22-09-2008, 10:14 AM
Introduction
There are many surround systems available in the market. They use different technologies for produce surround effect. Some Surround sound is based on using audio compression technology (for example Dolby ProLogic? or Digital AC-3?) to encode and deliver a multi-channel soundtrack, and audio decompression technology to decode the soundtrack for delivery on a surround sound 5-speaker setup. Additionally, virtual surround sound systems use 3D audio technology to create the illusion of five speakers emanating from a regular set of stereo speakers, therefore enabling a surround sound listening experience without the need for a five speaker setup.
A history of multichannel audio
Film sound, television audio, and music playback formats used to be distinctly different products of industries often working in isolation with out any standardization. In recent years, however, this has changed. The popularity of surround sound in the home has brought these industries and their sound formats closer together.
Origins of surround sound
The first commercially successful multi channel sound formats were developed in the early 1950s for the cinema. At the time, stereophonic sound, a concept new to the public,
was heavily promoted along with new wide-screen formats by a film industry feeling threatened by the rapid growth of television. Unlike the two-channel format later adopted for home stereo, film stereo sound started out with, and continues to use, a minimum of four channels.
With such film formats as four tracks Cinemascope (35 mm) and six-track Todd-AO (70 mm), multiple sound channels were recorded on stripes of magnetic material applied to each release print. To play these prints, projectors were fitted with playback heads like those on a tape recorder, and cinemas were equipped with additional amplifiers and speaker systems.
From the outset, film stereo featured several channel across the front, plus at least one channel played over speakers towards the rear of the cinema. At first the latter was known as the effects channel, and was reserved for the occasional dramatic effect-ethereal voices in religious epics, for example. Formats such as six-track 70 mm magnetic provided consistent signal-to noise ratios on all channels, so mixers could use the effects channel to envelop the audience in continuous low-level ambient sounds. The effects channel came to convey greater sonic realism overall, not just the occasional dramatic effect. This expanded, more naturalistic application came to be known as surround sound, and the effects channel as the surround channel. The extra speakers at the rear-and now along the sides of the cinema as well to create a more diffuse sound field-came to be known within the industry as "the surrounds." Which led to the evolution of "Home Audio".
