Acoustic Tape Measure

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Activity acousticmeasure.svg
Status alpha—under development
TST {{{1}}}
Translate-icon-55px.svg Localization

OlpcProject.png Benjamin Schwartz

Acoustic Tape Measure determines the physical distance between two XO's by measuring how long it takes sound to travel between them.

Development notes

In testing, accuracy was 1 cm or better at 2 m.

The measurements seem to be extremely accurate, under controlled conditions. In fact, accuracy seems to be limited by variation in the speed of sound with air temperature. The variation is approximately 0.2% per degree C. Code currently assumes an air temperature of 25 C, which means ± 3% over the whole range of reasonable temperatures.

Location+date+time might be enough to get temperature estimates? Is there another way?

Perhaps a known-distance measurement? Physically adjacent XO's? Could this be used as a air thermometer (Aa)ctivity? MitchellNCharity 15:45, 3 October 2007 (EDT)

Full-spectrum white noise is used to maximize accuracy.

The system only seems to work well when the laptops are facing each other and have uninterrupted line of sight. The measurements are unlikely to work well if the distance is changing during measurement.

Major computation is required for every measurement, to extract the distance from the audio data. This time was up to 30 seconds per measurement on B2s. On B4/MP's... ?

There's at least one more necessary feature, and two mysterious bugs that are either mine or Telepathy's. Typically, in emulation, the program runs for a few hundred measurements before hitting a networking bug. (2007-10-03)

Now uses aplay/arecord rather than gstreamer. And a delay to compensate for amplifier weirdness.

There may still be hidden race conditions, and the code still does not prevent more than 2 people from participating, even though this will break everything.

Ideas for use

2D Position determination

Even given full knowledge of the distances, determining the locations is a nontrivial nonlinear optimization. And the laptops have to face each other. And computation takes a long time.

Position graph matching game

The Boston Museum of Science has an exhibit which measures user distance from a computer along a long mat. The computer then horizontally scrolls a graph with a pattern, along with your current distance. Idea is you have to scurry back and forth to try and match your position to the pattern. Vaguely Digital Dance Revolution-like. I'm not really sure the concept works, but it could be easy to try, and with music added, and kids able to draw their own patterns, it might be interesting.

The algorithms would have to be greatly accelerated before something like this would be possible. Children would have to move with one laptop in hand, pointed at the stationary laptop. Also, the measurements are unlikely to work well if the distance is changing during measurement.

One possibility would be for the computer to say "go N meters away" and then have a competition to see how many tries it takes to get within, say, 5%. This could be a good distance-sense training exercise.