Surround Sound - Part I

Surround sound in movie theaters has been around much longer than most people realize. Many movies, beginning in the 1950s, used several tracks of audio on the film to give the sense of being surrounded by sound. These movies had discrete (separate) tracks for each of several speakers behind the screen, and one for an array of speakers to the sides and behind the audience, surrounding them in sound.

Surround sound lets the movie makers "recreate" the sound of the spaces and events they are portraying on screen.  By putting into the surround speakers the sound of chirping birds, wind whistling through trees, or a helicopter just about to appear on screen, the atmosphere is more realistic and believable, immersing us further into the movie.  In addition, having several separate channels of sound behind the screen lets the movie maker provide a much more convincing sonic image up front since the sound of a car whizzing across the screen can move with the image of that car.

Back then, multiple channels of sound were only possible by putting magnetic sound tracks (like there are on magnetic recording tape) to the side of the picture frame on the big 70mm film prints.  Because these prints were expensive, economies of scale drove them out of the mainstream by the late 60s.  Surround sound was only available on the biggest budget films and even then, only in big "first run" 70mm capable theaters in major cities.

In 1977, Star Wars brought us not only a new genre of science fiction film, but a new sound format that would impact motion pictures like no other. Dolby created a way to deliver four channel soundtracks optically on ubiquitous 35mm film.  Through their experience with noise reduction technology, Dolby was able to put two optical audio channels on the film in the space previously occupied by the Academy Optical Mono track. They embedded a center and surround channel into these left and right optical tracks. This is called a Matrix. When decoded in the theater, the system yielded three screen channels and one surround. The thrill of multiple screen channels and surround sound was now available for all movies at reasonable production costs. By the mid 80s, virtually every commercial movie release featured four-channel Dolby Stereo sound.

In 1992, with the release of Batman Returns, Dolby unveiled the next generation of motion picture sound, Dolby Digital (a.k.a. Dolby Stereo SR-D). Dolby Digital (DD) features three front screen channels, two surround channels, and one LFE (Low Frequency Effect) track. The sound is stored on the film in digital form, and all channels are discrete, meaning that each channel is a distinct or separate track in the recording, rather than the rear channel and front center channel having to be extracted from two stereo channels like it is with Dolby Stereo.  Today, just about every commercial movie release features a Dolby Digital Soundtrack.