It is a theory that tries to explain why our brain finds it a very pleasing sound when there are microvariations in pitch around a center value.
Examples are vibrato on violin or guitar etc.
The Leslie rotating speaker using the Doppler effect, transforming to beauty the sound of an electric organ. Perhaps even more so if the variation is produced by acceleration. For example accelerating from low to high speed as on Whiter Shade of Pale - Procol Harum or the subtle effect of angular speed constantly increasing or decreasing when hanging (and rotating) a loudspeaker from the selling.
Recording by a hand held microphone where the slight movement of the hand creates phase modulation.
Making a CD listenable by copying it to cassette and then listening to the cassette.
Listening to a digitally mastered classical music cassette.
Listening to Digitaly recorded Brothers in Arms - Dire Straits reprocuced from a record player.
Has frequency domain dithering been applied to digital recordings?
Examples of random dithering are the above.
It is easy to understand how this works by connecting a microphone to a preamplifier and then to an oscilloscope. If a Stationary loudspeaker plays a tone in the room and the microphone is hand held to capture the sound, as the hand moves towards or away from the source, the waveform in the oscilloscope moves to left or to right. This is phase shift.
Our basilar membrane operates in a similar way directly on frequency.
As frequency changes the basilar membrane is caressed by nearby hair cells which are connected to the brain.
The most boring scenario for the brain would be listening to a Digitaly produced tone through headphones, and it is very boring. The same hair cells are excited all the time which seems to be very boring for our brain.
If we listen to the same tone through a loudspeaker in the room the slight Doppler frequency shift microvariations (as our ear can't be in exactly the same place since our head moves) make the tone times more listenable.
Is this another reason why analog recordings of the 60s or even earlier sound so listenable on YouTube even on headphones? The microvariations are in the production as a reel to reel recorder no matter how good always has a very small amount. The small amount that our basilar membrane needs to be caressed.
It is frequency modulation but of a very small modulating index.
A large index means annoying wow and flutter.
It seems our brain likes small departures.
For example a double octave of 1000Hz as 4000Hz sound horrible. Increasing to say 4040Hz may sound great. But further increasing will sound horribly out of tune again.
More investigation is needed.
Reference:
Sound and Hearing - Stevens, Warshofsky - Time life science series
Analog vs Digital - euroelectron
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