On Sun, 15 Oct 2006 19:21:08 -0800, "Dana"
wrote:


"kony" wrote in message
.. .
On Sun, 15 Oct 2006 09:32:39 -0500, "Ken Maltby"
wrote:


You made a suggestion that was not resolvable to a
difference in operation of an MP3 player. With a constant
current and constant bitrate output, you'd essentially be
suggesting that from a distance you can discriminate which
bits are flowing on the bus to the memory, in what is likely
a shielded case. I find this highly unlikely.


I was suggesting no such thing. I find your idea that an
ungrounded MP3 recorder has any significant shielding,
very unlikely.

Define significant. Many have grounded copper foil in them.
It's not as though this is a high powered device to begin
with, though, and would commonly have to be detected at a
distance.

Still consumer electronics do not have very good shielding.

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.

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, let alone
recording. There is no one "sample clock" common to all MP3
players.

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


Manufacturer of the chip, yes, not the MP3 player, 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, the chip itself has a clock that can also vary
per chip. It is certianly not something that remains
constant over all MP3 players, and not a signal that appears
only when set to recording mode.



The recorder to be a threat and to respond
to sound must let sound waves through, even if it is a
contact microphone/sensor/transducer, and they require
significant amplification in their operation.

No, you are thinking of older devices. There needs be no
amplification prior to the digitization chip which can run
at constant current, very low voltage and no easily
detectable response to room noise from a distance.

You still have the sampling rate, which requires a clock at that rate,

No, it does not. Clock rates are divisible or multiplied
these days, and these rates are often common to process
sizes, or current targets, not a specific functional
requirement. In other words, it's a safe bet you cannot
detect a recording MP3 player with a universal "sampling
rate" detection scheme, even before considering they won't
all necessaril record at the same rate, further lacking
consideration for any possiblity of variable rate or spread
spectrum.

so at
a minimum that clock can be detected.
And most designs would include an amplification stage prior to digitization,
as the levels from most mics will not be sufficient,

Sufficient for hearing through earbuds, no, that'd be amp'd.

Sufficient for a microchip DESIGNED to use a mic input to
digitize MP3? It would be an incredibly poorly designed
chip if it had to have a preamp tacked on after the mic.

and also to add
isolation between the input stages.

You are thinking old-school multi-stage, possibly even
discrete audio designs. All-integrated single chip MP3
players (recording) isn't directly applicable.