Audio watermarking via phase modification

1. A method of conveying information using an audio channel, the method comprising modulating an audio signal to produce a modulated signal by embedding additional information into the audio signal, wherein modulating the audio signal comprises:

segmenting the audio signal into overlapping time segments using a non-rectangular analysis window function to produce a windowed audio signal;

processing the windowed audio signal for a time segment to produce frequency coefficients representing the windowed time segment and having phase values and magnitude values;

selecting one or more of the frequency coefficients;

modifying phase values of the selected frequency coefficients using the additional information to map the phase values onto a known phase constellation; and

processing the frequency coefficients including the modified phase values to produce the modulated signal.

2. The method of claim 1 , further comprising:

encoding the additional information using error correction coding to produce encoded information; and

using the encoded information as the additional information that is used to modify the phase values.

3. The method of claim 1 , wherein the phase constellation includes a quantizer offset to introduce an angular shift in the phase constellation.

4. The method of claim 3 , further comprising varying the size of the phase constellation to allow phase distortion to be reduced at frequencies where the phase distortion is more audible and to be increased at frequencies where the phase distortion is less audible.

5. The method of claim 1 , wherein modifying the phase values of the selected frequency coefficients includes setting the phase values to allowed phase quantization values, where the allowed phase quantization values are divided into multiple subsets and each subset corresponds to a different value of a component of the additional information.

6. The method of claim 5 , wherein setting a phase value to an allowed phase quantization value comprises setting the phase value to match a phase quantization value that (i) corresponds to a component of the additional information to be represented by the phase value and (ii) most closely matches the phase value.

7. The method of claim 5 , wherein a component of the additional information is represented by a group of phase values and setting the group of phase values to allowed phase quantization values comprises setting the group of phase values to match a group of phase quantization values that (i) correspond to a component of the additional information to be represented by the group of phase values and (ii) most closely match the group of phase values.

8. The method of claim 5 , wherein modifying at least some of the phase values comprises modifying only certain phase values corresponding to frequency coefficients between an upper and lower frequency bound.

9. The method of claim 8 , wherein modifying at least some of the phase values comprises modifying only certain phase values corresponding to a subset of the time segments.

10. The method of claim 5 , wherein modulating the audio signal comprises using an iterative approach in which a first iteration includes:

computing a DFT on a windowed segment to form frequency coefficients represented using magnitude and phase values;

modifying at least some of the phase values to embed information bits;

inverse transforming modified frequency components including the modified phase values using an IDFT;

applying a synthesis window to results of the inverse transforming; and

combining windowed results from neighboring time segments to produce a first iteration of the modulated signal.

11. The method of claim 10 , further comprising performing one or more additional iterations of the computing, modifying, inverse transforming, and combining steps using the first iteration of the modulated signal instead of the audio signal.

12. A method of conveying information using an audio channel, the method comprising multiple iterations of modulating an audio signal to produce a modulated signal by embedding additional information into the audio signal, wherein modulating the audio signal comprises:

segmenting the audio signal into time segments using an analysis window function to produce a windowed audio signal;

processing the windowed audio signal for a time segment to produce frequency coefficients representing the windowed time segment and having phase values and magnitude values;

selecting one or more of the frequency coefficients;

modifying phase values of the selected frequency coefficients using the additional information; and

processing the frequency coefficients including the modified phase values to produce an iteration of the modulated signal.

13. The method of claim 12 , wherein the modulated signal produced by a prior iteration is remodulated to reembed the additional information and produce a successive iteration of the modulated signal.

14. The method of claim 13 , wherein modifying the phase values of the selected frequency coefficients includes setting the phase values to allowed phase quantization values, where the allowed phase quantization values are divided into multiple subsets and each subset corresponds to a different value of a component of the additional information; and where setting a phase value to an allowed phase quantization value comprises setting the phase value to match a phase quantization value that (i) corresponds to a component of the additional information to be represented by the phase value and (ii) most closely matches the phase value.