"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.

"David Looser is ****ING MORON "


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%.


** Got nothing to do with how the frequency REALLY varies over intervals of
time like a few minutes.

Try MEASURING it instead of posting ABSURD ****ING BULL****.

**** HEAD !!



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.

** So each country now has its own characteristic mains frequency ??

When did that happen ????

ROTFLMAO !!

Wot a ****ing AUTISTIC MORON !!!!




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.


** But tells you absolutely NOTHING -

YOU RIDICULOUS ****WIT !!!




....... Phil

"Phil Allison" wrote in message
...


YOU RIDICULOUS ****WIT !!!


Glad to see that your utterly predicable response arrived promptly. It would
be a real shame if you were to come up with something new!

:-)

David.

In article , David Looser
wrote:
"Phil Allison" wrote in message
...


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.

From the published results I've seen in JAES and the accompanying comments
it seems clear it may be able to do far more that that. The key point is
that the pattern of small but measurable changes in frequency isn't the
same every day. So in a manner akin to 'tree rings' you can compare the
pattern with a library of recordings of ENF to find a match. This can then
be a way to tell you which time of day, which day, and in which mains power
network area the recording was made. More usefully it can be used to detect
when parts of a recording have been altered as that tends to cause
localised diversions from the pattern.


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.

Indeed. I must say I was both surprised and impressed by the reports I read
and the data presented on this. I'd known that we could expect variations
duing each day as the load varied. What I hadn't realised was how possible
it was to detect variations in the pattern from one day to another and then
match these with reference ENF pattern records that a forensic body might
collect for such purposes. Quite a neat use of what otherwise people regard
as an annoyance - the tendency for audio recordings to end up with some
background hum.

That said, I must say that the recent JAES paper by the two ex-FBI types
was writting in a stunningly boring way! Presumably to illustrate how
mind-knumbingly legalese reports may need to be written to avoid
nit-picking legal eagles from finding cause for objections! :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"Jim Lesurf" wrote

From the published results I've seen in JAES and the accompanying comments
it seems clear it may be able to do far more that that. The key point is
that the pattern of small but measurable changes in frequency isn't the
same every day.

I've just tried as an experiment displaying a broadcast TV waveform with the
'scope timebase locked to the mains frequency. This gives a much more
immediate indication of mains frequency stability than a frequency meter
does.

What was noticeable was just how fast mains frequency variations can occur.
Frequency variations of the order of 0.1% can occur in a matter of seconds,
an 0.5% variation within a minute or two. I'm sure that the pattern of this
sort of variation does not conform to a pattern, but is unique on each
occasion.

David.

David Looser wrote:

What was noticeable was just how fast mains frequency variations can occur.
Frequency variations of the order of 0.1% can occur in a matter of seconds,
an 0.5% variation within a minute or two.

http://www.nationalgrid.com/uk/Elect...ncy/Freq60.htm

When would be the correct time of the day to make perfect speed
recordings from my synchronous motor powered record deck?


I'm sure that the pattern of this
sort of variation does not conform to a pattern, but is unique on each
occasion.

It's related to the current appalling state of the weather and what's
just been suffered through on TV ...

--
Adrian C

So, is anyone gonna post a graph of UK mains frequency over a day or
something?

I'd like to see it.

Interesting sub-thread. For a change.


Martin

On Sun, 01 Nov 2009 19:37:28 +0000, Fleetie
wrote:

So, is anyone gonna post a graph of UK mains frequency over a day or
something?

I'd like to see it.

Interesting sub-thread. For a change.


There is a site that gives that sort of information, unfortunately I
can't find it at the moment, but this site
http://www.dynamicdemand.co.uk/grid.htm gives you instantaneous
frequency.

On Sun, 01 Nov 2009 19:37:28 +0000, Fleetie
wrote:

So, is anyone gonna post a graph of UK mains frequency over a day or
something?

I'd like to see it.

Interesting sub-thread. For a change.


Martin

http://www.nationalgrid.com/uk/Elect...ncy/Freq60.htm
gives frequency over the last 60 minutes.

Bill Taylor wrote:
On Sun, 01 Nov 2009 19:37:28 +0000, Fleetie
wrote:

So, is anyone gonna post a graph of UK mains frequency over a day or
something?

I'd like to see it.

Interesting sub-thread. For a change.


Martin

http://www.nationalgrid.com/uk/Elect...ncy/Freq60.htm
gives frequency over the last 60 minutes.

Cool! Thanks!

Martin