Re: Resolution switching on a monitor

From: Bob Myers (nospamplease_at_address.invalid)
Date: 08/02/04 

Date: Mon, 02 Aug 2004 18:29:39 GMT

"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.

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.

> 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).

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?

> What you drive it into is irrelevent.

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.

> 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.

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.

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