Detecting watermark modifications

1. An apparatus to detect watermark modifications, the apparatus comprising:

means for encoding a first watermark in a first media signal obtained from an output of a media device to obtain a second media signal encoded with the first watermark and a second watermark, the second watermark already encoded in the first media signal obtained from the output of the media device;

means for decoding the first watermark and the second watermark from the second media signal, the means for decoding to:

determine, from the second media signal, a first metric corresponding to the first watermark;

determine, from the second media signal, a second metric corresponding to the second watermark; and

means for outputting, based on the first metric and the second metric, an indication of whether the second watermark has been modified.

2. The apparatus of claim 1 , wherein the means for outputting is to determine whether the second metric exceeds the first metric by at least a threshold amount, and the indication is to indicate the second watermark has been modified when the second metric exceeds the first metric by at least the threshold amount.

3. The apparatus of claim 2 , wherein the indication is to indicate the second watermark has not been modified when the second metric does not exceed the first metric by at least the threshold amount.

4. The apparatus of claim 1 , wherein the first watermark is associated with a first watermark layer, the second watermark is associated with a second watermark layer different from the first watermark layer, the first watermark layer is associated with a first set of code tones to represent symbols of the first watermark, and the second watermark layer is associated with a second set of code tones to represent symbols of the second watermark, the second set of code tones different from the first set of code tones.

5. The apparatus of claim 1 , wherein to determine the first metric and to determine the second metric, the means for decoding is to:

determine, from the second media signal, symbol strength metrics for symbols of the first watermark;

combine the symbol strength metrics for the symbols of the first watermark to determine the first metric;

determine, from the second media signal, symbol strength metrics for symbols of the second watermark; and

combine the symbol strength metrics for the symbols of the second watermark to determine the second metric.

6. The apparatus of claim 5 , wherein the means for decoding is to:

average the symbol strength metrics for the symbols of the first watermark to combine the symbol strength metrics for the symbols of the first watermark; and

average the symbol strength metrics for the symbols of the second watermark to combine the symbol strength metrics for the symbols of the second watermark.

7. A system to detect watermark modifications, the system comprising:

means for encoding a second watermark in a sampled media signal obtained from a received broadcast signal, the sampled media signal already encoded with a first watermark that was included in the received broadcast signal;

means for decoding the first watermark and the second watermark from the sampled media signal;

means for comparing a first metric determined for the first watermark and a second metric determined for the second watermark to determine whether the first watermark was modified prior to being included in the received broadcast signal; and

means for revising ratings data corresponding to the received broadcast signal when the first watermark is determined to have been modified prior to being included in the received broadcast signal.

8. The system of claim 7 , wherein the means for revising is to discard the ratings data when the first watermark is determined to have been modified prior to being included in the received broadcast.

9. The system of claim 7 , wherein the received broadcast signal is a received radio station signal, and the sampled media signal is a sampled audio signal obtained from the received radio station signal.

10. The system of claim 7 , wherein the first metric is a first signal strength metric determined for the first watermark and the second metric is a second signal strength metric determined for the second watermark.

11. The system of claim 10 , wherein the means for comparing is to:

determine the first watermark was modified prior to being included in the received broadcast when the first signal strength metric exceeds the second signal strength metric by at least a threshold amount; and

determine the first watermark was not modified prior to being included in the received broadcast when the first signal strength metric does not exceed the second signal strength metric by at least a threshold amount.

12. The system of claim 7 , wherein the means for encoding is to encode the second watermark to overlap in time with the first watermark in the sampled media signal.

13. The system of claim 12 , wherein the means for encoding is to employ frequency multiplexing to encode the second watermark to overlap in time with the first watermark in the sampled media signal.

14. A watermark modification detector comprising:

means for encoding a second watermark in a sampled media signal obtained from a broadcast media signal received by a media device, the sampled media signal including a first watermark that was included in the broadcast media signal;

means for decoding the first watermark and the second watermark from the sampled media signal; and

means for comparing a first strength metric determined for the first watermark and a second strength metric determined for the second watermark to determine whether the first watermark was modified prior to being encoded in the broadcast media signal that was received by the media device.

15. The watermark modification detector of claim 14 , wherein the first watermark is encoded in a first watermarking layer of the sampled media signal, and the means for encoding is to encode the second watermark in a second watermarking layer of the sampled media signal.

16. The watermark modification detector of claim 15 , wherein the second watermarking layer is an unused watermarking layer of the sampled media signal.

17. The watermark modification detector of claim 14 , wherein the means for decoding is to:

sum energies of respective ones of a first set of code tones of the sampled media signal corresponding to symbols of the first watermark to determine the first strength metric; and

sum energies of respective ones of a second set of code tones of the sampled media signal corresponding to symbols of the second watermark to determine the second strength metric.

18. The watermark modification detector of claim 17 , wherein the respective energy of one of the first set of code tones is normalized based on a first average energy determined for a first code band of the sampled media signal including the one of the first set of code tones, and the respective energy of one of the second set of code tones is normalized based on a second average energy determined for a second code band of the sampled media signal including the one of the second set of code tones.

19. The watermark modification detector of claim 14 , wherein the means for comparing is to determine whether the second metric determined for the second watermark exceeds the first metric determined for the first watermark by at least a threshold value.

20. The watermark modification detector of claim 19 , wherein the means for comparing is to:

determine the first watermark was modified prior to being encoded in the broadcast media signal when the second metric exceeds the first metric by at least the threshold value;

determine the first watermark was not modified prior to being encoded in the broadcast media signal when the second metric does not exceed the first metric by at least the threshold value; and

report, via a network, information indicating whether the first watermark was modified prior to being encoded in the broadcast media signal.