Procedure for including digital information in an audio signal prior to channel coding

Claim 1

A method for combining a data signal with an audio signal prior to channel-coding the combined signal, in which - an audio signal entering in sample sequence mode is conducted to a first branch and divided into subbands, whereby in each subband an array of audio signal samples of equal size is obtained in one and the same time window, - a masking threshold is calculated simultaneously for said sample array in each subband, the sounds wherebelow being unaudible to the human ear, - the bits of the data signal are substituted for the bits of the samples of the sample arrays remaining below the masking threshold, - the subbands are combined, whereby a combined signal to be transmitted on an audio channel is obtained, and - all the information is gathered that is needed in re-separating the combined signal, and said information is transmitted in the form of side information on a separate data channel at the same time with the combined signal, whereby - an audio signal is conducted also to a second branch in which it is channel-coded and decoded, and thereafter it is divided into as many subbands as in the first branch, whereby in each subband an array of audio signal samples of equal magnitude is obtained in the same time window as in the first branch, - the audio signal conducted into the first branch is delayed for a time equivalent to the time required for channel-coding and decoding, - the audio signal samples of one and same point of time of the corresponding subbands of each branch are compared, - only the bits of the samples of the first branch are substituted for by data bits which are the same as in the second branch, and - information on the location of the substituting data bits in the sample is transmitted in the form of side information on said data channel.

Claim 3

An apparatus for combining a data signal with an audio signal before channel-coding the combined signal in the transmitter, said apparatus comprising: - a first filter means (311) for dividing an audio signal entering in the form of sample sequence mode into subbands, - a grouping means (315) to group in each subband an array of audio signal samples of the same size in one and same time window, - an analysing and calculating means (34,35), simultaneously calculating in each subband a masking threshold for a sample group, the sounds below which the human ear is not able to hear, - a substituting means (37) in which the bits of a data signal are substituted for the bits of the samples of the sample groups which remain below the masking threshold, - a second filter means (39) to combine the subbands, whereby a combined signal to be transmitted on an audio channel is obtained, - a data channel control means to gather all the information needed for reseparating the combined signal, which information is transmitted as side information on the data channel simultaneously with the combined signal, whereby the apparatus comprises further - a parallel branch to which the audio signal is also conducted, while the branch comprises in succession a channel-coder (325) and a decoder (316), a third filter means (31) to divide the output signal of the encoder into as many subbands as the first filter means (311), a second grouping means (33) to group within each subband an equal number of audiosignal samples in one time window, whereby in said subband an equal number of audiosignal samples are obtained in one and the same time window as in the first branch, - a delay means (317) to delay the audio signal entering the first filter means (311) for a period of time which corresponds to the delay of the channel coder (315) and the decoder (316), - a comparator means (313) which compares the same-moment audio signal samples of the corresponding subbands of the first (315) and the second grouping means (33) with one another, whereby the substituting means substitutes with data bits only for the bits of the samples of the first grouping means (315) which are the same as those in the samples of the second grouping means (33), and the comparator (313, 314) informs the control means of the side channel of the location of the substituting data bits in the sample.