"kony" wrote in message
...
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.

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.


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


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.

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.




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.

Without a sampling rate, there will be no conversion of analog to digital.
You have to take so many samples of the analog signal.

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,

and also to add
isolation between the input stages.