Re: Resolution switching on a monitor

From: Mjolinor (mjolinor_at_hotmail.com)
Date: 08/03/04 

Date: Tue, 03 Aug 2004 10:00:28 GMT

"Bob Myers" <nospamplease@address.invalid> wrote in message
news:1OyPc.7051$ZK1.1888@news.cpqcorp.net...
>
> "Mjolinor" <mjolinor@hotmail.com> wrote in message
> news:QmxPc.547$gQ3.103@newsfe4-gui.ntli.net...
> > > "Mjolinor" <mjolinor@hotmail.com> wrote in message
> > > news:PquPc.694$IE4.510@newsfe2-gui.ntli.net...
> > > > The scan frequencies do not necessarily change at all when you
change
> > the
> > > > resolution.
> > What I said was they do not necessarily change, "there is nothing in the
> > sync rates that denote the resolution of the image" is what I meant by
> that,
> > obviously you have to fit them into a frame so one or the other or both
> must
> > vary for that to happen.
>
> Let's put it this way - if you're changing "resolution" (pixel
> format) and/or refresh rate, then the vertical and horizontal
> frequencies almost always change. No, there is nothing in the
> sync rates that "denotes the resolution of the image", other than
> the total number of lines per frame (and from this, and the
> specific rates in question, the timing standard in use can usually
> be identified). But no, since the analog video standard for PCs
> does not include a true "blanking" or "display enable" signal
> (nor anything from which these can readily be derived), you cannot
> clearly identify the number of active lines per frame, which is
> part of the pixel format description. You (or rather, your monitor)
> are always basically guessing what standard timing is in use
> (under the assumption that it IS in fact a standard timing).
>
> > This is incorrect, there are 1024 discrete analogue values.
>
> No, there really aren't. IF you happen to be looking at an image
> which consists of, say, alternating vertical lines, then yes, you can
> tell where the "pixels" were supposed to be. Anything beyond
> that is just a guess; again, there is NO pixel-level timing information
> guaranteed in the VGA interface. You cannot unambiguously
> determine the pixel locations within the video signal for any and
> all video content. The best you can do is to try to generate a
> pixel sampling clock from what timing information you DO have
> (generally, just by multiplying up the horizontal sync rate) and
> taking your best guess at how it should align with the active video
> period.
>
> > You can show
> > this easily on an oscilloscope by looking at one line from a black field
> > with a one pixel width vertical white line.
>
> Sure, but that's a specific (and very fortunate) case. Again,
> there's no way to distinguish pixels within, say, a flat white
> field, or a single HORIZONTAL line, so there's really no
> guarantee of "discrete" values. This is one of the problems which
> has traditionally plagued analog interfaces for fixed-format
> displays (such as LCDs), since those DO require accurate sampling
> at the pixel times. There is a new analog video signal standard
> in the works which is designed to address this (the VESA NAVI
> standard), but since it's not published yet I can't go into the details
> of it here.
>
> If you really want to get into the details of all this (and I guarantee
> you that they're a LOT less interesting than you might think..:-)),
> it's covered in chapters 6-9 of my book, "Display Interfaces:
> Fundamentals & Standards," published by J. Wiley & Sons.
>
>
> Bob M.
>

quote you "frequencies almost always change"
quote me "frequencies do not necessarily change "

What is the difference between those two statements apart from maybe
pessimist versus optimist.

If there are not 1024 discrete analogue values in the signal what the hell
are there. There is an AtoD producing 1024 discrete voltage values and no
ammount "you can't see them" makes them something else. I am not saying you
do anything with them I am just saying that there lies the difference
between the two resolutions, in one there may be 800 and in the other, it is
probably faster (but not necessarily so) there are 1024 of them.

Maybe I will look at your book it would be interesting to learn about the
magic of a device that produces 1024 output voltages but miraculously when
they get to the end of the wire they are not there anymore. :)

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