27 Oct
2011
27 Oct
'11
1:40 p.m.
A variation on this is used to check the authenticity
of some
recordings in forensic audio. Almost all 'surveillance' audio
contains some low level 50/60 Hz. If the recording is done on
a digital device then its clock can be assumed to be more stable
than the mains frequency
At least in NA, mains frequency is very very stable. The entire
continent is in phase and clocked with very high precision. It's more
stable than a free-running digital clock.
...I used this fact to 'cheat' in a high school physics contest where
we had to build a clock and the winner was the most stable. Ours had
a CdS cell that watched the 120Hz flicker of the lights in the room.
Monty