The Science Behind Movie Sound

Imagine watching a movie at home without any sound. The absence of audio dramatically alters the experience. Sound, particularly dialogue, clarifies the narrative and enhances comprehension. Beyond that, it adds depth and emotional resonance to every scene. Without sound, most films would lose their appeal. When we visit cinemas, we anticipate audio that is as immersive and thrilling as the visuals on the screen.

In this installment of Mytour, discover the workings of analog and digital sound systems. Additionally, explore the three primary digital audio technologies:

The evolution of movie sound has been remarkable. As far back as 1889, Thomas Edison and his team were exploring ways to sync sound with moving images. Warner Brothers made history in 1926 with the release of "Don Juan," the first commercial film featuring synchronized recorded sound. While "Don Juan" included a musical score, it lacked dialogue. A year later, Warner Brothers introduced "The Jazz Singer," which featured music, sound effects, and a handful of spoken lines, marking the true arrival of sound in cinema.

Analog Sound

In the early days of cinema, the method for delivering sound was remarkably straightforward. Vitaphone, utilized in "The Jazz Singer," involved a record player that played a wax disc. This technique was referred to as sound-on-disc. Typically, the sound was recorded after the movie was shot. The disc was played on a turntable that synchronized the audio with the film by adjusting the projector's speed. It was a simple yet highly effective way to incorporate sound into movies.

By the early 1930s, sound-on-film started to replace sound-on-disc as the preferred method for adding soundtracks to films. A fascinating aspect of sound-on-film is that the audio is positioned several frames away from the corresponding visuals. This is due to the placement of the audio pickup, or reader, which is located either above or below the lens assembly of the projector. Most analog pickups are situated in the basement (below the lens), while digital pickups are typically in the penthouse (attached to the top of the projector). A test film is used to align the sound with the picture. Once calibrated, projectionists can splice the film confidently, ensuring proper synchronization.

Sound-on-film relies on one of two technologies:

The predominant method is an optical process, where a transparent line is recorded along the film's edge. This line changes in width based on the sound's frequency, earning it the name variable-area soundtrack. As the film moves past the audio pickup, an exciter lamp emits a bright light, which is focused through the transparent line by a lens. The light passing through the film then hits a photocell.

The photocell converts the light into an electrical current. The intensity of the current depends on the amount of light the photocell receives. Wider sections of the transparent strip permit more light, increasing the current produced by the photocell. As the strip's width fluctuates, it modulates the light, creating a variable electric current. This current is then sent to a pre-amplifier, which enhances the signal before passing it to the amplifier. The amplifier then routes the signal to the speakers.

An alternative approach is called the variable-density soundtrack. Instead of varying in width, this method uses a strip that changes in transparency. The greater the transparency, the more light passes through. However, this technique faces a significant drawback: the inherent graininess of the film can introduce considerable background noise.

During the 1950s, magnetic recording gained popularity. Magnetic sound-on-film offered several advantages over optical methods at the time:

However, there were also some drawbacks:

Despite magnetic recording offering up to six separate sound tracks on a film, the high costs made it impractical. Attempts were made with stereo optical tracks, but excessive noise rendered them ineffective. However, when Dolby Laboratories launched Dolby A in 1965—a noise reduction technique initially designed for professional studios—the film industry saw a chance to revolutionize optical soundtracks.

Dolby A divides the audio signal into four distinct bands. Using a method called pre-emphasis, each band's signal is amplified above 10 decibels, the threshold for ambient noise. Each signal then passes through a compander, where it is compressed to remove low-level noise and then expanded. The combined signals produce a much clearer sound.

The primary trade-off with Dolby A is a reduced frequency response, leading to a smaller dynamic range. Dolby noise reduction has advanced from Dolby A to Dolby Spectral Recording, a more refined process that cuts noise by twice as much.

In 1971, "A Clockwork Orange" successfully utilized Dolby A on magnetic sound-on-film. During the early 1970s, Eastman Kodak collaborated with RCA and Dolby to develop stereo variable area (SVA), an optical technique that enabled stereo sound by fitting two variable-width lines into the space previously reserved for one.

Surround Sound

Surround sound made its debut with Walt Disney's "Fantasia" in 1941. To present the movie with surround sound, theaters needed to invest $85,000 in a specialized setup featuring custom loudspeakers and two projectors—one for the film and one audio track, plus another for four additional audio tracks.

Due to the high cost, the full surround-sound system was only installed in two theaters: one in Los Angeles and another in New York. As magnetic-based sound gained popularity, many theaters adopted surround sound, supporting four or even six audio channels. While Dolby A noise reduction enabled stereo optical tracks, it couldn't handle the noise levels if more than two optical tracks were used. A significant advancement came with the development of Dolby Stereo.

Through an innovative technique called matrixing, Dolby found a way to use the two optical lines on the film to generate four separate sound channels:

Matrixing operates similarly to Boolean logic, analyzing the data on the left and right optical tracks to decide which speaker should receive the signal. For instance, if a signal on both the left and right tracks is entirely out of phase, it is identified as surround sound. When the projector's pickup reads the optical tracks, it deciphers this signal as surround sound and directs it to the rear and side speakers in the theater. If the in-phase signals from the left and right tracks match, the signal is sent to the center channel. Otherwise, the left track signal goes to the left front speaker, and the right track signal goes to the right front speaker.

It's fascinating to note that Dolby Surround and Dolby ProLogic are the home equivalents of Dolby Stereo. These home systems follow the same principle, compressing four audio tracks into the space of two. Without a surround-sound decoder, the tracks are processed as standard stereo (left and right) tracks. The main distinction between Surround and ProLogic lies in the center channel. Dolby Surround uses the left and right speakers to simulate a center speaker, which works well if you're seated precisely between them. ProLogic, however, sends the center channel sound to a dedicated center speaker.

With the rise of digital sound, the ability to provide discrete sound channels has expanded significantly. "Discrete" means each sound channel is encoded independently, unlike the averaging method used in matrixing.

To learn more about surround sound, visit How Surround Sound Works.

Digital Theater Systems

The first widespread commercial use of digital sound occurred with the release of "Jurassic Park." This system, known as DTS (short for Digital Theater Systems), was developed by the company that patented the technology. Essentially, DTS is a modernized version of the early sound-on-disc method used in cinema. DTS uses a unique optical time code embedded in the film, consisting of a series of dots and dashes located between the image and the analog optical soundtracks.

A specialized optical reader is attached to the projector. The film passes through this reader before entering the projector. Like the analog audio pickup, the DTS reader employs a light-emitting diode (LED) to project light through a lens, the film, and onto a photocell. This generates electrical pulses that the reader interprets as the time code. The information is then sent via a serial cable to a computer, which manages an audio system with three CD players. The movie's soundtrack, comprising six tracks (right, left, center, left-surround, right-surround, and subwoofer), is compressed onto one or two CDs, depending on the film's length. One CD can store approximately two hours of audio in DTS's specialized compressed format. The third CD player is reserved for previews of upcoming DTS movies.

Both the film and the soundtrack CDs feature unique identification codes. The computer verifies these codes to ensure the correct soundtrack is played for the movie. To prevent audio lag caused by CD access, the system buffers the audio in memory using the FIFO (first in, first out) method. Since the computer continuously analyzes the timecode and synchronizes the CD audio with it, the sound rarely falls out of sync with the visuals. Additionally, because the sound isn't directly encoded on the film, audiences avoid the occasional "pop" noise that occurs when the audio pickup encounters a splice.

However, DTS has its drawbacks:

If the DTS computer or CD players fail, the film can still rely on its analog tracks. DTS Stereo, which works with Dolby Stereo audio pickups, is the method used to produce these analog tracks. Like all digital formats, the optical analog tracks are only utilized under specific circumstances:

DTS has proven to be more enduring than anticipated. Initially seen as a stopgap solution during the transition to digital, its user-friendly nature, affordability, and the widespread adoption by theaters have ensured its continued relevance as an alternative to sound-on-film digital formats.

Dolby Digital

Dolby Digital is arguably the most widely used digital format, known by several other names:

Dolby Digital encodes data in the space between the film's sprocket holes. In the image below, you can see the gray dots between these holes. Upon closer inspection, you might even spot the Dolby Digital logo in the center of each segment!

The Dolby Digital reader is mounted on top of the projector (some modern projectors have it integrated). It scans the film as it moves through. Light from an LED passes through the film and hits a CCD. The reader sends the image, which contains tiny dots representing 1s and spaces representing 0s, to a DSP-based Dolby Digital Processor. This unit converts the binary data back into audio.

Similar to DTS, Dolby Digital utilizes six audio tracks:

This setup is often called 5.1, representing five primary channels and one effects channel. The effects channel, which uses a subwoofer, is frequently dubbed the boom channel due to its role in delivering impactful sounds like explosions and other intense audio effects.

If the Dolby Digital reader fails or encounters issues reading the digital data, the film can fall back on its Dolby Stereo analog tracks.

Sony Dynamic Digital Sound

The newest addition to cinema digital sound comes from a major player in the entertainment industry. Sony Dynamic Digital Sound (SDDS) encodes digital audio information along the outer edges of the film. Unlike other formats, SDDS includes error correction by using a duplicate redundant stripe on the opposite edge. SDDS enhances surround-sound capabilities by providing eight audio channels:

The SDDS reader employs a laser to project focused light beams. This light travels through the film, is magnified by a lens, and is then detected by an array of photocells. Dark areas on the film prevent light from reaching certain photocells, while illuminated areas cause the cells to emit a small current. SDDS interprets each photocell as a 1 or 0 based on whether it generates current. As the film moves, this creates a continuous stream of binary data sent to the digital processor.

SDDS is more costly to implement than DTS or Dolby Digital due to the need for additional digital sound equipment. While both formats convert digital signals to analog after decoding, SDDS uses a digital connection to transmit the decoded signal to a proprietary sound processor. Despite the higher cost, the inclusion of two extra audio channels makes SDDS a highly appealing format.

As seen in the film images in this article, multiple sound formats are often recorded on the same film. Since each format occupies a different section of the film, it is cost-effective for the distributor to include at least two digital formats. Nearly all commercial films today feature Dolby Stereo as the analog format, and some even include all three digital formats!

You might be curious why THX isn't mentioned here. Despite being commonly mistaken for a movie-theater sound system, THX is not a sound format—it's something entirely different. Check out How THX Works to learn more.