Synchronization with CRT refresh

[Markus Kuhn]

Jim Gettys wrote on 2005-05-04 15:40 UTC:
> When I move the window to the projector on the second video output (in
> this case, it happens to be an nvidia running TwinView), I start getting
> video tearing, as the second RAMDAC does not run at exactly the same
> frequency as the primary

Did you measure that (with an oscilloscope or frequency counter), or is
that just your conclusion?

I'm asking, because I recently learned that most flat-panel screens and
data projectors contain today a chipset that has its own full frame
buffer and that does scan-rate conversion between the DVI input and the
actualy display module.

The display module usually can only handle a single resolution and
refresh rate, but the customers expact that it "just works" with every
frequency, hence the use of scan-rate converters.

The curious observation is, that this scan-rate conversion on the
motherboard inside the monitor is often not switched off, even if the
input resolution and frequency matches that required by the actual
display module. So the 60 Hz that you feed in via VGA or DVI differs a
few hundred ppm from the 60 Hz that reaches the drive electronics of the
actual panel via its LVDS interface.

A widely used vendor for such display-internal scan-rate converters is

  http://www.gnss.com/

A "Samsung SyncMaster 170T" display that I took apart contained a
"GENESIS gmZ4S" scan-rate converter that was always on and not PLLed,
even in the displays native video mode.

Displays are not what they used to be ...

Markus

-- 
Markus Kuhn, Computer Laboratory, University of Cambridge
http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain