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This topic comprises 3 pages: 1 2 3
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Author
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Topic: Chasing hum bars?
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John Hawkinson
Film God
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Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-28-2009 09:19 PM
Background: Today I hung around and watched while a bunch of electricians, audio engineers, designers, consultants and technicians tried to troubleshoot a persistant problem with hum bars in a 400-seat auditorium that they recently installed here (no film, sadly). They failed.
I'm mostly curious what some tips and tricks might be for dealing with this.
To be clear, when I say "hum bar," I mean a horizontal bar of luma (lighter or darker than the image) that either scrolls up or down continuously, or just stays in a fixed position, in a video image.
This room has a particularly complicated setup. It's got video (and audio) inputs scattered around the room, both composite and VGA. Generally speaking they go into Extron distribution amps (or VGA extenders) and then run on 5-BNC cables up to the booth, where they and a bunch of other video sources go into an Extron switcher. Outputs from those switchers go to three seperate scalers, each of which drives the 3 DLP projectors in the room.
Some of the VGA inputs are in floor-mount boxes, several of them are in a rack at the front of the room with a Dell in it, etc.
The hum bars show up on all 3 projectors, and they show up differently. Sometimes they scroll up, sometimes they scroll down. Sometimes they scroll fast (2 or 3 seconds of period), usually medium (5 seconds), sometimes slow (12 seconds), and sometimes fixed. Of course, it is far worse with some sources than with others (especially laptop variation).
The per-projector scalers are all outputting 60Hz refresh signals to the projectors. One at 1280x1024, the other two at 1024x768.
AC power for all of these things are on isolated ground outlets that run to a dedicated panel for the A/V equipment in the room, though of course they don't all take the same runs to reach that point.
Most equipment in racks is not grounded with star washers.
Electricians have put power quality monitors on the 120VAC circuits and don't think there are any power quality issues.
Neutral/ground voltage is sometimes as high as 3V. Most circuits are on the same phase (not that it is clear that has an impact).
We obtained limited success troubleshooting these hum bars by running all three video projectors off of the same outlet strip, and plugging that outlet strip into an outlet adjacent the rack with switchers & scalers. Though this improved the hum bars, it did not completely eliminate them.
Ground-lifting the computer at the front of the room, along with it's DA and its local monitor, dramatically reduced the hum from that video source. But of course, that's not an acceptable thing to do beyond troubleshooting.
How do you fix a hum bar? In audio, you'd install transformers and forget about it. But what do you do about unbalanced video signals? Converting to HD-SDI or fiber is a really pricey proposition. Jensen sells a $600 5-BNC isolation transformer for video...installing boatloads of those might not be terrible, but is it one of the right answers?
How do you see a hum bar? We stared at (composite) video signal traces on some oscilloscopes for a while, and no one was able to see the hum bar on the scope, even though it was quite visible on-screen. Of course, no one was precisely sure what we were looking for in the scope trace, but nothing seemed to move at the same (or similar) rate that the visible hum bars moved at.
What are clever tricks to try? We began at the sinks (projectors) and started ground-lifting and isolating various devices, putting them all on the same outlet strip, bypassing components, etc. This was not very fruitful. In most cases, we could make small changes that would reduce the severity of hum bars, but not eliminate them entirely. And many of the changes were not practical from a permanent perspective ones, like "put everything all on the same circuit."
Trying to ground everything in a single-point fashion seems unmanageable in a large-scale installation like this, with lots of interconnected devices and many places where chassis grounds touch parts of the building, and 120VAC circuits for things at the front of the room don't run through the booth the way the video (and audio) signals must. Thanks for any thoughts.
--jhawk
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John Hawkinson
Film God
![](http://www.film-tech.com/ubbmembers/1135b.jpg)
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-28-2009 09:59 PM
Thanks!
I should stress, by the way, that this isn't my room/project and I'm mostly an interested onlooker, and not really a Video Guy. But that said:
Steve, I buy your argument for why IG outlets may not help. But why do they hurt?
Generally, speaking, most of the Extron extenders/DAs are mounted into the metal frame of the junction boxes and the various racks they are in. You say ground the interface with your video rack ground -- how? Run a seperate ground 100 feet from the box to the booth in the same conduit as the video signal wires? Why should that be any better than the shield of the BNC connectors?
I'm not sure how you bond everything throughout a large room-wide installation any better than, say, the chassis bonding that you already get through EMT, etc. (Perhaps that's less relevant if nothing is actually touching the non-IG ground of the various outlets?)
Privately, someone asked me about video humbuckers. I'm told they were tried and didn't help. Though I'm not really sure what they are. Chokes I guess? Though are they available for RGBHV (5-BNC) signals?
In re troubleshooting, maybe it's the same, but in video, you can't tell if you have a hum without a valid signal present, which makes it a fair bit more annoying than in audio, where you can unplug and dead short cables and boxes and just listen. And then add to that that 5xBNCs are a fair bit more effort than an XLR or a TRS to disconnect, and it quickly becomes quite time-consuming. Maybe they should focus on composite video to start with...
--jhawk
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John Hawkinson
Film God
![](http://www.film-tech.com/ubbmembers/1135b.jpg)
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-29-2009 11:41 AM
I assume you mean the 7.5IRE black in SMPTE color bars, not some setup area or sync pulse on any old signal? I definitely didn't zoom in and stare at that section.
Ironically, of course, on the projection screen, bars are most visible in WHITE areas, not black areas.
I don't understand your triggering comment -- in a composite video signal, if I was triggering on the hum, then I would see the entire rest of the signal wandering around with the period of the hum, right? If I see a stable waveform, I can't possibly be triggering on the hum, since the hum is a component of that waveform.
line lock? Like triggering on the AC Line? Or do you mean triggering on a particular NTSC Video Line number?
Thanks again. I wish there were some good references for debugging video. I'm sure there must be books and magazine articles, maybe I should spend some time at the library, the Internet is not-so-helpful for this stuff.
--jhawk
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John Hawkinson
Film God
![](http://www.film-tech.com/ubbmembers/1135b.jpg)
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-29-2009 04:43 PM
Interesting point, Bruce. Apparently this problem has been going on for months, (I only heard about it recently), and originally the circuits were more evenly distributed, and somebody went and changed them around to try to help solve the problem (of course it did not help).
(There's definitely a lot of belief in "on the same phase" out there; I can't say I've seen a good solid debunking of it...)
One of the electricians did show me his phase balance measurements and I can't remember exactly what they were, but I think they were something like 30, 17, and 9A.
That said, is the neutral/ground voltage that significant a problem? I guess it depends on how much leakage there is from neutral to ground in the various devices?
--jhawk
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