[LAU] Looking for Audio Watermarking Advice & Tools

[Fons Adriaensen]

On Thu, Oct 27, 2011 at 07:40:36AM -0400, Monty Montgomery wrote:

> > 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.

Yes, the long-term stability will be very good, as the frequency is
adjusted to keep clocks in sync. But there are small short-term 
variations all the time which you won't notice on a clock, but can
be measured easily.

> ...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.

And if the lights were switched off ... :-)

Ciao,

-- 
FA