Method and apparatus for quantisation index modulation for watermarking an input signal

Method for quantisation index modulation for watermarking an input signal x, wherein different quantiser curves Qm are used for quantising said input signal x and a current characteristic of said quantiser curve is controlled by the current content of a watermark message m, wherein in said quantising the difference between input value and output value at any position is not greater than T, and that said quantising curves Qm are reversible in that for any input value x there is a unique output value y,

• wherein ±T is a value defining the y shift towards y=0 of outer sections of said quantiser curves Qm and is determined by the current psycho-acoustic masking level of said input signal x, and y is the watermarked output signal,
• and wherein the different quantiser curves Qm are established according to the current value of m by different shifts of the complete quantiser curve in x direction.

Method for regaining an original input signal x which has been processed according to claim 2, said method including the steps:

• re-quantising according to y=Qm(x)+max(−T, min(T, α(x−Qm(x)))) the received watermarked signal using said quantiser curves Qm in a corresponding manner, wherein different candidate quantiser curves Qm are checked by applying different shifts of the complete quantiser curve in x direction, and wherein said re-quantisation is carried out with a bit depth that is greater than the bit depth that was applied originally;
• selecting that candidate quantiser curve Qm which matches best in the frequency domain;
• based on the current Qm so determined, removing the corresponding current watermark m from signal y so as to provide said regained signal x.