Re: VGA vs DVI connection to monitor

On Tue, 25 Jan 2011 01:30:52 -0900, floyd@xxxxxxxxxx (Floyd L.
Davidson) wrote:

Robert Coe <bob@xxxxxxxx> wrote:
On Mon, 24 Jan 2011 20:49:23 +1300, Eric Stevens <eric.stevens@xxxxxxxxx>
wrote:
: On Sun, 23 Jan 2011 18:25:34 -0900, floyd@xxxxxxxxxx (Floyd L.
: Davidson) wrote:
:
: >Robert Coe <bob@xxxxxxxx> wrote:
: >>On Fri, 21 Jan 2011 22:17:18 -0900, floyd@xxxxxxxxxx (Floyd L. Davidson)
: >>wrote:
: >>: DaveS <mnandds@xxxxxxxxx> wrote:
: >>: > Clearly, there is no cost for me
: >>: >to find out for myself if there is any visible difference with this monitor.
: >>:
: >>: Whether you think you can see it on any given displayed
: >>: image or not, use the DVI connection.
: >>:
: >>: I won't go so far as to say digital data is vastly
: >>: better than analog data, but it is certainly better and
: >>: you get significantly improved precision. Another point
: >>: is that with age the VGA interface will drift far more
: >>: than will the DVI interface.
: >>
: >>DVI might be slightly more resistant to RF interference, especially if the
: >>cable is long. But in normal use, it's very unlikely that you'll be able to
: >>see any difference in image quality. That said, there's no reason not to take
: >>Floyd's advice: if your card supports DVI, you might as well use it.
: >
: >In normal use it should be an obvious difference. A
: >digital interface sends a specific discrete value to the
: >monitor. It is the exact same value each time, and is
: >calculated from the value in the digital image file. It
: >doesn't change, and has the same accuracy each time.
: >
: >The VGA interface has to convert the digital value to an
: >analog value, and then the monitor has to using the
: >timing of a dot clock to pick out the precise time that
: >the right value is made available. It is not nearly as
: >precise as the process used by the digital interface.
: >It can never be as accurate.
: >
: >>I have two dual-monitor setups at work, one of which uses one monitor on DVI
: >>and one on VGA. On that setup, I can see a slight color difference between the
: >>two monitors, but not enough to be annoying.
: >
: >But it *is* different! The difference is error.
: >
: >>On the setup with two DVI
: >>monitors connected to the same video card, the colors look identical (given
: >>identical settings of the monitors, of course).
: >
: >No error.
:
: Unless the monitors are calibrated, it might be two different errors.

Probably neither. I suspect that the bulk of it is because two graphics
devices are generating the inputs. The fact that one is digital and the other
analog is a second-order effect.

If we are discussing two interfaces on the same card,
then it is a single device, not two. There are two
different types of data communications systems to send
identical data from the graphics device to the monitor.
It is a simple characteristic of the difference between
digital and analog data transport systems.

In either case the original data is digital. For a
"digital monitor", that processes the data internally as
digital data, sending the original data via a digital
system results in *precisely* the same data at the
monitor's CPU as was available at the CPU in the
graphics device. If it is converted to analog for
transport it has to be digitized again at the monitor.
That means it *can never be precisely the same data*.

The VGA interface was designed for an all analog monitor,
where the data is *never* digitized. For such a system it
makes no difference if the data is converted to analog at
the graphics device or in the monitor. And of course analog
only monitors never had DVI interfaces because of that.

DVI-A is specifically intended to carry only analog signals.

Virtually all DVI connectors will handle the analog signals of DVI-A

See http://en.wikipedia.org/wiki/Digital_Visual_Interface



Eric Stevens
.

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