VGA cable into RGB socket causes ghosting

PB · Post by PB » Wed Nov 12, 2008 12:23 am

I just bought a VGA cable that has pin 9 missing on both ends (see here:
http://en.wikipedia.org/wiki/VGA_connector), and when I plug it into my
LCD TV (which has an RGB input) from my laptop, the image appears to
be very ghosted, which is really noticeable on the Desktop but not so
noticeable with fullscreen games. Is that due to the pin missing?

Mistrel · Post by **Mistrel** » Wed Nov 12, 2008 12:28 am

VGA cables cause some degree of fuzziness on LCD panels because it's an analog signal.

SFSxOI · Post by **SFSxOI** » Wed Nov 12, 2008 5:01 pm

You are using the R-G-B out from the lap top, right?

PB · Post by PB » Wed Nov 12, 2008 5:10 pm

I'm not sure if it's actually an RGB out or just VGA (analog out); this is the manual:

tinman · Post by **tinman** » Wed Nov 12, 2008 5:46 pm

What screen refresh rates do you usually use for the desktop vs full screen games?

If your desktop has a higher refresh rate and resolution and the ghosting is worse then chances are the cable is too long for the higher frequency signals.

Try reducing your refresh rate to 60Hz or your resolution. Some laptops allow a different resolution for the built in display vs the external display, so it should not affect your use as a laptop too much.

case · Post by **case** » Wed Nov 12, 2008 6:59 pm

some cheap cables may produce goshting , for connecting my devices i don't mind paying more money because of the quality of the signal, torsaded pairs, shielded cables.

pdwyer · Post by **pdwyer** » Thu Nov 13, 2008 12:29 am

LCD TV? or LCD Monitor?

Some TV's don't handle this very well, or not with certain resolutions and sync rates. This maybe why games are ok. Try changing resolution size and sync and see what happens

SFSxOI · Post by **SFSxOI** » Thu Nov 13, 2008 11:13 pm

What your seeing is a time delay, phase, difference. lets see if I can say this without confusing anyone, I know what i want to say, but it might not come out right.

Ghosting in TV's is caused by phase differences. Thats the effect your seeing, something is arriving out of phase. From the textbook definition from my higher education days: "The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other. It is common for waves of electromagnetic (light, RF), acoustic (sound) or other energy to become superimposed in their transmission medium. When that happens, the phase difference determines whether they reinforce or weaken each other.

more specifically: "It is common for waves of electromagnetic (light, RF), acoustic (sound) or other energy to become superimposed in their transmission medium. When that happens, the phase difference determines whether they reinforce or weaken each other. "

VGA and RGB are RF signals (RF signals are basically oscillations being generated as a result of some source), the "superimposed in their transmission medium" part is a fancy way of saying ghosting for RF signals. You have something thats causing a phase difference and thus the ghosting. The first places i would look is making sure the source is correct for the input, then I would look at the cable.

There is also a possibility that there is an impedence difference between the laptop output and what the TV expects as impedance. Differences in impedence can also cause a shift in phase and thus a phase difference (the cable could be also causing an impedence difference.). The normal expected impedance for a video signal is 75 ohms. I'd start looking at the cable first, it could be a bad cable or it could possibly be too long (cable length can cause a phase difference in some cases). Remember also, your laptop output was designed for a monitor or projector, those are really different inputs then most TV inputs (depends on the TV) and the impedence might be different. I guess its also possible there is a problem with either the laptop outputs or the TV inputs, but I wouldn't go there first in this case.

the.weavster · Post by **the.weavster** » Fri Nov 14, 2008 10:32 am

Is it an HD TV?

HD devices are particularly fussy about receiving a 75 ohm signal and wont cut you much slack. I think SFSxOI is probably right, you're getting reflection loss from a low quality cable.

It's not just the length of the cable that's an issue but the quality of the cable, connectors and connector joints.

Tipperton · Post by **Tipperton** » Fri Nov 14, 2008 1:46 pm

case wrote:some cheap cables may produce goshting , for connecting my devices i don't mind paying more money because of the quality of the signal, torsaded pairs, shielded cables.

Agreed, it just doesn't pay to go cheap on cables, on the other hand what some places charge for cables is nothing short of robbery. For a good quality six foot cable I expect to pay about $20 but I've seen places charge as much as $40 for the exact same or similar quality cable! Ridiculous!

Irene · Post by **Irene** » Fri Nov 14, 2008 4:19 pm

Tipperton wrote:[..]Ridiculous!

As far as this cable discussion goes, be happy you don't need to pay 100€ for a couple of 1 metre SCSI cables and a 3 metre MIDI cable! Oh, and the coaxial S/PDIF cables I am using for my equipment cost around 25€ per metre O_O

Tipperton · Post by **Tipperton** » Sat Nov 15, 2008 12:06 am

Irene wrote:be happy you don't need to pay 100€ for a couple of 1 metre SCSI cables and a 3 metre MIDI cable! Oh, and the coaxial S/PDIF cables I am using for my equipment cost around 25€ per metre O_O

Ouch!

Of course just mentioning SCSI cranks up the price a ways.

MIDI I'm not too surprised at since it's a pretty specialized cable.

But S/PDIF? You'd think that with the popularity of home theater systems and digital audio that their prices would be better than that.

PB · Post by PB » Tue Nov 18, 2008 2:12 pm

Sorry for this late reply. It's an LCD HDTV. Here's what the ghosting looks like:

It's not too bad, but I would have expected that since it's an LCD TV, that it
would be ghost-free like an LCD monitor? I mean, what's the real difference?
They are both connected to PC's via the same type of cable. I don't get it.

The PC's refresh rate is 60hz and changing the resolution doesn't help either.

case · Post by **case** » Tue Nov 18, 2008 3:32 pm

can't you try another cable ? an old one from a monitor maybe ? i'm sure the problem comes from the cable.

SFSxOI · Post by **SFSxOI** » Tue Nov 18, 2008 8:07 pm

PB wrote:Sorry for this late reply. It's an LCD HDTV. Here's what the ghosting looks like:

It's not too bad, but I would have expected that since it's an LCD TV, that it
would be ghost-free like an LCD monitor? I mean, what's the real difference?
They are both connected to PC's via the same type of cable. I don't get it.

The PC's refresh rate is 60hz and changing the resolution doesn't help either.

A monitor and TV can be different in the way they process the video signal. Most TV's process the video based upon a seperation of the RF carrier from the video signal, monitors process the signal based upon the video signal only and there isn't an RF carrier involved to seperate it from. Resolution changes for the PC doesn't help in this case for the TV because your only changing how its displayed (how often basically), not what is displayed and the ghosting is part of what is displayed, resolution doesn't affect phase differences which is what ghosting is in a TV. If it displays fine on an actual LCD monitor but not on the TV then either the impedance is wrong, a bad cable, or the TV doesn't accept the output from the PC as you think. All types of video outputs and inputs are not created equal.