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Author
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Topic: Distortion / crackling in surrounds
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Fredrik Sandstrom
Film Handler
Posts: 63
From: Turku, Varsinais-Suomi, FINLAND
Registered: Mar 2014
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posted 04-11-2016 07:26 AM
We just finished a four day film festival. Mostly digital, but five features were presented in 35mm, and of those one had analog Dolby Stereo sound (A-type), and this revealed a problem that I'm currently investigating.
This particular film was run in changeover from 6000' reels, and after the changeover I started noticing some disturbing distorted crackling type sound in the right surround channel only, especially in conjunction with loud sound effects, but also (less noticeably) in loud music. Listening with headphones through the booth monitor confirmed what I thought I had heard in the auditorium: bad distortion in the right surround, and to a much lesser degree in the left surround. (Front channels, fortunately, sounded just fine all the time.)
Now, the fact that this occurs only with the second projector would indicate an A-chain problem, and it is indeed likely that there is a fault there, because this particular A-chain has always been problematic. But let's consider that a separate issue, because there is obviously something else wrong too: Analog Dolby Stereo of course has only one surround channel, so when format 04 is selected, the Ls and Rs output should be identical, no matter what happens in the A-chain! Right?
So, the general setup is that all sound to the auditorium is handled by a CP750. Film sound (analog and digital) is handled by a CP500, and goes from there to the 8-channel analog input of the CP750.
My first thought was that the B-chain equalization of the CP500 could be messed up, such that the noise (whether it is caused by a bad A-chain or something else) becomes much more pronounced in the right surround channel than in the left. This turned out not to be the case. The B-chain EQ setting is flat in all channels. (Room equalization is of course then handled by the CP750.)
My next step will be to disconnect the CP500 from the CP750 and find out if the difference in Ls/Rs output is already present in the CP500 output, or if it happens in the CP750. (But I need to find/make some cables that will let me do that...)
Ideas on other things to check would be much appreciated!
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Tony Bandiera Jr
Film God
Posts: 3067
From: Moreland Idaho
Registered: Apr 2004
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posted 04-11-2016 09:55 AM
Ok, first things first:
Since you have a CP750 (which means you also have D-Cinema installed, correct?) you need to conduct some tests with dcp material to see if the problem exists in the CP750, amplifiers or speaker array... there are test tones available that would be very helpful for this (Dolby's Channel Identification test for example. Otherwise, you can play back a dcp that you are familiar with.)
If the CP750 is at fault, you will hear the problem in the "processor" or "line" side selection of the monitor. (If your monitor has the ability to monitor the line side as well as the speaker (amplifier out) side of each channel.)
If the processor side is clean but the amplifier out is distorted, the amp is bad.
If that all checks out ok, then look at the CP500 or "A" chain of the affected projector.
My bet would be on a fault in the CP500 itself since, as you pointed out, the surround in Dolby analog is mono. Possible bad cards in the CP500 could be the optical preamp card (CAT661) (my #1 suspect), the 2ch input switch card/A to D convertor (CAT 681) or the first one of the CAT 675A cards. (To eliminate the CAT 675A card, you can power down the CP500 completely, and swap the two cards (in slots J12 and J14). After the processor reboots, test the film sound again. (The cards are electrically identical, it is the processor's software that sets the function of the CAT 675A to be either the Pro Logic Decoder or EQ in accordance to the slot it is in.)
The main reason I feel the CP500 is the problem is due to its age and it has been known to have some odd quirks.
Let us know what you find.
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Fredrik Sandstrom
Film Handler
Posts: 63
From: Turku, Varsinais-Suomi, FINLAND
Registered: Mar 2014
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posted 04-12-2016 01:00 AM
quote: Leo Enticknap
I can't see how the problem can be anything other than:
1 - The photocell or related electronics in that projector, e.g. alignment;
2 - The connection between the projector and the CP500's optical preamp, or;
3 - A fault in the optical preamp affecting the processing of the signal from one projector only (e.g. something to do with the changeover switching mechanism).
I'm sure the underlying reason for the noisy surround is related to one of these things. Like I said, this A-chain has always been problematic. I have discussed this with some of you in the past; we have ACL lasers that are a pain to align and don't sound very good even when all the alignment tests check out fine. I'm looking into replacing them with Kinoton reverse scan readers. But even if I do that, the real mystery here remains and needs to be resolved: How can there be a big difference between the Ls and Rs output? That, as far as I can see, cannot possibly be related to any of the points above. No matter how bad the A-chain is, you should get the SAME crap out of both surround channels. THAT is what I primarily want to get to the bottom of here.
Thanks for all your pointers so far, I'll let you know what I find out! Unfortunately the film that caused this to be discovered needs to be sent back, so for further testing I need to find a trailer or something that exhibits similar symptoms.
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Leo Enticknap
Film God
Posts: 7474
From: Loma Linda, CA
Registered: Jul 2000
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posted 04-12-2016 10:12 AM
quote: Fredrik Sandstrom
No matter how bad the A-chain is, you should get the SAME crap out of both surround channels.
Good point, and one that rules out 1 and 2, and narrows this pretty conclusively to the optical preamp and/or Dolby phase matrixing circuitry of the CP500. There are only two signals being read in the projector and sent to the CP500, from the left and right variable area tracks on the film. If there were a fault in the projector and/or connection affecting the pickup of one of the two tracks on the film, it would have to affect the left or right (front channels) as a bare minimum, and probably center and surround as well, given that they are phase matrixed from both LT and RT. In fact, if one of the tracks was not being read correctly, then only one of the four matrixed output channels (either left or right) should be totally unaffected.
It therefore sounds like (excuse the pun!) the gain issue mentioned by Sam and Louis is the obvious place to start looking.
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Randy Stankey
Film God
Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99
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posted 06-19-2016 03:19 PM
quote: Sam D. Chavez
First, I would run Dolby Tone film on each projector. Calibrate the levels for Dolby Level. I bet one is higher than the other.
I agree with Sam. Try the easy things first.
Run Dolby Tone, buzz track and crosstalk/alignment tests on both projectors to be sure that they are properly aligned and agree with each other. Use an oscilloscope if you have one. (You'll need one to do the crosstalk test.)
Maybe this won't solve the problem but it's a test that you can run in fifteen minutes, between shows.
If it works, you're done or, at least you can get through your show schedule until you have time to do a full set of tests on the projectors and processors.
If it doesn't work, or if it only partially solves the problem, you have eliminated one possible area so that you can better focus your efforts elsewhere.
Again, it's quick and easy. I consider it a first-in-line step for troubleshooting sound because nothing else in the processor will work right if your projector A-chains aren't right.
And... Yes, a bad A-chain can exhibit intermittent problems if something is loose or slightly out of alignment.
It could also be operator error if he didn't thread the projector correctly. I know that I've made mistakes like that.
Yes, it could also be some part or circuit board starting to go on the fritz.
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