On Sun, 15 Oct 2006 20:31:16 -0800, "Dana"
wrote:


Doesn't have to be *very good*, only has to further reduce
emissions which likely weren't at a level high enough to
discriminate recording mode even without the shield.

And most consumer electronics are not very well shielded, hence it is a snap
to pick up their emissions with off the shelf test equipment.


"Most" don't have any shield at all. MP3 players, commonly
do. Further, "most" consumer devices have an order or two
of magnitude, more active parts in them and use far more
power, stronger emissions.

Further, detecting a very faint signal is not the same thing
as having a strong enough detection and valid discrimination
method between recording MP3 players and all other consumer
electronics. Remember that we are not just trying to detect
that some "thing" using electricity is present, it has to be
identifed in function and is not just one device buy a
multitude of different MP3 player (or other digital
recorders too if you want to consider all types) recorders.

You will have to find a specific commonality, not just a
vague generalization, to discriminate them. Even this much
is premature- that commonality would have to exist which has
not in itself been established.




Hence it would be a very minor task to detect the sampling clock of the
recorder in question.

That does not indicate it is an MP3 player,

So what. It still indicates the presence of a device that can record the
persons converstaion,

No it does not. Did you think nothing but MP3 players have
clocks, or that all MP3 players have the same clock rate?
Neither is true.

and that is what is required. It can be a dictation
device some other kind of recorder, it would still be detected.

No, in some cases you might detect some devices, but it'd be
random, you'd far more often detect non-recording or devices
completely incapable of recording and wouldn't detect some
actually recording. In other words, random and useless.




most of the times the sampling rate is specified
by the MFG.


Manufacturer of the chip, yes, not the MP3 player

All you need is the chip, and usually the OEM will list what the chip MFG
states anyway.

You'll need ALL of the chips in existence, and you'd find
some are not putting out enough noise to be detected in a
typical scenario. Maybe if you put a scanner up against the
device. Is that really useful? If you had the device out
already, no further scanning is needed at all unless you
have far-fetched idea like if the MP3 recorder were built
into a shoe-heel or a clock, etc. Even then, it's a matter
of scenario. If that scenario doesn't allow getting the
scanner close enough to find the shoe is a source, you'll
never even know it was suspicious there was a noisey shoe.

I've gone off on a tangent though, for our purposes an MP3
player should be considered what is bought off the shelf.
OEMs do not "list what the chip MFG states". Most often you
have to tear open the specific player and examine it
yourself, or rely on reports from someone else who has.



and
"spec" really means, hardware support as it can't be
selected at random like with most computers running soft
codecs. Even so, this rate is not usually a separate
oscillator,

Usually you have an external clock needed to feed the codec. That clock can
be detected as well.

Again you are thinking of older electronics, today's
player/recorders are highly integrated. That doesn't mean
ALL devices will have a different or undetectable, or
indistinuishable clock signal, but it does mean you don't
have a commonality that allows detection as an MP3 player,
let alone one recording.


No, it does not.

Without a sampling rate, there will be no conversion of analog to digital.

The existence of a sampling rate does not suggest it is
always the same rate nor that it is measureable in any
particular scenario.

You have to take so many samples of the analog signal.

Yes, but this does not lead to any of the other conclusions.