Understanding the Audio of 3D

Sunday, July 15, 2012

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Rather than a simple speaker or two suspended above the audience, an array of top speakers plus the standard horizontal array are driven by up to 64 channels of source audio creating a multi-dimensional and fully immersive experience.

With all of the momentum to bring 3D video to consumers, Tim Carroll,  Founder and President of Linear Acoustic urges that we don't forget about audio.  

Certainly today there is enough to worry about with audio, from loudness to audio/video synchronization, and even integrating the enormous legacy of mono and stereo material. However, the history of sound is a roadmap filled with clues about what broadcasters might do to captivate consumers next.  

A (Very) Brief History of Multichannel Sound

The BBC made the first stereo broadcast as a test in 1925.  Stereo sound was further pioneered in the 1930’s by teams at Bell Laboratories and of course the venerable Alan Blumlein from EMI.  While Disney’s Fantasia was released with a stereo soundtrack in 1940 (to theatres with monstrously expensive and complex reproduction systems), it took until the late 1950’s for stereo to even begin to reach the masses via phonograph discs. Stereo sound provides a left to right horizontal dimension, and with good quality equipment, a good recording, and a chair exactly in the so-called “sweet spot” it can sound three dimensional as well.  

Film once again pushed the envelope and expanded the sweet spot to hundreds of seats. Incredible as it might seem, multichannel sound was part of most major motion pictures released in the 1950’s, although the reproduction systems in theatres were still quite large, complex, and expensive.  More practical 3-D sound arrived for consumers in the form of "Quad" in the early 1970’s and although a good stepping stone, was more a fad that a serious contender.  Through significant improvements, this eventually resulted in matrix-based consumer surround systems such as Dolby® Pro Logic becoming inexpensively available in the 1980’s.  

With the dawn of the digital audio age, the 1990’s brought higher performance discrete 5.1 channel sound first to movie theatres, of course, then to laser disc, DVD, and digital television for consumers.  Films were once again being mixed discretely and many of the original matrix encoded film soundtracks have been remixed for new discrete formats.  This time it truly caught on, and with prices and complexity driven to new lows, consumer adoption continues to be brisk.

Back to the Theatre
Discrete 5.1 channel sound delivers a convincing multi-dimensional image: left to right and front to back and the sweet spot is very wide so everyone in a room can enjoy the results. What is missing for an enveloping four dimensional aural experience is the vertical dimension, the so-called "Z-axis."  Solving this once again took its roots from film where formats such as IMAX® use the horizontal plane augmented by height channels.  

The new Dolby Atmos™ system takes this even further. Just like additional system channels and speakers in the horizontal plane results in a much smoother and more convincing 2-D surround experience, more channels and speakers in the vertical plane improves the 3-D experience. Rather than a simple speaker or two suspended above the audience, an array of top speakers plus the standard horizontal array are driven by up to 64 channels of source audio creating a multi-dimensional and fully immersive experience. These source channels can be manipulated to create compatible content for theatres not yet equipped with Dolby Atmos, and perhaps for eventual consumer release.

Figure 1 – Dolby Atmos speaker truss from the new Dolby Theatre in Los Angeles (photo courtesy of Dolby Laboratories).

Consumer Side
If history is in fact prologue, the next step would be towards the consumer. Unlikely that the full multi-array speaker system driven by 64 channels will hit home any time soon, it may not be surprising that there might be a way to capture some of that within the consumer space more immediately.

Whilst the Blu-ray Disc format supports 7.1 discrete audio channels, there is not yet a widely deployed method for transmission of additional height channels.  However, there are new ways to carry this height information within the ubiquitous 5.1 channel Dolby Digital (AC-3) system that do not require additional discrete channels, thus maintaining perfect compatibility with any plant or system capable of handling 5.1 channel audio today.  It is rare that a major dimension can be added to existing systems without requiring major infrastructure changes.

Most all new home theatre A/V receivers (AVRs) include Dolby Pro Logic IIx (PLIIx) which can create 7.1 surround from stereo or 5.1 channel inputs.  The additional channels are used to reinforce the existing surround channels enabling side-mounted or back-mounted surround speakers.  

Adding side or back surround speakers might not easily pass the so-called “household vote.” However, adding height channels can be aesthetically unobtrusive with the additional speakers being installed above the left and right front speakers. This could possibly as easy as placing speakers on top of an entertainment center and rehearsing this response to the inevitable questioning: “Those old things? They have always been there…”.  

A slight variation of PLIIx called Dolby Pro Logic IIz (PLIIz) is also found as a standard feature in most new 7.1-channel AVRs.  This format uses the two additional channels not for increased surround information, but instead for Lh and Rh (h=height), or LvRv (v=vertical).  The front channels remain completely discrete and the additional height channels are encoded into the Left and Right surround channels of a standard Dolby Digital 5.1 signal. After decode, the height information is then extracted from the left and right surround channels.  Although a PLIIz encoder is required to take best advantage of these channels, it will also work relatively unobtrusively with content that has not been specifically encoded for the format, making the system backwards compatible with existing content.

Figure 2 – Dolby PLIIz Encoding to compatible 5.1 channel output

The next step would be for a discrete method for carrying the vertical channel information. Matrix techniques can be very effective, but can be affected by channel conditions and always require a bit of compromise for compatibility. Discrete generally does not have these restrictions and thanks to metadata can be compatible all the way down to mono.

Enhanced AC-3 (Dolby Digital Plus)
Happily there is a codec that has envisioned the need for not only passing more than 5.1 channels of audio (there are several codecs which can do that). Under the auspices of carrying Descriptive Video programs as part of the main program, Dolby Digital Plus or E-AC-3 can carry at least 7.1 channels. Comprised of up to a 5.1 channel main program and an associated mono or stereo descriptive track, the audio can be mixed within the decoder to provide compatible 5.1 channel and stereo outputs. However, E-AC-3 also includes the metadata signaling required to indicate that the additional channels carry vertical plane information and not additional surrounds for the horizontal plane. 

E-AC-3 is capable of more than 5.1 channels and even more than 7.1 channels, but consumer systems continue to be released to the marketplace with support for at least 7.1 channels. Again, the intent was for more horizontal surround speakers but the capability for using them instead for vertical height channels is also present. Setting the metadata in the E-AC-3 encoder presents at least an initial challenge, and much of the content will have to be created with these parameters preset during file-based encoding, or an E-AC-3 encoder will have to be told to set the parameters live. Luckily, both methods can be accomplished today, although quite a bit of testing lies ahead.

Figure 3 – Encoding of 7.1 channel discrete audio 

The Future
Broadcasters looking for a compatible edge to their offerings today should investigate the possibilities of adding the vertical dimension to their high profile events.  With advances in signal processing and encoding technologies, taking audio to the next level with or without 3-D video could be a very cost effective choice. Encoding height information into a standard 5.1 channel program is possible today and will not affect viewers with mono, stereo or 5.1 channel playback systems, and can provide a unique treat for those who install height speakers.

On the discrete front, with the advent of Dolby Atmos, producers are beginning to amass the audio sources for 3-D content and developing the production experience for this new format. The industry will have the tools to produce consumer versions that might well take advantage of the barely hidden treasures contained within a large number of consumer devices. 

International broadcast standards already document newer transmission systems such as Dolby Digital Plus which supports fully discrete 7.1 channel audio and a large majority of consumer devices inherently support decoding this format. It is only a matter of time before someone figures out a way to make the path from vertically enabled content to the home, and although it might not be as sophisticated as the theatre-based systems of today, remember what history hath wrought.

Detailed technical information regarding E-AC-3 can be found within the ATSC A/52-2012 standard available for free at www.atsc.org. Information about the new Dolby Atmos system can be found at www.dolby.com. 


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