Detecting media watermarks in magnetic field data

1. A media monitoring apparatus comprising:

means for transforming magnitude values of magnetic field data to a frequency domain to determine transformed magnetic field data, the magnetic field data associated with a first sampling rate, the magnetic field data obtained from a sensor; and

means for detecting an audio watermark in a portion of the transformed magnetic field data associated with a first frequency, the audio watermark encoded in an audio signal, the audio watermark to have a frequency component associated with a second frequency different from the first frequency, the first frequency to be aliased relative to the second frequency based on the first sampling rate.

2. The apparatus of claim 1 , wherein the sensor is a magnetometer.

3. The apparatus of claim 1 , wherein the first sampling rate is less than twice the second frequency.

4. The apparatus of claim 1 , wherein the first frequency is based on a difference between the first sampling rate and the second frequency.

5. The apparatus of claim 4 , further including means for selecting the first frequency based on the difference between the first sampling rate and the second frequency.

6. The apparatus of claim 1 , further including means for outputting an indicator when the audio watermark is detected.

7. The apparatus of claim 1 , wherein the audio signal is output by audio circuitry of a media device, the audio circuitry including speaker leads to electrically couple a speaker to a speaker driver circuit.

8. The apparatus of claim 7 , wherein the watermark is included in a first frequency range of the audio signal, the first frequency range below a threshold of audio frequencies capable of reproduction by the speaker, the first frequency range including the second frequency of the audio signal.

9. The apparatus of claim 1 , further including means for determining the magnitude values of the magnetic field data from a plurality of directional components included in the magnetic field data obtained from the sensor.

10. A media monitoring apparatus comprising:

means for transforming magnitude values of a magnetic field measured by a sensor over time into a frequency domain to determine transformed magnetic field data, the magnitude values associated with a first sampling rate; and

means for detecting a watermark included in an audio signal based on the transformed magnetic field data, the audio signal output by audio circuitry of a media device, the watermark to modulate a first frequency of the audio signal, the first sampling rate to be less than twice the first frequency, the means for detecting to evaluate the transformed magnetic field data at a second frequency different from the first frequency to detect the watermark, the second frequency selected to compensate for aliasing caused by the first sampling rate associated with the multidimensional magnetic field data.

11. The apparatus of claim 10 , wherein the sensor is a magnetometer.

12. The apparatus of claim 10 , further including means for determining the magnitude values from a plurality of directional components included in multidimensional magnetic field data obtained from the sensor, the multidimensional magnetic field data representative of values of the magnetic field measured by the sensor.

13. The apparatus of claim 10 , wherein the audio circuitry includes speaker leads to electrically couple a speaker driver circuit to a speaker that is to reproduce the audio signal, the watermark is included in a first frequency range of the audio signal, the first frequency range below a threshold of audio frequencies capable of reproduction by the speaker, the first frequency range including the first frequency of the audio signal.

14. The apparatus of claim 10 , further including means for selecting the second frequency based on the first frequency and the first sampling rate.

15. The apparatus of claim 14 , wherein the means for selecting is to select the second frequency based on a difference between first frequency and the first sampling rate.

16. The apparatus of claim 10 , further including means for outputting an indicator when the audio watermark is detected.

17. A media monitoring apparatus comprising:

means for determining magnitude values of a magnetic field measured by a sensor over time, the means for determining to determine the magnitude values from a plurality of directional components included in first magnetic field data obtained from the sensor, the first magnetic field data representative of values of the magnetic field measured by the sensor;

means for detecting a watermark included in an audio signal based on the magnitude values of the magnetic field, the audio signal output by audio circuitry of a media device, the means for detecting to:

correlate the magnitude values with a reference sequence to determine second magnetic field data; and

process the second magnetic field data to detect the watermark.

18. The apparatus of claim 17 , wherein the sensor is a magnetometer.

19. The apparatus of claim 17 , wherein the audio circuitry includes speaker leads to electrically couple a speaker driver circuit to a speaker that is to reproduce the audio signal, and the means for detecting is to process the second magnetic field data to detect variations in the second magnetic field that are induced by an electrical signal carried by the speaker leads.

20. The apparatus of claim 17 , further including means for outputting an indicator when the audio watermark is detected.