Film-Tech Forum ARCHIVE: Buzzing during trailers and blue band

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Posted by Ken Lackner (Member # 1002) on 11-03-2003, 12:31 AM:

I screened Borther Bear Thursday night. During almost every singe trailer, there was a brief low-pitch buzz just before the green band. It also happened just before the blue rating band at the end and right as the changover closed and the sound switched back to non-sync. The fact that this occured during the trailers and on the feature print leads me to believe that this was a processor issue and not a print issue. However, this did not happen with the two other prints we had playing in this house on Friday, nor did it happend again with Brother Bear on Saturday. The processor is a CP-650. Since it is digital, does it ever develope glitches that are fixed simply by rebooting, much like a PC? That's the only thing I can think of. This isn't a problem anymore, I would just really like to know what it was.

Posted by Carl Martin (Member # 1146) on 11-03-2003, 04:51 AM:

did it happen at the reel changes too? maybe something goofy when the splices go through? is there an end of show cue covering the digital track on the blue band?

carl

Posted by John Pytlak (Member # 331) on 11-03-2003, 06:00 AM:

Don't know what would cause that symptom in digital.

But if the problem had occured while playing analog sound, you may have had images of the perforations or framelines close to the scanned area of the analog soundtrack. If your reader was even slightly misaligned, it may have picked up the extraneous image, resulting in a "motorboating" buzz. I blame the reader mainly because all the trailers were affected. If only one was affected, it could have been a printing problem.

Posted by Aaron Garman (Member # 1653) on 11-04-2003, 02:12 AM:

We tend to have this problem with buzzing at reel changes or during trailers in one of our Dolby houses (DA20). At some reel changes and the trailers, the Ultra Stereo drops to stereo and the buzz occurs. I did a test and ran a film in just stereo with the DA20 off and the buzz did not go away. My guess is it has something to do with the alignment of the soundhead. I've let people know at our place, but since the films run digital all the time, they don't see it as "important." Of course, when the buzz happens, it is extremely loud and distracting. Apathy is going to be the death of the commercial movie exhibition industry! [Mad]

AJG

Posted by Scott Norwood (Member # 30) on 11-04-2003, 04:53 AM:

What sound format do you use for trailers? If digital, then does the buzzing happen with trailers/features which don't have digital tracks (or which are intentionally run in optical)?

It sounds like an A-chain alignment issue.

Oh, and I have seen a few random issues with the CP500 which require reboots to fix...don't know about the CP650.

Posted by Manny Knowles (Member # 1171) on 11-04-2003, 11:54 AM:

Have you measured the frequency of the buzzing sound? I once had a buzzing sound that lit up the 60Hz band on my RTA. I forget what *exactly* the problem was but I do recall having to bring the electrician back into the building.

Do you hear this noise when NOTHING is playing in either digital, analog or non-sync? If the problem affects everything then I would soonest suspect a problem with grounding or electrical interference.

As John P. pointed out...the odds of having trailers and a feature with the same defect are slim. However, it is entirely within the realm of possibility that the entire print may have suffered some damage due to a misthread or equipment defect. In such a case, the damage would be uniform throughout the entire program. A close physical examination of the soundtrack could be a great help.

Does the problem happen with other prints played on this system? If so, then the system is suspect. If not, then the "noisy" print itself is suspect.

Does the problem follow the print if you play it elsewhere? If so, then the problem *definitely* resides with the print.

Does the noise affect Digital or Analog playback? I agree that it sounds like an analog A-chain issue but that is merely an assumption. If this is happening on a Dolby Digital soundtrack then I would be inclined to rule out the reader because my understanding is that the SRD track will either play or fail-and-revert. I would start looking for the problem further down the line if you are hearing this in digital.

If you are certain that the problem happens ONLY at the tails of things then you should check to see if the CP650 tends to revert to analog at these points. This would take us right back to assuming a problem with your analog A-chain.

Can this buzzing sound be observed during quiet passages during the actual feature? You might attempt to measure the frequency of the noise -- after all, you know where to look for it -- and then monitor playback of the entire show using the RTA to see if that frequency stays locked "up" throughout.

I recently responded to a report that described a noise which supposedly only affected "parts of the movie" and it turned out that the noise was present throughout the entire show -- trailers and all -- but louder sounds were masking the offending noise. As you might have guessed, there was a problem in the analog sound head. A small clump of dirt on the edge of the impedence roller was causing a rhythmic "thump-thump" sound which was only noticeable during quiet passages, even though it was there all the time.

There was another case where a constant "crackling" sound was caused by thousands of tiny horizontal cinch marks all through the print. Again, it was only a problem during quiet passages.

In the situation that Ken is describing, the ends of trailers and the blue-band area are usually silent. It could simply be that these quiet passages afford a better listening opportunity for a noise that is actually constant.

Posted by Jeffry L. Johnson (Member # 453) on 11-04-2003, 02:30 PM:

Carefully examine the analog soundtrack. Sometimes I see very low modulation audio in some of the black leader sections that should be silent.