Re-mastering classic films for digital audio formats presents several difficult challenges. Old films decay with time and deteriorate from use; they get scratched, pick up dirt or discolor. These films were created with the resources and technology that was available at the time. As such their imperfections and short comings become more apparent on modern digital systems.
Two prevalent methods of sound recording involved using light to expose a section of film to the side of the picture frame. This is visible on the film as a continuous strip or track that runs along the edge of the film. (Now you know why its called a sound track)
One method of sound recording used the intensity of the sound to control the intensity of the light. This varying light intensity would result in a varying level of exposure onto the film that upon developing would appear as varying shades of gray. This was known as the variable density method as the sound track appeared cloudy and ranged from clear to opaque and all shades of gray in between.
The other method was called Variable area. It to used sound to control the amount of light striking the film. However the difference here was that the light intensity remained bright and a mechanical aperture was moved . The gates of the aperture cast a shadow blocking some of the light. The result is a clear sound track where the width of the clear section varies with the sound.
Both of these sound recording systems could be used on the same projector. A light sensor on the projector would see variations in light intensity and pass this variation to an audio amp.
The Variable Area method was better as the Variable density method reproduced sound with less quality. (sort of a static sound because of the cloudiness of the sound track due to film coating imperfections would be interpreted as sound)
Both of these systems were not purely electrical but also relied upon the mechanical systems in the camera. Vibrations from the mechanical movements in the camera would be transferred onto the film despite the best efforts to avoid this. As the sprocket in the camera moved the film the teeth of the sprocket engaging the holes in the film would cause a vibration. This is most noticeable as a 96Hz flutter. The frequency being based on a combination of frame rate and the number of sprocket holes per frame.
It wasn't until the 60's that noise reduction systems entered into use to remove unwanted background noise such as Wow, Flutter and Hiss. The most notable and most recognizable name in noise reduction is Dolby which was introduced in 1966.
So from this point on (1966) there is an ever expanding use of noise reduction and improvements to audio recording in the film industry. Contemporary audio systems provide a high fidelity experience that the older recording systems cannot come close to meeting.