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Author Topic: Horizontal timing and pixel-perfect resolutions  (Read 43886 times)
Rik Wang
Posts: 8833

« on: September 14, 2003, 06:35:42 AM »

A common question among, e.g., plasma and LCD/TV owners, is why horizontal timings cannot be specified in terms of individual pixels - i.e., why 848x480 instead of an optimal 852x480, or 1368x768 instead of the native 1366x768?

The reason is that graphics cards almost invariably do horizontal timing in terms of character clocks of 8 pixels, rather than in terms of individual pixels - a legacy of the original Motorola 6845 CRT controller. At the hardware register level, 848x480 is actually programmed as 106 character clocks x 480 lines - 106 characters of 8 pixels each equals 848 total pixels. Likewise, 1366 pixels isn't possible - the closest possible values are 1368 (171 character clocks) or 1360 (170 character clocks).

Hence a horizontal resolution that isn't evenly divisible by an 8 pixel character clock is not possible on display hardware that claims compatibility with VGA (and pre-VGA) standards. You either live with a couple of pixels of overscan, or settle for a couple of pixels blank border.

Exceptions to this horizontal timing rule include ATI new X1K and Matrox's Parhelia and P-series, which use pixels rather than character clocks to generate horizontal timings.

Another exception is the old Kyro 2 - still used on special purpose graphics cards from some manufacturers. The Kyro 3 is also capable of pixel-perfect resolutions at the hardware level, but the generic drivers do not allow this.

Also: for digital (not analog) timing, NVidia cards have always used individual pixels rather than character clocks of 8 pixels, even though effective changes have tended to be in character clocks anyway. Again: this only applies to DVI-D/HDMI, not analog VGA or DVI-A connections.

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