Dolby Sound at the Movies

The Evolution of Dolby Surround Sound

Going to the movies today is more exciting and involving than ever, thanks in large part to a continuing effort to improve film sound started by Dolby Laboratories in the early 1970s. The following outlines a history of cinema sound over the past two decades. For a more in-depth look, see Surround Sound: Past, Present and Future.

Optical Soundtracks

The photographic, or "optical," soundtrack was the first method of putting sound on film. Today it remains the standard, in both analog and digital forms.

The optical soundtrack is located on an opaque area adjacent to the picture. As the film is played, a beam of light from an exciter lamp or LED in the projector's soundhead shines through it. Variations in the width of the soundtrack cause a varying amount of light to fall on a solar cell, which converts the light to a similarly varying electrical signal. That signal is amplified and ultimately converted to sound by loudspeakers in the auditorium.

 

The optical soundtrack offers economy, simplicity, and durability, all of which has contributed to its universal acceptance. Since the soundtrack is printed photographically on the film at the same time as the picture it can last just as long as the film itself.

Magnetic Striping and Multichannel Sound

In the early 1950s, as the film industry sought to woo viewers away from their fascinating new television sets, magnetic striping was introduced to carry the soundtrack on film prints. After the picture was printed, narrow stripes of iron oxide material (similar to the coating on magnetic recording tape) were applied to the release print.

The sound was then recorded on the magnetic stripes in real time. In the cinema, magnetic prints would be played back on projectors equipped with magnetic heads similar to those on a tape recorder.

Magnetic sound was a significant step forward, and at its best provided much-improved fidelity over the conventional optical soundtrack. It also enabled the first multichannel sound reproduction, dubbed "stereophonic sound," ever heard by the public. The voice of an actor appearing to the left, center, or right of the picture could be heard coming from speakers located at the left, center, or right of the new wide screens also being introduced at this time. Music took on a new dimension of realism, and sound effects could emanate from the rear or sides of the cinema.

By the 1970s, however, the magnetic stripe fell into decline. Its expense, short lifespan, and high maintenance costs were the culprits. Moviegoers were again hearing low fidelity, mono optical releases most of the time, with only an occasional multitrack stereo magnetic release. Ironically, home hi-fi stereo systems of the time offered better sound than most movies.

Enter Dolby

By the late 1980s, however, almost all major titles were being released with wide-range multichannel stereo soundtracks thanks to new technology and a turnaround in the industry.

The technology breakthrough was the development by Dolby Laboratories of a highly practical 35 mm stereo optical release print format called Dolby Stereo. In the space allotted to the conventional mono optical soundtrack are two soundtracks that not only carry left and right information for stereo sound, but are also encoded with a third center-screen channel andÑmost notablyÑa fourth surround channel for ambient sound and special effects.

This format not only enabled stereo sound from optical soundtracks, but higher-quality sound as well. Various techniques were applied to the soundtrack during both recording and playback to improve fidelity. Foremost among these was Dolby noise reduction to lower the hissing and popping associated with optical soundtracks, and loudspeaker equalization to adjust the cinema sound system to a standard response curve.

The result was multichannel sound capability equaling that of four-track magnetic 35 mm (which soon became obsolete), with consistently higher fidelity, greater reliability, and far lower costs.

The Next Step: Dolby SR

In 1986, Dolby Laboratories introduced a new professional recording process called Dolby SR (spectral recording) that provided more than twice the noise reduction of Dolby A-type, and, moreover, permitted loud sounds with wider frequency response and lower distortion. The 35 mm optical soundtracks treated with Dolby SR not only sounded superb in cinemas equipped with new Dolby SR processors, but also played back satisfactorily in all cinemas. As a result, today, the analog soundtracks on virtually all prints are Dolby SR tracks.

The Digital Age Begins

The next film sound development from Dolby Laboratories was Dolby Digital, introduced in 1992. Dolby Digital puts a six-channel digital optical soundtrack (in addition to the four-channel SR analog track) on 35 mm prints.

 

Yet another significant step forward, this format provides independent Left, Center, Right, Left Surround, and Right Surround channels, plus a subwoofer channel for low-frequency effects.

 

The SR analog track allows prints to play conventionally in any cinema not equipped for Dolby Digital.

Dolby Digital provides extraordinary dynamic capability, wide frequency range, low distortion, and relative immunity to wear. Its combination of high quality, reliability, and practicality has been proved in cinemas around the world, and today it is the most popular digital format, with the most cinemas equipped worldwide and the most releases.

Dolby Digital Surround EX

The latest Dolby sound technology, Dolby Digital Surround EX, was introduced in 1999, and adds a third surround channel to the Dolby Digital format. Enabling improved realism, more precise sound placement, and exciting special effects, the new channel is reproduced by rear-wall surround speakers, while the left and right surround channels are reproduced by speakers on the side walls.

As with all Dolby soundtrack improvements, Dolby Digital Surround EX is backward-compatible, with prints playable in all Dolby Digital cinemas, whether or not they are equipped to decode the additional surround track.