Archived from groups: comp.sys.ibm.pc.hardware.video (
More info?)
Thanks for the explanations, I have some follow up questions:
I want to determine as precisely as possible, when an image is
actually displayed on the LCD. If the software is set to 'wait on
vsync' before it writes into the video buffer, then I should be able
to predict the time when the image is displayed, if I know what
scanlines the image is on, when the vsync happened, and the pixel
response time.
Could I do this with the DVI 'display-enable' signal you mention?
Could you tell me what pin(s) you are talking about?
http://sunsolve.sun.com/handbook_pub/Devices/Monitor/MONITOR_Connector_Pinouts.html
Chris G
"Bob Myers" <nospamplease@address.invalid> wrote in message news:<9U1wc.2991$hg2.1628@news.cpqcorp.net>...
> "Chris G" <cgrebeld@hotmail.com> wrote in message
> news:74c403e9.0406040707.779a372d@posting.google.com...
> > Can someone explain to me how the meaning of the vsync interrupt for
> > an LCD monitor will differ from that on a CRT?
> >
> > Does an LCD update it's pixel values in the same way (timing-wise) as
> > a CRT? IE. does it paint the screen line by line and then issue the
> > vsync to mark the start of a blanking period?
> >
> > And would the updating technique change if you are using DVI-D vs.
> > VGA?
>
> Taking the last question first - no, not really. The LCD panel itself
> sees pretty much the same data at the same timing in either case; once
> the analog video signal from the VGA input is A/D converted, the
> contents of the video data streams (assuming the same timing in both
> cases) should ideally be indistinguishable. What difference there IS
> between the "analog" and "digital" inputs almost all comes from errors
> in the sampling clock generation in the A/D conversion process, owing
> to the fact that the analog video signal we have today doesn't carry
> any pixel clock information along with it (this may change in the future).
>
> VSYNC does not mark the start of the vertical blanking period in
> either case - it typically occurs one to three lines time into the blanking
> period. It is difficult to unambiguously determine the start of the
> blanking periods in analog video (at least per the PC standards), as
> there is no difference between the "blank" and "black" levels in the
> video (this is not the case in "TV" video). In the digital standards,
> such as DVI, a separate "display enable" signal is produced which
> is effectively the blanking. VSYNC in both cases means "get ready
> to start the next frame; in CRT displays, it triggers the vertical retrace,
> while in others it basically is used just to reset the internal counters
> in the panel controllers such that the next valid video data received
> is written to the first line.
>
> Since the non-CRT types don't need a "retrace time" as such - it
> doesn't take much time to reset the thing to writing the first line of
> the panel - there has been a recent move to generate "reduced
> blanking" timings for these (which affects both the horizontal and
> vertical blanking intervals - most of the blanking time is actually in
> the horizontal). The reason for this is to permit slower pixel rates
> for a given pixel format and refresh rate combination - often as
> much as 20% or more lower than the "CRT" timing requires.
>
> LCDs aren't exactly updated the same way as a CRT - they are
> NOT written one pixel at a time, although the data comes into the
> panel more or less in this manner. What typically happens is that
> an entire line of pixels is moved into the row drivers, and then that
> entire row (the equivalent of a CRT "scan line") is written at once.
> This is all under the control of the internal panel electronics, though,
> so you don't have to worry about it in terms of how you drive the
> panel.
>
> Bob M.