How does DVI-D work

Hi,

I tried googling for a answer to that question, but didn't find any answer.
I used a DVI-D connected TFT for the first time and wondered how it
could be that there is no refresh rate.
I thought may be it just transpheres changed pixels, but that won'T work
as then the protocol has to be s fast as the video memory itself (could
ever get).

So how does DVI-D work? And how can I find explanations like that on the
Web?

Thanks!

Greetings
Christoph

Christoph Vogelbusch wrote:

I tried googling for a answer to that question, but didn't find any answer.
I used a DVI-D connected TFT for the first time and wondered how it
could be that there is no refresh rate.
I thought may be it just transpheres changed pixels, but that won'T work
as then the protocol has to be s fast as the video memory itself (could
ever get).

So how does DVI-D work? And how can I find explanations like that on the
Web?

I tried entering 'DVI "refresh rate"' into Google, and about halfway
down the page, came across this link, which looks promising:
http://howstuffworks.lycoszone.com/monitor2.htm.

On that, it says "The DVI specification is based on Silicon Image's
Transition Minimized Differential Signaling (TMDS) and provides a
high-speed digital interface. TMDS takes the signal from the graphics
adapter, determines the resolution and refresh rate that the monitor is
using and spreads the signal out over the available bandwidth to
optimize the data transfer from computer to monitor. DVI is
technology-independent. Essentially, this means that DVI is going to
perform properly with any display and graphics card that is DVI
compliant."

This seems like a little bit of the answer to your question. It also
indicates that "refresh rate" may not be the most common word for this
with regard to DVI interfaces, and so there are probably better search
terms. It looks like a decent explanation of the process will refer to
TMDS, so I do a google search on TMDS. No luck there; it's a rather
common acronym, and most of the results aren't relevant. Instead, I try
the phrase "Transition Minimized Differential Signaling" (in quotes),
which on the first page has a link to a pdf entitled "DVI White Paper":
http://www.proxima.com/downloads/pdf/DVI-WhitePaper.pdf

That seems to explain some of it, but it doesn't really have a direct
answer to your question. So, I try again, this time adding "DVI" and --
instead of "refresh rate", "60Hz" -- to the search list. The reason for
"60Hz" is that maybe they're not calling it "refresh rate" in some of
the useful papers, but that's a very common number for it and likely to
show up anyhow. That results in a couple of pages:
http://www.extremetech.com/article2/...1189976,00.asp
http://www.anandtech.com/video/showdoc.html?i=1577&p=2

The second of those, incidentally, explains the difference between a
"DVI-D" and a "DVI-I" connector; the latter also has pins for an analog
monitor (and thus can support a DVI to VGA adaptor), whereas the former
doesn't. You'll note that I've been searching on "DVI", not "DVI-D" all
along; this is because I'd never heard of "DVI-D" before, and figured
the suffix was likely an irrelevant thing that would limit the search
results without being useful. It appears I was right.

Those still don't help that much to answer your question, though, so I
again try "refresh rate" instead of "60Hz", but with all the other
search terms, and find this:
http://www.cyberresearch.com/content...tutorial19.htm

That includes the note, "Digital CRT displays with a DVI interface
eliminate the need for costly multisync technology. With selective
refresh, a CRT display can maintain the high refresh rate that reduces
eye strain, while only sending data to the display when changes in data
need to be displayed."

Ok, so that's a phrase to look for: "selective refresh". I'll try a
search on that and "DVI" and see what pops up. That doesn't find much
of use, though, although the results seem to mention a DPVL standard
that's based on DVI but with the modification that it can send only
changed regions of the screen rather than the whole screen, which sounds
interesting.

Somewhere in there -- I forget where -- I came across the full DVI spec
as PDF form, at:
http://www.gbe.com.tw/New_Folder6/dvi_10.pdf.pdf

At this point, I'm going to call it quits. It sounds like that the DVI
standard is very definitely transmitting full-frame video, rather than
just changed pixels (otherwise, the DPVL wouldn't be something new), and
that it does this at fairly standard frequencies of refresh rates.
Also, it's required to query the monitor, and only output refresh rates
that the monitor can support -- there doesn't seem to be anything that
requires it to do the highest supportable refresh rate, though, so there
is at least the possibility that the monitor driver could let the user
specify the refresh rate.

Now, as it happens, you've got DVI-D, not DVI-I, so it's only going to
be connected to flat-panel monitors. And, further, I've never seen a
flat-panel monitor with anything other than a 60Hz refresh rate, since
they don't need higher rates to prevent flicker. Thus, my guess is that
the driver for your video card was written by people who figured that if
the refresh rate was always going to be 60Hz with no reason or option
for changing it, then there was no need to put it in the driver dialog
boxes and such.

- Brooks


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