Digital communications

A method of marking a digital signal with a digital fingerprint, the digital signal comprising at least one of an audio or video signal, wherein the digital fingerprint is an ordered rotation in an abstract vector space constructed from principal components of the digital signal, the rotation being determined using families of patterns having low off-peak autocorrelation and low cross correlation, the method including:

• transforming a plurality of samples of the digital signal onto a discrete orthonormal basis;
• altering n principal components by a marking angle to generate marked principal components;
• converting the marked principal components and unmarked non-principal components into Cartesian coordinates to generate converted, marked principal components and converted unmarked, non-principal components;
• combining the converted, marked principal components with the converted unmarked, non-principal components of the signal to generate combined principal and non-principal components;
• and
• applying an inverse of the transform function to the combined principal and non-principal components to generate a fingerprinted digital signal comprising the digital fingerprint.