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Author
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Topic: Stage speaker strange noise
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Mike Olpin
Chop Chop!
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Posts: 1852
From: Dallas, TX
Registered: Jan 2002
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posted 05-04-2008 09:12 AM
quote: Muhammad Husni 1. There were humming sound on the stage speaker. What could be the actual cause and how to overcome it?
This is probably a ground loop. It tells you that one of the components is not properly grounded. Make sure all equipment is plugged into a 3-prong outlet and no three-to-two prong adapters are being used. If you only hear it during the non sync, you can purchase an inexpensive RCA ground loop isolator.
quote: Muhammad Husni 2. Hall #1 is now showing a movie and the output can be heard on stage speaker of hall #6. The distance is about 3-4 halls and the sound can be heard clearly, only the level is a bit low. How can this happen?
That is very strange indeed, and if true, would indicate an epic wiring problem. But I have seen this illusion occur if two theatres are positioned directly across from one another. The sound passes through the door of the auditorium, across the hall, then into the door of the theatre across from it. The way the sound funnels in and then reflects of the screen really makes it sound as though a faint bit of the other theatres audio is coming through the speakers even though it is not. Theres an easy fix for this, just close the auditorium doors.
quote: Muhammad Husni 3. While running a DTS trailer in SR mode, there were popping sound on the subwoofer. But if we were running it on a DTS mode, there's no such sound.
DTS is a digital format, and not subject to much extra noise beyond what is present in the soundtrack. SR is analog, and is subject to noise, especially if the print is damaged or warn out, contains improper splices (make sure your splices aren't leaving a small gap) and alignment issues. Make sure you clean your sound head with a dry q-tip periodically and have your technician check its alignment.
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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 05-04-2008 11:52 AM
Hi, Husni, welcome to Film-Tech!
I'd suggest you'd be better off posting seperate questions in seperate threads (maybe not all on the same day!), otherwise some of your questions may get lost amidst the discussion of the others.
1. Hum can have many causes. Grounding problems can be one, but there can be others. Usually it is because unbalanced wiring is not robust over long distances, and most equipment uses unbalanced wiring. The absolute best answer is to convert your unbalanced signals to balanced signals, ideally with a transformer. How far are your amps from your processor? Does the hum happen when the processor is off? Does it happen when you change sources? Does it happen if you turn off your analog sound power supply?
2. This is definitely weird. If the wiring runs go nowhere near each other, and it's not crosstalk, when perhaps you are having some kind of RF-related problem. Are you near a powerful radio station transmitter? In any case, you should debug the problem scientifically. Which channel of Hall 1 is causing the problem? Can you hear the problem with Pink Noise? Is it frequency dependant? Is the signal originating before the amplifiers or after them? Turn off the amplifiers in Hall 1 and see if the sound continues to travel to Hall 6. Where in Hall 6 is the sound being picked up? Is it in the cinema processor? Is it between the processor and the amps? Is it between the amps and the speakers? Does it happen in non-sync or just in film? In DTS as well as SR? Turn off and disconnect individual components in the system until you isolate the source. Then report back and we can hopefully give you some better recommendations on where to procede.
You mention the sound appears in the Center (LF *and* HF sections?) the Right LF and the Left HF. Tell us more about your crossovers.
Also, are these problems recent? Did they start when other changes were made? Maybe there is a hint as to what is wrong based on other factors.
3. DTS is absolutely a digital format. When we talk about "digital," we mean many things, but usually we mean any of Dolby Digital (aka SRD), Digital Theatre Systems (DTS), or Sony Dynamic Digital Sound (SDDS) -- three ways of encoding film sound without using the analog optical track of the film. But the point really is that SR and DTS use a different signal path for sound to get to the processor. So if you have a problem with noise in the subwoofer in SR, it tends to suggest that the problem is not with the amplifier or the subwoofer itself, but in the cinema processor or beforehand. But it's strange that you would see analog problems manifest only in the subwoofer, and not in the other channels, because as you correctly point out, SR is not discrete.
It is misleading to say that SR is only two channels. There are two channels encoded on the film, but your cinema processor decodes those channels into 4.1: left, center, right, mono surround, and a subwoofer channel.
Are you sure that your subwoofer is actually working in DTS? Do you hear the problem if you play the DTS Empiral Test CD? Do you hear it with pink noise on the subwoofer channel?
--jhawk
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