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This topic comprises 2 pages: 1 2
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Author
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Topic: Question on terminology
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Tim Reed
Better Projection Pays
Posts: 5246
From: Northampton, PA
Registered: Sep 1999
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posted 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. ------------------
Better Projection Pays!
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Tim Reed
Better Projection Pays
Posts: 5246
From: Northampton, PA
Registered: Sep 1999
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posted 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." ------------------
Better Projection Pays!
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 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!
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 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.
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