Film-Tech Forum ARCHIVE: Soundtrack ticks and hums

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Posted by Kyle Thiessen (Member # 1337) on 06-28-2002, 06:47 PM:

Hey, gang...

Long-time reader, first-time poster.

I'm currently working in a non-union IMAX booth, and per IMAX Co.'s contract, this booth must have its own designated projection staff apart from the other 20 35mm projectors also in this building. Management here seems to resent us for this, so we make sure that everything runs flawlessly -- not only because we actually have pride in what we're doing, but also to validate our postion here.

Long story short, we too have a 35mm projector in here. Now, it's bad enough they have us running a 7,000 watt bulb in a 3,000 watt Big Sky lamphouse, but I've noticed recently we're also having sound issues.

The problem is this: We run in Dolby Digital. And I've been noticing when I walk down into the auditorium that there are faint ticks and warbles in the soundtrack. They are quick, sporatic and subtle, but are most prominent during long, soft notes in the musical score. The best comparison I can offer is that the noises are similar to those found in poorly-encoded mp3s. It's not an issue with the print; I've noticed this in "Lord of the Rings," "Spider-Man" and now "Minority Report." Does anyone happen to have a clue what this is?

For all practical purposes, I'm self-taught, so forgive my ignorance.

kyle t.

Posted by Darryl Spicer (Member # 711) on 06-28-2002, 06:58 PM:

The issue may be in the Digital reader. Is it a penthouse reader or a basement reader. Seems we talked about an Issue similar to that sometime back. Did you try to do a search using Dolby Digital as the search words. I know this may pull up a lot of information to look for.

Posted by Brad Miller (Member # 2) on 06-28-2002, 07:24 PM:

That sound is your SRD dropping out. When you are at the extreme high end of the error rate (starting at 7) you will hear that, even though the sound will not actually drop to analog. In this instance, you are better off running in another digital format or in SR analog.

Do note that your entire problem may only be the alignment of your SRD reader. Definitely have your tech come in with his oscilloscope and re-align it.

Posted by Steve Kraus (Member # 476) on 06-29-2002, 01:21 AM:

You might not even be having a high average error rate (as shown by the number) but frequent severe errors causing instantaneous failures (watch the error display for momentary F flashes). It's not enough to drop the sound into analog but enough to put a sort of tic into the sound, especially audible during music with long held notes.

Do you play Dolby Digital from a soundhead atop the projector (a penthouse) or from a scanner built into the regular soundhead below the projector (a basement reader)? Especially in the latter case a realignment by itself may not cure the problem. The F flash syndrome is generally caused by film motion instability and you may need some soundhead components replaced.