One simple question [LWN.net]

One simple question

Posted Dec 10, 2007 15:17 UTC (Mon) by alankila (subscriber, #47141)
In reply to: One simple question by tialaramex
Parent article: A pair of small Linux system reviews

My experience with DVI compared to VGA is that DVI produces noticeably crisper picture. It's
important especially for text, where contrast seems better with DVI. (It's easy to understand
that two unnecessary conversions in the signal path are not good for picture quality.) That
alone is surely worth something.

Second, related matter is that the flat panels I have been using tend to get fuzzier with time
and need a resync once in a while. I do not know why this happens, but it's an annoying extra
bother. It's often required when I get to work on monday, for some reason.

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Situation is not so simple, but...

Posted Dec 10, 2007 15:26 UTC (Mon) by khim (subscriber, #9252) [Link]

It IS possible to produce crisp picture with VGA if you are using good video card (from Matrox, for example) and LCD panel with very good ADC. But cheap models are almost always unusable and in reality I'm yet to see onboard video with VGA connector I can tolerate. So I gave up trying and just refuse to use anything with analog video connection...

Situation is not so simple, but...

Posted Dec 13, 2007 9:57 UTC (Thu) by zdzichu (subscriber, #17118) [Link]

And a good cable. I have Matrox card and VGA only LCD. With good cable (1cm thick, filters on
both ends) image on LCD is crisp. With cheap cables bundled with some videocards/LCDs (thin,
about 4mm diameter, one or no filters) image is blurred.

One simple question

Posted Dec 11, 2007 0:57 UTC (Tue) by endecotp (guest, #36428) [Link]

> My experience with DVI compared to VGA is that DVI produces 
> noticeably crisper picture.

This should only become an issue at higher pixel rates.  These "mini" systems have relatively
low maximum resolutions, so it shouldn't be much of an issue.  I think that both of the ones
in this story use an AMD Geode which could only manage 1280x1024 last time I looked.  The
VIA-based boards can get up to 1600x1200 (and a few have DVI).

If you're seeing non-crisp images at lowish resolutions (say 1280x1024 or below), something's
broken.  Maybe a crap cable or wonky connector?  Maybe your X server has chosen a
high-frame-rate mode, e.g. 120 Hz?

My experience - and I have spent some time working with the ADCs that LCD monitors use - is
that unless you're unlucky analogue works OK at 1600x1200@60Hz, but you wouldn't want to go
higher than that.

One simple question

Posted Dec 11, 2007 2:07 UTC (Tue) by zlynx (subscriber, #2285) [Link]

I've got a pretty nice ViewSonic VX900 at home and I use the analog port to connect to a
Belkin KVM for a few servers.  The desktop machine uses the DVI.

The analog display really is crap, even at server resolutions of 800x600.

Every time I switch machines on the KVM the sync changes.  Even stopping X and restarting it
on the same machine (which does a resolution switch from console and back) makes it lose sync.
The alternating pixel gray X start screen is horribly fuzzy until I re-run the analog
auto-adjust.

VGA has always been a very silly idea for LCD.  Digital to analog to digital?  What is this,
display over a modem?

One simple question

Posted Dec 16, 2007 0:00 UTC (Sun) by endecotp (guest, #36428) [Link]

> analog port to connect to a Belkin KVM ... is crap

The KVM is almost certainly the problem.  Maybe you also have long cables?

> Every time I switch machines on the KVM the sync changes.

You might like to investigate some other brands of KVM gear.  To some extent, you get what you
pay for.

Disclaimer: I have worked for one of Belkin's KVM competitors.

One simple question

Posted Dec 16, 2007 21:35 UTC (Sun) by zlynx (subscriber, #2285) [Link]

I do not think the KVM is the problem.  It always worked well with a CRT display and the LCD
loses perfect sync even when switching from 800x600 to text console then back to 800x600.

It could be the video card, it could be the cables, it could be the LCD.  

But the point is that DVI does not have any of those problems because it is much less
complicated to do digital->digital->digital than digital->analog->digital.

Something is broken ? Of course !

Posted Dec 11, 2007 7:58 UTC (Tue) by khim (subscriber, #9252) [Link]

If you're seeing non-crisp images at lowish resolutions (say 1280x1024 or below), something's broken.

Yum. But unfortunately for small systems with high-density boards in these conditions more often then not in these conditions "something" is analog path of video signal on the board itself.

Plus constant problems with sync, etc. No, the VGA is very bad idea for LCDs...

Something is broken ? Of course !

Posted Dec 20, 2007 12:31 UTC (Thu) by endecotp (guest, #36428) [Link]

> for small systems with high-density boards in these conditions more often
> then not in these conditions "something" is analog path of video signal on
> the board itself.

This would also be true for DVI signals, where careful routing of the LVDS pairs from the
transmitter chip to the connector is needed.