VGA over CAT5e

On Mon, 6 Oct 2008 16:27:41 -0700 (PDT), Bod43 RemoveThis @hotmail.co.uk wrote:

>There of course may be commercial solutions available now that
>avoid the problems entirely. Perhaps by combining the
>signals into one pair.

The most common solution now is to use an ethernet KVM (keyboard,
video and mouse) switch. All signals get packetized.

"philo" <philo.TakeThisOut@privacy.net> writes:

> <BigAl.NZ.TakeThisOut@gmail.com> wrote in message
> news:8fdab6cc-b21a-4ed7-9bac-2ed3b666c46c@x16g2000prn.googlegroups.com...
> > Hi Guys,
> >
> > I need to get a VGA signal from the Nav Station in my boat to the
> > monitor in the bridge. I could just run a VGA cable (about 4m worth)
> > but I need to pass the cable through some small holes for the looming
> > (sp?) and the DB15 connectors wont fit unless I make the hole bigger.
> >
> > I am thinking about running the VGA singal over CAT5e, which a Google
> > search seems to suggest is possible. I was wondering if anyone had any
> > better ideas, keeping costs fairly low.
> >
> > Has anyone here had any success cutting the end off a VGA cable and
> > wiring on a new DB15 for example?
> >
> > Thanks in advance,
> >
> > -Al
>
>
> VGA cable is shielded
> and CAT5 is not, so you would get some horrible ghosting.

Thethe wrong cable impedance and lack of shileding between
RGB signals will not cause ghosting. Most ghosting will
be caused by the impedance mismatches.

The lack of shield in cable will cause that that cable
will pick up more easily external interference
and will radiate out more RF interference and properly
shielded VGA cable.


> BTW, there may still be some ghosting even with VGA cable

True. The VGA cables vary in quality. The good ones are good
but there are also bad ones. One thing to keep in mind
in VGA connections is that it is a good idea to keep the
number of VGA connectors along the link minimum (ideally
only at source and destination), because the VGA connector
impedance is not exactly 75 ohms as the system is designed
for, and having many such wrong impedance connectors on
the way will cause impedanc mismatches that cause
reflections. For VGA cables is best to use a correct
length cable in the beginning, and avoid using
orignal cable + extension cable combinations.


--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

In article <xcCdnUH-t6705XfVnZ2dnUVZ_qTinZ2d RemoveThis @comcast.com>,
glen herrmannsfeldt <gah RemoveThis @ugcs.caltech.edu> wrote:

> Rich Seifert wrote:
> (snip)
>
> > You can use a transformer, but wired as a *balun* (coils in series with
> > the lines instead of across them); this provides the required impedance
> > transformation while still passing DC.
>
> So that is how they do it.
>
> TV baluns, from 300 ohm balanced to 75 ohm coax, don't do that.
>
> http://en.wikipedia.org/wiki/Balun
>
> Does that restrict which impedance transformation you can make?
>

The impedance transformation is determined by the winding ratio,
regardless of whether the coils are in series or parallel with the
lines.

Putting the balun in series provides better low-frequency response (down
to DC), but of course this configuration does not provide any electrical
isolation. It functions very much like a common-mode choke, with an
impedance change.


--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

"Alfred" <invalid.DeleteThis@invalid.co.uk> wrote in message
news:mv0ke4d8sag8dndkd0c0i3rq52l99ldjp5@4ax.com...
> On Mon, 6 Oct 2008 17:23:03 +1300, "PeeCee" <abuse.DeleteThis@local.host> wrote:
snip
>
> I used to make up VGA cables for control room applications. 15M using
> individual screened conductors was about the recommended limit



mmm wouldn't surprise me.
I've seen VGA extension cables as short as 2M exhibit ghosting.

P.