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Topic: A-chains and B-chains

Carey Barber
Expert Film Handler

Posts: 143
From: Newport News, VA, USA
Registered: Jan 2003

posted 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?

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Louis Bornwasser
Film God

Posts: 4441
From: prospect ky usa
Registered: Mar 2005

posted 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 ]

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Ken Lackner
Phenomenal Film Handler

Posts: 1907
From: Atlanta, GA, USA
Registered: Sep 2001

posted 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.

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Monte L Fullmer
Film God

Posts: 8367
From: Nampa, Idaho, USA
Registered: Nov 2004

posted 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 ]

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 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

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Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 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.

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Ken Lackner
Phenomenal Film Handler

Posts: 1907
From: Atlanta, GA, USA
Registered: Sep 2001

posted 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.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 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

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Gerard S. Cohen
Jedi Master Film Handler

Posts: 975
From: Forest Hills, NY, USA
Registered: Sep 2001

posted 09-15-2007 07:51 PM

PROJECTIONISTS OF THE WORLD, UNITE!

YOU HAVE NOTHING TO LOSE

BUT YOUR A & B CHAINS!!!

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Louis Bornwasser
Film God

Posts: 4441
From: prospect ky usa
Registered: Mar 2005

posted 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

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 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

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 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?

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001

posted 09-23-2007 03:58 PM

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

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 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

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 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

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