Fantasound

Ever wondered how sound works? I still don't know how it works, but this is what I learned reading.

Fantasia, 1940

The soundtrack was recorded using multiple audio channels and reproduced with Fantasound, a pioneering sound reproduction system that made Fantasia the first commercial film shown in stereophonic sound.

So...before Fantasia all sounds where 'mono' sounds meaning that the audio is coming from one channel. To make the sound in the film more diverse,

The Disney brothers contacted David Sarnoff of RCA regarding the manufacture of a new system that would "create the illusion that the actual symphony orchestra is playing in the theater." Sarnoff backed out at first due to financial reasons, but agreed in July 1939 to make the equipment so long as the Disneys could hold down the estimated $200,000 in costs.

The collaboration led to the development of Fantasound, a pioneering stereophonic surround sound system which innovated some processes widely used today, including simultaneous multi-track recording, overdubbing, and noise reduction.

Fantasound employed two projectors running at the same time. With one containing the picture film with a mono soundtrack for backup purposes, the other ran a sound film that was mixed from the eight tracks recorded at the Academy to four: three of which contained the audio for the left, center, and right stage speakers respectively, while the fourth became a control track with amplitude and frequency tones that drove variable-gain amplifiers to control the volume of the three audio tracks.

 In addition were three "house" speakers placed on the left, right, and center of the auditorium that derived from the left and right stage channels which acted as surround channels.

 As the original recording was captured at almost peak modulation to increase signal-to-noise ratio, the control track was used to restore the dynamics to where Stokowski thought they should be. For this, a tone-operated gain-adjusting device was built to control the levels of each of the three audio tracks through the amplifiers.

The illusion of sound traveling across the speakers was achieved with a device named the "pan pot", which directed the predetermined movement of each audio channel with the control track. Mixing of the soundtrack required six people to operate the various pan pots in real time, while Stokowski directed each level and pan change which was marked on his musical score. To monitor the recording levels at lower frequencies, Disney ordered eight three-color oscillators from the newly established Hewlett-Packard company,  a predecessor from VU meters used today. Between the individual takes, prints, and remakes, approximately three million feet of sound film was used in the production of Fantasia. Almost a fifth of the film's budget was spent on its recording techniques.

To think of how much input this Disney spent his time effort I can't but respect the film.

Source: Wikipedia