EMBEDDING CODES IN AN AUDIO SIGNAL

1 . A method of communicating data imperceptibly in an audio signal, the method comprising:

for each sub-band of the audio signal, identifying the tone in that sub-band having the highest amplitude;

scaling an audio code comprising the data to be communicated by a frequency mask profile, the frequency mask profile having maxima at the frequencies of the identified tones;

aggregating the audio signal and the scaled audio code to form a composite audio signal; and

transmitting the composite audio signal.

2 . A method as claimed in claim 1 , wherein the sub-bands are frequency Barks.

3 . A method as claimed in claim 1 , wherein within each sub-band, the frequency mask profile decays from the maximum towards the lower frequency bound of the sub-band at a first predetermined rate, the first predetermined rate being such that the frequency mask profile is imperceptible to human hearing exposed simultaneously to the audio signal and the frequency mask profile.

4 . A method as claimed in claim 3 , wherein the first predetermined rate is 25 dB/Bark.

5 . A method as claimed in claim 1 , wherein within each sub-band, the frequency mask profile decays from the maximum towards the higher frequency bound of the sub-band at a second predetermined rate, the second predetermined rate being such that the frequency mask profile is imperceptible to human hearing exposed simultaneously to the audio signal and the frequency mask profile.

6 . A method as claimed in claim 5 , wherein the second predetermined rate is 10 dB/Bark.

7 . A method as claimed in claim 1 , the maxima of the frequency mask profile matching the amplitudes of the corresponding identified tones, the method comprising scaling the audio code by:

reducing the amplitude of the frequency mask profile by an offset to form a reduced amplitude frequency mask profile, and

multiplying the audio code by the reduced amplitude frequency mask profile.

8 . A method as claimed in claim 1 , the maxima of the frequency mask profile having amplitudes reduced from the corresponding identified tones by an offset, the method comprising scaling the audio code by multiplying the audio code by the amplitude frequency mask profile.

9 . A method as claimed in claim 1 , further comprising for a subsequent frame of the audio signal, scaling a further audio code by the frequency mask profile by:

reducing the amplitude of the frequency mask profile by a further offset to form a further reduced amplitude frequency mask profile; and

multiplying the further audio code by the further reduced amplitude frequency mask profile.

10 . A method as claimed in claim 1 , further comprising for a subsequent frame of the audio signal:

reducing the amplitude of the frequency mask profile by a further offset to form a further reduced amplitude frequency mask profile;

for each sub-band of the subsequent frame of the audio signal, identifying the further tone in that sub-band having the highest amplitude;

for each sub-band,

if the further identified tone has a lower amplitude than the maximum in that sub-band of the further reduced amplitude frequency mask profile, scaling a further audio code by the frequency mask profile, and

if the further identified tone has a higher amplitude than the maximum in that sub-band of the further reduced amplitude frequency mask profile, scaling the further audio code by a further frequency mask profile, the further frequency mask profile having a maximum in that sub-band at the frequency of the further identified tone.

11 . A method as claimed in claim 1 , further comprising embedding the audio code in each of several frames of the audio signal according to the method of claim 1 .

12 . A communications device for communicating data imperceptibly in an audio signal, the communications device comprising:

a processor configured to:

for each sub-band of the audio signal, identify the tone in that sub-band having the highest amplitude;

scale an audio code comprising the data to be communicated by a frequency mask profile, the frequency mask profile having maxima at the frequencies of the identified tones; and

aggregate the audio signal and the scaled audio code to form a composite audio signal; and

a transmitter configured to transmit the composite audio signal.

13 . A communications device as claimed in claim 12 , wherein within each sub-band, the frequency mask profile decays from the maximum towards the lower frequency bound of the sub-band at a first predetermined rate, the first predetermined rate being such that the frequency mask profile is imperceptible to human hearing exposed simultaneously to the audio signal and the frequency mask profile.

14 . A communications device as claimed in claim 12 , wherein within each sub-band, the frequency mask profile decays from the maximum towards the higher frequency bound of the sub-band at a second predetermined rate, the second predetermined rate being such that the frequency mask profile is imperceptible to human hearing exposed simultaneously to the audio signal and the frequency mask profile.

15 . A communications device as claimed in claim 12 , configured to, if the maxima of the frequency mask profile match the amplitudes of the corresponding identified tones, scale the audio code by:

reducing the amplitude of the frequency mask profile by an offset to form a reduced amplitude frequency mask profile, and

multiplying the audio code by the reduced amplitude frequency mask profile.

16 . A communications device as claimed in claim 12 , configured to, if the maxima of the frequency mask profile have amplitudes reduced from the corresponding identified tones by an offset, scale the audio code by multiplying the audio code by the amplitude frequency mask profile.

17 . A communications device as claimed in claim 12 , further configured to, for a subsequent frame of the audio signal, scale a further audio code by the frequency mask profile by:

reducing the amplitude of the frequency mask profile by a further offset to form a further reduced amplitude frequency mask profile, and

multiplying the further audio code by the further reduced amplitude frequency mask profile.

18 . A communications device as claimed in claim 12 , further configured to, for a subsequent frame of the audio signal:

for each sub-band of the subsequent frame of the audio signal, identify the further tone in that sub-band having the highest amplitude;

for each sub-band,

if the further identified tone has a lower amplitude than the identified tone, scale a further audio code by the frequency mask profile, and

if the further identified tone has a higher amplitude than the identified tone, scale the further audio code by a further frequency mask profile, the further frequency mask profile having a maximum in that sub-band at the frequency of the further identified tone.

19 . A communications device as claimed in claim 12 , further configured to embed the audio code in each of several frames of the audio signal.