Projection causing hum on sound

[SSMattL]

Hi guys,

 

Completely flummoxed by a problem we have.

 

We are running a projection system involving VGA over CAT-5e and have come across a problem which has caused a lot of head scratching.

Right, here goes.

 

Running the sound from a laptop (yes, we have tried different laptops) and all is ok. Out of the laptop and direct linked into a set of QSC K10s and K8s, the sound is clean.

 

If we connect the VGA out of the laptop via the installed CAT-5e and a set of Kramer converters to the ceiling mounted projector but don't send a signal, the sound is also clean.

 

As soon as we introduce a video signal, a hum appears on the sound system. If we change the image to projecting black, the hum disappears, if we project white, the hum is at its worst, and a coloured image has a hum somewhere in the middle!

 

If we project an image only and disconnect the sound, the hum disappears.

 

Basically, audio alone is fine, video alone is fine, put them together the hum appears!

 

The Audio does not travel via the Kramers, it is an installed direct link between the input point (3 pole mini-jack) and the speakers on XLR

 

We have tried everything we can think of, any weird and wonderful suggestions, please???

 

Cheers

 

Matt

[timsabre]

It is an earth loop, very common problem when projecting from a laptop.

 

You need to run the audio through an isolated DI box (i.e. one with an earth lift switch)

 

A good test is to run the laptop off battery, this should also fix the problem temporarily as it removes the earth.

[Dave m]

I have had something similar and it's something to do with earthing.

 

do you have a video hum eliminator?

EDIT just read VGA so no good.

 

As said above, 2 different mains supplies are the issue

Edited March 27, 2014 by Dave m

[Tow]

Ground loop. Put a cheap isolation transformer between the laptop and the amps.

[Rob Harris]

I had an issue like this a little while back. Projection was causing hum on the sound system. Putting a DI box in made the hum worse!

It turned out to be a impedance mis-match between the laptop output and the mixer input.

[SSMattL]

Hi guys,

 

Thanks for all these, DI box didnt help, neither does unplugging the laptop. We've tried all these suggestions and more, which is why we're so confused!

 

As a temporary "work-around", we've put in an external ARX Audibox USB DI, which has cured it for the moment (a standard DI didn't help) but we are looking for a permanent solution if possible?

 

Cheers

 

Matt

[Hilary Watts]

I seem to remember having a similar problem a year or two ago with a desktop computer. I think I eventually minimised the problem by moving the audio PCI card as far away as possible from the video PCI card (ie moved them to different PCI slots with another card or cards between them) and came to the conclusion that somehow the video card was radiating an rf signal which was being picked up by the audio card. It may be that the audio and video circuits in the laptop are very close together but by moving the audio to an external USB device you've taken it away from the rf generated by the video card. If this is the case then the only solution may be an external audio card.

[Whiskers]

The easy solution

 

http://www.play.com/.../-/Product.html

 

Only 2ch basic PC sound but a total lifesaver, mine is not for sale EVER

 

The hard solution understand and fix

 

Is the "hum" 50Hz or 60Hz ? Make some bright picture hum and ...

 

1) play some genuine 50Hz out of a separate sound system, if there is a noticeable pitch difference compared to the target hum then its 60Hz*

 

2) Up the laptops refresh rate to 75Hz (projector may not sync) does the target hum go up further in pitch?*

 

If NO to 1 & 2 ie "hum pitch is not video dependent" Lightly you have a regular vanilla 50Hz noise boy hum loop > not my department > skip to end.

 

If YES to either 1 or 2 then its Video getting into audio, most lightly a combination of missing earths and a common earth perhaps in cat5,

 

next move

 

3) run laptop to projector with a flexible HD15 lead removing cat5 and Kramer and converters from the game.

 

if 1) or 2) is yes and 3) removes the problem then the escape is in the infrastructure

 

Now bell through from the HD15 male at the laptop all the way to the projector checking earth paths, confirm how the projector straps or commons the various "earths"

 

(explanation so far VGA / UXGA is a collection of unbalanced analogue video signals, most often at 60Hz )

 

In HD15 VGA/UXGA the "earths "on each of the RGB signals are separate, pins 6, 7 & 8 and then there are one or two Sync signals with separate earth conductors again, and some signaling and ID lines

 

can you feel the standard being creatively modified as we near cat5 with less pins than HD 15 ? signal earths might be joined to save cores, then fanned out somewhere else or not or lots of other fun stuff.

 

laptops are often double isolated with a dummy earth pin on the wall wart PSU but you have earthed it to the projector, laptops have various opinions but often strap vid earth to frame.

 

we have a LOOP ALERT, would give you 50Hz in UK plain vanilla noise boy hum loop. Disregard if you scored YES to either 1) or 2)

 

Suspicion now is an incomplete earth path which could cause the laptop frame to assume video signal potential 60/75 Hz keep belling, or look at Kramer and converter diagrams and finger follow the earth and signals

 

Oh gosh the laptop frame is the audio common, unrequited video could run to earth via the sound system this way causing induction.

 

Might be worth checking if there is one of the three primary colours causing the buzz, project pure green then pure red then pure blue, (VGA/UXGA is a Component system), if yes it narrows down where to bell.

 

 

/end of long twopennyworth sorry its a load more questions

 

 

*except if you need to confirm Hz in 1) & 2) a reading bass player is required,bring beer or gig oil. 50Hz (mains) is a shade above G, 60Hz (Standard VGA) is 1/4 tone under B 75Hz is 1/4 tone above D,

[Pete McCrea]

Now that's a comprehensive response!

[SSMattL]

Indeed it is, thanks Whiskers!! I've passed this on to our Technicians who are going back in to see what happens! I'll keep y'all posted :)

[idm]

1396347883[/url]' post='496761']

Indeed it is, thanks Whiskers!! I've passed this on to our Technicians who are going back in to see what happens! I'll keep y'all posted :)

 

Any update on this? I have the same problem and I’m looking for possible fixes

 

Thanks also to Whiskers for very helpful post, especially re pitch change matching refresh rate...

[kitlane]

Indeed it is, thanks Whiskers!! I've passed this on to our Technicians who are going back in to see what happens! I'll keep y'all posted :)

 

Any update on this? I have the same problem and I'm looking for possible fixes

 

Thanks also to Whiskers for very helpful post, especially re pitch change matching refresh rate...

 

Welcome to Blue Room, Ian. Don't be too surprised if you don't get a response to this. The thread is 6 years old. It might be better if you start a new thread with your specific circumstances.

Edited July 13, 2020 by kitlane