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From: Laurence Payne on 24 Jun 2008 12:34
On Tue, 24 Jun 2008 11:13:51 -0500, walkinay(a)nv.net (hank alrich)
wrote:

>I've just met a situation where a DMX lighting control signal is being
>sent down an audio snake (Whirlwind Medusa series). I'm just starting to
>look into some sound anomalies with this particular system,
>
>Reflex behavior on my part is to want that DMX signal out of the audio
>snake. But is it really a concern?

What are the anomalies?
From: Soundhaspriority on 24 Jun 2008 13:00

"hank alrich" <walkinay(a)nv.net> wrote in message
news:1ij18xc.1t83hbs1iqo76mN%walkinay(a)nv.net...
> I've just met a situation where a DMX lighting control signal is being
> sent down an audio snake (Whirlwind Medusa series). I'm just starting to
> look into some sound anomalies with this particular system,
>
> Reflex behavior on my part is to want that DMX signal out of the audio
> snake. But is it really a concern?
>
From http://en.wikipedia.org/wiki/DMX512-A , I think so. A digital signal
like this has a lot more voltage than audio, and square rise/fall that will
leak badly. Get it out.

"DMX512 data is sent using RS-485 voltage levels and cabling practices. The
DMX specification refers the reader to RS-485 for information about the
electrical signal. Data are transmitted serially at 250 kbit/s and is
grouped into packets of up to 513 bytes, called 'slots' in DMX512-A. Data
are sent with 1 start bit and 2 stop bits, LSB first. "

Bob Morein
(310) 237-6511


From: RDOGuy on 24 Jun 2008 13:17
I routinely run DMX down a snake return (it happens to be a Medusa)
with no trouble. I also routinely run line level signals down those
same returns. Theoretically, neither is a good idea. But have I
noticed problems from doing this? No. Is the audio degraded by doing
this? Could be. In my mind, it's a trade-off between the possibility
of some degradation (which I have never noticed) versus the
convenience of having those lines in the same snake. If I were
putting in an installed system, I'd run them seperately. But when I'm
setting up in the bar we're playing tonight, I run the signals down
the returns.

But none of that means that the DMX in your snake isn't causing a
problem, and it's easy enough to find out if that's the case.

One other thing, though... if you're using snake returns for both DMX
and the line level signals to your main amplifiers, double check that
you've connected everything properly before you power up the mains...
every time! DMX into the PA is something you'll only have to
experience ONCE before you make this a habit! :)
From: Scott Dorsey on 24 Jun 2008 13:34
"hank alrich" <walkinay(a)nv.net> wrote in message
> I've just met a situation where a DMX lighting control signal is being
> sent down an audio snake (Whirlwind Medusa series). I'm just starting to
> look into some sound anomalies with this particular system,
>
> Reflex behavior on my part is to want that DMX signal out of the audio
> snake. But is it really a concern?

Probably not. But plug a pair of headphones into a DMX line and listen
to it when you move some faders on the lighting board. It's a very
distinctive sound. It's hard to mistake it for anything else.

If you hear that noise in your audio, you have a DMX leakage problem.

Oh yes, and make sure the audio and lighting systems don't share a common
ground... the external shield on the snake should not be connected to the
lighting system ground.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
From: Les Cargill on 24 Jun 2008 18:38
hank alrich wrote:
> I've just met a situation where a DMX lighting control signal is being
> sent down an audio snake (Whirlwind Medusa series). I'm just starting to
> look into some sound anomalies with this particular system,
>
> Reflex behavior on my part is to want that DMX signal out of the audio
> snake. But is it really a concern?
>

A quick Google says it's 250Kbit. Other than some weird "subharmonic"
thing ( perhaps caused by improper termination ) it should be well
out of the audio band.

But there's more than one DMX protocol out there, apparently.

I also find it curious that 250Kbit would travel well
on long runs of an ostensibly audio snake. It's
terminated to 100 ohm, so maybe that termination takes care of the
matching.

--
Les Cargill
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