Recently, I cleaned up a 1933 motion picture sound track. There was a continuous "whine" sound throughout the movie which was happening at 96 Hz (basically sinusoidal in shape). It turns out that the frame rate for motion pictures became standardized in 1927 at 24 frame per second (FPS). The 4th harmonic of that would be 96 Hz, and that is apparently what I observed. It was easily eliminated with a notch filter set for 96 Hz with 0.1 Octave bandwidth. My theory about why is was occurring at 96 Hz and not 24 Hz is that the camera probably had a governor that was running at a higher rate than the film itself and some of that noise transferred to the film sound track.
Craig
Craig
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