cellphones as room bugs

[Bill Stewart]

At 11:26 AM 12/9/2006, Daniel F. Fisher wrote:
>Ian Farquhar (ifarquha) wrote> The other problem for this technique is 
>battery life.
>
>Suppose this worked by recording from mic to memory and then transmitting 
>later. This leads to a bunch of questions:
>
>By what factor could transmission time/power be reduced sending such a 
>recording later? How many minutes could a typical phone buffer? How much 
>does a typical conversation compress? Are such algorithms within the power 
>of a typical phone's processor? How much power is used in recording to 
>memory and compressing?

Cell phones already compress voice, to reduce spectrum needs,
and that's done in hardware rather than wasting CPU.
If the phone's design is sufficiently general, it can easily grab
the compressed voice bits and store them in memory instead of transmitting
(assuming there's enough memory, which isn't necessarily the case.)
Voice compression rates are typically 5.6 - 6.5kbps, or 13 on some GSM flavors,
and you may gain a bit from silence suppression depending on
whether the microphone can adequately hear the other speaker.

If the phone doesn't have data networking features,
or only has the slow types (CDPD, etc.) used to handle text messages,
there's probably no big advantage to doing this.
But if you've got faster data service, say 50-60kbps or the newer
~~200-300kbps stuff, then you can transmit faster than real-time speech,
and if you can buffer enough data, say 1 MB for 20 minutes of talk time,
you might save some battery.

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