This is topic Question on terminology in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Dave Cutler (Member # 515) on 07-13-2000, 02:39 PM:
Alright, not to sound like an idiot or anything but what exactly are A-chain and B-chain? I have heard them refered to, but have never been told what they are. I check Dolby Level and SPL with Pink noise periodically, but what else should I be checking in the sound system? I have only been doing projection work for just over 5 years so I am pretty new, and have had to 'formal' training. Just the 'on the fly' kind. Is there any publication that has all the terminology difined?
Posted by Gordon McLeod (Member # 33) on 07-13-2000, 03:34 PM:
The A chain is the solor cell slit lens excitor (reverse scan LED and reader) its focus azimouth and lateral alignment or the mag head and the preamp slit loss and gain or for mag gap compensation and eq and gain
The B chain is the House Equalizers,Fader,Amplifiers, Speakers and room accoustics
Posted by Tim Reed (Member # 137) on 07-13-2000, 04:42 PM:
Read the Dolby manual. There's a lot in there. If you can find a CP-50 or CP-200 manual, that's even better, because things were explained to a greater degree. In fact, read everything you can get your hands on. You'd be surprised how much knowledge is in the manuals. The A-chain is defined as everything in the sound system up to the fader. B-chain, everything after.
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Better Projection Pays!
Posted by Scott Norwood (Member # 30) on 07-13-2000, 05:52 PM:
The A-chain includes the fader? Somehow, I'd been thinking that A-chain covers only the exciter lamp to the preamp and that the B-chain was everything after (fader, EQ, amps, etc.).
And where does the NR and matrix stage fit into this? Now I'm confused.
Hmm...gotta go re-read that Dolby manual.
Posted by Gordon McLeod (Member # 33) on 07-13-2000, 08:35 PM:
The eq and fader tim are part of the b chain
dolby defined the matrix for years as the divison unitl you start playing with mag in a cp200 and then it gets a bit grey
Posted by Tim Reed (Member # 137) on 07-13-2000, 09:27 PM:
Let me check my manual here... {open mouth, insert foot}Boy, I sure look like an ass on that one (moreso than usual)! Sorry everybody! For some reason, I was thinking the VCA's were ahead of the EQ cards. And I have that part about 'up to the fader' being the transistion, emblazoned in my memory somehow. I swear I remember reading that a long time ago.
Maybe I'm hallucinating in my old age, and beginning to believe things that never happened :O
I'll go for the safe response next time:
"I don't know."
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Better Projection Pays!
Posted by Rick Long (Member # 213) on 07-13-2000, 10:36 PM:
I always beleived if that ALL signals go through it (regarless of the source - non/sync, digital,mag or optical), it is a part of the B-chain.This would then make the fader part of the B-chain.
Posted by Russ Kress (Member # 461) on 07-13-2000, 10:46 PM:
Hey Tim! Don't be so hard on yourself! You DID read it!Remember Dolby Stereo in its earliest form? Your cables came out of the CP-50 into a stack of four faders (Radio Shack potentiometers) and then on to the amps.
I think the 55 was the first to incorporate the VCAs, but I'm not sure.
Russ
Posted by Russ Kress (Member # 461) on 07-13-2000, 10:49 PM:
Oops! The fader is to "the right" of the EQ cards either way......sorry, it's late here!
Russ
Posted by John Walsh (Member # 168) on 07-13-2000, 11:14 PM:
The entire projector sound system (from optical solar cell to the loudspeakers) is divided into two sections; A-chain and B-chain. The A-chain starts at the solar cell and ends at the input of the 2:4 matrix. The B-chain starts at the output of the matrix and ends at the loudspeakers. (I thought it was at the output card, but Gordon knows more than I!)Each section has it's own setup procedures. Generally, you have to set each of them to a specified volume level and equalization. (Equalization is a fancy name for setting the "bass" and "treble" controls for a pre-specified tone.) The A-chain uses Dolby tone/pink noise test film with an oscilloscope to set the solar cell and preamp. The B-chain uses a special card that plugs in and generates the pink noise that is heard in the auditorium. Calibrated microphone(s) feed the pink noise into a real-time analyzer (RTA.) The technician observes the display on the RTA, and adjusts the "bass" and "treble" controls (only there are 27 controls) for a pre-specified tone. (If I remember, pink noise is rapidly and randomly-generated tones, ie; 1k, 1.2k, etc. who's amplitude decreases at a set rate as the generated frequency increases. White noise is the same, except the amplitude stays at a constant level.) This makes every auditorium sound the same.
Why divide the system in to two halfs? There was probably no difficult technical reason. They could have designed the processor such that you ran Dolby tone/pink noise test film and adjusted everything with that. But, dividing the system makes sense for practical reasons.
EQ'ing the B-chain can take a long time (I used to allow at least a half-hour.) The test film would probably break, especially in the acetate days.
Also, EQ'ing the B-chain requires turning on/off each channel. Dolby probably did not want to pay to put on every unit the extra signal switching circuits, pink noise generator and operator controls that would be needed to do this, especially for a infrequent procedure. Requiring a seperate pink noise generator card (Cat. 85 or eqv.) means only buying those circuits once, and you can also add troubleshooting options (+ pink noise or -pink noise.)
Lastly, if you run magnetic-striped film, (or any other audio input) that's an A-chain by itself. You can just set it up, without disturbing anything else. It works the other way, too; if a speaker blows out, after you replace it, you can (re)set the B-chain without messing with the A-chain.
Sorry if I said stuff you already know.... I typed too much again..... If I goofed, anyone, please jump in!
Posted by John Wilson (Member # 269) on 07-14-2000, 03:16 AM:
Tim said>>Let me check my manual here... {open mouth, insert foot}<<Blame that damn hot sun in Florida...that'll do it. (You might need these, Tim... 
)
Posted by Tim Reed (Member # 137) on 07-14-2000, 08:30 AM:
Let's see... 'get these on... Wow, THANKS!
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Better Projection Pays!
Posted by Bill Purdy (Member # 202) on 07-14-2000, 11:01 AM:
The original CP-100 had VCA's and the CP-50 could be had with the Cat. No. 117 output card which had VCA's or the Cat. No. 111 which did not and required the user to furnish a 4 gang potentiometer as a house fader. The lack of a fader built into the CP-50 was in a very real sense what got Component Engineering into the theatre equipment business. Our first serious monitor amplifiers had in them a house fader (either a single for the VCA's or a 4 gang pot) to compensate for Dolby's omission.
Posted by Scott Morrison (Member # 165) on 07-14-2000, 02:58 PM:
I think the cp200 manual is very poorly written for a novice. The CP65 Manual is the best explanation of A-Chain and B-Chain set up I've seen.
Posted by John Walsh (Member # 168) on 07-14-2000, 11:25 PM:
It's true that the CP200 manual is not too good. Actually, it's not that it's written poorly, it just looks like it was never finished. In my copy, there's a page that is supposed to have a picture, but instead there's a hand-written note that says something like; "Put picture of Cat.xx here..." Although it was witten in the late 1970's, I feel it was enough of a money-maker to justify the expence to finish it.Fortunately, it does have one thing CP65, 500, and 650 manuals do not have, and that's a complete set of schematics.
Posted by Ari Nordström (Member # 288) on 07-19-2000, 04:05 PM:
Speaking of the CP50, wasn't the omission of the fader because it was at first intended to be installed as an "add-on" in theatres that already had 4-channel mag systems? These theatres already had multi-channel faders and weren't about to get rid of them because of some stupid invention in the much neglected field of optical sound. At least that was what somebody claimed years and years ago...