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Posted by Paul Looker (Member # 5420) on 09-17-2009, 10:38 PM:
 
I might get booed right out of here, but what is an a chain? How do I do it? What's the worst that could happen if its not done right? Should I get a tech in straight away to do it if I can't manage on my own or can it wait a couple of weeks for their regularly scheduled visit?
 

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Posted by Scott Norwood (Member # 30) on 09-17-2009, 10:49 PM:
 
Film sound systems are typically divided into two logical "chains": the A-chain relates to the mecanical and electronic parts that read the sound off the film, preamplify the signal, and, optionally, process it through an Academy filter or noise-reduction stage. The B-chain relates to the process of reproducing sound in the auditorium, including level settings, equalization, amplification, and loudspeakers.

Note that most people referring to an "A-chain" alignment refer to the optical alignment, although the term also applies to digital and magnetic sound.

The consequence of a poor A-chain alignment is bad sound. A mono system can be aligned by ear with acceptible (not optimal) results, but you really need the proper tools and test films to do the alignment for a Dolby system. Unless you have these items, it's probably best to leave everything as-is unless the sound is so bad that it can't be made worse.
 

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Posted by Joe Redifer (Member # 3) on 09-17-2009, 10:50 PM:
 
The A chain is basically anything before the B-chain. The B-chain is basically the equalization and amplification of the sound. The A-chain consists of the focus and alignment of the optical reader as well as its levels. A Dolby Digital unit or a DTS unit are also A-chain devices being that their sound is input to the B-chain device for equalization and amplification. And SDDS unit includes it's own equalization in order to make it more expensive.

Messing with the A-chain could really mess up your sound. Your optical analog track could be too quiet or you could hear a buzz, etc. It could sound muddy. A whole bunch of things can affect the A-chain and that is why there are many different test loops to properly adjust it. You can use a scope as well to really nail it down. If you mess with the digital devices your sound could be out of sync, cut in and out frequently or not work at all.

EDIT: Scott posted one minute before me!
 

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Posted by Frank Angel (Member # 248) on 09-18-2009, 02:00 AM:
 
One thing to keep in mind, Paul (and no one will boo you here UNLESS you DON'T), that unless you have learned the fairly sophistocated testing and adjusting procedures with the proper tools and testing gear to properly align either the A or B chain, or the adjustments of any other piece of equipment in the booth, stay away from tinkering with anything.

I know it's tempting, especially if you are young and curious and have an innate aptitude for mechanics and electronics, but you can take what you might think is a not-so-great sounding sound system and in short order turn it into a completely non-functional one with the turn of the wrong screw. If a room sounds less that what you think it should or could or has been in the past, call the service tech. Most are fairly accomodating and may be willing to show you a trick or to that you can do without mucking up his 2 hours of painstaking work....which, if you do it, will get some really thunderous boos.

And READ - Dolby CP manuals are a wealth of information in describing the theory and the practical operation of the sound system. More than likely there will be manuals in the booth or somewhere in the theatre -- get them and study them. You will come out on the other side knowing much more than just what the A and B chains are.

Whatever you do, don't loose that curiosity and thirst for knowledge of all things cinema. Learn how to put on a flawless presentation and you will be the last craftsman at the end of the long chain of artists and technicians from the those in front of the lens to the final result of that picture on the big screen and the sound in the theatre. You will be part of the magic.

Also I encourage you to read the wealth of information here on this most excellent forum. Sometimes you will have to listen to some of us older windbags, but listen, learn and ask questions.

And put your picture up here too. I am sure more than just me wants to see what a guy named Looker looks like!
 

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Posted by Gavin Lewarne (Member # 5397) on 09-18-2009, 04:44 AM:
 
Paul, it might be easier to think of the sound chains in the following manner:>

A-Chain:
Everything in the projector up to and including the inputs of the sound processor

B-Chain:
Everything from the sound processor to the speakers

Getting the B-Chain messed up is not as serious as the A-Chain. The B-Chain can usually be done quite adequatly by ear until you either have the necessary equipment to do it properly or you get a service tech in.

You can NOT align the A-Chain by ear though, that does really need to be scoped and test loops run for proper function (as the posters above have excellently explained)

For example, on my (ancient and crappy) equipment,
if i have either the Jaxlite LED, Slit Lens, Lateral guide roller or Solar cell improperly positioned by even 0.05mm i will get rubbish sound. And those are my considerations before the sound even gets to the processor!

Once again, i highly reccommend you read a Dolby CPxx manual - it explains everything very well and will give you a good grounding for the future. I am sure that when you get your service tech in he can show you how to dolby tone, that is the single most important regular maintenance you can do to maintain proper sound. Once he has everything working properly, of course!

So, what do you think the problem you have is?
 

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Posted by Pete Naples (Member # 761) on 09-18-2009, 05:13 AM:
 
IIRC the 'A' Chain is everything up to the matrix decoder section of the CP, including the mechanical and optical parts of the rep-set, the pre-amps and the NR circuitry.
You can do a passable job of aligning the 'A' chain by ear, but that takes a; good ears, b; experience c; patience. To do it correctly needs the relevant test films, an oscilloscope, a real time spectrum analyser and our previously mentioned friend, experience.

'B' Chain is everything after the matrix decoder, so that's Eq, level control, subwoofer filtering, amplifiers, loudspeakers and one could easily argue, even the room itself.

Paul, if you're having sound issues that you think are 'A' chain related, the first piece of advice I or any other engineer would give you is to give the optics and the sound head a good clean, then set the Dolby level using a good loop of Cat69T. You'd be amazed at how many problems this cures... If you don't know what you're doing either find someone who does, or have a read in the manuals for your equipment, which are probably here on Film-Tech if you don't have them.
 

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Posted by Randy Stankey (Member # 64) on 09-18-2009, 10:38 AM:
 
If you have sound, leave well enough alone. Messing up the A-chain can not only make the sound worse, you could totally LOSE all sound if you mess up.

If your sound is so bad that people are complaining you might certainly want to take matters into your own hands but even if you have marginal sound it's better than NO sound. Consider switching to "MONO" or "BACKUP" modes until you can get a tech to come check it.

At my old job, all projectors were supposed to have their A-chains checked ever 90 days or less. At the place I work now, I do it whenever I think it needs it.

If you haven't had somebody come do a checkup on your projector(s) in a year or more, it would be worth it to call somebody to come do it.

If you are inclined to do it yourself, you should do two things first:

1) READ and STUDY! Learn everything you can about how the projector and sound systems work. Take manuals home and read them if you can. Actually study how the projector is assembled and how it operates. (Just don't tinker!)

2) Find somebody who knows how to do the job right and have them lead you through it a couple-few times. Once you've seen how it's done it's not really hard. It's all about attention to detail more than anything.

If you've got good sound and there's a tech who does preventive maintenance on a regular basis, don't tinker!
Some adjustments inside a movie projector (namely, the intermittent) can be affected by even sub-micron changes in alignment!
 

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Posted by Tim Reed (Member # 137) on 09-20-2009, 08:48 PM:
 

quote:

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the Jaxlite LED, Slit Lens,

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quote: Paul Looker

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How do I do it?

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