Wednesday, August 22, 2007

Surround Sound Beyond 7.1

Dolby Surround Sound [Dolby Stereo] was first heard in cinemas for the release of the first Star Wars film [Episode 4 - A New Hope] in 1977. The film soundtrack contained a matrix where Left, Centre, Right and a mono Surround channel were encoded into two channels and then stored as the optical Left [Left Total] and Right [Right Total] stereo tracks on the film strip. This technology is also used in home theatre today for both over the air broadcasts and packaged media. A Matrix decoder like Dolby ProLogic is used to recover the four channels of sound.

Dolby Pro Logic
Dolby Pro Logic is a Phase/Amplitude matrix decoder and can output up to 4 channels from any 2 channel source. Being a matrix system, it has limitations that only a fully discrete system can surpass.

Dolby Digital
Dolby Digital was first heard in cinemas with Batman Returns in June of 1992 and allows 5 full range channels [shown above] as well as a dedicated Low Frequency Effects [LFE or .1] track to be stored as a digital data stream. Today there are three competing formats based on the same layout - Dolby Digital, DTS and Sony Dynamic Digital Sound [SDDS is only in cinemas].

Dolby Digital Surround EX

NOTE: The above diagram shows the Back Surround as a single channel. In the home we play this single channel over 2 Back Surround speakers - hence the name 7.1, but it is in fact driven by 6.1 program.

Whilst SDDS upgraded their system from the conventional 6 channels to 8 channels [allowing playback of special mixes of soundtracks], not much changed until 1999 with the introduction of Surround EX - a joint venture between Dolby and THX.

By combining the older Dolby ProLogic technology with the newer Dolby Digital format, it became possible to matrix encode a Back Surround channel into the standard Left and Right surrounds of the 5.1 format retaining full backwards compatibility with all 5.1 systems using Dolby Digital.

If we are to sit between the speakers of a good 2CH system, we will hear parts of the program as a phantom image. The centre channel of ProLogic simply replaces that phantom with a real channel so that if we are not seated in the centre, the sound remains fixed in the middle of the two speakers and this is the purpose of adding Back Surround in Dolby Digital EX. But why should it end there?

Like EX, it is possible to further enhance the existing surround sound system by adding additional channels through a matrix - however the end results are unpredictable unless the soundtrack is specially mixed to support these extra channels.

Tom Holman's 10.2 System
Tom Holman [now TMHLabs] created what he calls the "10.2 Switcheroo" allowing the system to be configured to either 7.1 or 10.2 channels over analogue connections. His demonstrations use discrete audio but it might be possible to create a matrix version using 2 out board decoders like Smart Device's Circle Surround CS3xJr.


The CS3xJr was one of the first outboard matrix decoders released in 2000 for the intended purpose of allowing Surround EX soundtracks to be decoded and played in the home. It is designed to go between the pre-outs and amp in of separate components and can process any 2 channels to create 4 [Smart claims up to 5.1] channels of sound.

Plan View Of One Possible System
The proposed plan above is based on THMLab's 10.2 system, but would use two Smart Device CS3xJr matrix decoders that would be employed so that their outputs would produce two additional channels per side - a WIDE channel [for off screen audio cues] and a HEIGHT channel [the subject of much debate].

The purpose of the "WIDE" channels is actually to compensate for the fact that whilst we can easily hear phantom images in both the front and back of the room, we can not hear them at the sides very well. We can however localize the sound cue to any point if originating from a real source, so adding a real channel to replace the phantom has merit.

Whilst the screen [including the two new WIDE] channels would all be at the same height [seated ear height], the surround [including the new HEIGHT] channels would all be elevated. Whilst I've drawn the front channels at 22.5 degrees apart, they could be as far apart as 30 degrees.

A Quassi SDDS System
Another way to add more channels would be to split the Centre Channel and combine one half with the Front Left and the other with the Front Right to create Left Centre and Right Centre as is used with SDDS 8CH systems

Like TMH Labs 10.2 system, the SDDS 8CH system is also fully discrete. The system originally supported 6 tracks [5.1] like the two competitors [DTS and Dolby Digital], but was upgraded to add two extra centre channels increasing the number of screen channels from 3 to 5 - the extra channels are Left Centre and Right Centre.

DTS 5.1[Core]
When DTS was first released to the home, it was stored in the PCM portions of Laser Disc and on CD. It was able to deliver much higher data rates than Dolby Digital.

DTS 6.1 [Discrete]
Whilst both Dolby EX and DTS ES offer a Back Surround channel that is matrix encoded in the Left and Right surrounds of 5.1 program, DTS also offers a fully discrete 6.1 channel system for the home. The system uses CORE data for the normal 5.1 program, and EXTENSION data for the discrete Back Surround. A clever phase inversion technique cancels out the matrix version from the Left and Right Surrounds leaving three fully discrete surround channels.

The latest versions of DTS [DTS-HD and MA] carry additional data that is added to the core data for true high definition surround of up to 7.1 channels. The core data allows the format to be backwards compatible with even the oldest DTS decoders.


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