Sound Formats

Sound is of huge importance to any home cinema. This page guides you through the numerous sound formats used in the home and in a movie theatre, including Dolby Digital, DTS, Dolby Surround and THX, plus some background about lesser used formats.
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This page describes the history of cinema sound and how it evolved into the home cinema formats we use today. Use the links below to learn more about the sound formats.  
  MONO DOLBY DIGITAL SURROUND-EX  
STEREO DTS
QUADRAPHONIC STEREO DTS-ES
DOLBY STEREO MPEG-II MULTICHANNEL
DOLBY SURROUND SONY SDDS
DOLBY PRO-LOGIC THX SOUND SYSTEM
DOLBY DIGITAL SOUND TIMELINE

 

INTRODUCTION

We take sound in the cinema for granted as if it has always been there, but the technology has been changed and developed over a relatively short period. Film before the 1920's contained no sound, with these silent 'flicks' being accompanied by music played live from within the theater as the picture was shown on the screen. The 1920's heralded the birth of a real soundtrack played along with the picture. This was done by recording an optical soundtrack on the actual filmstrip at the edge of the film itself. A linear audio 'picture' was recorded in the film. As the film passed through a projector, a beam of light was passed through the optical strip and created an image on a solar cell behind the film. This image was converted into an electrical signal corresponding the variations in the optical track and ultimately amplified and passed to the speakers as a mono signal. Movies with sound became the the normal standard by the mid-1930's, and for the next few years sound engineers had an impossible task upgrading existing theaters and adding the new sound technology to new venues within a very short time period. For a few years, a new technology was used using magnetic audio tracks. Two iron oxide strips were added to the edges of the filmstrip onto which audio information could be recorded and was read by magnetic heads, much the same way as audio tape cassettes. Magnetic sound was a big leap forward over previous methods and offered higher fidelity and overall sound quality. It also allowed recording of multiple tracks required for stereo sound, including tracks for a centre and rear speakers.

 
   
In the late 1970's, new technology was being developed that would change the movie industry forever and revolutionize cinema sound forever. Dolby Laboratories developed a way of recording a stereo optical soundtrack onto the filmstrip in the space previously occupied by the optical mono track. The encoding and decoding process was simple and practical, allowing recording of true cinema four track audio, and eventually replaced magnetic sound completely during the 1980's. This new technology was the first incarnation of what we know as Dolby Stereo today. This new stereo format was adopted by theaters almost universally and more than 25,000 venues have made the necessary conversions so far. The format was further improved in 1987 with the introduction of Dolby SR, or Spectral Recording, that provided technology for both the encoding and decoding of four channel stereo sound and could easily be added to existing theaters at very little cost. Dolby SR used a much improved noise reduction system that allowed sounds to be recorded louder with a higher frequency response but less audio distortion.  
   
The latest development in cinema sound came in 1992 with the introduction of the six channel (5.1) digital optical standard known as Dolby Digital. It uses a similar method of recording and decoding to that used in stereo optical tracks, but because a digital signal takes up less space on the film due to data compression, audio tracks could be encoded for the front left and right speakers, a centre dialogue speaker, two stereo rear surround speakers and a separate bass channel for feeding a sub-woofer. The new digital technology had to be implemented in a way that allowed an analog stereo soundtrack to be encoded on the same film - theaters could not be expected to operate two incompatible sound systems for digital and for analogue (not to mention the fact that two versions of every film would need to be released!). The problem was solved by recording the digital tracks on the filmstrip in the gap between the sprocket holes, allowing the normal stereo track to be recorded in the existing way beside the picture area.  
   
There are currently a number of sound formats in use today for both theater sound and home cinema. Use the menu for an overview of all the major types.