This is topic What's a normal dolby digital reading? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Dave Thompson (Member # 1242) on 05-09-2002, 05:25 AM:
I've been looking at past posts on this forum, to try and find out what is a normal dolby digital reading, but had no luck.Our dolby digital error correction reading on the 7seg display usually ranges between 3-7, and occasionally drops out. It varies greatly from print to print.
We are a student cinema, so most of the films we get have usually "done the rounds", (and hence the digital track may be damaged) but all of our trailers are brand new, and sometimes we have brand new prints, but I've never seen it read less than 3. Does this mean there is something wrong with it?
I've seen from previous discussions that any reading 0-7 results in complete reproduction, as it means that the error correction has been successful. However, it must be safer to have the system running at 0-3 rather than 5-7.
Our reader is mounted at the top and is approximately 3 years old from new.
It would be useful to know some sort of average value that people tend to get.
Dave
[this my first new topic submitted on this forum 
]
Posted by Jon Bartow (Member # 241) on 05-09-2002, 07:08 AM:
Dave Said: "It varies greatly from print to print"exactly.
You say that your reader is mounted "at the top" Therefor I am assuming that you have either a cat 699 or cat 700 (possibly a cat 701) digital reader.
If it is a 699/700 when was the last time the bulb was changed? The last time a tech aligned the reader?
How often do you clean the optics on the reader?
Also, what make of projector is it sitting on?
One excellent way to improve your digital performance is to start treating your prints with Film-Guard. (Click on the FilmGuard link in the upper left corner of this window)
Jonathan
Posted by Dave Macaulay (Member # 813) on 05-09-2002, 07:32 AM:
The lowest number you ever see indicates the potential of the reader, the variaition is usually caused by the print although mechanical issues with the reader or its mounting are possible.
3 is a good reading although a penthouse reader will sometimes get 0-1 with a really good print.
From those numbers I'd assume you have a good reader amd worn prints, but keep on top of the reader maintenance!
Make sure the reader optics are kept clean.
If you have a Dolby reader with the white-light source how old is the bulb? Before they burn out the bulbs darken, and the reader gets less tolerant of worn prints plus the error rate in general rises. This is less of a problem with the penthouse LED source but they age too; a tech can check that and whether all segments are alive.The SR-D information is located in a high wear area of the film and is known to degrade, some types of projector "hurt" the sprocket area more than others but a worn or defective machine can destroy the SR-D and SDDS info in very few passes. Analog sound is in a safer film area and isn't usually damaged by a projector but degrades with dirt and handling-related scratches. DTS data is at the edge of a wear area but the data is only low bit rate timecode - much harder to make unusable than the very dense data image of SR-D or SDDS. (DTS would seem to be the ideal digital format for rep houses - but the damn disks are rarely with the prints rep houses end up with. Is there a solution to that?)
Posted by Darren Briggs (Member # 1071) on 05-09-2002, 08:00 AM:
Another thing to look at is if the print plays in SRD on the first run wothout failing, but on the second it fails, you might have something in your film path that is scratching the film, ie a stuck roller etc. Also like Jon said above, when was the last time you had the lamp changed in it?? It could be covered in dust giving a poor light. Especially as you say new trailers play at high error rates. Even old prints I show which are scratched here and there play fine at 5. or 6. Im playing a real dirty and oiled up print of Gosford park thats being around the block a few times, and thats running at 6. 7 throughout without failing, same on our basement reader and a brand new BACP penthouse which im testing for the company.
Darren
Posted by Rob Jones (Member # 902) on 05-09-2002, 08:22 AM:
The reader in question is a cat 701 installed in '99 - I wouldn't have thought its an optics issue as it has the LED light source and I've have seen it tracking 2-3 on some occasions, but I don't think its had a good clean for quite a while. Maybe it would be worth getting a tech to hook up DRAS to it next time we have someone over and check its calibrated/focussed on the CCD properly?Do these things have a habit of needing calibration or are they usually set to go straight out of the factory?
Posted by Gordon McLeod (Member # 33) on 05-09-2002, 09:29 AM:
LED illumination also drops with time as does mechanical alignment drift
Your engineer should use a scope and measure the actual output of the video and set the illumination accordingly
Dave non of the DTS rephouses I service have had problems getting disks
Posted by Benjamin J. Kepner (Member # 1241) on 05-09-2002, 01:46 PM:
Just wondering... I know where to find these numbers on a DA-20, but we've got a couple of readers paired with CP-500's and was wondering if there is anywhere the tolerance numbers can be viewed on them.
Posted by Scott Norwood (Member # 30) on 05-09-2002, 02:39 PM:
The error count is displayed on a single-digit LED on one of the cards (I forget which one...it's in the manual) _inside_ the CP500. You have to open the front of the unit to see it. Why they didn't think to make the error count visible from the front panel is beyond me.
Posted by Josh Jones (Member # 447) on 05-09-2002, 02:59 PM:
I have *never* seen our cat 700/CP500 setup go below a 3 reading on the error display. I would assume that is just as good as it will get. 0-2 readings? what kind of hooch are you guys poring into those readers to get them to read that well? I would certainly like to have the recipy 
Josh
Posted by Steve Kraus (Member # 476) on 05-09-2002, 03:17 PM:
I've seen 0's and 1's. That's with a Kelmar Basement reader on a 5-Star + CP500D. Funny thing was that the first time I ever got numbers that good was right after replacing the LED (2nd time in the space of about a year) as the culmnation of replacing a lot of components trying to locate the source of unstable film motion that was giving me middling numbers (4-5-6) with frequent interspersed F flashes on that machine.
The main culprit there was the sound drum but I didn't get the good numbers til I re-replaced the LED which was very uneven on the scope, despite position adjustments. After I changed it I began by adjusting the position that gave me normal analog Dolby level (on Kelmar the positions are not independent). Then before even looking at the scope or running DRAS I put up an SR-D reel and there I was at 0's and 1's. Turned out it was an exceptionally good print and normally, even with full tweaking, I'm in the 2-3 area but that's decent for a basement reader on a 5-Star.
Posted by Karen Hultgren (Member # 395) on 05-09-2002, 07:43 PM:
Dave in Canada - if you don't get discs with the print:1) Be sure to order the film in DTS.
2) If no discs arrive, call the film distribuator and request them.
3) If you get no where with the distribuator, contact me and I'll help you. But always try the distribuator first because we always have to get permission (the discs belong to them, not DTS) before sending discs to theaters.
Karen at DTS
khultgren@dtsonline.com
Posted by Brad Miller (Member # 2) on 05-09-2002, 10:18 PM:
I think half of this is due to misaligned readers and the other half is due to prints that are not handled well. I have a variety of readers from 699 to 701 that I service and 0s and 1s are NOT an uncommon thing to see on any of them. In general, on a typical print the 701s tend to read 2-3, but the 699 and 700s tend to read 0-1. Lately I've been watching the numbers and I've been noticing the 699 and 700 units have been sitting firmly on 0 the entire movie and the 701s have been flipping between 0-1. Someone at the labs have been getting better. Josh, note the error reading on a reel of a print while it runs. Then clean the film with a media cleaner and FilmGuard. By that next show you will almost certainly see 1-2 points worth of improvement. This can be easily tracked with DRAS if you don't want to stand there and watch the display.
Posted by Josh Jones (Member # 447) on 05-09-2002, 11:14 PM:
I would but I doubt the theatre will splurge 700 for a film cleaner. What sort of tweeking is involved to get the levels that low?Josh
Posted by Brad Miller (Member # 2) on 05-10-2002, 12:49 AM:
Brand new Kelmar media cleaning machines are only about $500. You can get them used for $150.You'll need an oscilloscope, an laptop running DRAS, the appropriate tools and some loops made from known good trailers printed at Deluxe Hollywood.
To obtain a good reference loop, here is what I do. I assemble half a dozen brand new trailers (all from Deluxe Hollywood) and run them. I watch the DRAS and make notes of each one and then use the one on the roll that is the most *average/centered* of them all. At that point, I label it and can make tons more Dolby loops from that one trailer to align with as I need them.
I've found using the actual loops produced by Dolby to align an SRD head is a bad idea. The alignment is never as good as it could be. Also, for some reason when I use loops produced at Technicolor or Deluxe Toronto I end up with high error readings when I play prints out of Deluxe Hollywood. Using Deluxe Hollywood prints to align by, everything plays nicely. Why? No idea. (Don't know, don't care, it works, I get zeros and that's good enough for me! )
Instructions on how to perform the alignment are in the manuals.
Posted by William Phillips (Member # 1162) on 05-10-2002, 01:03 PM:
The digital failed on my vic 5 in February of this year. The digital had been fine up until the last show. Started the ads and trailers, DA20 was sluggish getting going, it got there after first few ads, it was playing quite happy. Got a call to stop from the duty manager, and when restarted the digital did not want to play in any shape or form. The tech came in a few days later, had a look at the led, out of the 20-24?? segments there were only 10 working as soon as the led came on, the others flickered on and off as they pleased.
The led was changed and have seen 0.`s on a few ocasions. Ads and trailers sit at about 2.3. and the features at 4-6 depending on wear. The digital is the cinemeccanica basement reader and led set to 350 ma.
Posted by Dave Thompson (Member # 1242) on 05-12-2002, 11:59 AM:
Would you recommend us re-aligning the dolby digital head ourselves (assuming it's that that is a bit out), or would it be better to get an engineer to do it?I know how to use an oscilloscope, but I've never done it or seen the alignment done before.
Dave
Posted by Brad Miller (Member # 2) on 05-12-2002, 03:51 PM:
Definitely and absolutely NO. Let your tech do it. The smallest adjustment will throw those things so far off of alignment you won't have ANY playback.
Posted by David Kilderry (Member # 142) on 05-12-2002, 05:54 PM:
Most has been stated already on this topic, mis-aligned heads and wear on prints are the major culprits with high readings, not to mention the LED's or Cat 699 - 701 exciters.
The reason why a penthouse Dolby reader is generally better is that it is an environment designed by Dolby for the best film tracking and reading of a digital track possible. Look at the amount of rollers on a cat 700 compared to a basement reader that was designed for optical tracks. Dolby can control all specs on one of their own products but can only "recommend" to projector manufacturers.
Basement readers are a compromise, but both the basement environment precisly aligned and fresh high quality tracks will provide low error readings.
David