[lug] Monitor/Card tweaking - was Re: Color tweaks

D. Stimits stimits at idcomm.com
Thu Feb 22 00:02:48 MST 2001

John Starkey wrote:
> I changed the subject line hoping to promote more people, I know there are several
> experts on here who I'd also love to hear from (if they can forgive me for my
> inadvertent spam, the move of the week for me, sheeesh :} )  I think this is pretty
> important since, for one, the fatigue from HSync can send you home ready for bed. I
> felt it at 60Hz and didn't really know why the screen had become so annoying until i
> saw it in my peripherals. :}

I'd attempt to get at least 85 Hz.

> > There is a modeline FAQ or HOWTO out there somewhere, which I haven't
> > looked at in a VERY long time (though I see below you have found it).
> > I'm not sure what each timing is, but think of them in general as a
> > specification of things like compensation for lag (e.g., an artifact of
> > inductance), periods in which things must be blanked, and correction for
> > general distortions. Timing issues in general to fine tune your
> > particular monitor/video card setup. Anything closer in description I
> > couldn't comment on, but there IS a FAQ out there somewhere, I remember
> > seeing it a long time ago. You might find more help and information at:
> > http://www.hut.fi/Misc/Electronics/faq/vga2rgb/calc.html
> This is a great link. Just playing with it for a couple minutes taught me a lot
> about the relationships between the vars. Thnks.
> >
> > Up until you reach monitor limitations, there is a tendency to gain a
> > better picture as greater bandwidth is supported. Bandwidth is a product
> > of horizontal scan rate, vertical scan rate, color depth, and
> > resolution. Increasing any one item decreases how much total bandwidth
> > is available for the others. The dot clock, that faster it is able to
> > run, I think increases the possible resolution for a given scan rate.
> > However, as a dot clock approaches its limits, its signal might lose its
> > nice square form, and start distorting or rounding, which degrades the
> > quality of what you see. So you would consider your maximum resolution
> > based not just on whether or not you can reach a particular resolution,
> > but also on whether it starts losing quality...a better dot clock means
> > you get get closer to the max bandwidth before it distorts. Take this
> > whole paragraph with a grain of salt, I haven't investigated too closely
> > what I think is probably correct.
> I follow you. Audio is the same. (as I said, it's the only thing i have to compare
> to). I guess you can't distort the input of the card right. It's a set voltage and a
> square wave. But once it's analog going out it's possible to distort the input of
> the monitor? My monitor is 65MHz. So anything more from the card is gonna bloat the
> signal. But distortion in the sense that your talking about is dynamics not
> frequency based. Is it possible to create a frequency based distortion by cramming
> too much, a harmonic distortion? I know you said you weren't sure.... just thought
> I'd ask.

Forgive the babble about to occur, if it seems a bit out in limbo land.
The short answer is that "I think so". Read the long explanation of why
at your own risk.

The signal isn't just about spectrum, but also about how cleanly the
rising and falling edges are shaped. Sync simply needs to trigger a
timing, while individual dot intensities need to be sharply turned on or
off to some extent, instantly (sync will usually succeed longer than an
undistorted picture succeeds...artifacts will occur prior to loss of
sync). There is definitely a distortion that takes place, especially if
there is inductive or capacitive reactance, which alters the phasing
(phase-shift modulation, which fights the correct placement and timing
of pixel intensity changes). The tolerance against phase shifting is
more critical as frequency goes up, or as capacitive or inductive
reactance increases (imperfect cables and CRT)...so as one means to scan
faster and cleaner, you can reduce reactance (capacitive and inductive
content)...if reactance is held constant, you'll begin to see more
problems as the sweep speed and signal complexity goes up (and you can't
use a speed faster than your dot clock can run), since complexity of
mixing signals means more rising and falling edges in a given time
period. Imagine a screen that has only one pixel intensity, you could
probably get pretty close to sync failure without noticing artifacts
(constant intensity means no rising or falling edges except at the start
and end of a scan line). Compare with a signal that alternates pixel
values between completely on and completely off, which is the maximum
transition (equivalent to cramming in more data); I would guess that the
latter would show artifacts sooner than the
single-intensity-on-all-pixels view. The long answer is that cramming in
more data means more rising and falling edges over a shorter time
period, causing flaws to show up in the cable, monitor, and dot clock.
Feel free to forget what I just said, and take a few aspirin.

> > > 3) Sync pulses, positioning them.... This just determines where the new line
> > > will start and end right?
> >
> > Yes.
> So my monitor has an adjustment for screen width and height. Is it changing the sync
> pulses? Makes sense that it is. So i could easily defeat the purpose by raising the
> horizontal sync and closing the viewing area in with the monitor. Would the monitor
> be more efficient at this than X? Take a load off by lowering the HSync in X and
> up'ing it on the monitor?

Don't know. But I suspect that the less the physical hardware has to
compensate for, the better the quality will be. If your monitor gives
values between 0 and 100 for each control, I would bet the best signal
would be found by setting the controls to 50, then adjusting modelines
to get that picture as close as possible to your preferred picture. Once
you'd done all you can that way, it'd be time to use the monitor's
centering and shaping controls.

> Thanks again Dean, you wrote a lot, much appreciated.
               ^^^^ Actually it is "Dan" :P
D. Stimits, stimits at idcomm.com

> John
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