeRocco
> Sent: Wednesday, March 06, 2002 1:27 PM
> To: scan@leben.com
> Subject: OT: DAC glitch (was: Recommended specs for new system)
>
>
> DAC glitch is one problem that often goes unnoticed in video
> cards. I'd like
> to hear from people whether they can identify this problem with their card
> or not. Just create a gradient in PS that goes from black on the left to
> white on the right, and see if there are any faint vertical lines
> in it. The
> most prominent one would likely be somewhere around the middle. I
> can report
> that my Matrox G450 has none, the original Matrox Millenium didn't, but my
> Number Nine had a real nasty case of it.
>
> --
>
>
>

-
Turn off HTML mail features. Keep quoted material short. Use accurate
subject lines. http://www.leben.com/lists for list instructions.