Anyone experienced this type of sound problem before? (1 Viewer)

[Mike Huey]

Saw "The Dark Knight" yesterday in IMAX again. The first time I saw it the whole production was flawless. Yesterday, there were sections of the film where the audio seemed to dropout with loud stuttering/popping sounds most prominently in the surrounds. It didn't seem to be a speaker problem, but I've never experienced this before with digital sound in a theater. It was almost like the CD's were skipping. Does IMAX run sound seperately like DTS or is it included on the film strip?

Anyone know what it could have been?

 

[Phil A]

I've actually had a tour of the projection room at the theater at the Smithsonian as a friend I was with used to work there. This should give you some good info:

IMAX - Wikipedia, the free encyclopedia

“In order to use more of the image area, IMAX film does not include an embedded soundtrack. Instead the IMAX system specifies a separate six-channel 35 mm magnetic tape synchronized to the film. (This original system--35 mm mag tape locked to a projector--was commonly used to "dub" or insert studio sound into the mixed soundtrack of conventional films.) By the early '90s, a separate digital 6-track source was synchronized using a more precise pulse generator as a source for a conventional SMPTE timecode synchronization system. This development presaged conventional theatrical multichannel sound systems such as Dolby Digital and Digital Theater System. This digital source came in the form of a unit called a DDP (Digital Disc Playback) in which the soundtrack was recorded onto multiple CD-ROM discs which would play the sound which was recorded to the discs as a digital audio file. This DDP system has been replaced in almost all theaters with the newer DTAC (Digital Theater Audio Control) system which utilizes a computer running the IMAX's proprietary DTAC software. The software works in a similar style as the DDP except that instead of the audio file being based on discs, it is instead played directly off a hard drive in the form of a single uncompressed audio file containing the 6 channels which are distributed directly to the amplifiers rather than using a decoding method such as Dolby Digital. Many IMAX theaters place speakers directly behind the screen as well as distributing the speakers around the theater to create a three-dimensional effect.
“

 

[Leo Kerr]

In short, which isn't terrifically clear from the Wiki quote, the audio in most updated theaters resides on a hard-drive.

The first 35mm tape machines I saw in the early 1990s - first time I was up in the NASM booth - it was SMPTE locked to the projector's spindle drive. As was the 8-track 1/2 inch tape machine playing translations and assisted-listening audio to the RF transmitters.

At that time, and at least the last time I was up in a booth, there is no time-code or tracking info of any sort on the film, except for the "start frame" that gets loaded into the gate, and then the whole SMPTE system is locked to the drive shaft. (Which is a good thing, actually. The drive shaft won't "slip" a frame or a fractional frame without anything pretty disasterous happening to the projector.)

My understanding of the original DDP system was that they were three CD players - not CD-ROM players - that had some basic SMPTE tracking. Except you couldn't regenerate the code and have it work. This mean that if the program was interrupted, the only sound that (a) work and (b) have a chance of being in sync -- AT ALL -- with the picture was the 6-track backup mag tracks.

It sounds more like a amp/power/signal processing chain problem to me, but they're all fairly stable systems...

Leo

 

[Mike Huey]

I talked with the theater, and apparently it was an amp problem. One of the amps was dying and they apparently stayed up till 5am to try and isolate the problem. Wonderful service and dedicated people. Despite the somewhat soured second experience, the first time I saw it there was flawless.

Another interesting tidbit is that they told me that pretty soon all IMAX theaters will be switching to digital from film over the next year or so. Does that have any impact on the sound? I can't imagine the image having more impact in digital than film, especially considering the size of the screen...

Any thoughts?

 

[Leo Kerr]

They will be going from a 65mmx15 perf absolutely rock solid frame during projection, which, in the glory days of Imax, was generally beautifully shot, edited, and released, to low resolution digital projection.

This is an improvement?

It'll certainly be cheaper. But going from a frame that had the potential for about 13,000x10,000 pixels to a frame that's hard limited to 1920x2160 (that's what I've heard, indirectly,) doesn't seem like a real good idea to me.

Heck, NHK is doing better than that with their UHDTV, at 7680x4320, with 22-track sound.

Leo

 

[JeremyErwin]