Film-Tech Forum ARCHIVE: Analog Dolby SR and Surround decoding

This is topic Analog Dolby SR and Surround decoding in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Marin Zorica (Member # 1716) on 04-23-2004, 06:17 AM:

In analog dolby processors like CP-45 what are principles of SR sound processing and decoding of surround chanels?

I'm interested what sound in SR cat going to, and what is procedure for decoding it to surround(how we get sound from that), another thing is if we from stereo solar cells put pre-amp and from pre-amp record sound to pc, can we get L, R, S, C, chanels or SR NR editing sound what is recorded in programs like Adobe Audition (cool edit), Wave Lab or similar.

Can you tell me this in detail if somebody know this.

Thanks, Marin

Posted by Steve Guttag (Member # 268) on 04-23-2004, 08:29 AM:

With respect to the surround channel, there is nothing special about which NR is used (if any).

The way Dolby SVA films work is by a complimentary process. You start with 4 distinct channels, Left, Center, Right and Surround.

The Surround channel is then encoded with a Dolby-B process and then matrixed into two channels, Lt and Rt (stands for Left-Total and Right-Total). At that point, depending on the recording medium, the Lt, Rt may be encoded with noise reduction. Nowadays, virtually optical stereo tracks are encoded with Dolby-SR.

So, when you receive a print that has a Dolby-SR optical soundtrack. It is at the stage of being an Lt, Rt mix with Dolby-SR encoding. Step-by-step all of that encoding must be decoded in the exact reverse order.

First, the Dolby-SR must be decoded (in the case of the CP-45, by the Cat. 222 "SR"/A.

The soundtrack is still matrixed into the Lt, Rt configuration and must also be decoded.

In the case of the CP-45 it decoded on the Cat. 511 Main board. The decoder is a single chip solution on the Cat. 511. The decoder will turn the Lt, Rt channels back into the individual Left, Center, Right and Surround channels. However, the Surround channel was encoded with Dolby-B as well so it must have a Dolby-B decode applied to it and a bandpass filter applied (100Hz-7KHz). The bandpass filter dates back to far inferior encoder/decoders where it was impractical to have surrounds with real bass real high frequencies. That legacy is with us to this day but when they record the films, they use a 4:2:4 encoder/decoder (encodes the soundtrack and immediately decodes the soundtrack) to evaluate the mix. As such, they can apply boost to the mix to nearly overcome the bandpass filter. This special device also lets them know how the stereo spread is going to sound in theatres and they can compensate for any narrowing of the stereo image from the matrixing process.

Did that answer you question?

Steve

Posted by Christopher Seo (Member # 7) on 04-23-2004, 10:20 AM:

I have a question, Steve... where is the Dolby-B located, on the matrix card? Thanks.

Posted by Gordon McLeod (Member # 33) on 04-23-2004, 10:41 AM:

On the older piggybacked cards it was part of the delay line just after the bucket brigade delay

Posted by Steve Guttag (Member # 268) on 04-23-2004, 01:02 PM:

Depending on the processor and/or card, the Dolby-B decoding is done in different places. In the case of the CP-45, it is done in the matrix decoder chip (it is an Analog devices chip, part of their SSM line).

Steve