Film-Tech Forum ARCHIVE: A-chains and B-chains

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Posted by Carey Barber (Member # 1586) on 09-15-2007, 11:45 AM:

Okay, I have a general sense of what the A-chain and B-chain refer to but I wanted a more clear-cut definition.

- What exactly defines these terms?

- Why are they called a-chain and b-chain? How did they get these names?

- What distinguishes the two?

Posted by Louis Bornwasser (Member # 3063) on 09-15-2007, 01:46 PM:

In the days of Western Electric, the A chain was film to fader and B chain was fader to speaker. Alignment, just as now with Dolby, was divided (and tested) seperately.

(Note that in A chain; B chain alignments, no one can check the matrix card without an additional step.) Louis

[ 09-16-2007, 07:44 AM: Message edited by: Louis Bornwasser ]

Posted by Ken Lackner (Member # 1002) on 09-15-2007, 03:08 PM:

The A- and B-chains are portions of the path of sound from the film all the way out to the speakers. The A-chain includes everything from the exciter, through the reader, through the preamp (in the case of an reverse scan reader), and through the preamp card in the processor. (Or, at least the preamp "section" on modern processors such as the CP650.) "Doing an A-chain" involves checking and, if necessary, aligning, all the components in this section of the system. Typically it would refer to physically adjusting the focus, azimuth, and lateral positon of the reader assembly, as well as setting the optical gain ("Dolby Level") and slit-loss EQ on the preamp card.

The B-chain is everything after the preamp, from the equalizer circuitry, through the fader, amplifiers, and out to the speakers. "Doing a B-chain" usually refers to performing house equalization and setting levels.

I am sure someone can offer a more precise explanation, but that's my $.02. I, too, have long been curious as to where these terms got their names, and I would like to know the answer to that question.

Could someone please clarify whether the NR circuitry is part of the A-chain or B-chain? I would think it's the B-chain.

Posted by Monte L Fullmer (Member # 2797) on 09-15-2007, 04:25 PM:

I would agree that NR is part of the "B" chain since it's after the optical pickup head and the optical preamp, but before the SR decoder, EQ and Split Surround and Subbass cards - being a part of the analog sound processing.

[ 09-15-2007, 06:36 PM: Message edited by: Monte L Fullmer ]

Posted by John Hawkinson (Member # 1135) on 09-15-2007, 07:12 PM:

This is really debatable, but it's also not a very interesting question (why are we asking?). Essentially the NR takes place in-between the A chain and the B chain. In the formal historical definition Louis offers, of course there was no NR.

If you forced me to pick, I would place the NR in the A chain for two reasons:

1) Pragmatically, when you do the A chain, you're paying close attention to the optimizing the NR. You are ensuring that you make Dolby Level -- that your gain is appropriate for proper NR decoding. Yes, you are not adjusting the NR circuitry, but you are watching the output of the NR circuitry (e.g. when you look at LEDs on a Cat. 222, etc.)

2) If you feed in external signals, like with a Cat. 85C pink noise generator, they feed in post-NR. Since pink noise is the basis for B chain calibration, you cannot really argue that the B chain includes circuitry that is not involved in the calibration.

For instances, a broken NR card could show in the A chain alignment, but it would not show up in the B chain alignment.

(Now, some might observe that in analog cinema processors, SR decoding has no place in the A chain alignment. While this is true, it's an artifact of some special historical optimizations Dolby made. Formally speaking, you ought to align Dolby A with Dolby Tone and align Dolby SR with Dolby Noise, and do this for every film seperately. But conveniently all films are (ostensibly) recorded at the same levels for us, with the A level and the SR level set equivalently.)

However, to play Devil's Advocate, each A chain is specific to a particular sound reproducer. If you have a changeover system, both projectors share the same NR. You do two A chain calibrations and one B chain calibration. There is only one set of NR circuitry, so it can't be part of the A. Furthermore, if you run mag, you may use the processor's NR circuitry. The mag A chain and the optical A chain are entirely seperate.

I think, again, that it is best to consider the NR to be inbetween the A chain and the B chain.

Dolby says:

quote: Dolby Model CP65 Cinema Processor Installation and Alignment Instructions

The system alignment procedure is divided into two parts—the A-chain alignment which covers the projector, optical preamplifier, and Dolby noise reduction adjustments—and the B-chain alignment which covers the portions of the system from the room equalization circuits to the CP65 fader through to the loudspeakers.

Any input from the tapeheads? I think they really set the definition of these terms...

--jhawk

Posted by Scott Norwood (Member # 30) on 09-15-2007, 07:26 PM:

So, where does the matrix card fit? A-chain or B-chain?

To my way of thinking, the A-chain would stop at the matrix card and the B-chain would start with the room EQ (optical decoding vs. room setup common to all formats (optical, digital, non-sync, video, etc.)), but I would be interested in the "official" answer.

(I should add that I'm thinking of analog processors here, but I don't see why digital processors would be any different, except that the NR and matrix may be implemented in software rather than on discrete physical cards.)

quote: John Hawkinson
In the formal historical definition Louis offers, of course there was no NR.

No Dolby NR, but the Academy filter served (serves) the same purpose.

Posted by Ken Lackner (Member # 1002) on 09-15-2007, 07:43 PM:

quote: John Hawkinson
This is really debatable, but it's also not a very interesting question (why are we asking?)

Because inquiring minds wanna know! Who says it's not interesting?

quote: Monte L Fullmer
I would agree that NR is part of the "B" chain since it's after the optical pickup head and the optical preamp, but before the SR decoder....

Um...SR is noise reduction.

After I posted, I thought about it and it does make sense that NR would be part of the A-chain because it is specific to the optical formats, and has nothing to do with the EQ.

Posted by Steve Guttag (Member # 268) on 09-15-2007, 07:51 PM:

Historically, in Dolby training seminars, Dolby defined the transition from A to B chain was at the matrix decoding (Cat 150 from 1980 onwards). I would say that is pretty accurate in that the A-chain would naturally include what it takes to get the channel configuration into its final form. In a system that supports magnetic and optical, if one wants to make a clear division between the two chains, one has to to cut it after the matrix decoding for optical which would mean after the noise reduction for both. It can be said that both optical and magnetic share the same B-chain at that point.

Steve

Posted by Gerard S. Cohen (Member # 994) on 09-15-2007, 07:51 PM:

PROJECTIONISTS OF THE WORLD, UNITE!

YOU HAVE NOTHING TO LOSE

BUT YOUR A & B CHAINS!!!

Posted by Louis Bornwasser (Member # 3063) on 09-16-2007, 07:43 AM:

Steve has it right. In effect, the "break point" has shifted from the old fader to the new matrix.

Note that "checking the matrix card" cannot be done during A chain/B chain setup unless you add a step. Louis

Posted by Gordon McLeod (Member # 33) on 09-16-2007, 10:40 AM:

Actually it should be defined as the input the switching to the eq as there is a "A chain" to both digital and mag inputs

Posted by Stephen Furley (Member # 1277) on 09-23-2007, 05:50 AM:

Could you say that the A-chain is everything that is used only for optical film tracks, and the B-chain is everything that is also used for non-sync, digital, mag/aux etc. inputs, or is there anything which would end up in the wrong chain using this definition?

Posted by Demetris Thoupis (Member # 833) on 09-23-2007, 03:58 PM:

Where would one place the Digital Reader alingment!!? A or B Chain!!! ??? [Confused]

Posted by Gordon McLeod (Member # 33) on 09-23-2007, 04:01 PM:

The clear division is that the input to the EQ is the dividing point
Other input sources to the cinema processor have A chains to do not just optical
in fact it is reffered to in many dolby manuals as optical A chain mag a chain etc

Posted by John Hawkinson (Member # 1135) on 09-23-2007, 04:43 PM:

Stephen asked, "Could you say that the A-chain is everything that is used only for optical film tracks, and the B-chain is everything that is also used for non-sync, digital, mag/aux etc. inputs"

No.
To restate what Gordon said:

There is an A chain for optical.
There is an A chain for mag.
You could even argue there is an A chain for digital sound alignment, though that terminology is not typically used.

I still don't understand why there's interest in trying to shoehorn so much stuff into these definitions. A chain typically refers to the optical sound setup, B chain to the EQ (and post), and if you try to fit other things into these definitions, you are likely to just confuse people you are talking to.

--jhawk

Posted by Stephen Furley (Member # 1277) on 09-23-2007, 05:13 PM:

quote: John Hawkinson
No.
To restate what Gordon said:

There is an A chain for optical.
There is an A chain for mag.
You could even argue there is an A chain for digital sound alignment, though that terminology is not typically used.

That actually isn't too far off what I said; the A chain is the part where the various sources are handled separately (perhaps I should have said 'chains'), but the B chain is common to everything.

quote: John Hawkinson
I still don't understand why there's interest in trying to shoehorn so much stuff into these definitions. A chain typically refers to the optical sound setup, B chain to the EQ (and post), and if you try to fit other things into these definitions, you are likely to just confuse people you are talking to.

Probably true.

Posted by Monte L Fullmer (Member # 2797) on 09-24-2007, 01:08 AM:

quote:
Where would one place the Digital Reader alingment!!? A or B Chain!!! ???

you're key word here is "alignment"- being optical magnetic, digital...as what Louis said before .. anything before the fader is "A" chain - any source before it hits the "target" preamp section.

Posted by Jeffry L. Johnson (Member # 453) on 09-24-2007, 03:29 PM:

SMPTE 202M

quote:
A-chain (transducer system): That part of a motion-picture audio system...extending from the transducer to the input terminals of the main fader.

B-chain (final chain): That part of a motion-picture sound reproduction system...commencing at the input terminals of the main fader and terminating in the listening area...at which sound pressure measurements are taken.

Posted by Ken Lackner (Member # 1002) on 09-24-2007, 05:38 PM:

Now I'm confused. I thought the EQ cards (or EQ section, in a digital processor) were part of the B-Chain, but I also thought said section was before the fader. Also, by common practice, isn't the optical tube part of the A-chain, and yet it is before the transducer? [Confused]

Posted by Louis Bornwasser (Member # 3063) on 09-25-2007, 10:04 AM:

In the olden days, the fader was post preamp and pre output amp. In the Dolby the fader has moved over to the point after the EQ, probably for noise reasons.

I would consider all "common to all formats" areas as the B chain in a processor. I would consider the A chain to be each individual format: Dolby optical; magnetic, Dolby Digital, etc. Louis

Posted by Viktor Marosi (Member # 4489) on 10-26-2007, 02:00 PM:

A chain:

CAT 240A
CAT 222 or CAT 350 Dolby® SR/A
CAT 150 2:4 Dolby® MP

B chain:

CAT 64 EQ
CAT 441 surround + LFE card (Ls, Rs, SW)
CAT 242
CAT 443
CAT 249, CAT 250