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Surround playback can be grouped into two types: LtRt and Encoded Multitrack
LtRt is the "traditional" surround format. It is audio as opposed to the Encoded Multitrack which is a digital stream and needs decoding software or hardware to be heard. LtRt will play through a LR stereo speaker system, but when played through a cinema processor the surround audio is extracted and sent to the surround speakers. The processor does this by analyzing the phasing between the two channels. Straight LR stereo will also contain phasing and audio will be sent to the surrounds, but with the LtRt designation (left total right total) it means audio has been mixed down from a 5.1 mix. The most common encoding decoding processor for LtRt is called Dolby Pro Logic. A newer version, which makes a distinction between left surround and right surround, is called Dolby Pro Logic II. Ultra Stereo and Circle Surround are two other companies that make the hardware and software for LtRt encoding and decoding. You will find LtRt used on 35mm, HD, DVD, file based mediums like QuickTime and many other analog and digital mediums.
For Encoded Multitrack (5.1, 6.1 and 7.1) there are three main encoding digital stream formats: 1) Dolby Digital/Dolby Digital Plus which is used on 35mm, HD, DVD, Blu-ray and Quicktime. 2) Dolby E which is used on HD and 3) Dolby True HD which is used only on Blu-ray. Other than Dolby there are two additional encoding formats: DTS (Digital Theater Systems)and SDDS (Sony Dynamic Digital Systems). The encoding of a six and eight tracks is essential for saving space on the various playback mediums and for enabling a multitrack mix to play when available tracks are limited. An example is the case of HDcam with only four audio tracks: the LtRt is placed on tracks 1 and 2 and either the Dolby Digital or Dolby E stream is placed on tracks 3 and 4. The digital streams also enable theater and home cinema processors to do many other things like set proper listening levels and playback only LR if the speaker system does not have 5.1 capability.
The Surround Formats at Soundcrafter
All projects at Soundcrafter are mixed in 5.1and LtRt. We can also do 6.1 and 7.1 if desired. The LtRt is automatically collapsed down from 5.1 during the mix so both formats can be monitored and recorded at the same time. Each completed project will have at least 8 tracks: six for the 5.1 mix and two for the LtRt mix. The digital streams are created from the six tracks in either real time using hardware encoding or software plug-in encoding or through stand-alone encoding software. At Soundcrafter we are capable of encoding all formats except Dolby Digital for 35mm. (See the next paragraph.) All surround formats, either audio files or digital streams, are provided as files on a hard drive for the filmmaker to transfer at a video post facility or for making DVD or Blu-ray disks. The Final Cut Pro software, Compressor, can be used by the filmmaker to create either a Dolby Digital stream for DVD or a Dolby True HD stream for Blu-ray.
Dolby Licensing for 35mm
A Dolby license is not needed for an LtRt to be included on a 35 print but for a 5.1mix to be included it is required. The license involves a fee and the requirement that the printmastering of the Dolby Digital stream be checked at a Dolby approved stage. The stage must be no less than a certain size, with specific acoustics and speaker management. By doing this Dolby assures the filmmaker that the audio will play properly in a theater. The cost of the fee (between $3000 and $11000) depends on how many prints are made. The procedure involves hardware encoding of the multitrack mix to an AC3 digital stream and a LtRt PCM. Both formats are recorded to a magneto optical drive which is delivered to the 35mm optical lab.
Soundcrafter Procedure for Dolby Digital 35mm Printmastering
What do you do if you want a 5.1 mix to be included on a 35 print and the room is not large enough for Dolby approval? This is the case at Soundcrafter. Our procedure involves moving the last two days of the mix to the Austin Film Society/Austin Studio's screening room or to a Pro Tools Dolby approved stage either in Los Angeles, New Orleans or New York- which ever the filmmaker prefers.
So why is it that Dolby specifies that the mix room be a certain minimum size?
This has to do with the way volume and frequency output change as one moves in relationship to the speaker. The smaller the room, the smaller the listening position. Outside this position the frequency output can change dramatically. Low frequencies are a special problem- they tend to congregate against walls. Because of this dramatic frequency change Dolby specifies that the listening area be at least a certain size and that the mixer and client seating positions be within this area. Both the Soundcrafter Room 1 and Room 2 listening positions are too small for Dolby approval and that is why the end of the mix must be moved to a larger stage. On the other hand we have learned how to utilize the small space and many films mixed in Room 2 without Dolby licensing, have gone on to theatrical distribution and played wonderfully in large theaters. The plus side of completing most of the mix in a small room is the huge savings in room rental costs. Daily rental rates for a Dolby approved stage will be at least four times the rate for Room 2. The firm "Bernie" was recently mixed in the larger Room 1 and played perfectly in the King Soundworks Dolby approved stage in Los Angeles.
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Theatrical Presentation and Dolby Licensing Issues
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