Induction loops and VGA leads

[pritch]

Ever since we had our induction loop installed, the CRT monitor that I use has developed a bit of a wobble, especially if there's anything particularly loud going on. Not a huge problem, I'm not really worried about it.

 

However, we've got a show coming up where we'll be using projection. The projector itself will be well away from the loop, but the computer running it will be right next to the loop, and there's not an awful lot we can do about it.

 

So, what I'm wondering is if anyone has had any experience with this before? Do you think that I'm going to have interference with the projector, or perhaps it's just the monitor that was picking up interference, and the projector will be fine? If I'm going to have problems, apart from turning the loop off or keeping everything away from it, what options do I have to cut down on the interference?

 

I'm very much hoping that this is just a problem with operating a CRT in such close proximity to the loop cable - is it?

[dave_m]

can someone correct me if I'm wrong, but the induction loop works by generating a magnetic field?

This means that the generated field can bend the electron rays generated in the CRT, in the same way holding a magent on the screen discolours the image.

So, and don't hold me to this, it shouldn't cause any problems with and LCD projector/screen.

Just my thoughts

[dunk_1984]

We had this issue when working in the Scarborough Spa complex Grand hall, They have an in house induction loop, and from what we can gather it is fitted running around the insides of the balcony... Exactly the same place as the VGA cables we layed were, this meant that both projectors, FOH monitors and on stage screens developed interference. There was no way other than turn the induction loop off, to combat the problem; We offered the organisers that we could order in a small induction loop and create a small area which was covered by an induction loop, but budget cuts meant this wasn't possible. We could reduce the problem by lowering the gain going to the induction loop in the X-overs as we had already cut the Amp output to an acceptable level.

 

I don't know if this was because the cables were layed side by side, or because the induction loop was so big and powerful that it affected the screen's and projectors, it certainly affected the CRT's the worst but I believe all the screens were affected including the projectors (as lighting and AV control - at the back of the hall - could see the effects). So there probably is an effect on the cables too, but there were other factors to take into account.

[dbuckley]

It is magnetic induction. For cables running alongside or within the inductive loop area, use VGA/Cat5 converters, as then the signal is balanced and wont get interfered with.

 

Just wait till you get induction loop picked up by bass guitar DI'd into PA :D

[PoppaDom]

It is magnetic induction. For cables running alongside or within the inductive loop area, use VGA/Cat5 converters, as then the signal is balanced and wont get interfered with.

 

Just wait till you get induction loop picked up by bass guitar DI'd into PA :D

 

Happened to me at the RSC in the Swan with a lead guitar....talk about walking a knife edge!!

 

Was funny to watch guitarist holding guitar in the air saying "It's not me man!"

 

 

Poppadom

[Andyb]

After being present when a reputable company could not solve a similar problem; I have some observations

 

CRT / LCD / Plasma monitor made no difference. The interference was picked up on the lengths of VGA cable.

 

Having an electronics background, I would agree the theory that VGA cable is screened so should solve the problem - it did not, and it was not cheap cable being used.

 

Any number of Humbugs, Line Filters, Distribution Amplifiers did not help the problem at all. The only solution was to turn the induction loop off - or right down to the point of being useless.

 

I would be very interested to hear if anyone has come across the same problem - and been able to fix it.

Has anyone had any luck with using vga to cat5 adaptors? The theory that the pairs are all twisted should eliminate the interference. Any experience of costs / quality of converters? I am after driving a number of large plasma screens.

 

I look forward to hearing other comments / suggestions.

 

cheers

andrew

[Olie]

We had this problem and managed to solve it by buying a high quality shielded VGA cable. They are very expensive but certainly do the job.

 

I discovered it was the cable as we had two projectors running from seperate VGA feeds but split at the source. The cables where both side by side. When we switched the cables over we found the problem switched to the other projector.

 

Most cheap CGA cables you buy are not shielded correctly and so are susceptable to magnetic fields such as 100V loops. If you use a correctly shielded cable then thanks to an effect known as faraday's cage they are protected from the field.

 

We bought high spec Lindy VGA cables and all our 100V loop problems went away :-)

 

EDIT

 

VGA to CAT5: The VGA to CAT5 adapters I have seen do not make use of the balanced signal that you need to use to get any benefit from twisted pair. All they seem to do is match the impedence for transmission down the cable. That is not to say that some do use a balanced signal, just be careful.

[pritch]

Cheers folks,

 

This sounds as if it could be amusing. Given our (lack of) budget (the projector is being borrowed), I think we may end up trying it out and seeing how bad it looks, and then going with 35mm slides if it's unusable.

[dbuckley]

Having an electronics background, I would agree the theory that VGA cable is screened so should solve the problem - it did not, and it was not cheap cable being used.

 

It's really really hard to screen your way out of magnetic coupling, and if you do it successfully for an assisted hearing loop you'll probably be disrupting the loops magnetic field. For example, thick wall steel electrical conduit has good screening properties, but it will disrupt the loop.

 

On the other hand, running balanced cables (which VGA isn't) makes the interfering signal a common mode signal, so the interfering signal goes away.