Film-Tech Forum ARCHIVE: Dolby Digital audio splats from film

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Posted by Mike King (Member # 6307) on 05-31-2011, 01:11 PM:

Hi All,

Been reading many posts on here for a while and have always been amazed by the wealth of knowledge flying around! I am essentially a noob when it comes to actual film projection and have only had to handle mostly mute prints in separate reels for audio printmastering (I work for an audio post facility).

We do however have a screening theatre and unfortunately, our projectionist left recently so the maintenance is up to me now (we get freelancers in to do the actual film screenings which are not that common).

OK, on to my question: last week we found that the Dolby Digital output from both projectors (using CAT701 units) where causing loud "splats" which sound a bit like digital audio losing sync to me. As it was happening on both projectors (no platter system) I assumed it was the CP500. I borrowed another one from a contact of mine but it was still happening. Grabbed a CP650 from one of the dubbing theatres and the audio played fine (no splats). I am now totally stumped and was hoping someone may have a suggestion as to what I can try next.

The CAT701 units are pretty old and we do not know when last the LEDs were replaced so the first thing I did was turn up the gain (inside the units). This helped as the reader went from 7-F to 4-6 which admittedly is still not great. The bit that gets me is that with an error rate of 4-6, the CP650 played fine. Is there a difference in the tolerance between a CP650 and a CP500?

Posted by John Wilson (Member # 269) on 05-31-2011, 03:43 PM:

Hi Mike. Welcome to FT.

Did you turn up the gain on the reader before you swapped to the 650?

When you say 'splats' my thought is that it was the sound of the processor faulting (7-F as you say you were getting). The CP500 will not handle them anywhere near as seamlessly as a CP650 as the 650 handles even analog sound in the digital domain.

If you turned up the wick after you'd swapped to the 650 go back to the 500 and see how it goes now.

If you are running a new print and only getting 4-6 error rates it does seem a tad high but it's only a real problem if it goes higher and faults. Many installs will never see a lower rate than that. If the problem persists and you're still unhappy you could have both the LED's changed out (seeing you have no history as to how old they are) and have an A/B chain alignment service performed (again, since you have no date as to the last one).

Posted by Mike King (Member # 6307) on 05-31-2011, 05:12 PM:

Hi John! This is why this forum rocks - thanks for the quick response!

Yes, I turned up the brightness/gain in the 701 units before swapping the CP units. Am I understanding you correctly that a 650 is effectively "more forgiving"? That would explain why both 500s are splatting whereas the 650 isn't.

The weird thing is that until last week we would get digital playback fine through the 500 on 4-6 error rates. It's just recently that this problem has arisen (on both projectors) so not sure what has caused it.

I have a CAT700 which is in very good condition so I might swap out one of the 701s just as a test (would obviously revert to the red LED 701 as a long term solution)...

May well have to consider new LEDs and an A/B chain alignment - just trying to eliminate the obvious first...

Will report back once tested further - thank you so much for your help!

Posted by John Wilson (Member # 269) on 05-31-2011, 05:28 PM:

I'm not sure the 650 handles it in actuality any better than the 500, but it is certainly WAY less noticeable on the 650 when it faults.

Is this the same print you were having no troubles with last week or something else?

Posted by Mike King (Member # 6307) on 05-31-2011, 05:58 PM:

Mmmm.. there's me getting hopeful. These are really loud splats, so I am wondering if there is something else at play especially as we are achieving error rates that were no problem a few weeks ago. I may video some of it (including a view of the CP500's cards when the splats happen) and post it somewhere if that helps...

The problem was brought to my attention last week by a freelance projectionist who was checking a new print for a screening. I am currently using my test film which is a nearly-new 6 reeler that played fine before.

About four weeks ago I was checking the sound on a short for someone, switching between the digital, analogue and DTS for comparison and all was well then too.

Even though I have tried swapping CP units, is it worth me checking the video card with a scope and anything else obvious? Never done this before but up for a challenge...

Posted by Steve Guttag (Member # 268) on 05-31-2011, 09:38 PM:

CP500s were notorious for sporadic sounds. Try swapping the EQ DSP with the 2:4 Decoder DSP (I'm drawing a blank on the CAT number at the moment CAT 671A, maybe? There are three of them in there). The one in J14 has a decent probability of issues and some boards do better than others. No recalibration is needed.

-Steve

Posted by Monte L Fullmer (Member # 2797) on 05-31-2011, 11:49 PM:

..are you using opaque tape? If so, you're covering up 4 squares of information which is one frame of digital sound.

When the system looses that one frame, it wants to swing back to optical. But, since the transaction happens so fast, the system can't swing back and forth that quick and you get that "boop" in the sound - it's like a gap in the sound.

Also, CP500's were known for backplanes to go bad where the entire backplane have to be replaced - been there and done that many times...