Fingerprint extraction
Fingerprints are bit strings extracted from a media signal (e.g. an audio or video clip) to identify said media signal. Typically, they are derived from a perceptual property of the signal, for example, the spectral energy distribution of an audio fragment or the luminance distribution of a video image. A method and arrangement for extracting a fingerprint is here disclosed which is robust with respect to shifts of the perceptual property. Such shifts occur, inter alia, when the fingerprint is derived from a logarithmically mapped spectral energy distribution of an audio signal and said audio signal is subjected to speed changes. According to the invention, the fingerprint is not derived from the perceptual property as such, but from its auto-correlation function.
1 . A method of extracting a fingerprint from a media signal, comprising the steps of extracting from said media signal a sequence of samples of a given perceptual property of the signal, and deriving from said sequence a binary sequence constituting said fingerprint, characterized in that the method comprises the steps of:
subjecting the sequence of property samples to an auto-correlation function ( 202 ) to obtain a sequence of auto-correlation values;
comparing ( 105 ) said auto-correlation values with respective thresholds; and
representing the results of said comparisons by respective bits of the fingerprint.
2 . A method as claimed in claim 1 , wherein said step of subjecting the sequence of property samples to an auto-correlation function comprises correlating a sub-sequence of property samples with the complete sequence of property samples.
3 . A method as claimed in claim 1 , wherein said step of subjecting the sequence of property samples to an auto-correlation function further includes down-sampling ( 204 ) the sequence of auto-correlation values to obtain a desired number of auto-correlation values.
4 . A method as claimed in claim 1 , wherein said step of deriving from said media signal a sequence of perceptual property values comprises dividing an audio signal into sub-bands and computing the energies of said audio sub-bands.
5 . A method as claimed in claim 1 , wherein said step of deriving from said media signal a sequence of perceptual properties comprises dividing an image into blocks and computing the luminances of said image blocks.
6 . An apparatus for extracting a fingerprint from a media signal, comprising means for deriving from said media signal a sequence of samples of a given perceptual property of the signal, and means for deriving from said sequence a binary sequence constituting said fingerprint, characterized in that the apparatus comprises:
means for subjecting the sequence of property samples to an auto-correlation function to obtain a sequence of auto-correlation values;
means for comparing said auto-correlation values with respective thresholds; and
representing the results of said comparisons by respective bits of the fingerprint.
7 . A computer program comprising instructions to cause a programmable device to perform the steps of:
deriving from a received media signal a sequence of samples of a given perceptual property of the signal;
subjecting the sequence of property samples to an auto-correlation function to obtain a sequence of auto-correlation values;
comparing said auto-correlation values with respective thresholds; and
representing the results of said comparisons by respective bits of a fingerprint.