Suppose you wanted to plant a hidden camera for some long period of time and capture photos of all that went past. You'd like to never again have to enter the place where it's hidden, and only visit it rarely; you'd like it to be small; and you'd like it to last a long time. For example, the book "The Social Life of Small Urban Spaces" was based on a few years of research in this vein using Super 8 cameras for time-lapse photography. It appears to me that this equipment should now be incredibly cheap. USB "webcams" that capture 100-kB 640x480 JPEGs are on the order of $10. I think 4-port USB hubs (again, on the order of $10) contain all the hardware necessary to act as USB host controllers; one could imagine integrating the USB hub hardware with a small single-board computer with SD/MMC and Bluetooth interfaces, for a total cost on the order of $50 plus up to 4 cameras and their USB cables, and an MMC card ($50-$110). This device would presently be limited in smallness only by the size of its power supply, USB ports, and multi-chip integration, so it could be concealed in many places. You could probably run it on 200mW when running (for less than a second) and <1mW when idle. You could drop by periodically with an inconspicuous Bluetooth device, such as a cellphone or laptop, to download the pictures (say, 4 cameras * 100kB/shot/camera * 4 shots / minute * 60 minutes/hour * 24 hours/day = 2.3GB/day; but one shot per minute is only 144MB/day). Anyone snooping over Bluetooth at the time could tell that a lot of data was being sent over Bluetooth (1megabit/sec? not sure; but at that speed you'd have to spend 2300 seconds in the vicinity.) Alternatively, you could use a directional antenna from hundreds of meters away (the "Bluesniper" folks managed to do 1km.) An adaptive surveillance algorithm could shoot four times per minute until the data card was full, followed by twice a minute (replacing every other old shot, starting with the oldest) until the data card was all full at twice a minute, then once per minute (thinning out old shots to once a minute) until it was full again, etc. Supposing that USB 12Mbps transfers were the limiting factor, you'd need about 67ms of "on time" per shot, or (according to my 200mW estimate above) 13.4 mJ. My laptop's Li-ion battery supposedly holds around 46Wh, or 165kJ (abridged info below): $ cat /proc/acpi/battery/BAT1/state present rate: 1227 mA remaining capacity: 2579 mAh present voltage: 11300 mV $ cat /proc/acpi/battery/BAT1/info design capacity: 4500 mAh last full capacity: 4067 mAh design voltage: 10800 mV model number: XM2018P02 battery type: Li-ION 11.3V * 4.067Ah = 46Wh. On that basis, my laptop's battery could power 12 000 000 invasions of privacy by this system --- saving that many camera shots to an MMC card. It might only be able to power 4 000 000 invasions of privacy if it had to transmit them all over Bluetooth. Still, that's nearly six months in the four-shots-with-four-cameras-per-minute maxi configuration described above, where you'd have to come download up your photos at least once a day, and at one camera shooting once per minute, it would last 8 years. (I'm assuming that the webcams power up instantly. This may be unreasonable.) Obviously you could do a similar job with audio surveillance, but ironically, this may consume more storage and power; minimally comprehensible speech is 10kbps under the best of conditions, so you'd need at least 108MB/day, and probably several times that to get anything useful. You'd need some very-low-power constantly-on device to buffer the audio so you wouldn't have to run the CPU all the time. A similar system, but without the cameras or other transducers, could serve as a maildrop or backup server (for data with high value per byte, obviously). We can anticipate that the power and monetary cost of data storage and transmission will decrease considerably more before Moore's Law runs out. ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> ______________________________________________________________ ICBM: 48.07100, 11.36820 http://www.leitl.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE [demime 1.01d removed an attachment of type application/pgp-signature which had a name of signature.asc]