long vga runs

[kierong99]

Hello everybody.

 

I keep having problems with long VGA runs (and those those technician's, I keep having problems running RGBHV over 15 pin D sub etc etc.) I know you can only run certain distances. I understand how it works, I do it for a living. What I'm looking for is somewhere to find a little more data on why. I want to know excatly why some laptops are ok and others won't work at all particularly on projectors.

 

Also, when you have dual monitors, cloned and out PowerPoint for example with embedded video and on the second display, the monitor appears black. Again, I'm looking for more details on 'excatly' why that is.

 

If anybody knows where I can get that type of data, please let me know.

 

Thanks

 

Kieron

[soundo26]

Here we go again!!!!

 

If you run from a computers' 15 pin HD output you MUST amplify the signal if you wish to run any appreciable didstance, you can get away with longish runs to some extent but if you look closely you'll see deterioration in various forms which affect the image, to some people this is acceptable, to a professional it is not!

 

The signal Via the 15 pin d-sub was only ever designed to run to a local computer monitor, not over longer distances, use a splitter/amplifier which will usually allow you to send a signal up to about 250ft (assuming you use decent quality cable etc)!

 

The black screen on your laptop when outputting video is usually due to an under-powered graphics card having to prioritise where best to use its' output, you will need to upgrade your graphics card to one with larger memory.

[kierong99]

Thank you very much for this. But if you read back, my original quote does say that I do this for a living and was looking for some in-depth data that might help me. For example, is the data projector image affected by the voltage of the signal? Is the volt drop the cause of on image not showing on a projector? What voltage does RGBHV run and and what are the tolerances.

 

Thanks for your help anyways.

 

Kieron

[mi-ul]

Also, when you have dual monitors, cloned and out PowerPoint for example with embedded video and on the second display, the monitor appears black. Again, I'm looking for more details on 'excatly' why that is.

 

Usual solution for me is to go into the advanced monitor settings and turn down the hardware acceleration to just left of centre.

I haven't done this for a while so whilst I could do it with a machine in front of me I'm a bit vague here. Also this doesn't seem to help Vista

 

Mi-ul

[kierong99]

You can also resolve it by using just the external monitor i.e. scroll through until you get just the external and not the actual laptop monitor. Which is great, but again, looking for more details on excatly why this happens as there has to be an easy solution to these common problems.

[Jivemaster]

The data rate of video relies on good cable and good driver chips to get a quality signal out at the receiver end. Laptops tend towards lesser chips to keep power consumption down on the board and from the battery. Use a minimum spec chip and min spec cable and the signal may not get to the far end intact, due to power losses in the cable and unmatched cable impedance. Especially if the cable is too long also.

[Bobbsy]

The VGA spec is .7v p-p for each of the colour signals and, I believe .3v p-p for the horizontal sync signal. The tolerances (and therefore maximum cable runs) are fairly dependent on the screen resolution in use: the greater the resolution, the higher the frequencies required and high frequencies roll off much more quickly, particularly in the "skinny" coax used in a VGA multicore.

 

As has been said, the chipsets used to drive the VGA out (especially on laptops) are designed for minimal power consumption and this, coupled with the lossy cables, accounts for the limited distance. At best you will rapidly lose screen resolution but, as the run extends, the signal will get to the point where timing information can no longer be extracted accurately at the display end of the run.

 

Bob

[Jivemaster]

Read a few posts about DMX and cable! The situation is similar - Trying to get a high frequency signal down a cheap wire. A good signal over full spec cable will the useable at a much greater distance than a poor spec signal over off spec cable, even connectors can afflict the signal.

 

All the receiver chipsets will recover some signal degradation BUT there is a point where the signal becomes so weak that no amount of restoration can recover the data accurately and the picture closes down.

 

The mode of failure is different for analog and digital. - Analog just degrades image quality til there is nothing to see but noise. Digital will "mute (what is mute in picture terms?)" whenever the signal quality degrades. Manufacturers have an interest in killing the picture before degradation impinges on picture quality to prevent their monitor being labeled as poor picture quality for a poor feed signal.

 

A really good line driver at the source end helps, as do repeaters at sensible intervals. Truly top spec cable will solve many problems BUT separating specification from sales adjectives and purchase price comparisons is like distilling the truth out of snake oil.

[Bobbsy]

Jivemaster gives a good description of the difference between analogue and digital failure modes. However, there is one additional wrinkle I've encountered a few times that can confuse this issue.

 

I've used some projectors which, when the analogue quality drops below a certain level, just black (or sometimes "blue") the picture rather than projecting a noisy image. This can be annoying as it tricks you into thinking there's no input rather than just a poor one!

 

Bob

[Nick Mawer]

Jivemaster gives a good description of the difference between analogue and digital failure modes. However, there is one additional wrinkle I've encountered a few times that can confuse this issue.

 

I've used some projectors which, when the analogue quality drops below a certain level, just black (or sometimes "blue") the picture rather than projecting a noisy image. This can be annoying as it tricks you into thinking there's no input rather than just a poor one!

 

Bob

 

The main issues with laptops that we come across are

low levels of sync

distorted sync shapes

one very well known laptop manufacturer is a particular bete noir - I don't know if it is because they have so much of the market share, or because they generally push the envelope of acceptable standards more than other manufacturers.

 

Some of the modern laptops look to handshake with the EDID of the display regardless of whether the signal is DVI-D or analogue VGA-UXGA. I don't think that there is an industry standard for how the laptop should respond if there is no path for EDID handshaking

 

There are also issues regarding the audio output of laptops. Some of the new small laptops have cut back on the screening, and others are prone to distorted audio due to low battery, or poorly placed power supplies.