While reading an article about the Academy Curve (a standard playback equalization curve for pre-1975 mono, non-Dolby® optical soundtracks), I came across this statement:

The severe reduction of the high end of the spectrum was specifically designed to conceal the high frequency crackling noise inherent in early film sound production.

Why in your opinion early movies had crackling noises in background? Were they introduced during the recording phase or in the mixing/mastering one (if any)?

3 Answers 3


I think the statement you quoted is misleading. I'm pretty sure the Academy Curve is not designed to remove the crackling of playback or at least this is not the main purpose. This is an interesting queston though, and although I can't remember much from my lectures past The Jazz Singer, I thought I'd do some research and add a fuller answer anyway.

Academy Mono Optical Soudtrack

At the time when optical sound-on-film was developing, devices like the Movietone sound system were able to use the electrical signal from a microphone to modulate the width or height of a narrow band of light. This modulated light would be directly captured on the edge of a filmstrip, creating a soundtrack that is in-sync with the film. Due to the fact that this recording process used 'primitive' components and devices, quite a lot of noise was also present in the recording.

Academy Mono Optical Soudtrack Film A vintage Fox movietone motion picture camera

This optical recording process allowed advances in multitrack recording, and as a result, the noise present in each audio track would accumulate to a point where it would be significant in the final mix. At the same time, sound systems in movie theatres were suffering from a problem with high frequency response. These problems along with others lead to the need for a process to attenuate the high and low frequency noise apparent during theatrical playback.
So a process of pre-emphasis and de-emphasis was developed. During the recording of each audio track, pre-emphasis was added to increase the level, particularly at higher frequencies. During playback, de-emphasis would be applied, restoring the original EQ to the sound track, and also removing the high and low frequencies that the theatre system had trouble reproducing. The EQ curve used in this de-emphasis process was known as the Academy Curve. This process of pre-emphasis and de-emphases was successful in bringing down the level of noise in the theatre, and removing any troublesome frequencies.

Sound Recording: The Electrica Era
Cinema Sound and EQ Curves
The Dolby Film Sound Revolution (Ioan Allen)
Academy Curve Info
Sound-On-Film: Analog sound-on-film recording
Pre-emphasis and De-emphasis(Dolby NR Example)


I'd be inclined to think it would be from scratches/dust in the optical medium that sound was carried on in cinema film, up until 1976.

See this image from Wikipedia…

enter image description here

Edge of a 35mm film print showing the soundtracks. The stereo optical sound strip is located on the right, with waveforms for left and right channels. To the far left is the SDDS digital track (blue area to the left of the sprocket holes), then the Dolby Digital (grey area between the sprocket holes labelled with the Dolby "Double-D" logo in the middle), and to the right of the analog optical sound is the DTS time code (the dashed line to the far right.)

Source : https://en.wikipedia.org/wiki/Optical_sound


Sound was recorded on magnetic bands, this kind of medium is not really tough also electronic circuits weren't isolated as much as today against interferences.

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