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I'm currently digitizing a series of Odeon 78 rpm discs of recordings made
in the early 1950s of one particular ensemble and I'm surprised to find that
while most discs render A=c440Hz when replayed at 78rpm (77,92) certain
exceptional examples deviate by 2% or more. Can anyone suggest a likely
cause? In some cases, discs ostensibly recorded on the same day whose two
sides bear consecutive matrices, even differ from each other. 
More puzzling still are a couple of discs where the pitch drops gradually by
around a quarter of a tone (roughly 3%) during playback of the side. If
anyone can suggest the likely or possible cause of this, I'd be delighted to
read about it. 
Many thanks