DMX Lighting Signal in Audio Snake?

On Tue, 24 Jun 2008 11:13:51 -0500, [email protected] (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?

Laurence Payne <[email protected]> wrote:

> On Tue, 24 Jun 2008 11:13:51 -0500, [email protected] (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?

There's a weird distortion over certain types of signals, at levels well
below what should be possible without distortion.

We're going through the system step-by-step to try and pinpoint the
source. I'll likely be back with questions as we proceed.

An A&H MixWiz and outboard kit feeds some signal processing, Crown
CDi1000 amps, and finally QSC HPR 152i's (will double-check that model #
this morning). What I'm hearing strikes me at first as crossover
distortion.

It's taking some time to get into the system as the heart of the
processing is all computer-managed. Now have availble the software, the
right laptop, etc., to try to dig in further.

Thanks to everyone for the helpful comments, and I'll be reporting back
as we dig further into it.

--
ha
Iraq is Arabic for Vietnam

hank alrich <[email protected]> wrote:
>
>There's a weird distortion over certain types of signals, at levels well
>below what should be possible without distortion.
>
>We're going through the system step-by-step to try and pinpoint the
>source. I'll likely be back with questions as we proceed.
>
>An A&H MixWiz and outboard kit feeds some signal processing, Crown
>CDi1000 amps, and finally QSC HPR 152i's (will double-check that model #
>this morning). What I'm hearing strikes me at first as crossover
>distortion.

Disable all the signal processing, then swap out the amps. Check the
diaphragms in the speakers by attaching a known-good amp and a CD player
to them.

Amplifier crossover distortion, bad connections causing rectification at
low levels, and damaged diaphragms in compression drivers, would be my
first guesses.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"hank alrich" <[email protected]> wrote in message
news:1ij18xc.1t83hbs1iqo76mN%walkinay@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

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! :)

Soundhaspriority wrote:
> "hank alrich" <[email protected]> wrote in message
> news:1ij18xc.1t83hbs1iqo76mN%walkinay@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
>
>

The fundamental of the bitrate is an
order of magnitude over the audio passband, so *so
long as everything is terminated properly*, there shouldn't
be much crosstalk. But it is certainly not best
practice. And any crosstalk might end up
eaten by any CMRR in the audio equipment.

But then again, it could easily be intermittent. Not best
practice, for sure.

--
Les Cargill

"Les Cargill" <[email protected]> wrote in message
news:4861784d$0$18113$9a6e19ea@unlimited.newshosting.com...
> Soundhaspriority wrote:
>> "hank alrich" <[email protected]> wrote in message
>> news:1ij18xc.1t83hbs1iqo76mN%walkinay@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
>
> The fundamental of the bitrate is an
> order of magnitude over the audio passband, so *so
> long as everything is terminated properly*, there shouldn't
> be much crosstalk.

Not that all things are equal, but the fundamental of GSM is about ten to
the seven over audio, yet it makes audible interference.

Bob Morein
(310) 237-6511

"Soundhaspriority" <[email protected]> wrote in message
news:sp-dnXT784lBDvzVnZ2dnUVZ_h7inZ2d@giganews.com...

> Not that all things are equal, but the fundamental of GSM is about ten to
> the seven over audio, yet it makes audible interference.

Thats because of the frame rate of GSM, which is around 200 Hz.

Meindert