On 27/11/2015, Aaron Z Snyder wrote:

> The most reliable, although not always perfect, way to determine the
> pitch is to set the mains hum (and harmonics) frequencies (almost
> always captured in the recording process) to 50 or 60 Hz (and
> multiples for the harmonics). Hum is the only real historical
> information regarding recording speed, and, hence, pitch.
> 
I believe the mains frequency is allowed to vary over a certain range,
so long as the total number of cycles per day is correct. This allows
mains operated electric clocks to keep in time.

And in France in the early '50s, the power supply might well have been
unreliable. The country was still recovering from the War.

Regards
-- 
Don Cox
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