audio location accurate low-cost location sensing james scott intel research cambridge boris...
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Audio Location Accurate Low-Cost Location Sensing
James ScottIntel Research Cambridge
Boris DragovicIntern in 2004 at Intel Research Cambridge
Studying for PhD at University of Cambridge
Background
Fine-grained location systems have been built using ultrasound e.g. Bats (AT&T), Cricket (MIT) Achieving ~3cm 3D accuracy 95% of the time
Many end-user devices have audible-range I/O built in, but few have ultrasound
Can we use integrated/off-the-shelf audio hardware for location?
Audio-based location withoff-the-shelf hardware
Many txSpeakers in environment as txMobile phones or PDAs as rxPrivacy-preserving
Many rxComputer microphones as rxCan use mobile devices for txBUT can also use human sounds for tx
Locating human sounds
Need coverage from at least 4 micsUnknowns: X,Y,Z,t (t=time of sound) More is better since occlusion happens
Users do not need special “tag”No per-user setup requiredLowers costs and increases simplicity
User identity is not providedMany apps do not need identityAnonymity is good for privacyCould fuse with identity e.g. from RFID
Aims of prototype
Fine-grained location sensing with hardware accessible to end users
What accuracy can we obtain for locating human sounds, e.g. finger clicking, hand clapping?
Application area: 3D user interfaces using human sounds
Prototype
Use standard PC Add 6 PCI sound
cards and 6 mics Total cost of sound
hardware ~£100 From dabs.com
Fedora Core 2 Linux distribution
Java software
SignalDetection
System Architecture
PositioningTimingSignalDetection
SignalDetection
Signal Detection
Problem: identify the same part of the same sound in audio streams from multiple mics
Amplitude-threshold algorithm Keep track of current noise floor Mark sample as “significant” when amplitude is
is at least F times noise floor. (F ≈ 2.5) Properties
Very good at detecting sharp sounds Equally important: ignores other sounds Robust to noisy environments and cheap mics
Timing
Need time sync for sound streams 1ms error ≈ 30cm in space
Problem: Linux/Java introduce delays Buffering and scheduling result in variable
delays of >1ms Solution: hacked sound driver
Timestamp taken at interrupt made available to Java app (via /proc)
Does not account for interrupt delay Around 200 lines of C code
Positioning
Survey of microphone positions is currently done manually See orthogonal work on self-surveying
Use well-studied Levenberg-Marquardt technique to find 3D position (and sound generation time)
1D evaluation
First evaluated 1D performance for relative distance
Use two mics and a 6x7 grid of test points
20 hand claps and 20 finger clicks at each point
Microphones
Y
X
60cm
60cm
1D results: hand clapping
1D results: finger clicking
Implications of 1D results
Our mics are usable ~60º either side of axis Our mics have a maximum range of 4m
Drops to ~2m in very noisy conditions
Implications for deployment Density of microphones required to sense
location in a space
Finger clicking has median 1D error <5cm At least some of this due to human error
3D experiment setup
20 finger clicks at 4x4x3 test points on 60cm grid Total clicks: ~1000. Very sore fingers.
Microphones at 2 heights, and much more spread in X,Y than in Z This might be typical for real deployments
Y
X
60cm
60cm60cm
Key:Microphone at 60cm high
Microphone at 120cm high
Lollipops!
3D distance error
What do I think it’s good for?
3D user interfacesWhen I click here in future, do thisExtend computer input beyond desk/lap
Situated interfacesAdd a light switch by the bedRemote control without a losable device
Inspiration: SPIRIT (AT&T) which allows Bats to be used as 3D pointers
What do you think it’s good for?
Accessible user interfacesElderly, disabled
Activity inferencingFusion of location with sound recognition
Performance artSpotlights follow sounds
Tracking planes in an air show!Well, maybe not…
Visualisation
To help deploy and demo it UbiComp, Mobisys
Allows placement of mics creation of
“buttons” in 3D By mouse or finger
Used to create an mp3 player demo
Demo video – Accuracy
Demo video – User Interface
Conclusions
3D location sensing for under £100 of consumer sound peripherals
Accuracy: better than 28cm (3D) for 90% of finger clicks Improves to 10cm for 2D and repeated clicks
Sound-based user interfaces
Happy to provide source and specs [email protected]