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This topic comprises 7 pages: 1 2 3 4 5 6 7
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Topic: Dolby Digital WOW at reel change?
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Ken Jacquart
Film Handler
Posts: 82
From: San Francisco, CA, USA
Registered: Mar 2000
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posted 02-08-2006 09:10 PM
I want to follow up on this thread regarding the evidenced problems that started this discussion. I wish I could’ve posted sooner. I waited until we received the specific print in question along with the specific cards involved in the scenario. We tasked a few key engineers here at Dolby to look into this. First, I believe it best to post their explanation of what is going on inside a Dolby Digital film decoder (DA20 / CP500 digital section / CP650 Cat. No. 773 board). During a previous post, Lonny also nicely described its functionality. Maybe this one will also shed some light on to what is going on inside the workings of the decoder. I’ll follow up at the bottom with my own comments.
From Engineering:
The DA10, DA20, CP500, and CP650 all handle the data from the readers in the same way.
In order for the sound to stay in sync with the picture, the audio sample clock, which determines the speed of audio playback, must be synchronized to the projector speed. Since penthouse readers do not have the same inherently low wow and flutter as the analog sound head, the measured short-term projector speed will vary due to splices and platter tension variations. The playback systems therefore incorporate a low-pass filter that prevents short-term rapid speed changes from having an audible effect on the clock speed but allows the long-term average projector speed to control it.
For this filtering to work, the input data with speed variations must be stored so that it can be read out at a steady rate even though it came in at an unsteady rate, containing wow and flutter. The digital storage device used for this is called a FIFO buffer, which stands for First In, First Out. A FIFO buffer is like a bucket of water that is filled from the top and has a pipe coming out of the bottom. So long as you keep the bucket from running dry, water will always be available at the bottom and can be metered out at a steady rate. Of course, you don’t want the bucket to overflow, either, and as long as the average fill rate equals the emptying rate, the bucket will neither run dry nor overflow.
The buffer stores about a tenth of a second of audio. As long as data is taken from the output end at a speed whose average is the same as the average input speed, the buffer will neither fill up nor empty out, just like the bucket of water.
Since the output speed is low-pass filtered so that it is steady, wow and flutter are removed but the long-term speed is matched, preserving sound-to-picture synchronization.
Ultimately, however, the projector speed does govern the playback speed. Gross disturbances in input data speed, such as those caused by splices, are greatly reduced but cannot be totally eliminated by the filtering process. If a steady musical note is held across a reel splice boundary, it is possible for a small wow effect to become noticeable. Most films are mixed so as to avoid critical picture or sound material crossing reel boundaries, but sometimes the length of particular scenes or events in the film will require sensitive program material segments to cross a reel boundary. In these rare cases, small wow effects may become audible.
Back to me:
Okay, according to the above mentioned explanation, “gross disturbances in input speed are greatly reduced, but ultimately may not be completely eliminated by the filtering process”. Key word: “gross”.
Under the hopefully unlikely circumstance when a film is mixed with steady toned type audio across a reel boundary, for optimum performance we need to assume a few things.
One, there are no “gross disturbances in input speed”.
Two, the film splice is a good one. If the perforations are not cut out cleanly, and there are “hanging chads” into the perf holes, the front end PLL may lose its sync. This is because the video front end PLL obtains its projector speed sync from the rising edge of the square wave produced by the light coming through the holes of the perforations. The audio output sync (the spigot flow of the buffer bucket of bits) bases its flow synchronization by identifying the Dolby Digital block corner targets of each digital block (Each and every block contains within it four sync targets in the corners). If the splice tape being used is zebra tape and is blocking the data blocks, this also creates problems. The audio sync is momentarily lost because it cannot find those sync targets. If 4 successive blocks are not found at a reel splice, then splice cache data begins dumping into the bucket rather than the “bad” data. (If this happens outside of the reel boundary area, then the processor simply reverts to Analog Dolby SR).
Three, the dashpot is not broken. If the dashpot assembly is broken, then the entire dampening mechanism around the sound drum is not doing its job properly. Film splices and platter payout tension problems will magnify themselves into front end problems. If you have a damaged dashpot assembly, you have two choices. One, you can RXO the entire reader unit. Somewhat costly, yes. But it will be repaired perfectly. Two, if budgets constrain you, then you may choose to replace the assembly yourself. We’ve conferred with our reader design engineer here and have written instructions which should help get you close to a proper tension adjustment. It won’t be perfect, but it’s better than nothing if you don’t have a choice…
I hope this information helps the understanding of what is going on here.
I know that I have a headache now <g>
-ken
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Mark J. Marshall
Film God
Posts: 3188
From: New Castle, DE, USA
Registered: Aug 2002
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posted 02-10-2006 10:15 AM
Thanks for the explaination, Ken. And thanks to Dolby for looking into the issue. What was the result of the testing of the unit returned to you? Did the engineers certify that it was working as designed? Or did they determine that there was a problem somewhere?
I'm wondering... when the system notices a change in the speed of the perforations going by, how long does it wait to see if the speed stablizes back to the average before adjusting the "spigot" on the bottom of the FIFO "bucket"?
Remember that while the sound going across reel changes may be mixed to not include noticeable jumps in the audio, the end of the film almost always ends with some sort of "TAA-DAAAAAAAAAA" musical note, and that is where just about every theater puts some piece of cue tape. The ones that use edge sensing cue tape along the outboard edge of the film, like us, are introducing what would probably constitute a "gross disturbance" since that cue tape is about five frames in length.
Also, during musical scenes, especially near the end of the movie in the credits for example, there is a clear constant, although very slight, warbling sound for us. Most people can't detect it unless it's pointed out to them. It would be interesting to put about 200 feet of SRD Dolby tone on the end of a fully built up print and go down and listen to it.
Thanks again.
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Ken Jacquart
Film Handler
Posts: 82
From: San Francisco, CA, USA
Registered: Mar 2000
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posted 02-10-2006 12:29 PM
Mark,
The Cat. No. 773 (CP650 Dolby Digital film decoder) boards that we received and tested were from a customer up in Canada. They evidently were aware of this discussion thread and got in touch with us regarding the same type of symptom that Robert had reported on his first post here that started this discussion. We also went to the studio and received the same film print “Rent” that the customer had used. We tested the Cat 773 cards that were returned to us. We used the film "Rent", Dolby Tone loops, CP650's, DA20’s, o-scope leads soldered into test points on the 773 cards to watch signals, etc… At the end of testing, the results of what we saw, supports the statement made by engineering (which I posted a few posts up this thread). Again, in order for this digital system to work well and be robust with various projector motor speed differences, we are able to filter out all of wow and flutter except under certain extreme circumstances. For example, if you yank on the film, you will cause enough disruption to possibly hear a subtle wow affect. While maintaining normal best practices in the projection booth, this scenario should not occur.
You asked about the FIFO bucket spigot response time. Basically, the system is designed to never have a FIFO underflow or overflow. It tracks the speed closely depending upon the FIFO.
As you mentioned, edge sensing cue tape will likely cause some disturbance through the reader and may or may not cause an audible effect depending upon the state of condition of the reader itself. Certainly if enough data blocks are covered there will be trouble. If the cue tape is truly along the outside edges, it will only cause bounce in the reader.
-ken
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Mark J. Marshall
Film God
Posts: 3188
From: New Castle, DE, USA
Registered: Aug 2002
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posted 02-25-2006 05:20 PM
I filmed a little video of our CP650 problem and hopefully I can get some feedback from the Dolby folks and other experts here. I apologize for the crudeness, but I was operating the camera and the projector at the same time. I wish I could have done this sooner, but I'm officially on a leave of absence from the theater while I work on other things.
In this video, we run a loop of Cat 69T SRD formatted Dolby Tone film through our projector setup. This includes a Simplex 1050, a Cat 702 reader, and a CP650. The first thing we do is run the loop through the projector and analog sound head to show that the projector itself is running at a constant speed.
Then we move the loop up to the digital setup. The loop goes through the Cat 702, around the outer rollers of the SDDS reader (not through the gears of the reader), down around the upper feed sprocket of the projector, back up out of the projector and up to the upper speco roller that is mounted on the upper arm of the projector. During this run, you'll hear a very slight warble. To me, it doesn't even sound like it's the right frequency. Should it be different? After a few seconds, I'll move the rollers on the Cat702 up and down to exagerate the warble.
Next we add a piece of edge sensing cue tape to the loop. This is accutape pre-cut foil tape. I trimmed the width of the strip so it doesn't even come close to the digital data on the SRD track. What you'll hear over and over during this demo is basically what happens at the end of every movie when the "house lights up" cue goes through the projector. I also show the CP650 display so you can see that the error correction is around a 4, which in theory should mean that everything is still fine.
Finally, note what happens when I turn off the motor. The warbling tone relaxes back to a constant tone the way it should sound until it slows down to the point where it drops out completely.
I'd like to hear some analysis of this from some of the experts here. If there's something else you'd like me to try, let me know and I'll see if I can set it up. Also, I'm hoping someone can grab this and archive it here on the Film-Tech server.
Thanks.
Video Here (Archived on FT server)
Edit: Thanks, Adam. --Mark
[ 02-25-2006, 07:00 PM: Message edited by: Mark J. Marshall ]
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