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Topic: Audible effect of high SRD error rate?

David Stambaugh
Film God

Posts: 4021
From: Eugene, Oregon
Registered: Jan 2002

posted 02-16-2002 11:06 AM

If the SRD reader is experiencing a high error rate, but still getting enough data to work with to avoid a hard dropout, will the high error rate cause audible "defects"? I'm talking about tiny, split-second "blips" in the sound. This is especially noticeable during music containing sustained notes from string instruments. There's just the slightest audible blip in the sound. Might occur as often as once every couple of seconds.

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- dave
Look at this! His chin strap has been cut!

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 02-16-2002 11:56 AM

Yes. Absolutely. I have heard this many times before. You will hear a teeny tiny "blip" like effect at super high error rates, even if it never drops to analog. You will hear a much harder blip when it actually drops out even for a split second. This is one way that I can always tell a film is playing in Dolby Digital. Of course if it is getting good readings, then you won't hear the blip and all will be fine.

Good ears! You are the only person besides myself and Erika (another member on the forum from long ago) who has seemed to notice this. Once again I'd like to stress that what we are hearing is not SR-D dropping out of digital. This has been confirmed by Erika staying in the booth watching the processor and me listening to the blips in the auditorium. Usually the Dolby processor just reads "F" briefly when the blip occurs. Never goes to analog. I always like to say "I can hear the Dolby Digital F'ing!"

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Jesse Skeen
Phenomenal Film Handler

Posts: 1517
From: Sacramento, CA
Registered: Aug 2000

posted 02-16-2002 12:33 PM

Yes, I've noticed this too. The LED on the reader was turned up and that seemed to take care of it. I also always notice that funny "buzzing" sound it sometimes makes when a splice goes through.

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David Stambaugh
Film God

Posts: 4021
From: Eugene, Oregon
Registered: Jan 2002

posted 02-16-2002 12:58 PM

I've noticed this in at least 2 of the local Cinemark's SR-D auditoriums. It's as if the error correction system is momentarily overwhelmed and has to throw out a very small sample of sound, and can't conceal it. Kind of like a break in continuity.

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- dave
Look at this! His chin strap has been cut!

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 02-16-2002 02:06 PM

Yes, I noted too. When the "blip" occurs, the "block error" red led into the CP500 will light on for a moment.

I was in a theater that had continuing "blip" during the show because they had oil on the reader.
I wonder why CP500 allow that kind of stupid thing: I believe that after few "blip" it is better to drop to SR: if the "blips" cannot be recognized during dialogue, they are very annoying during music scenes.

Bye
Antonio

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 02-16-2002 04:01 PM

How many block errors can be tolerated before you take the rather drastic and far more audible step of reverting to analog would have been a software design consideration for the Dolby engineers. It is likely that they did not regard a rate of block errors below a certain level as bad enough to warrant reversion.

Has anyone noticed if this problem is software version related? If it is only on say, later versions, there is always the possibility that it is a bug and Dolby don't know about it.

It is also very important for projectors to run as close as possible to correct speed because, unlike the optical soundtrack which just shifts up or down in pitch, a digital system is designed around a constant data rate and so must use some method such as software "splicing" to correct for the speed difference. This can cause audible artefacts if the speed variation is too great.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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David Stambaugh
Film God

Posts: 4021
From: Eugene, Oregon
Registered: Jan 2002

posted 02-16-2002 04:15 PM

Ray, "splicing" is a good description of the artifact. It sounds kind of like a bad analog tape splice. But if it were a digital problem caused by the projector running too fast or slow, wouldn't these blips be more likely to repeat at a constant interval? What I'm hearing is basically random.

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 02-16-2002 08:05 PM

This problem seemed to vary from print to print with me. I'd start checking out a new print, smooth music would come on and then I'd hear "blip" "blip" "blip" and I would go insane! I then ran the movie in SR since it sounded so much better that way until I could move it to a DTS house. Realigning the readers seemed to minimize or eliminate the problem.

I can sometimes hear a similar effect when I am watching UPN. Not sure if it's just my station or the network itself, but it was very prominent during Star Trek Voyager. Drove me up the wall because there was no "SR" button I could press. I'm sure it was a digital error in whatever low-end system UPN uses.

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David Stambaugh
Film God

Posts: 4021
From: Eugene, Oregon
Registered: Jan 2002

posted 02-16-2002 08:35 PM

The severity of the SR-D problem seems to vary from one reel to another. If the reader is marginally out of whack, maybe normal lab processing variations between reels would make the problem better or worse?

I've noticed the same thing with UPN and a couple other cable networks. My theory there is they are running the show through some kind of "time compressor" so they can squeeze in more commercials.

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Ken Jacquart
Film Handler

Posts: 82
From: San Francisco, CA, USA
Registered: Mar 2000

posted 02-17-2002 12:55 AM

What you guys are hearing are "block repeats". If a Dolby Digital block cannot be read, even after all error detection/correction, then the previous block will be repeated in its place. A Dolby Digital block occurs every 1/96 per second (or every 10 ms). A repeated block will consume 10 ms of time and can barely be heard if you are paying attention to it. If two blocks are repeated in a row, then it is a little bit easier to discern (albeit very slight). If the processor cannot read 3 blocks in a row, it will revert to SR (3 strikes and you're out!). So, whenever you see that brief "F" on the Cat 673 card, you will hear a block repeat (or two). If this occurs on a regular basis, then you probably have a problem with alignment... namely, most likely, video amplitude. Drag out the scope and check it...

good ears on those block repeats!

ken

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 02-17-2002 01:49 AM

Like everything else in software, it seems that there is always a senerio that beats the system...

I once had a bad reader/print situation (a penthouse reader at that!!!) where the block repeat was just enough to be annoying yet not enough to have the DA-10 drop to analog. So it sounded like one was virtually under water for the last reel of Harry Potter. Using DRAS...looking at r-9 the azimuth jumped from say -1 to about -8 and stayed there! One reader did ok with it, the other, did not!

I've recently taken a liking to the skinny splicing tape either 16mm or 19mm...no drops and no seemingly ill affect regarding print longevity.

Steve

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 02-17-2002 01:57 AM

I still fail to see why you need to use the skinny clear splicing tape to prevent your SRD from dropping out when I have used full opaque tape before with no dropouts at all.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 02-17-2002 03:18 AM

Yes, you can use opaque tape and not revert but the fewer blocks you cover, the fewer repeats, the splice also has less hop on the screen. Personally, I was sad when cement spliced stopped working (read polyester)...between perf splices worked for me!

Steve

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"Old projectionists never die, they just changeover!"

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 02-17-2002 03:47 AM

Hey, just get an ultrasonic splicer for those ultrasonic prints. It's even cheaper than a Kinoton OR a 2nd projector!!!

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 02-17-2002 05:20 AM

The projector that had the block repeat had some dirty on the sound drum and the print jumped regularly on the digital reader. On the CP500 I had 5-f-5-f-5-f.
I was surprised because 5 was not a bad number and I was wondering how can the reader travel between 5 and f.
Then, I found the dirty and I resolved the problem.

Pls note: there were not my projectors, there are no dirty on my projectors...
Bye
Antonio

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