"Phil Allison" wrote in message
...

"Jim Lesurf"

Electric Network Frequency. In effect, allowing the sound recording to
have a small amount of 'mains hum'. (Which tends to happen anyway, even
when you *don't* want it. ;- )

The frequency of mains hum varies with time,


** It wanders a small amount, like 0.1%, around the nominal frequency.

The tolerance within the EU is +/- 1%. It's unlikely to be much less
elsewhere.


and differs from time to time and with the location (i.e. which mains
network covers your area).

** Mains networks cover huge geographical areas, sometimes whole
countries - within which the frequency is locked in phase.

Even if it only tells you which country it was recorded in, that may well be
significant.

Hence
it can and has been used as evicence that recordings were recorded when
and where claimed and have no breaks or edits - or not! Bit like an audio
forensic version of dating a tree by its growth rings. Surprisngly, this
technique has been used a number of times in court, etc, and apparently
works well. Even for recordings made on battery systems where the mics
picked up the hum as interference.


** Sure - you can say if it was in a 50Hz or a 60 Hz country.

The difference between 50Hz and 50.5Hz (or even 50.05Hz) is easy to detect
and measure these days. Discontinuities in the 50Hz waveform, caused by
editing, can also be readily detected.



David.