The development of synchronised audio and moving images can be traced back to Thomas Edison's Kinetophonograph of 1895.

Edison originally designed the motion picture to accompany his phonograph, to enhance entertainment by pairing sound with images. Edison believed that sound combined with a moving picture would offer more immersive experiences than sound alone (Kellog, 1955).

The Kinetophone was a modification of the earlier Kinetoscope, which utilised ratchet wheels and electrical impulses to synchronise the film with a wax cylinder player (Dickson, 1933). However, the Kinetophone did not achieve commercial success, and the system was discontinued after a few years.

The drive for synchronised sound persisted for decades despite initial challenges.

The silent film era, which dominated the early 20th century, was seen by some as a significant hindrance to the commercial success of sound films. As silent films were already highly successful and culturally embedded, there was significant resistance to sound-on-film technologies.

Sound-on-disc systems

Early sound-on-film systems struggled with the issue of amplifying sound sufficiently to make it viable for commercial applications. This led to the development of sound-on-disc systems, where the audio was recorded separately from the film but synchronised mechanically.

The Vitaphone system, introduced in 1926, became one of the most recognised early sound-on-disc systems. The Vitaphone system used a 400mm shellac disc played at 33 1/3 rpm, differing from the 78rpm systems used by contemporaneous disc systems. The disc player was mechanically connected to the projector by gears and spindles to ensure synchronisation of audio and video.

However, the mechanical connection caused issues, including wear on the disc and stylus and poor synchronisation due to inaccurate handling by projectionists (De Forest, 1927).

Despite these issues, the impact on audiences was profound, as it represented a significant step forward in integrating sound with moving images.

Although synchronisation issues plagued the initial use of sound-on-disc, the concept experienced a revival in the 1990s with the development of Digital Theatre Systems (DTS).

The DTS system utilised high-quality sound distributed on a CD format, with synchronisation maintained through an optical control track printed on the disc. This setup enabled seamless synchronisation of sound, providing an efficient and effective way to pair high-quality audio with the film's visual elements.

Optical sound-on-film processes

The subsequent significant development in sound synchronisation was the introduction of optical sound-on-film processes, which enabled sound to be directly recorded onto the film itself.

Early challenges included the inability to amplify the sound signal sufficiently for commercial applications. A breakthrough in amplification occurred with the invention of the triode vacuum valve by Lee De Forest, which enabled higher amplification with minimal distortion, thereby facilitating the development of practical sound-on-film technology.

In 1922, De Forest gave the first successful public demonstration of his Phonofilm system. The core of the Phonofilm system utilised a gas-filled tube, called a photon, that varied in brightness according to the voltage variations from the microphone.

These variations in brightness were recorded on the film as fluctuations in density, forming the first viable optical soundtrack.

There were two primary methods for recording sound: variable density (VD) and variable area (VA). VD utilised variations in the density of the recorded track, while VA involved modulating the width of the light beam that exposed the film, creating variations in the area exposed (Wilkins, 2018).

Both VD and VA soundtracks could be played back using modulation of light through the film, where a fixed light source illuminated the soundtrack and a photo-sensitive cell detected fluctuations in light, converting them into an electrical signal.

Over time, optical soundtracks improved with the introduction of noise reduction technologies, which mitigated the effects of film grain and electronic noise. Additionally, advancements such as stereo sound and digital encoding of audio tracks (e.g., Cinema Digital Sound, Sony Dynamic Digital Sound, and Dolby Digital) have further enhanced the capabilities of optical sound systems (Barker et al., 2021).

Magnetic sound recording

Magnetic sound recording played a critical role in film sound throughout the 20th century. Magnetic tracks were used to record both the original audio tracks and the final mixed soundtracks on projection prints.

From the early 1950s onwards, magnetic soundtracks were commonly added to films. The magnetic stripe can be applied either before exposure and development or after development, with pre-striped film often used for news gathering on reversal stock.

Magnetic soundtracks became a standard feature for major film formats, including 35mm, 16mm, 8mm, and 70mm.

One of the significant advantages of magnetic sound is its flexibility. For example, it allowed for synchronised sound to be recorded independently of the film, which was essential for high-quality audio.

Magnetic soundtracks were recorded by applying a slurry of binder and magnetic particles (pigment) onto the film surface or via adhesive tape that applied the stripe to the film.

However, the addition of the magnetic stripe caused an increase in film thickness, which required a balanced stripe on the opposite edge of the film to ensure proper winding on the film reel (Meyer et al., 2020).

While magnetic striping enabled easy integration of sound with the film, it introduced several preservation concerns. Films with magnetic soundtracks were often stored tightly wound, which is not ideal for archival storage. Over time, magnetic soundtracks can suffer from 'blocking', a condition where the adhesive layer of the magnetic stripe adheres to the adjacent layers of film, potentially causing damage when the tape is unwound.

This condition highlights the importance of careful film handling and storage practices (Lucas et al., 2020).

Current trends in film sound

Modern film sound technologies have continued to evolve, with advancements in both recording and playback systems. Digital technologies have revolutionised film sound recording and playback, enabling higher fidelity, more dynamic soundscapes, and greater flexibility in editing.

In addition to digital sound formats such as DTS, Dolby Digital, and SDDS, advancements in immersive sound technologies, such as Dolby Atmos and DTS:X, are enabling new forms of auditory experiences that complement visual storytelling (Brouard et al., 2021).

The digital revolution has not only enhanced the quality of sound but has also streamlined the production process. With the transition from analogue to digital technologies, the need for complex mechanical synchronisation systems, such as those used in earlier sound-on-disc technologies, has been eliminated.

However, challenges remain in the preservation and archival of both analogue and digital film soundtracks. Continued research into the longevity and stability of digital formats, as well as their integration into modern archival strategies, is crucial for the future of film sound preservation (Smith & Herring, 2020).