Buzz buzz and arrggghhhhh.....

[KevinE]

I have once had interference from DMX cables being mysteriously induced into the PA, at a club where I was programming a Martin system. It wasn't a Buzz though, just a sort of, well, data sound, which changed its tone depending on what the mirrors were doing (the old Martin RS485 system in vector mode.)

 

If it's mains hum from the dimmers, that's another matter.

[wide-load]

If by Shield you mean the XLR shell, then this should not be connected to PIN 1 at either end. If you mean the shield in the cable, then this should be connected to pin 1 at both ends (but not to the shell)

 

Ben - I do mean the shield in the cable. I have had PA buzz problems in some venues when using a cable that has the cable shield connected to Pin1 at both ends - simply removing the connection at one end resolved the problem - consequently my 3 to 5 pin adapter cable that I use at my desk end does not have Pin1 connected. Whilst it may not be the DMX standard (and I haven't checked this out) surely as long as you have Pin1 connected at one end for the signal ground then this is OK?

 

The other thing I should point out (before everyone goes cutting Pin 1 connections) is that this has worked for me at a few venues but this does not always remove the PA buzz :) as this can be caused by a multitude of things as posted earlier...

 

Andrew.

 

 

u have said here that you have had the problem in "some venues" does that mean u havn't had it in others?

 

it could be a case of "DIRTY MAINS" where phases could be un balanced and receiving bad signal! (in affect)

 

try hunting for a mains filter, and signal earthing unit, they are purley a way of lifting earth on your whole system out put and has dug people I work with out of nasty problem very affectivly! I will have a look for some links and post them for you!

[dbuckley]

99.9% of the time dimmer buzz is heard due to deficiencies in the sound system configuration. And its often the lack of a single ground point in the sound system, often as trivial as plugging the amps in at the stage and plugging the mixer in at the back of the hall.

 

The other 0.1%...? Well, I haven't found one yet, but I keep looking :)

[Wuddy]

I'm a lighting guy not a sound guy but I work with a sound guy and occasionally cause him problems with dimmer hum.

 

He says principal cause is the magnetic field from the dimmer chokes being picked up by the audio system.

 

Certain mics he uses, I think ribbon or cardiod but I'm not sure, have this problem, guitar pick-ups and earth loops within the audio system itself, that is when there is a parallel earth connection between items of audio equipment, an earth connection via each mains lead and through the building earth and an earth path down the signal cable screen, effectively creating a loop which picks up the interference and trasmits it onto the signal cable, this loop can be broken by removing the screen connection from one end of signal lead, NEVER remove the earth from the equipment mains plug.

[ben.suffolk]

Whilst it may not be the DMX standard (and I haven't checked this out) surely as long as you have Pin1 connected at one end for the signal ground then this is OK?

 

The spec certainly does state that you need pin1 connected at both ends. In fact in this post, in a current running thread, part of the (draft) spec is published. Have a look at 6.2.1 and 6.3

 

If disconnecting pin 1 has helped you, it would only be because of a an earth loop, which could only have been caused if the equipment you were connecting to had pin 1 connected to earth, (ie. pin 1 effectively connected to the shell internally in the equipment, since it was not connected in your cables), which is clearly wrong. So whilst you may have fixed the problem, its only because of a bad design in the equipment you were using.

 

Also disconnecting pin 1 could cause you DMX problems, as there is no reference for the data pins (although because they are differential, it generally copes ok)

 

Regard

 

Ben

[daemon]

Whilst it may not be the DMX standard (and I haven't checked this out) surely as long as you have Pin1 connected at one end for the signal ground then this is OK?

 

The spec certainly does state that you need pin1 connected at both ends. In fact in this post, in a current running thread, part of the (draft) spec is published. Have a look at 6.2.1 and 6.3

 

Also disconnecting pin 1 could cause you DMX problems, as there is no reference for the data pins (although because they are differential, it generally copes ok)

 

Regard

 

Ben

 

I believe that the new spec calls for the receiver to not connect the earth. Certainly there is no reason for a RS-485 signal to have a ground.

 

Think of what can happen connecting 2 seperate racks together fed from different power ... personally I don't like any current in a supposed shield.

 

Ben.

[Tomo]

I believe that the new spec calls for the receiver to not connect the earth.

Not quite there:

The shield must be connected to the receiver, but it should not be connected to protective earth.

Thus the RX chip should have an isolated power supply, such as an isolating DC-DC converter.

 

Good shielding is acheived by having the sheild connected to ground at exactly one location - because there's only one source of DMX data, it makes sense for it to be done there.