Method of embedding a digital watermark in a useful signal

Method of embedding a digital watermark in a useful signal, in particular an audio signal, using one of a programmable computer, a programmable computer network, and a further programmable equipment, wherein the useful signal (xn) represents the evolution of a spectrum comprising useful signal frequencies, over time, and the digital watermark includes a watermark bit sequence, each bit of the watermark bit sequence representing one of a first state and a second state, characterized in that for imprinting the first state on the useful signal, the ratio of a first spectral amplitude (Af) of the useful signal at a first imprinting frequency (f1) to a second spectral amplitude (Ag) of the useful signal at a second imprinting frequency (g1) is established to be higher or equal to a first value of a threshold parameter (ys), and for imprinting the second state, the ratio of the second spectral amplitude (Ag) of the useful signal at the second imprinting frequency (g1)to a first spectral aplitude (Af)of the useful signal at a first imprinting frequency (f1) is established to be higher or equal to a second value of the threshold parameter (ys) forming the digital watermark containing auxiliary data, and embedding the digital watermark in the useful signal to generate a modulated useful signal incorporating the auxiliary data,

• characterized in that a bit time length (Tbit) is established indicating a time length within which the state represented by a single bit is imprinted to the useful signal, a segment time length (Tcode) is calculated, using the number of bits of the watermark bit sequence and the bit time length, indicating the time length within which the watermark bit sequence is imprinted to the useful signal, a segment of the useful signal with a time length of at least a segment time length is selected to imprint the watermark bit sequence.

Method of detecting a digital watermark in a useful signal, using one of a programmable computer, a programmable computer network, and a further programmable equipment, wherein the useful signal (y) represents the evolution of a spectrum comprising useful signal frequencies, over time, and the digital watermark is represented as a watermark bit sequence, each bit of the watermark bit sequence representing one of a first state and a second state, characterized in that the ratio of a first spectral amplitude (Âf) of the useful signal at a first detection frequency (f1) to a second spectral amplitude (Âg) of the useful signal at a second detection frequency (g1) is calculated, and in case the ratio (Δi) is equal to or larger than 1, the first state is detected, otherwise the second state is detected, processing the useful signal to detect the digital watermark, and indicating when the digital watermark has been found,

• characterized in that a bit time length (Tbit) is established indicating a time length, within which the state represented by a single bit is detected from the useful signal, for each bit time length, an indication of the detected state (Bi) and a value of a ratio parameter (Δi) is stored in association to each other in a detection bit sequence, wherein the ratio parameter indicates the value of the calculated ratio, if this value is equal to or larger than 1, and indicates the reciprocal value of the calculated ratio otherwise.