Film-Tech Forum ARCHIVE: How to do an analog A-chain alignment

This is topic How to do an analog A-chain alignment in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Chris Hipp (Member # 1788) on 09-17-2003, 05:52 PM:

What is the best way to do an a chain alignment? Also, how often should it be done?

I'm running Simplex 5 star soundheads (reverse scan), comopnent engineering solar cells and Ultra stereo JS200 processors.

Thanks

Posted by Manny Knowles (Member # 1171) on 09-17-2003, 05:58 PM:

Caution: The A-chain is a critical alignment.

First things first -- what's your level of experience in this area? Your topic header "How to do..." suggests that you've never done an A-Chain Alignment but your question "What's the best way..." suggests that maybe you're just looking for tips to improve upon your technique. Which is it?

If you are a complete novice, then I would personally recommend that you get some hands-on instruction with an experienced tech to guide you until you can do this alone.

The whole process made perfect sense when I read about it in the Dolby manuals and elsewhere...but actually doing it turned out to be a whole different ballgame. I was overwhelmed by all the test film, the equipment, the sheer number of steps involved and the hand-eye coordination needed. I did not achieve good results until an experienced technician showed me how to do it. Two or three practice sessions was all it took -- less than an hour in all -- but the personal assistance, the advice and the feedback (as I worked) was invaluable. And I have to say I've gotten better at it over time -- can do it faster and get a better alignment -- all with practice.

So my point is this: The A-Chain Alignment is a "doing" thing, not a "reading" thing, in my opinion. You could easily make your theatres sound worse despite the fact that you will probably get some EXCELLENT instruction and tips here on the site.

You will need to have an oscilloscope, RTA and some test films. Do you have those already?

A classic example of something you can't get out of the manuals -- You need to be able to connect your o'scope and your RTA at the same time. So in addition to the scope and the analyzer, you're also going to need extra cables and a signal splitter. I don't know about USL processors but all of my Dolby's made it necessary for me to get little pin adapters for my BNC cables. By the time my visit with the tech was done I had a complete shopping list of absolutely everything I would need.

In the meantime -- while you wait for further postings here -- you might take a look at www.smartdev.com and click on "Technical Tips" and then follow the "Theatre Sound Installation" link. They have a description of the A-chain procedure there that was easy to understand when I was starting although, as I said, none of the written stuff really helped me at first.

Good luck to you.

Posted by Jack Ondracek (Member # 1466) on 09-17-2003, 06:22 PM:

Not wanting to set off a firestorm here (though I may be sticking my big size 10 in my face), I'd think you shouldn't have to do A-chains very often at all... especially since you're using a reverse scan pickup. Once an A-chain alignment is done properly, you shouldn't have to do much other than check your dolby levels now and then.

I recently checked up on a CP55 that had been installed about 5 years ago. The reverse scanner was conservatively set, since we had plenty of gain in the preamp (read: LED not being hammered). When I looped the soundhead, I found that the levels were almost exactly where they should be.

Here's where the debate probably lies, because I wouldn't doubt there are those who feel the need to make this a part of their weekly routine. Personally, I don't know why you'd need to check A-chain more than once a year at most, unless you notice something out of whack when you do your auditorium checks...

If you have meddlers in your booth, all of this goes out the window. If not, get it done right then leave it alone. Worry about alignment issues if you change or repair things in your soundheads or amplifiers, or if you need to work on your speakers or auditorium treatments. Loop your soundhead often enough to satisfy yourself the Dolby's ok. That isn't complicated, and will give you an idea what kind of "drift" your system will exhibit over time.

As for how to do a full alignment, there should be information about that in the manual for your processor. I'd agree with Manny... if you've never actually seen it done, go find somebody to watch for a while. It's not totally a "by the numbers" technique, and you need to understand the judgment calls you need to adjust for when actually listening to your work in the auditorium. Tackling this cold is probably not a good idea.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-17-2003, 06:26 PM:

Chris' problem is that he can't seem to get management or his tech to do the alignment of a poorly aligned analog pickup -- which is unfortunate.

My suggestion would be not to touch it. It sounds like you're in the situation that you'd probably lose your job, or catch great hell if you touch it.

Another suggestion, which may get your A-chain done... get your friends to complain to management / head office about the sound quality in that auditorium. Either you'll get the alignment done or your friends will end up with some passes. [Smile]

Posted by Richard Fowler (Member # 893) on 09-17-2003, 06:33 PM:

The A chain is best learned "hands on". If you have had previous experience with magnetic recording equipment, some A chain procedures are identical. Phasing on a scope must be learned since you can put the A chain out of phase. Properly maintained A chain with reverse scan should need less checking than exciter type systems....B chain ( auditorium ) should be checked more often since aging of high frequency drivers, amplifier and crossover points may be more easily tampered or have effects from aging.

Posted by Chris Hipp (Member # 1788) on 09-17-2003, 06:35 PM:

Well, I have test films (dolby tone and buzz track)

My engineer showed me how to do the buzz track alignment and how to set the led levels.

The main reason I am asking is that in some houses I can hear a slight buzz in the dead spots between trailers. I'm sure it is there when there is actually sound but I'm half deaf and can't tell .

So, everytime I call him in to fix it he tells me he just adjusted the buzz track, which I always do before I call but it never seems to make a difference one way or the other.

Posted by John Westlund (Member # 913) on 09-17-2003, 06:43 PM:

Do these speakers buzz when the film is not running as well? From what you describe it sounds to me like you could have a ground loop problem.

Posted by Richard Fowler (Member # 893) on 09-17-2003, 06:45 PM:

The film guide roller pass the sound drum on a 5 star has been known to drift side to side which will introduce buzzing.

Posted by Chris Hipp (Member # 1788) on 09-17-2003, 06:47 PM:

John:
No, it is just when running in analog it's not there in non sync or when running digital. However, if a trailer goes through that is not digital it is very noticable compared to the digital.

OK, I guess there is a lot more to an A-chain alignment than I realized. I really just want to know if anyone has any tips on aligning the lateral guide roller.

I'm not going to be fired over anything or get into any trouble. If I messed something up then it would run for maybe one show, two at the most, wich would be in digital, before my engineer would come in and fix it.

[ 09-17-2003, 10:33 PM: Message edited by: Chris Hipp ]

Posted by Gordon McLeod (Member # 33) on 09-17-2003, 09:33 PM:

The bottom line is you need more than a dolby tone and buzz track
You also need pinknoise test film a cat97 left/right alignment film (I usually splice it and the buzz track togather into an alternating loop)
and a illumination test film
All requie a dualtrace scope and a spectrum analyzer