Methods and apparatus to detect spillover in an audience monitoring system

1. A method to distinguish local media from spillover media content comprising:

sampling a first audio signal from a first source received by a first microphone;

sampling a second audio signal received by a second microphone from the first source, wherein the second microphone is separated from the first microphone by a first distance;

calculating, with a processor, an origin direction of the first source based on a time difference between receipt of the first and second sampled audio signals;

determining, with the processor, whether the origin direction is within a threshold angle;

when the origin direction is within the threshold angle, retaining the media associated with the first source; and

when the origin direction exceeds the threshold angle, ignoring the media associated with the first source.

2. A method as defined in claim 1 , further including:

calculating a first absolute value audio signal corresponding to an absolute value of the first sampled audio signal;

calculating a second absolute value audio signal corresponding to an absolute value of the second sampled audio signal; and

applying a first and second decimation threshold to the first and second absolute value audio signals, respectively.

3. A method as defined in claim 2 , further including:

removing samples from the first absolute value audio signal with intensities below the first decimation threshold; and

removing samples from the second absolute value audio signal with intensities below the second decimation threshold.

4. A method as defined in claim 3 , wherein the first and second decimation thresholds are determined such that removing samples from the first and second absolute value audio signals removes a majority of the samples from the first and second absolute value audio signals.

5. A method as defined in claim 1 , further including:

selecting a first set of samples from the first sampled audio signal;

selecting a second set of samples from the second sampled audio signal; and

determining a first correlation value between the first set of samples and the second set of samples.

6. A method as defined in claim 5 , further including calculating a moving average of the first and second sampled audio signals, wherein each point of the moving average of the first and second sampled audio signals includes an average of two samples from the first and second audio signals, respectively.

7. A method as defined in claim 5 , wherein the first set of samples includes samples from the first sampled audio signal that are equally spaced between a first point in time and a second point in time.

8. A method as defined in claim 7 , wherein the second set of samples includes samples from the second sampled audio signal that are equally spaced between the first point in time and the second point in time.

9. A method as defined in claim 8 , further including:

selecting a third set of samples from the second sampled audio signal, wherein the third set of samples is offset from the second set of samples by a first number of samples; and

determining a second correlation value between the first set of samples and the third set of samples.

10. A method as defined in claim 9 , wherein the time difference is determined by determining the greater of the first correlation value and the second correlation value.

11. A method as defined in claim 7 , further including selecting a fourth set of samples from the first sampled audio signal, wherein the fourth set of samples is offset from the first set of samples by a first time.

12. A method as defined in claim 1 , wherein the first source is a media device.

13. A method as defined in claim 1 , wherein the threshold angle is based on the physical location of the first source associated with the media.

14. A tangible machine readable storage medium comprising instructions that, when executed, cause a machine to at least:

sample a first audio signal from a first source received by a first microphone;

sample a second audio signal received by a second microphone from the first source, wherein the second microphone is separated from the first microphone by a first distance;

calculate an origin direction of the first source based on a time difference between receipt of the first and second sampled audio signals;

determine whether the origin direction is within a threshold angle;

when the origin direction is within the threshold angle, retain media associated with the first source; and

when the origin direction exceeds the threshold angle, ignore the media associated with the first source.

15. A machine readable storage medium as defined in claim 14 , wherein the first source is a media device.

16. A machine readable storage medium as defined in claim 14 , wherein the threshold angle is based on the physical location of the first source associated with the media.

17. A machine readable storage medium as defined in claim 14 , wherein the instructions cause the machine to:

calculate a first absolute value audio signal corresponding to an absolute value of the first sampled audio signal;

calculate a second absolute value audio signal corresponding to an absolute value of the second sampled audio signal; and

apply first and second decimation thresholds to the first and second absolute value audio signals, respectively.

18. A machine readable storage medium as defined in claim 17 , wherein the instructions cause the machine to:

remove samples from the first absolute value audio signal with intensities below the first decimation threshold; and

remove samples from the second absolute value audio signal with intensities below the second decimation threshold.

19. A machine readable storage medium as defined in claim 18 , wherein the machine determines the first and second decimation thresholds such that removing samples from the first and second absolute value audio signals removes a majority of the samples from the first and second absolute value audio signals.

20. A machine readable storage medium as defined in claim 14 , wherein the instructions cause the machine to:

select a first set of samples from the first sampled audio signal;

select a second set of samples from the second sampled audio signal; and

determine a first correlation value between the first set of samples and the second set of samples.

21. A machine readable storage medium as defined in claim 20 , wherein points of the moving average of the first and second sampled audio signals correspond to an average of two samples from the first and second sampled audio signals, respectively.

22. A machine readable storage medium as defined in claim 20 , wherein the first set of samples includes samples from the first sampled audio signal that are equally spaced between a first point in time and a second point in time.

23. A machine readable storage medium as defined in claim 22 , wherein the second set of samples includes samples from the second sampled audio signal that are equally spaced between the first point in time and the second point in time.

24. A machine readable storage medium as defined in claim 23 , wherein the instructions cause the machine to:

select a third set of samples from the second sampled audio signal, wherein the third set of samples is offset from the second set of samples by a first number of samples; and

determine a second correlation value between the first set of samples and the third set of samples.

25. A machine readable storage medium as defined in claim 24 , wherein the machine determines the time difference by determining the greater of the first correlation value and the second correlation value.

26. A machine readable storage medium as defined in claim 22 , wherein the instructions cause the machine to select a fourth set of samples from the first sampled audio signal, wherein the fourth set of samples is offset from the first set of samples by a first time.

27. An apparatus to distinguish local media from spillover media comprising:

an audio sampler to sample a first audio signal from a first source received by a first microphone and sample a second audio signal received by a second microphone from the first source, wherein the second microphone is separated from the first microphone by a first distance;

a direction detector to calculate an origin direction of the first source based on a time difference between receipt of the first and second sampled audio signals; and

a filter to determine whether the origin direction is within a threshold angle, the filter to retain the media associated with the first source when the origin direction is within the threshold angle, the filter to ignore the media associated with the first source when the origin direction exceeds the threshold angle, at least one of the audio sampler, the direction detector, or the filter being implemented by hardware.

28. An apparatus as defined in claim 27 , wherein the first source is a media device.

29. An apparatus as defined in claim 27 , wherein the threshold angle is based on the physical location of the first source associated with the media.

30. An apparatus as defined in claim 27 , further including:

an absolute value calculator to calculate a first absolute value of the first sampled audio signal, the absolute value calculator to calculate a second absolute value of the second sampled audio signal; and

a threshold processor to apply a first and second decimation threshold to the first and second absolute value audio signals, respectively.

31. An apparatus as defined in claim 30 , wherein the threshold processor is to remove samples from the first sampled audio signal with intensities below the first decimation threshold, and remove samples from the second sampled audio signal with intensities below the second decimation threshold.

32. An apparatus as defined in claim 31 , wherein threshold processor removes a majority of the samples from the first and second sampled audio signals based on the first and second decimation thresholds.

33. An apparatus as defined in claim 27 , further including:

an audio sample selector to select a first set of samples from the first sampled audio signal;

an audio signal delayer to select a second set of samples from the second sampled audio signal; and

a correlation engine to determine a first correlation value between the first set of samples and the second set of samples.

34. An apparatus as defined in claim 33 , further including a moving average calculator to calculate a moving average of the first and second audio signals, wherein the moving average calculator is to calculate the moving average of the first and second audio signals such that points of the moving averages include an average of two samples from the first and second sampled audio signals, respectively.

35. An apparatus as defined in claim 33 , wherein the audio sample selector is to select samples from the first sampled audio signal that are equally spaced between a first point in time and a second point in time.

36. An apparatus as defined in claim 35 , wherein the audio signal delayer is to select samples from the second sampled audio signal that are equally spaced between the first point in time and the second point in time.

37. An apparatus as defined in claim 36 , wherein the audio signal delayer is to select a third set of samples from the second sampled audio signal offset from the second set of samples by a first number of samples, and the correlation engine is to determine a second correlation value between the first set of samples and the third set of samples.

38. An apparatus as defined in claim 36 , further including an offset selector to determine the time difference by determining the greater of the first correlation value and the second correlation value.

39. An apparatus as defined in claim 35 , wherein the audio sample selector is to select a fourth set of samples from the first sampled audio signal offset from the first set of samples by a first time.