Detecting media watermarks in magnetic field data
Methods, apparatus, systems and articles of manufacture (e.g., physical storage media) to detect media watermarks in magnetic field data are disclosed herein. Example media monitoring methods disclosed herein include accessing magnetic field data determined by a magnetometer, the magnetic field data associated with a first sampling rate. Disclosed example methods also include transforming magnitude values of the magnetic field data to a frequency domain to determine transformed magnetic field data. Disclosed example methods further include evaluating a portion of the transformed magnetic field data associated with a first frequency to detect a media watermark having a frequency component associated with a second frequency different from the first frequency, the first frequency being aliased relative to the second frequency based on the first sampling rate.
1. A media monitor comprising:
a magnetic field estimator in communication with a magnetometer, the magnetic field estimator to obtain magnetic field data determined by the magnetometer, the magnetic field data associated with a first sampling rate;
a frequency transformer to transform magnitude values of the magnetic field data to a frequency domain to determine transformed magnetic field data; and
a watermark decoder to process a portion of the transformed magnetic field data associated with a first frequency to detect an audio watermark encoded in an audio signal, the audio watermark having a frequency component associated with a second frequency different from the first frequency, the first frequency being aliased relative to the second frequency based on the first sampling rate.
2. The media monitor of claim 1 , wherein the first sampling rate is less than twice the second frequency.
3. The media monitor of claim 1 , wherein the first frequency is based on a difference between the first sampling rate and the second frequency.
4. The media monitor of claim 1 , further including an output indicator to be asserted when the watermark decoder detects the media watermark.
5. The media monitor of claim 1 , wherein the magnetic field estimator is in communication with the magnetometer via an application programming interface of a portable device.
6. The media monitor of claim 1 , wherein the first sampling rate is based on a measurement interval associated with magnetic field measurements performed with the magnetometer.
7. The media monitor of claim 6 , wherein the first sampling rate is further based on a power-down interval that is to occur after respective ones of the magnetic field measurements performed with the magnetometer.
8. A media monitoring method comprising:
accessing, by executing an instruction with a processor, magnetic field data determined by a magnetometer, the magnetic field data associated with a first sampling rate;
transforming, by executing an instruction with the processor, magnitude values of the magnetic field data to a frequency domain to determine transformed magnetic field data; and
evaluating, by executing an instruction with the processor, a portion of the transformed magnetic field data associated with a first frequency to detect an audio watermark encoded in an audio signal, the audio watermark having a frequency component associated with a second frequency different from the first frequency, the first frequency being aliased relative to the second frequency based on the first sampling rate.
9. The media monitoring method of claim 8 , wherein the first sampling rate is less than twice the second frequency.
10. The media monitoring method of claim 8 , wherein the first frequency is based on a difference between the first sampling rate and the second frequency.
11. The media monitoring method of claim 8 , further including asserting an output indicator when the watermark decoder detects the media watermark.
12. The media monitoring method of claim 8 , wherein the magnetic field data is accessed via an application programming interface of a portable device.
13. The media monitoring method of claim 8 , wherein the first sampling rate is based on a measurement interval associated with magnetic field measurements performed with the magnetometer.
14. The media monitoring method of claim 13 , wherein the first sampling rate is further based on a power-down interval that is to occur after respective ones of the magnetic field measurements performed with the magnetometer.
15. A non-transitory computer readable medium comprising computer readable instructions that, when executed, cause a processor to at least:
access magnetic field data determined by a magnetometer, the magnetic field data associated with a first sampling rate;
transform magnitude values of the magnetic field data to a frequency domain to determine transformed magnetic field data; and
evaluate a portion of the transformed magnetic field data associated with a first frequency to detect an audio watermark encoded in an audio signal, the audio watermark having a frequency component associated with a second frequency different from the first frequency, the first frequency being aliased relative to the second frequency based on the first sampling rate.
16. The non-transitory computer readable medium of claim 15 , wherein the first sampling rate is less than twice the second frequency.
17. The non-transitory computer readable medium of claim 15 , wherein the first frequency is based on a difference between the first sampling rate and the second frequency.
18. The non-transitory computer readable medium of claim 15 , wherein the computer readable instructions, when executed, further cause the processor to assert an output indicator when the watermark decoder detects the media watermark.
19. The non-transitory computer readable medium of claim 15 , wherein the first sampling rate is based on a measurement interval associated with magnetic field measurements performed with the magnetometer.
20. The non-transitory computer readable medium of claim 19 , wherein the first sampling rate is further based on a power-down interval that is to occur after respective ones of the magnetic field measurements performed with the magnetometer.