Method and apparatus for increasing the strength of phase-based watermarking of an audio signal

1. A method for increasing the strength of phase-based watermarking of an audio signal, which watermarked audio signal is suitable for acoustic reception and watermark detection in the presence of surrounding noise, said method including:

receiving said audio signal;

determining a masking threshold for a phase change based watermarking of a current frequency bin in a frequency/phase representation of said audio signal, wherein said masking threshold determination is controlled by a received audio quality level value representing the audio quality following said audio signal watermarking;

determining an allowed phase change value for the phase of said current frequency bin, according to a reference angle to be embedded in that current frequency bin, which reference angle is derived from a watermark pattern;

changing the phase of said current frequency bin according to said allowed phase change value;

based on said masking threshold and said allowed phase change value, calculating an allowed magnitude change value for said current frequency bin, and calculating from the audio quality level value a magnitude change scaling factor;

calculating a scaled allowed magnitude change values from said allowed magnitude change value and said scaling factor;

increasing the magnitude of said current frequency bin by said scaled allowed magnitude change values;

embedding said watermark into said current frequency bin with said changed phase and said increased magnitude; and

providing the correspondingly watermarked current frequency bin suitable for acoustic reception and watermark detection in the presence of surrounding noise.

2. The method according to claim 1 , wherein no phase changes are carried out for frequency bins representing a frequency smaller than a first frequency threshold value and for frequency bins representing a frequency greater than a second frequency threshold value that is greater than said first frequency threshold value.

3. The method according to claim 1 , wherein a magnitude change value for said current frequency bin is denoted δX[i] and δX[i]=√{square root over (LT g [i] 2 −X[i] 2 +(X[i] cos(δφ[i])) 2 )}−X[i]+X[i] cos(δφ[i]), where LT g [i] is said current masking threshold, X[i] is the original magnitude of said current frequency bin, and δφ[i] is said current phase change value.

4. The method according to claim 1 , wherein said magnitude change scaling factor is denoted ƒ and f=10 −maskingCurveOffset/20 , where

maskingCurveOffset

=

100

-

level

100

×

30

⁢

[

dB

]

and level has a value between ‘0’ and ‘100’ and is said audio quality level value, with level=100 for the best audio quality.

5. An apparatus for increasing the strength of phase-based watermarking of an audio signal, which watermarked audio signal is suitable for acoustic reception and watermark detection in the presence of surrounding noise, said apparatus including means adapted to:

receiving said audio signal;

determining a masking threshold for a phase change based watermarking of a current frequency bin in a frequency/phase representation of said audio signal, wherein said masking threshold determination is controlled by a received audio quality level value representing the audio quality following said audio signal watermarking;

determining an allowed phase change value for the phase of said current frequency bin, according to a reference angle to be embedded in that current frequency bin, which reference angle is derived from a watermark pattern;

changing the phase of said current frequency bin according to said allowed phase change value;

based on said masking threshold and said allowed phase change value, calculating an allowed magnitude change value for said current frequency bin, and calculating from the audio quality level value a magnitude change scaling factor;

calculating a scaled allowed magnitude change values from said allowed magnitude change value and said scaling factor;

increasing the magnitude of said current frequency bin by said scaled allowed magnitude change values;

embedding said watermark into said current frequency bin with said changed phase and said increased magnitude; and

providing the correspondingly watermarked current frequency bin suitable for acoustic reception and watermark detection in the presence of surrounding noise.

6. The apparatus according to claim 5 , wherein no phase changes are carried out for frequency bins representing a frequency smaller than a first frequency threshold value and for frequency bins representing a frequency greater than a second frequency threshold value that is greater than said first frequency threshold value.

7. The apparatus according to claim 5 , wherein a magnitude change value for said current frequency bin is denoted δX[i] and

δ X [ i ]=√{square root over (LT g [ i ] 2 −X [ i ] 2 +( X [ i ] cos (δφ[ i ])) 2 )}− X [ i ]+ X [ i ] cos(δφ[ i ]),

where LT g [i] is said current masking threshold, X[i] is the original magnitude of said current frequency bin, and δφ[i] is said current phase change value.

8. The apparatus according to claim 5 , wherein said magnitude change scaling factor is denoted ƒ and f=10 −maskingCurveOffset/20 , where

maskingCurveOffset

=

100

-

level

100

×

30

⁢

[

dB

]

and level has a value between ‘0’ and ‘100’ and is said audio quality level value, with level=100 for the best audio quality.

9. A non-transitory processor readable storage medium that contains or stores, or has recorded on it, a digital audio bitstream, said digital audio bitstream including a watermark embedded therein according to:

determining a masking threshold for a phase change based watermarking of a current frequency bin in a frequency/phase representation of said audio signal, wherein said masking threshold determination is controlled by a received audio quality level value representing the audio quality following said audio signal watermarking;

determining an allowed phase change value for the phase of said current frequency bin, according to a reference angle to be embedded in that current frequency bin, which reference angle is derived from a watermark pattern;

changing the phase of said current frequency bin according to said allowed phase change value;

based on said masking threshold and said allowed phase change value, calculating an allowed magnitude change value for said current frequency bin, and calculating from the audio quality level value a magnitude change scaling factor;

calculating a scaled allowed magnitude change values from said allowed magnitude change value and said scaling factor;

increasing the magnitude of said current frequency bin by said scaled allowed magnitude change values;

embedding said watermark into said current frequency bin with said changed phase and said increased magnitude to produce a watermarked digital audio bitstream suitable for acoustic reception and watermark detection in the presence of surrounding noise.