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» Film-Tech Forum ARCHIVE   » Operations   » Film Handlers' Forum   » Optical track matrixing. (Page 1)

 
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Author Topic: Optical track matrixing.
Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

- posted 10-17-2002 09:56 AM      Profile for Tim Reed   Author's Homepage     Send New Private Message       Edit/Delete Post 
Any movie sound mixers on the forum? I have a question.

What condition delivers a good solid center channel signal on the optical track? Here's my understanding of the Dolby/Sansui 2:4 matrixing scheme, as used on movie optical soundtracks.

A signal appears on one of the two optical tracks. That sound is reproduced from the left (or right, as the case may be) channel only. When the same signal appears on the opposite track (only), the sound is reproduced from the other side of the screen. When on both, in phase, it comes from the center.

My question has to do with the level of this center signal ON THE FILM, as compared to individual striations of L-only or R-only signals. What are the optical track conditions that reproduce the center channel in the theatre, to the same level as a lone individual track (L or R-only) would at the screen?

Is there something in the encoding that raises the level or modulation of the L and R striations of the optical track that specifically steer it to the center channel?

Here's what I am observing. I have a track with two equal (in phase) signals which seems to come, not from the center channel in the theatre, but from the left and right speakers simultaneously.

This isn't a reproducer or processor problem, it's a print master recording condition that I'm researching.

Thanks!

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Gordon McLeod
Film God

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

- posted 10-17-2002 10:27 AM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
The Dolby patent on there encoder lists
left as left
right as right
centre is placed in phase at .7 and .7 on the each channel
The surround is placed +90 degree .7 on the left and -90 degrees at .7 on the right channel

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Mitchell Dvoskin
Phenomenal Film Handler

Posts: 1869
From: West Milford, NJ, USA
Registered: Jan 2001

- posted 10-17-2002 10:52 AM      Profile for Mitchell Dvoskin   Email Mitchell Dvoskin   Send New Private Message       Edit/Delete Post 
> Is there something in the encoding that raises the level or modulation of the L and R striations of the optical track that specifically steer it to the center channel?

There is nothing encoded in the soundtrack striations to steer the sound. The front channel steering is entirely a matter of volume as you have stated. Identical L/R (in phase sounds) should be routed to the center channel. If this is not happening, either something is malfunctioning in your Dolby decoder or the stereo photo cell is out of alignment.

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

- posted 10-17-2002 11:09 AM      Profile for Rick Long   Email Rick Long   Send New Private Message       Edit/Delete Post 
Tim, any possibility that the Dolby is ok but the amps or speakers are out of phase with each other?

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

- posted 10-17-2002 11:50 AM      Profile for Tim Reed   Author's Homepage     Send New Private Message       Edit/Delete Post 
Thanks, guys, for the quick responses. It's not a theatre problem, it's the print master.

Gordon, does the .7 value refer to 70% below a L-only / R-only signal? That would explain why the sound appears to eminate from L and R simultaneously as it stands.

Thanks again!
-Tim

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Gordon McLeod
Film God

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From: Toronto Ontario Canada
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- posted 10-17-2002 01:53 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Yes it is .7 below
Also if there was a phase error in encoding the or in the camera it could decorillate the signals enough to keep it as hard lrft and right say equal level but a 90 degree shift

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Mark J. Marshall
Film God

Posts: 3188
From: New Castle, DE, USA
Registered: Aug 2002

- posted 10-17-2002 02:34 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
If the thing was recorded with the phase reversed for some reason ('+' & '-' crossed somewhere), then the sound should be coming out of the surrounds, correct? Sounds like the tracks are slightly out of phase, but not fully reversed. Sounds like a recording equipment alignment problem to me, but I'm no expert. Is it possible the track was recorded with a bit of synthesizing which would take a mono signal and make it 'sound like stereo'?

Mark

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Tim Reed
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From: Northampton, PA
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- posted 10-18-2002 12:27 PM      Profile for Tim Reed   Author's Homepage     Send New Private Message       Edit/Delete Post 
I think Gordon solved the problem. Thanks all!

Tim

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

- posted 10-18-2002 02:31 PM      Profile for Michael Schaffer   Author's Homepage   Email Michael Schaffer   Send New Private Message       Edit/Delete Post 
Two questions: I still don`t understand why the two channels are played back seperately in Tim`s setup. I know that the C signal is attenuated by 3dB when it is added to L+R to compensate for the channel addition. But what does this have to do with Tim`s problem.
Secondly, what are the specifications behind the 2-channel print master format as found for example in the CP500?
Michael

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Gordon McLeod
Film God

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From: Toronto Ontario Canada
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- posted 10-18-2002 10:25 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
The basic decoder is phase sensitive and if a small lead lag phase shift occurs the signals will not appear as hard centre but hard left and right
The principle is taken advantage of in the EX encoding to allow better panning and flyover effects

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

- posted 10-18-2002 10:46 PM      Profile for Michael Schaffer   Author's Homepage   Email Michael Schaffer   Send New Private Message       Edit/Delete Post 
So Tim`s master was simply not properly encoded? What about the 2-ch printmaster playback format?
Michael

I just reread Tim`s post. It had slipped my intention that the soundtrack wasn´t encoded, but supposed to be mono. So there must have been a phase shift or slight level discrepancy when the sound negative was recorded. OK.
Still, what about the printmaster format?


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Tim Reed
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From: Northampton, PA
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- posted 10-19-2002 01:02 PM      Profile for Tim Reed   Author's Homepage     Send New Private Message       Edit/Delete Post 
The print master wasn't created properly. It is a 2:4-channel track, but the parts that were supposed to be hard center sounded like they were coming from L and R. Left is left, right is right, and the surround worked great. Center just wasn't very well defined.

The problem was noticed in an answer print.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

- posted 10-19-2002 03:29 PM      Profile for Michael Schaffer   Author's Homepage   Email Michael Schaffer   Send New Private Message       Edit/Delete Post 
How did that happen? What equipment did you use to create the Lt/Rt-Tracks?
By the way, there is Dolby Surround Mixing Manual available from Dolby as pdf. Since the theatrical Dolby Stereo format and Dolby Surround are to a large extent similar (minus the noise reduction), this is quite informative. There is also a 5.1 mixing guide.
Michael

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Scott Norwood
Film God

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

- posted 10-19-2002 05:08 PM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
I, too, am curious about the printmaster formats on the CP-500:

26 - printmaster 2-ch. no NR
27 - printmaster 2-ch. a-type
28 - printmaster 2-ch. SR

I had always assumed that these formats were intended for 35mm mag tracks to be played through an external preamp (MPU-1, perhaps) and then fed through the matrix/eq/output and, optionally, the NR stages in the processor. Thus, the only difference between, say, 27 and 04 (Dolby A-type) would be the input source (external mag. vs. optical preamp), correct? Or is there more to it than this?

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

- posted 10-19-2002 05:50 PM      Profile for Michael Schaffer   Author's Homepage   Email Michael Schaffer   Send New Private Message       Edit/Delete Post 
I don`t know (obviously, since I asked about it), but that doesn`t seem likely to me as 35mm magnetic is not matrixed but discrete channels. It would seem logical to me that this option is for mixing studios to check the matrixed 2-channel magnetic tape before recording the sound negative on film, but if so, couldn`t that tape be played back by the standard film formats? Probably the only difference between film and printmaster formats is the input selected for the format (should be accessory rack), but I can`t verify that since I don`t have a CP500 within reach. The way to check that would be to select custom format, copy one of the pre-configured printmaster formats and then check the parameters of the format. Maybe someone who has a CP500 could do that.
Michael


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