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major lighting problems

Discussion in 'Lighting and Electrics' started by mr_sound, Aug 3, 2004.

  1. mr_sound

    mr_sound Member

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    so saturday night was the first real show i've done with my lighting rig. i had it setup for weeks before the show when i was programming everything and it all worked fine...with only a few minor exceptions. but during the show, the lights would all flicker insanely at certain times, but then work fine at others, and then without warning start flickering again. i managed to push it through the entire show, but now i have to figure out why it was doing this.

    after the sound crew loaded out, we tried it again, and the lighting was still doing that, so i know it had nothing to do with them..although it did seem to coincide with the heavy bass. i came in last night to spend some time and just take things out one by one until i can figure out what it is......except the problem didn't come back, at all. i still tried reworking some things, i tested all the cables, and i still couldn't get the problem to come back.

    i'm 99% sure it's a control issue, since the entire rig is on 4 separate circuits, and all the lights would flicker. although a followspot i had on one of the same circuits did not flicker, since it wasn't controlled from dmx. i didn't have a dmx terminator on, but the rig had worked before without one. i think the problem may have been the fact that i'm using mic cable. it's high quality mic cable, belden, and i made sure the sleeve of the connector wasn't connected to the cable sheild. but just for the hell of it, i went out and bought a resister, and rigged up a terminator, put that on, and everything still worked fine. i'm already planning to buy real dmx cables. do you think this could be the problem or might it be something else? the sound guy thought it was a loose cable, but i checked all the connections and nothing seemed to change the situation at all.

    i'm using a behringer board, with american dj dimmers. i know behringer and ADJ aren't the best...but do you think these could be the problem? has anyone had a problem like this with these before?

    another thing i've noticed is that even when i've got the master fader down, some lights on stage will still flicker, often to the bass beat. the dmx line is running right next to a whole bunch of heavy duty power cables...but that's pretty much the only route. all the cables have to be hung over the doorway, and there's really no other route not going along the power cables. i'm running the dmx on a separate mic cable, and not through the snake...although we have had other people bring in lighting and run it through the snake, and on regular mic cable, and they've never had a problem. of course they also only have 1 dimmer...i have 5.

    any ideas? i need to get this figured out without having to setup the rig again, since that takes a lot of time, and it's a waste if i'm not doing a show.
     
  2. DMXtools

    DMXtools Active Member

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    I see several problems, starting with the lack of termination. DMX signals are high-frequency pulses. Pulses behave differently than DC or low-frequency (audio) signals. One of the things pulses DON'T like is a change in the cable impedance. DMX cable is supposed to be 120 ohms, but generally anything between 90 and 150 will do. The problem is that when the pulses get to the end of the cable, i.e. the last dimmer, if it's un-terminated, the pulses see that as a change from 120 ohms to near infinity. They hit the end of the line like a rubber ball hitting a brick wall, and bounce! They start traveling back up the cable toward the controller, interfering with the pulses the controller is still sending out. How much interference will depend on the total length and where, along that length, each dimmer is located. Put a terminator at the end of the line and it looks, to the pulses, like the cable just goes on forever. The pulses don't bounce, so there's no interference from reflections.

    Interference is cumulative. In a clean environment, with the system set up in your basement for programming, there might not be enough to screw up your lights. Run the signal next to power lines and it adds some interference. Heavy bass out of really big amplifiers can cause the power-line voltage to sag a little, decreasing noise margins. And neither American DJ nor Behringer have very large noise margins to begin with.

    I think terminating the DMX line is an excellent first step, and may be all you need. If that doesn't completely solve the problem, I'd invest in a small computer-backup power supply - the kind that switches to batteries if the power falls below the normal voltage - to run the Behringer board.

    John
     
  3. mr_sound

    mr_sound Member

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    would there be any complications with putting the board on a separate circuit from the lighting? i can probably manage to get it on the same one as all the sound FOH gear, which doesn't use up nearly as much power.

    the programming was all done in the same environment, with the dmx run next to the power cables. only difference between that time and the show, was that we did add more cables for the sound power, but like i said, it was still doing the same thing even after the sound was loaded out. i even tried it with the a/c on and off to see if that made a difference....but it didn't. the power in this place is not the best....since the building is about 80 years old, but most of the power has been upgraded since.

    if it was a terminator issue...what exactly would this look like? i've heard it being called mirroring, and the interference i get doesn't look like it's being mirrored. another odd thing......if i'd periodically turn all the lights off for a couple seconds, and bring them back on, it seemed to help a little.

    i've already made a terminator, and bought some real dmx cables, but i want to be sure this solves the problem before i set it up for another crucial show...and make a fool out of myself. it's hard to get people to hire you when you can't get your own rig to work.
     
  4. DMXtools

    DMXtools Active Member

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    What it would look like depends on how far from the controller you are. I've only got pictures of the signal right at the controller, and only driving 100 feet of Belden mic. cable. DMX is a differential signal, so the pictures were taken with the scope channels summed and the channel B input inverted. Channel A was connected to XLR pin 3 and channel B was connected to XLR pin 2, so the scope trace above center means pin 3 is more positive than pin 2. With the terminator in place, the leading edge of a single pulse looked like this. When I removed the terminator, it looked like this. The thing is, the farther from the controller you go, the worse it gets.

    John
     
  5. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    There is the chance that you could introduce some noise into the audio system but I doub't that there would be much of a problem. As a general rule I run a line filter for my audio to protect against this. However, where ever possible I run them on seperate phases. Try it an see.

    Before I learned the importance of using terminators I had an experience with some scanners in which the mirror would "twitch" when in a static position. It drove me nuts until someone suggested that I terminate the DMX signal at the last unit. I did and the problem went away.

    This is the only time I have actually "seen" the problem.
     
  6. mr_sound

    mr_sound Member

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    my scanner used to twitch as well. i figured since i was only using it sporadically, and eventually i did away with it completely, it wasn't a big deal. i always thought that was because of the quality of the cable i was using.

    well, quick question about the terminator...i've heard that i should get a 120 ohm 1 watt resister...but all i could find was a half a watt i beleive. will this make it an issue? it's still 120 ohms.
     
  7. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    From what I understand that should be fine. I am pretty sure that a 1/4W resistor is fine.

    Do a Google search and see what others are saying/using. If you do a search of the forums here for "terminator" you will probably find some info or links
     
  8. DMXtools

    DMXtools Active Member

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    Worst case, the terminator will see a maximum of 0.208 watt... and that's only if things go dreadfully wrong. Typically, it'll be about half that, so a quarter-watt resistor is perfect.

    John
     
  9. rgsw

    rgsw Member

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    no dmx-terminator can cause crazy things however mad and far-fetched it can be.
     
  10. Les

    Les Well-Known Member

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    Since you are using mic cables, I would switch to true DMX cables just for good measure. Some mic cables have the ground tied in with pin 2 while others ground through pin 3. Also there is a definate problem with running DMX next to any power supply or extension cord, even on a temporary basis; because the higher voltage can actually jump to the DMX cable and interfere with the signal.
     
  11. DMXtools

    DMXtools Active Member

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    Unless the insulation on both cables is pretty well shot, the voltage won't "jump" from one cable to the next (an arc). However, if the power cable or extension cord is carrying a lot of current, there is a possibility of electromagnetic coupling - the two parallel cables acting like a transformer. Such coupling follows an inverse-square law. That's a fancy term meaning if you double the distance between the cables, you get 1/4 the interference. It doesn't take a major move to get a big decrease in interference, if that's indeed the problem.

    In my system, I'm running DMX through one channel of my snake. The snake runs 100 feet, right next to an extension cord I use to get the band's instrument amps on the same circuit (and ground) as the mixer. No interference problems ever. But If I ever forget to terminate the DMX line, the lights go crazy... and moving the snake away from the extension cord doesn't help.

    John
     
  12. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Hi John – just wondering if I understand this scenario correctly.

    Is the reason fro running the extension lead to overcome the possibility of the equipment sourcing the same ground through different paths? My understanding is that this is one of the ways in which a ground loop can be created.

    Also, can this occur when differences in audio/signal ground and power potential ground exist- or am I now confusing myself :?

    Cheers.
     
  13. DMXtools

    DMXtools Active Member

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    The answer is C - all of the above. In my case, the audio signals from the mixer, effects rack, etc. to the power amps are transformar coupled. I don't have to worry too much about ground loops if the amps are on a different circuit. However, if I take a direct output from an instrument amp on-stage as an input to the mixer, I can pick up some ground noise there.

    But the main reason is safety. Some of the venues I work are pretty old. For electrical upgrades they simply added on, leaving existing outlets on existing service and running a new service into the building for the new outlets... in one case on the opposite side of the building from the existing service. The earth ground is in a physically different location. This provides the opportunity for a fairly large potential difference between the two grounds. If a musician, grounded to one service through his instrument, then touches a microphone, grounded to the other service through the mixer, the show can get a little more exciting than I'd like. By making sure all the instrument amps and the mixer are on the same circuit and ground I reduce the possibility.

    I've gone to wireless vocal mics for the most part. Because they aren't connected to ground at all, that eliminates the possibility entirely. However, there are one or two singers who think they're Roger Daltrey and insist on a wired mic. Keeping to my "same circuit-same ground" rule lets them touch the other musicians in relative safety.

    John
     
  14. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Thanks for the response John. My first thoughts when reading you initial post was "hang on, what am I missing here?" I thought that all circuit grounds were connicted to the same point. I didn't consider that there could be more than one ground. I think (pretty sure) that it is illegal to do that here.
     
  15. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Was doing a bit of reading on the subject and came across a rather interesting post:

    “What's this stuff I hear about "noise/hum" on the ground from the Sound Guys. I was just speaking to a Sound Guy in LA yesterday (they call him the Hum Buster) and he was telling me something that scared me; to solve the "hum" they disconnect the equipment ground wire from the electrical system and connect it to a separate ground rod.”

    There are several follow up posts by electricians etc, which are interesting in themselves. The full URL is: http://www.mikeholt.com/safety.php?id=unformatted/7-9b-99&type=u&title=Audio%20Hum%20(07-09-99)

    I have seen people over here cut off the earth pins on the plugs of (all but one piece of) equipment mounted in a rack. The theory was to force all the equipment to earth via the rack, thus eliminating the problem. I didn’t post this initially as I think it is dangerous and I don’t actually see how it would make any difference. Most people who have equipment in a rack will have all the plugs in a power board, thus sharing the same earth. Again – perhaps I am missing something here.

    However, after reading the post quoted, I am wondering how much we know and how much is actually based on misinformation or myth.
     
  16. mr_sound

    mr_sound Member

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    I hate to beat a dead horse, but for those who may have had this problem (I can't be the only one) I have figured out the problem. After getting real dmx cable, and putting a terminator in, I had my rig setup early for another show, and as soon as I started playing music the lights started to flicker again. I finally figured it out....a faulty dimmer pack. It's probably just a problem with the connection, but either way it's been taken out of service. Everything worked fine from then on.
     
  17. avkid

    avkid Not a New User Fight Leukemia

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    can i use a backup power supply to power my board long enough to get in there to shut it down?
     
  18. mr_sound

    mr_sound Member

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    I don't know if that was directed at me....and I've never actually used a UPS...but I'm pretty sure most of them can power a computer long enough to shut it down properly. I know I did see some that used USB to communicate with the computer and have the computer shut itself down in the event that the power went out....but I don't know if that would apply to your lighting board.
     
  19. zac850

    zac850 Well-Known Member

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    Yes, it should give you enough power.

    At the regional theater that I worked at over the summer they had a UPS on there Obsession light board, and I believe they said they would get at least 2 minutes of power out of it. Also, the UPS that they had would make this really shrill beeping to tell you that power had died, got on our nerves a bit when the battery started dyeing half way through the summer....
     
  20. Nephilim

    Nephilim Active Member

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    Mayhem, ship's probably able to answer this better than I am but over here venues will generally have multiple ground points, and different disconnects (and in some cases different panels) will be grounded individually. Not good if you find yourself in a situation where you absolutle have to bridge disconnects. Thus the obsession over iso-ground. I know that the HS I used to go to in SF had two seperate GPO systems, one with an iso-ground and one on the building ground.
     

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