Articles and whitepapers

1/6/2003

Surround Sound Formats - Part One - A Background

By Mark F. Davis

The goal of surround sound audio is to convey the spatial aspects of an audio performance more accurately than is possible with stereo or mono reproduction. The importance of preserving spatial audio cues was appreciated long before audio reproduction was possible, as indicated, for example, by the care and skill that has traditionally been brought to bear in the design of concert halls.

Since human beings have but two ears, it was initially hoped that two channels of audio would suffice for full spatial reproduction, and for headphones this has proven to be nearly the case. But loudspeaker-based stereo presentation is simply incapable of reproducing sounds from all directions, leading recording engineers to have to compromise between clearly defined direct imaging and a pleasing sense of ambience.

By using additional channels and speakers all around the listener, the compromises of stereo can be avoided, resulting in a presentation with good direct imaging, a compelling sense of ambience, and a much wider listening area (sweet spot).

A Little History

Investigations of reproduced spatial audio actually began not long after the invention of the telephone in the late 1800s. Alexander Graham Bell found that using a pair of telephone transmitters and receivers imparted a more natural quality to the sound, and in 1881, Clement Adler carried out a widely noted demonstration in which pairs of microphones were set up across the stage of the Paris Opera, with the crude binaural signals auditioned by guests in nearby hotel rooms using pairs of telephone receivers.

Clement Adler's 1881 Binaural Demonstration

Still, the primary focus of sound system development for the next few decades was on devising and improving basic methods of monaural sound recording and transmission, and it was not until the 1930s that serious attention was again paid to using more than one channel of audio. In 1931, British engineer Alan Blumlein developed a comprehensive infrastructure to support stereo sound reproduction (it was, alas, way ahead of its time), and in the years following, researchers at Bell Labs carried out a series of experiments and demonstrations with two- and three-channel stereo, leading to some initial tests of surround systems.

With the task of delivering multiple channels of sound to the consumer still economically unfeasible, it fell to the cinema industry to forge ahead with surround audio, culminating in the 1940 release of the film 'Fantasia.' For premiere and road show presentations, this film used a sound system called 'Fantasound' which employed a second, synchronised strip of film carrying multiple audio channels fed to as many as 10 speakers distributed around the theatre.

Further progress was interrupted by the Second World War, but the early 1950s saw the introduction of a number of cinema surround formats, including Cinerama, Perspecta-Sound, Cinemascope, and 70mm films with multiple channels conveyed on magnetic stripes placed along the edge of the film. Few films using these formats actually attempted to present a coherent surround sound field, and instead used the additional channels for spot effects, such as airplane flyovers. In any case, none of these systems was economical enough to be used for all releases, and most were eventually abandoned.

Cinerama Presentation Using Multiple Projectors and Sound Channels

Over on the consumer side, things heated up for a while in the 1970s, with the rise and fall of Quadraphonic sound systems, which really did not work terribly well, but for cinema use, Dolby Laboratories' matrix surround system finally achieved mainstream acceptance with the release of the first 'Star Wars' film. With the subsequent introduction of consumer video formats such as VCRs and laserdiscs, and an improved decoder dubbed 'Pro Logic,' the Dolby surround format migrated into homes, and consumer surround sound was finally established.

In the early 1980s, the digital audio Compact Disc was developed by Sony and Philips, and a decade later, movie companies adopted the use of discrete digital multichannel soundtracks employing Dolby Digital Surround, DTS Surround, and the Sony SDDS system. This in turn led to the use of digital multichannel soundtracks in consumer media such as laserdiscs, DVDs, and digital television, and the list of such formats is still growing.

Recording Surround Sound

The availability of these various surround formats has led to a flurry of activity on how best to record and present surround audio.

Cinema soundtracks are still prepared in layers, with separate dialogue tracks, usually mono and presented via the centre front speaker; music tracks, with the instruments usually spread across the front; and effects and ambience tracks, which may derive from stereo sources, but are processed into full surround. Movies still make extensive use of surround channels for spot effects, plus some low-level ambience, sometimes referred to as 'room tone,' but the emphasis in most cases is on keeping the viewers' attention focused on what's happening on the screen - leading to a somewhat restrained use of the surround channels.

Music recordings have fewer a priori constraints, which has led to something of a dichotomy in the techniques used. On the one hand, the more traditional approach is to emulate the typical live presentation, and place the performers in front of the listener, and use the surrounds for hall ambience and possibly audience sounds. A more aggressive approach is to make fuller use of the available channels, and array the performers all around the listener. An oft-cited advantage of the latter arrangement is that it allows some listeners to hear 'farther into' the mix, and be aware of melodic lines or other details that might otherwise be masked in a front-only presentation. At this point, there are few hard and fast rules on the matter, and the choice usually represents an aesthetic consensus of the producer, recording engineer, and performers.

By Mark F. Davis is the Senior Engineer at Dolby Laboratories, Inc.

www.dolby.com