Georgia Hilton MPE CAS MPSE Prod/Dir/Editor

Film Services Professional ( Producing / Editing / Delivery )


Dolby Surround LtRt info DOLBYsurround

Posted on February 28, 2010 at 12:55 AM

Dolby Surround LtRt info DOLBYsurround. LtRt Left total, Right total vs LoRo L only R only.. itstarted as a consumer version of Dolby's multichannel film format.

LtRt is a L C R S ( left center right and mono surround ) mix that isencoded down to a , and i'm going to use this very loosely, "stereo"mix. The "stereo" mix is really the four tracks encoded to 2 tracks.Then upon playback the LtRt can be heard in just plain mono or stereoor decoded back to L C R S. LtRt or Dolby Surround was the first ofDolby's multichannel film format. ( 1980 something ) The encoded filefits nicely on VHS or other 2 track device and then you get surroundoff of these 2 track systems.

The encoding process relies heavily on phase to encode and decode the positional information within the mix.

you can use a DOLBY SEU 4 or Dolby DP563 hardware devices to encodeLtRt, or you can create an LtRt optical track starting with a Dolby DMUor LtRt files... You can use the DOLBY surround tools in protools tocreate LtRt ( both encode and decode )...

remember that an LtRt track is just analog audio vs an AC3 or Dolby-Eencode which is a digital bit stream. You can create an AC3 encodeusing only L C R S very similar to an LtRt for DVDs with A-Pack orother AC3 encoder, but again it will create a digital bit stream, notan analog audio file.

to mix... just mix in L C R S... paying close attention to any couplingor phase issues with similar materials in multiple speakers... music,ambience etc. then downmix to LtRt and LoRo and mono checking each forpositing of material. When mixing you can use the Dolby Surround toolsto create 2 stereo aux tracks with the encoder on them Master( L,R )slave( C,S ) and them place the decoder (master, slave) after theEncoder so that you can monitor thru the encoder/decoder and simply hita button to hear whats going on in LCRS, LR, mono... the details willbecome obvious when you look at the software plugins...



PS: i think today the whole this is called DOLBY ANALOG by Dolby...sorry I haven't had my morning coffee so this reply is a littledisjointed.. ;)



some additional from Neil Wilkes across the pond...

There is Dolby ProLogic and Dolby ProLogic II, which are utterlydifferent systems despite the incremental naming protocol - DPL hasdual mono rear channels & feeds the same information to both Ls& Rs and DPL II uses proper separate Ls/Rs where available. Inaddition, all DPL II en/decoders are fully backwards compatible withthe earlier systems.

SRS Circle Surround is also a Matrix Lt/Rt technology.

Lt/Rt is basically a generic term taken to mean any multichannel mixthat is matrixed down into a stereo compliant stream & will playback on a stereo system, but if the stream is fed through the correctdecoder you will get the surround mix back out of it.

There are Pros & Cons to this.

Pros -

1 - you get to supply a single stream that will play back in stereo if no surround setup is present.

2 - It is widely used in TV/Broadcast.

3 - You can even take the Lt/Rt stream & further reduce to AC3(Dolby Digital) as long as you set the metadata flag (in a softencoder) or push the switch/button (in a hardware encoder) for "DolbySurround Encoded"

Cons -

1 - It is a compromise for both types. If you start with a fullydiscrete 5.1 mix, then it will *not* sound the same when decoded again.Neither will the stereo mix be as good as one that was especially mixedfor stereo.

2 - It's a matrix system - not discrete - and as Georgia points out there are all manner of possible pitfalls involved.

3 - It makes delivering an M&E mix much more complex as you wouldneed to completely reset the encoder resulting in a vastly differentsounding mix on a dubbed language version.

When I have to deliver an Lt/Rt stream I use a VST plugin en/decoder sothat switching between the various modes is extremely easy(source/encoded/decoded).

A good place to start research on this is at the Dolby Labs technical library where you will find a lot of helpful material. __________________

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