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 monitors disclosed herein include a magnetic field estimator to determine first magnetic field data, the magnetic field estimator in communication with a magnetometer. Disclosed example media monitors also include a correlator to correlate the first magnitude field data with a reference sequence to determine second magnetic field data. Disclosed example media monitors further include a watermark decoder to process the second magnetic field data to detect an audio watermark encoded in an audio signal.
1. An apparatus comprising:
a magnetometer to detect a magnetic field induced by circuitry of a speaker, the speaker to communicate wirelessly with a media device;
machine readable instructions; and
programmable circuitry to execute the machine readable instructions to at least:
transform multidimensional magnetic field data from the magnetometer to determine transformed magnetic field data, the multidimensional magnetic field data associated with a sampling rate; and
process a portion of the transformed magnetic field data associated with a first frequency to detect an audio watermark encoded in an audio signal to be output from the speaker, 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 sampling rate.
2. The apparatus of claim 1 , wherein the sampling rate is less than twice the second frequency.
3. The apparatus of claim 1 , wherein the first frequency is based on a difference between the sampling rate and the second frequency.
4. The apparatus of claim 3 , wherein the programmable circuitry is to select the first frequency based on a difference between the sampling rate and the second frequency.
5. The apparatus of claim 1 , wherein the programmable circuitry is to cause an output to be asserted when the audio watermark is detected.
6. The apparatus of claim 5 , further including a display, wherein the output is at least one of a light, a tone or a symbol to be displayed on the display.
7. The apparatus of claim 1 , wherein the audio watermark is included in a first frequency range of the audio signal that is below a second frequency range of the speaker, the first frequency range including the second frequency.
8. The apparatus of claim 1 , wherein to transform the multidimensional magnetic field data, the programmable circuitry is to:
process a plurality of directional components included in the multidimensional magnetic field data to determine magnitude values representative of a magnitude of the multidimensional magnetic field data over time; and
perform at least one of a fast Fourier transform, a discrete Fourier transform, a discrete cosine transform of a Hadamard transform on the magnitude values to determine the transformed magnetic field data.
9. The apparatus of claim 1 , wherein the apparatus is structured to be positioned adjacent to the speaker.
10. The apparatus of claim 1 , wherein the audio signal is to correspond to media presented by the media device.
11. An apparatus comprising:
a magnetometer to detect a magnetic field induced by circuitry of a speaker, the speaker to communicate wirelessly with a media device;
machine readable instructions; and
programmable circuitry to execute the machine readable instructions to at least:
process multidimensional magnetic field data from the magnetometer to determine first magnetic field data representative of a magnitude of the multidimensional magnetic field data over a time domain;
correlate, over the time domain, the first magnitude field data with a reference sequence to determine second magnetic field data; and
process the second magnetic field data to detect an audio watermark encoded in an audio signal to be output from the speaker.
12. The apparatus of claim 11 , wherein the reference sequence is a pseudo-noise sequence.
13. The apparatus of claim 12 , wherein the audio signal is a first audio signal, and the audio watermark is encoded in the first audio signal by:
modulating the audio watermark with the pseudo-noise sequence to determine a modulated audio watermark; and
combining the modulated audio watermark with a second audio signal to produce the first audio signal.
14. The apparatus of claim 11 , wherein the programmable circuitry is to process the second magnetic field data with a matched filter to detect the audio watermark.
15. The apparatus of claim 11 , wherein the programmable circuitry is to cause an output to be asserted when the audio watermark is detected.
16. The apparatus of claim 15 , further including a display, wherein the output is at least one of a light, a tone or a symbol to be displayed on the display.
17. The apparatus of claim 11 , wherein the audio watermark is included in a first frequency range of the audio signal that is below a second frequency range of the speaker.
18. The apparatus of claim 11 , wherein the multidimensional magnetic field data includes a plurality of directional components.
19. The apparatus of claim 11 , wherein the apparatus is structured to be positioned adjacent to the speaker.
20. The apparatus of claim 11 , wherein the audio signal is to correspond to media presented by the media device.