Resolution switching on a monitor

I know you can change your monitor resolution in Windows from say 800x600 to

1024x768. The dot pitch of the monitor does not change however, so what is happening in the electronics to make this magic happen? Does the width of the electron beam change? Is it the scan rate that changes when you change resolutions? I'm curious for an explanation of what happens in the hardware when you make a resolution change. I am talking about the CRT monitors.

Thanks!

[Tim]
Reply to
Tim Benner
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hardware

It's just the scan frequencies that are changing to accomodate the new timing/pixel format. The focus (which is sort of the electron beam width, at least as it is seen at the screen) MAY be altered slightly as well, if the monitor has the capability of storing adjustments for that and other parameters (geometry, convergence, etc.) for specific timings, although it is VERY unusual for focus to be included in this.

You're right, the dot pitch can't change - that's a fixed physical parameter of the CRT itself - but the physical dots on the screen (or the holes in the shadow mask) really have nothing at all to do with the logical pixels of the image, other than being one of the things which ultimately limits the resolution (in the proper sense of the word, the amount of detail which can be resolved per unit distance or area) of the product.

Bob M.

Reply to
Bob Myers

800x600

of

other

parameter

which

The scan frequencies do not necessarily change at all when you change the resolution. What happens is that the signal as seen on the VGA plug has (for example) 1024 discrete values between 2 consecutive line syncs as opposed to

800 discrete values and 768 line syncs between frame syncs as opposed to 600 (assuming non interlaced). What you drive it into is irrelevent. The way your particular monitor handles this change can vary but generally the more you pay for the monitor the more changes will take place inside when it detects the faster data rates including thing like the dynamic focus and dynamic beam acceleration.

On TFT monitors they specify a "recommended" resolution that the TFT works best at and when not run at this resolution they get seriously blocky and in some cases unreadable text.

Reply to
Mjolinor

Remember, we're talking about CRT monitors here, not LCD; it is very rare for the scan frequencies NOT to change when changing the pixel format. (There are a few of the original "VGA" modes, for instance, that all use the same horizontal rate, but differ in the vertical.) Changing the "resolution" (pixel format) and not changing either the horizontal or vertical rates can only come by packing more (or fewer) pixels into a given scan line (since you can't possibly have changed the lines per frame if neither scan rate changes, except trivially by altering the blanking period). But since a CRT monitor doesn't know anything about "pixels" in the first place, there's no real change from the monitor's perspective.

to

600

First, there aren't "1024" or "800" discrete values on the VGA video in any case; the analog VGA interface provides absolutely no information that permits "pixels" to be clearly distinguished. It carries a continuous analog video signal. (Which is not to say that this video can't be sampled at what you BELIEVE are the correct "pixel" times - analog-input LCD monitors do exactly that - but there is nothing on the interface itself that identifies the individual pixels for you.) Thought experiment - try showing one line of video from a VGA interface, running 1024 x 768 @ 60 Hz, on an oscilloscope - and point to pixel #483 on that line. This is especially fun when the image in question is a full white raster...:-)

Second, and more importantly - if you've changed the number of line syncs between the number of frame syncs (i.e., changing the H sync rate vs. the V sync rate), then you HAVE changed the line (horizontal scan) rate by definition (assuming the same frame rate), right?

I have absolutely no idea what you mean by this. But since a CRT monitor (except for a VERY few highly specialized designs) runs the horizontal and vertical deflection at the H and V sync rates of the incoming video, it's most definitely relevant to the original question.

in

That would be the native pixel format (and frame rate) of the panel. This is recommended, since when the incoming video matches the requirements of the panel, no scaling or frame-rate conversion (both of which can result in visible artifacts in the image) is needed.

Bob M.

Reply to
Bob Myers

To be simple, the rate or speed of the scanning changes to match the resolution. More lines are scanned, therefore there is more information per line, and more information in total on the screen, because there are more lines.

The detailed explanation of what happens is very long.

LCD and Plasma monitor displays are different in this approach.

Jerry G. ========

Reply to
Jerry Greenberg

the

monitor's

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.

opposed

continuous

what

especially

This is incorrect, there are 1024 discrete analogue values. 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.

I don't think I am understanding your point here at all. What I meant by that was that the video signal source and destination have no closed loop properties. It is an open loop system. There is no control directed back to the source from the monitor.

works

and

Reply to
Mjolinor

that,

must

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).

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.

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.

Reply to
Bob Myers

Not 1024 values. Take this example: with some older, cheaper video cards, the image becomes fuzzy with a very high dot clock. This is sometimes down to little ferrite beads on the output lines, which slichtly low-pass filter the outpur signal. That means that even if the DAC is outputting 1024 discrete values, what you see on the line is certainly not that. You also get slewing in the DAC and any amplifiers which may also reduce the bandwidth to below that of the dot clock.

Often the bad picture is caused by low-quality RAM instead.

It's called a low pass filter.

:-)

-Ed

--
(You can't go wrong with psycho-rats.)       (er258)(@)(eng.cam)(.ac.uk)

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Reply to
E. Rosten

change

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. :)

Reply to
Mjolinor

hell

you

it is

the

when

OK I'll concede the practical point but I was not talking about bandwidth limited devices rather I was talking about the theory of it. :)

Reply to
Mjolinor

Just want to say thanks for all the info. But my million dollar question is, does the size of a pixel change with different screen resolutions? I was staring at the screen yesterday as I was switching resolutions, and it appears like it to me. But what causes this? Are more of the color triads being lit up(or skipped) at the lower resolutions then at the higher resolutions? This would make sense to me since the number of color triads on the monitor does not change. If you go from 1600 pixels per line down to 640 pixels per line, then either the lower resolution has to take up more color triads per pixel, or skip some color triads to fit the smaller resolution across the monitor screen. Can it do both?

[Tim]

"Bob Myers" wrote in message news:1OyPc.7051$ snipped-for-privacy@news.cpqcorp.net...

change

Reply to
Tim Benner

Well, from the above, you apparently believe that when a DAC is clocked, the output instantaneously reaches precisely the nominal voltage level intended, with zero rise time, no overshoot, undershoot, ringing, etc., and then does this again exactly one pixel time later. I would submit to you that this does not, and in fact CANNOT, occur. Further, you cannot possibly identify the pixel periods with any real certainty, given just the information carried over the VGA interface. So I guess I'm still having some problems understanding just what "1024 discretre analogue values" would actually mean in any practical sense.

In simpler terms, in analog video the notion of a "pixel" as a distinguishable thing simply does not make sense. In analog video, all you can really talk about is the video bandwidth, which is what really corresponds to "resolution" here.

Bob M.

Reply to
Bob Myers

question

triads

The bottom line is that on a CRT display, there's really no such thing as a readily-identifiable "pixel." For faster timings, certainly the duration of a pixel period (and therefore the width of "single-pixel" features such as vertical lines) will change, but there is not generally an intentional change in the CRT spot size made by the monitor. (In fact, and this is going to seem rather counter-intuitive, the spot size of a typical CRT monitor is almost always considerably larger than the "pixel size" you'd get by simply dividing, say, the image width by the number of logical "pixels" per line.)

With "higher-res" timings, yes, less screen area (number of phosphor triads, whatever) corresponds to a given logical pixel. But it's not as neat and tidy a relationship as you might think at first.

Bob M.

Reply to
Bob Myers

hell

It has no meaning at all in the practical sense but I wasn't talking in a practical sense. The original question wasn't aimed at understanding things that deeply he was concerned at to what the difference was in the signals that allowed the monitor to "know" and react to what was going on in the changed signal from the PC. I attempted to explain the changes in this signal without the complex analysis that a full explanation would require. I don't think it would serve any purpose for basic understanding to go into the finite bandwidth of devices and the instantaneous voltage when examined several orders of magnitude faster than the pixel rate involved.

Reply to
Mjolinor

things

Exactly. And since the monitor does NOT see any information that identifies individual pixels within the video signal (even though that may be a convenient way for the video to be thought of in some cases), this has absolutely NOTHING to do with "the signals that allowed the monitor to 'know' and react to what was going on..." The only change in the signal set that the monitor sees, recognizes, and acts upon are the changes to the horizontal and vertical sync timing, PERIOD. To bring up the notion of "discrete pixels" in such a discussion is irrelevant, misleading, and simply incorrect.

I

examined

There's not even a need to examine it THAT fast; for "single-pixel" details, the output of the ADC may NEVER be at the nominal intended voltage level corresponding to the ADC input, except for the briefest of instants during transition.

Bob M.

Reply to
Bob Myers

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