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There is a familiar audio effect in old films where the music (especially if a note is held a bit longer) bends in pitch and sounds damaged or slightly worn out. The only example I could find is someone recreating the effect:

VHS Pitch Bend Effect

Although this sounds similar to a normal Doppler effect where the soundwaves are being stretched/compressed as an object moves away/toward you, the sound in this case is coming out of something stationary (e.g. the VCR), so it can't be the Doppler effect. It is my understanding that pitch is also not directly equivalent to wave frequency, but is a collective result of loudness, timbre, and duration.

From Wikipedia: "Pitch may be quantified as a frequency, but pitch is not a purely objective physical property; it is a subjective psychoacoustical attribute of sound."

So what causes this pitch bending effect? Is it a physical property or perceptual/sensory property?

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  • $\begingroup$ Audio is recorded as a simple linear analog signal, so if the tape is stretched due to being worn, playback pitch will be lower, while the condensed spot will play higher pitch. I don't know how this is affected by the control track in the VHS, but at least worn audio cassette tapes have a similar distortion where pitch varies in time. $\endgroup$
    – Ruslan
    Nov 25 at 14:27
  • $\begingroup$ I think it is definitely not a perceptual or psychoacoustical effect. I almost never experienced this effect on proper tapes (e.g. films bought on VHS) played on proper machines. Therefore I always thought it comes from a tape transport mechanism that does not run smoothly, either when recording, or playing back. $\endgroup$
    – Koschi
    Nov 25 at 15:17
  • $\begingroup$ @Ruslan Okay so it's a result of physical tape damage. I guess years of playback and winding just cause subtle damage which can drag audio up or down in pitch. Thanks! $\endgroup$
    – jinks908
    Nov 26 at 12:33
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In the audio world, this phenomenon is called wow and is caused by cyclic variations in the playback speed of the tape: when the speed goes up, the pitch increases, and when it goes down, the pitch decreases.

The root cause of playback wow is imperfections in the playback speed control system- and in the tape itself, caused by anisotropic stretch while stored on a spool with uneven tension. Stretched tape plays flat compared to unstretched tape. This effect is strong enough that when mylar is used as the tape base, it must be prestretched ("tensilized") in the factory.

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