Patent ID: 7760790

Claim:
A method performed by an apparatus for transmitting watermark data bits using a spread spectrum, said method including the steps: modulating said watermark data bits on an encoder pseudo-noise sequence; modulating said modulated encoder pseudo-noise sequence on a carrier frequency; determining whether at one or more different candidate frequency band positions in a current frame of said audio signal the energy or amplitude level of said audio signal is such that it can mask one or more, respectively, of said modulated carrier frequencies and, if this is true, notch filtering said audio signal at the corresponding frequency band positions and inserting at this frequency or at each of these frequencies, respectively, said carrier frequency or one of said carrier frequencies, respectively; checking at which candidate frequency band positions in a frame following said current frame of said audio signal the energy or amplitude level of said audio signal is such that it can mask one or more, respectively, of said modulated carrier frequencies, and providing information about the locations of these frequency band positions; and transmitting or transferring data of said current audio signal frame carrying said watermark data bits together with the information about the locations of the frequency band positions to be used in said frame following said current frame of said audio signal, wherein, in the frame following said current frame, no watermark signal carrier is transmitted in the frequency band or bands which have been occupied in said current frame, in order to decrease watermark data bit errors caused by echoes following reception of said audio signal.