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Author
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Topic: Stray Pick-Up on Magnetic Sound Heads
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Steve Klare
Film Guy
Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003
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posted June 12, 2017 10:31 PM
It seemed like a good idea at the time!
The last couple of weeks I'd noticed this thin, high pitched squeal coming out of the audio on my ST-1200HD here and there. Immediately I started trying to isolate where it was coming from. I unplugged my external sound system, and it still came out of the internal speaker. I tried the standard Elmo trick of working switches, and it even disappeared but came back.
This was getting to be a desperate situation. I don't have schematics for this machine, so troubleshooting would probably come down to replacing entire assemblies and hoping for the best. This could take months!
Just then, I thought about the neighborhood this machine lives in. I basically have the southern third of the dining room table to project from. Some times of the year when I run to a third projector this might become the southern half.
My wife surprised me with a video projector for Christmas. We're enjoying it a lot and I'm working out how to integrate it into my theater. There's the projector, there's the HDMI cable, there's the video player, and the twin RCA audio cables that go to my mixer. (plus power...)
The projector sits on a low table close to the screen, but the player wants to be up with the mixer because of the cables I have.
This puts another module on the table, and puts me in danger of violating the Neutral Zone!
-simple solution: put the player under a projector.
So after the squeal started, and after all the usual tricks failed, I began to wonder if the machines were talking to each other. First thing I noticed is even with the squeal absent, turning on the player worsened the hum on the projector. The squeal also disappeared when the player was turned off.
I demounted the two machines and all is well ever since. The real solution is a long set of audio cables so the player can live up with the VP.
I should have remembered: there is a ceiling lamp above the machines. When the lamp is on and a the dimmer is used, I can hear the harmonics in the projector audio.
-those heads are listening and ready!
The field strength of a mag sound stripe is a spider fart: it takes a very sensitive head and a lot of amplification to get a usable audio signal out of it.
-sometimes you have to be a little careful what's going on close by!
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All I ask is a wide screen and a projector to light her by...
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Rob Young.
Phenomenal Film Handler
Posts: 1633
From: Cheshire, U.K.
Registered: Dec 2003
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posted June 13, 2017 02:52 PM
Steve, I think you are right in that the ST-1200 is particularly sensitive to interference regarding the audio via the monitor outputs.
I'm sure you know this, but one thing I found very helpful was to try and effectively isolate it from the dirty mains electricity supply in your house, and therefor other appliances attached to the mains.
I use an old film set 240v to 240v transformer designed to isolate big old HMI lights from mains interference.
I'm sure the thing probably isn't legal any more as it basically provides a floating earth...and I'm sure you'd know a lot more about this than me, but it was recommended by a technician at my university back in the day and I bought it for £30.00 from a film house auction.
When you power it up it vibrates and hums into life! Scares me actually, but I've come to love it.
Certainly before using it, every mains crackle, such as the oven
switching on would send bangs and hum through the soundtrack from the Elmo.
With it in action...background silence. ![[Smile]](smile.gif)
Probably a very expensive mains smoothing unit designed for hi-fi would also do the trick. Last time I looked though, around £300.00 at the very least...
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Rob Young.
Phenomenal Film Handler
Posts: 1633
From: Cheshire, U.K.
Registered: Dec 2003
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posted June 14, 2017 01:32 PM
Steve, you need the most robust and isolated output from your Elmo ST-1200/HD variant to your amplifier or mixer with good shielding.
If that means making up a home made, self soldered 2 x 2.5mm jack to 2 x Phono input cable to amplifier, with a quality a quality shielded cable just try it for a few $. With shortest possible length.
I did this.
Then isolate the projector primarily from the mains.
Then run it as you do, through EQ, in my case a 7 channel equaliser, in your case a mixer.
You can get pretty much noise free Dolby Surround from a good stereo track on an Elmo-ST1200D and with a modern day surround amplifier and some care....
I'm really not kidding either, I've made super 8 prints such as "Lion King", "Raiders of the Lost Ark", "Predator" make Blu-ray 5.1 sound amateur.
Best,
Rob.
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Steve Klare
Film Guy
Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003
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posted June 14, 2017 07:01 PM
Generally if I'm careful what else is plugged into the outlet strip and watch out for stray fields I do very well.
What I do is operate everything at the local ground(s). I avoid ground loops through the mixer by operating only one machine with an unbalanced connection. Everything almost all the way back to the amp is operating grounded by this machine. Everything else into the mixer gets a balanced connection: still grounded, yet independently.
I have 30 feet of shielded cable between the mixer and the amp (-just the way the house is laid out). The amp is ultimately grounded to our cable TV network, so I have a wicked ground loop between the two ends of the system.
So I built this interface:
-and put it in between.
The 3.5mm jack is stereo in from the mixer (or just a projector). The first stage is a differential amp. It is two terminals per channel, neither of them grounded to the amp. It just subtracts their voltages and shifts the difference to the amp's ground level. This busts the ground loop and also takes out any pickup gathered in the cable along the way.
While I was at it I built a 60 Hz. notch filter as the second stage. This doesn't really kill the projector hum, but severely wounds it. Even with a fairly hummy machine, the hum in the speakers is very low. With a quieter machine you have to crank the volume up to fairly deadly levels to hear it at all.
After that comes a low pass filter around 10 kHz to take out hiss and assorted nasty high frequency stuff. There's a rule of thumb you can't use modern speakers with cinema sound. This filter lets me get away with doing that.
After that it's just two RCA connectors so this can plug into my amp with a short cable. Basically the idea is from the point of view of the amp, this is just another component like a tape deck or a CD player. (I won't tell it if you don't!) All the cabling and the mixer and the projectors and whatever else are only isolated signal sources and their distance away is of no consequence.
[ June 14, 2017, 08:03 PM: Message edited by: Steve Klare ]
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All I ask is a wide screen and a projector to light her by...
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Steve Klare
Film Guy
Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003
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posted June 29, 2017 03:12 PM
-actually I m going to fix this by making up some cables (because that's what I know how to do! When all you have is a hammer...)
The real issue is the fact that I have this 10 foot long HDMI cable but like a 3 foot long RCA stereo cable, so I'm stuck to keep the Blu-ray player close to my mixer, and therefore the projectors.
Part of what's bad here is the RCA inputs into my mixer are kind of auxiliary ones: no per channel volume control, no equalizer either. If I don't like the sound, I really can't do a ton about it. We were watching a Blu-ray the other night and the sound was a little muddy. If this was a regular mixer channel I'd just dial back the bass and boost the midrange and treble. Instead I tried to concentrate on the movie. (It didn't work!)
These are unbalanced inputs too, so (for now) I'm stuck with plugging the video components into the same outlet strip as the projectors to avoid a ground loop.
What I want is a stereo cable, maybe 10 feet long: one end two RCA plugs, the other end two balanced 1/4" plugs (Right&Left). After that, the video system will have just as good a audio channel as the film projectors.
-plus: I'll be able to move the player up with the video projector, and plug them both into the local outlet without worrying about the room breaking out in ground hum.
It's the same every time you build some new setup: there is always something goofy you have to figure out. That's why it pays to not cast anything in stone until you've lived with it a while.
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All I ask is a wide screen and a projector to light her by...
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