Re: DMX Lighting Signal in Audio Snake?

On Wed, 25 Jun 2008 23:25:48 -0400, Les Cargill <lcargill@xxxxxxxxxx>
wrote:

Ben Bradley wrote:
On Tue, 24 Jun 2008 18:38:49 -0400, Les Cargill <lcargill@xxxxxxxxxx>
wrote:

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.

Not so. You're confusing a square wave at a certain frequency with
a data line at the "equivalent" bit rate. A line with data on it has a
very different spectrum than a square wave.
Generate a 250kHz square wave and it won't be audible (barring
mixing with another ultrasonic signal that generates one or more
difference frequencies in the audible range, that kind of stuff). But
a random string of 1's and 0's at a 250kHz clock rate WILL have lots
of "signal" in the audio range, and just about everywhere below the
clock freuency.

I'd be a lot more concerned if they were not differential signals.

No doubt that helps a whole bunch (that the data lines AND the
audio lines are differential/balanced).

It'd
take... 12 or so consecutive symbols of the same state to hit
the top of the passband. Not impossible by any stretch, but ...

But you don't even need that. How about a string of 12 consecutive
bits with seven 1's and five 0's, followed by 12 bits with seven 0's
five 1's? The "signal" generated by such strings will be a lower level
than with a string of 12 1's followed by 12 0's, but such a sequence
will happen much more often.

422/485 also tristate to hiZ in quiescence.

And, to clarify - I'm not endorsing this - I'm pondering "can
you get away with it?"

Tell me the capacitances between the pairs in the snake, and I'll
put it into LTspice and get a crude approximation...

But the only real test is to try it and hear. From Hank's latest
post, it looks like it's not a problem, or at least not the problem
he's trying to fix.

.

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