Detecting multi-touch inputs

1. A system for determining a touch contact location, comprising:

a communication interface configured to receive a signal that has been disturbed by a touch input on a surface, wherein the signal was propagating in a medium of the surface prior to the touch input; and

a processor coupled with the communication interface and configured to:

correlate the received signal to determine a time domain signal encoding a time delay represented in the received signal;

transform the time domain signal to determine a spatial domain signal, wherein transforming the received signal includes translating the time delay represented in the received signal disturbed by the touch input to a physical distance encoded in the spatial domain signal;

compare the spatial domain signal with an expected signal associated with a potential location of a source of a disturbance caused by the touch input, wherein the spatial domain signal encodes the physical distance traveled by the received signal due to the disturbance caused by the touch input and the expected signal encodes a variable with respect to a measure of distance; and

determine the touch contact location of the touch input based at least in part on the comparison.

2. The system of claim 1 , wherein the received signal was detected by a transducer.

3. The system of claim 1 , wherein the received signal is an active signal that has been propagated through a medium of the surface.

4. The system of claim 1 , wherein the received signal is an ultrasonic signal.

5. The system of claim 1 , wherein the received signal includes a pseudorandom binary sequence.

6. The system of claim 1 , wherein the time domain signal captures a time delay caused by the disturbance of the touch input.

7. The system of claim 1 , wherein correlating the time domain signal includes cross-correlating the time domain signal with a reference signal.

8. The system of claim 1 , wherein the spatial domain signal captures a distance traveled by the received signal through a medium of the surface due to the disturbance of the touch input.

9. The system of claim 1 , wherein transforming the time domain signal to determine the spatial domain signal includes compensating the time domain signal for dispersion through a medium of the surface.

10. The system of claim 1 , wherein the processor is further configured to match filter the spatial domain signal using a template signal.

11. The system of claim 1 , wherein the processor is further configured to generate the expected signal to simulate an ideal spatial domain signal that would result if a touch contact was received at the potential location.

12. The system of claim 1 , wherein the expected signal is associated with a plurality of potential simultaneous touch contact locations of the touch input.

13. The system of claim 1 , wherein the expected signal is one of a plurality of expected signals compared with the spatial domain signal.

14. The system of claim 1 , wherein comparing the signals includes determining a mean square error of the spatial domain signal and the expected signal.

15. The system of claim 1 , wherein comparing the spatial domain signal with the expected signal includes determining whether a hypothesis number of simultaneous touch contacts of the touch input is likely an actual number of simultaneous touch contacts of the touch input.

16. The system of claim 1 , wherein comparing the spatial domain signal with the expected signal includes determining a result of a cost function comparing a plurality of spatial domain signals with corresponding expected signals for a plurality of pairs of transmitters and sensors coupled to a medium of the surface.

17. The system of claim 1 , wherein determining the touch contact location includes selecting a potential location among a plurality of potential locations as the touch contact location.

18. A method for determining a touch contact location, comprising:

receiving a signal that has been disturbed by a touch input on a surface, wherein the signal was propagating in a medium of the surface prior to the touch input;

correlating the received signal to determine a time domain signal encoding a time delay represented in the received signal;

transforming the time domain signal to determine a spatial domain signal, wherein transforming the received signal includes translating the time delay represented in the received signal disturbed by the touch input to a physical distance encoded in the spatial domain signal;

using a processor to compare the spatial domain signal with an expected signal associated with a potential location of a source of a disturbance caused by the touch input, wherein the spatial domain signal encodes the physical distance traveled by the received signal due to the disturbance caused by the touch input and the expected signal encodes a variable with respect to a measure of distance; and

determining the touch contact location of the touch input based at least in part on the comparison.

19. A computer program product for determining a touch contact location, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions for:

receiving a signal that has been disturbed by a touch input on a surface, wherein the signal was propagating in a medium of the surface prior to the touch input;

correlating the received signal to determine a time domain signal encoding a time delay represented in the received signal;

transforming the time domain signal to determine a spatial domain signal, wherein transforming the received signal includes translating the time delay represented in the received signal disturbed by the touch input to a physical distance encoded in the spatial domain signal;

comparing the spatial domain signal with an expected signal associated with a potential location of a source of a disturbance caused by the touch input, wherein the spatial domain signal encodes the physical distance traveled by the received signal due to the disturbance caused by the touch input and the expected signal encodes a variable with respect to a measure of distance; and

determining the touch contact location of the touch input based at least in part on the comparison.

20. The method of claim 18 , wherein the received signal includes a pseudorandom binary sequence.