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How detect if MP3 player is recording in your room? [OT]
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. |
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