MXPA00007002A - Embedding and extracting supplemental data in an information signal - Google Patents

Abstract

A method and an arrangement are disclosed for embedding supplemental data in an information signal such as a sigma-delta modulated audio signal. The encoded signal includes two or more channels, e.g. a left channel bit stream (z1) and a right channel bit stream (z2). The same supplemental data (w) is accommodated at corresponding predetermined bit positions, e.g. every Mth bit position, of the bit streams. This allows detection and extraction of the supplemental data without the need for accommodating a sync pattern in the signal. At the receiving end, a sequence of Mth bits from the first channel is compared with a corresponding sequence of Mth bits from the second channel. If they are identical, the sequence is a candidate supplemental data sequence. This is done for different sequence positions (m) until the supplemental data has been found.

Description

INCLUSION AND EXTRACTION OF SUPPLEMENTARY DATA IN AN INFORMATION SIGNAL FIELD OF THE INVENTION F This invention relates to a method and an arrangement for including supplementary data in an information signal, comprising the step of including supplementary data samples in predetermined positions of the information signal. The invention also relates to a method and an arrangement for extracting supplementary data from such an information signal.

BACKGROUND OF THE INVENTION There is a growing need to accommodate watermarks in audio and video signals. Watermarks are supplementary data messages included in multiple media assets, preferably in a perceptually visible way. They comprise information, for example, about the origin or status of copyright of documents and audiovisual programs. They can be used to provide legal proof of the owner of the copyright, and to allow tracking piracy and support the protection of intellectual property.

A known method for including supplementary data in an information signal as defined in the opening paragraph is described, inter alia, in International Patent Application WO-A-98/3332. In this prior art method, a watermark pattern is included in a modulated audio signal (sigma) delta. The watermark is included in an encoded audio signal by modifying said selected ones. For example, every 100 is? or bit is replaced by a bit of the watermark pattern. The step of modifying the encoded audio signal is carried out within the feedback loop of the encoder to compensate for the effect of the modification in subsequent coding steps. The prior art method was devised to record high quality audio in the audio version of the Digital Versatile Disc (DVD). A sampling frequency of 2,822,400 Hz (64 * 44,100) will be used to produce a signal-to-noise ratio of 115 dB. Replace each? Oo? Mo bit of the sigma-delta modulated audio signal by the bit of a watermark at the expense of only 1 dB and increase the quantization noise. This corresponds to a bit rate of the watermark of about 28000 per second. The aforementioned patent application O-A-98/33324 also describes an arrangement for extracting the watermark. A synchronization bit pattern is accommodated (here subsequently synchronization pattern for brevity) in the bit stream to identify the positions of the bits of the supplementary data. The arrangement comprises a division stage and a synchronization detector. The division step divides the bit rate by the number of bits by which the bits of the watermark are separated (for example 100 if every I00th bit of the signal is a bit of the supplementary data). The synchronization detector changes the phase of the division stage until the synchronization pattern is found. Such a synchronization detector includes a relatively large offset or shift register or a serial to parallel converter to store a portion of the bit stream. If each Month? Bit of the signal is a bit of the supplementary data and the synchronization pattern comprises N bits, the synchronization guard must necessarily store (N-1) »M + 1 bits. German Patent Application DE-A-37 17 315 discloses such known synchronization detector in more detail. In this publication every 15th bit of the signal is a supplementary bit and the synchronization pattern is a 4-bit word. Accordingly, the offset or offset register (serial to parallel converter 5 in Figure 2 of DE-A-37 17 315) contains 46 bits. To reduce the length of the deviation or displacement record, it is proposed in the Patent Application copending, unpublished, of the applicant PHN 17.148 to make the separation between the bits of the synchronization pattern considerably smaller than the separation between the watermark bits. However, this solution affects the coding performance and increases the signal to noise ratio of the sigma-delta modulator.

BRIEF DESCRIPTION OF THE INVENTION It is an object of the invention to provide a method for including supplementary data in an information signal, which allows the positions of the bits of the supplementary data to be detected in an alternative form. Up to this point, the method according to the invention is characterized in that the information signal comprises at least two signal channels, the method includes the step of including the same supplementary data samples in the corresponding positions of the channels. The transmission of a synchronization pattern can now be omitted because the position of the included watermark bits can easily be found by looking for the positions in which at least two samples of the channel are identical. The corresponding method for extracting supplementary data from the information signal comprises the steps of comparing a first sequence of the first predetermined channel signal samples with a corresponding second sequence of the channel signal samples, shifting the first and second frequencies at a position of the sample, and repeating the comparison steps for the first and second second displaced sequences as long as the sequences are not identical.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic diagram of a preferred embodiment of an array for including supplementary data in a sigma-delta modulated audio signal according to the invention. Figures 2 and 3 show waveforms to illustrate the operation of the sigma-delta modulators shown in Figure 1. Figure 4 shows waveforms to illustrate the operation of the array to include supplementary data in a signal in accordance with the invention. Figure 5 shows a schematic diagram of a modality of an array for extracting supplementary data from a signal. Figure 6 shows a flow diagram of the steps of the method to illustrate one embodiment of a method for extracting supplementary data from a signal.

DESCRIPTION OF A PREFERRED MODE The invention will be explained with reference to Figure 1, which shows a schematic diagram of a preferred embodiment of an array to include supplementary data in a sigma-delta modulated audio signal according to the invention. The array receives a stereo audio signal comprising the signal from a left channel i and a signal from a right channel x2. The channels i and x2 are applied to conventional sigma-delta modulators 1 and 2, respectively. Both sigma-delta modulators are identical, so only one of them will be described. The sigma-delta modulator 1 comprises a subtracter 11, a loop filter 12, a polarity detector 13 and a feedback path 14. The subtractor 11 subtracts a modified output signal Zi (having a level of + 1V or -IV) of the input signal xi. The circuit filter or loop 12 filters the signal of the difference. The filtered signal is applied to the polarity detector 13 which produces, at a speed determined by a sampling frequency fs, an encoded signal and i having the value of the bit "1" (+ 1V) or "0" (-1V) . Figure 2 shows waveforms to explain the operation of the conventional sigma-delta modulator. More particularly, the Figure shows an input signal x and the encoded signal and. The sigma-delta modulator produces more positive samples since the input signal becomes larger. As the Figure shows, an input voltage of -0.5 V is encoded as a sequence of bits 0001 (three pulses of -IV and one pulse of + 1V), an input voltage of OV is encoded as a bit pattern of high frequency 01010 (alternating pulses of -IV and + 1V), and an input voltage of + 0.5V is encoded as a sequence of bits 1110 (three pulses of + 1V and a pulse of -IV). The encoded signal is decoded at the receiving end by reforming the received pulses and passing them through a low pass filter. In this simplified example, it was assumed that the encoded signal is demodulated by averaging 13 samples of the signal. The demodulated signal x 'is also shown in Figure 2, in addition to a time delay caused by the operation of the low pass filter. In the Figure, the demodulated signal x 'is thus aligned by time with the input signal x. The modification circuits 3 and 4 are connected between the polarity detector 13 and the feedback path 14 of the respective sigma-delta modulators. In response to a control signal c released by a control circuit 5, the modification circuits (multiplexers) replace each month? m bit signals coded yx and y2 by a bit of a watermark w. The modified coded signals zx and z2 are combined by a multiplexer 6 to form a single bit stream for transmission to a receiver or register on a storage medium. The message of the watermark w is stored in a register 501 of the control circuit 5. It is important to note that the same watermark is applied to both modification circuits 3 and 4. Figure 3 shows waveforms to explain the operation of the sigma-delta modulator if the respective modification circuit is active. The Figure shows the same input signal x shown in Figure 2 and the modified coded signal z. In the example, a sample "-1" 20 (Figure 2) of the signal codes and has been replaced by a sample "+1" 30 to represent the bit of a watermark w = l. Because the modification is fed back into the input, the adverse effect of the modification will be later compensated by the coding step. Thus, a portion of the output signal z that immediately follows the bit of the supplementary data 30 differs from the corresponding portion shown in Figure 2. Accordingly, the demodulated signal x 'in Figure 3 is also temporarily different from the same signal in Figure 2. Note that the temporal alignment in the Figures makes the difference becomes already manifest before the supplementary data bit is included. As will be appreciated from a comparison of Figures 2 and 3, the difference is strongly noticeable in practice. A sigma-delta modulator for encoding high-quality audio signals at a sampling frequency f2 = 2,822,400 Hz (64 * 44,100) has a signal-to-noise ratio of 115 dB. It has been found that replacing 1 sample with 100 samples increases the quantization noise by only 1 dB. Figure 4 shows a simplified example of a multi-channel audio bit stream which is produced by the array shown in Figure 1. In this simple example, each? Or is? Bit of the bit streams of channel z2 and z2 is a bit of supplementary data. The data bits of the watermark included w are shaded in the Figure. They are the same in both channels. The bits of the encoded signal are generally not the same. This is still true in practice if the audio signal is a mono signal. The positions of the M bits within each series of M bits were numbered 0 ... M-1. It will be appreciated that the objective of a receiver array is to identify the positions of the bit bits of the supplementary data (m = 2 in Figure 4). This is achieved by looking for the positions of the bits of the bitstream of multiple channels in which the samples of the signal zx and z2 are identical. Once this objective has been formulated, an arrangement for searching for those positions of the bits can easily be designed by a person skilled in the art. For illustration, two modalities will now be described. Figure 5 shows a schematic diagram of an implementation of physical computing components of an array for extracting supplementary data from a bitstream z that is produced by the encoder shown in Figure 1. The array comprises a demultiplexer 50 for demultiplexing the stream of audio bits z in the bitstream Zi representing the left audio channel and the bitstream z2 representing the right audio channel. The array further comprises a counter divided by M 51. Each period Més? M0 bit, and the counter produces a timing pulse representing the current position of a bit m (m = 0..M-l) that is being tested. The bit streams zx and z2 are applied to a comparator 52. In response to the timing pulse m, the comparator produces an EQ pulse if the bits currently applied are the same, or a NQ pulse if they are different. If the bits are the same, they are bits of potential watermarks. In that case, the bit of the potential watermark z2 (or zi) is diverted or displaced in aDeviation or shift register 53 in response to the EQ pulse. The next timing pulse now occurs M bit periods later. In this way, as long as each Month? M0 bit of zi and the corresponding bit of z2 are not identical, that particular bit is diverted or moved in the shift or offset register. If the bits are not the same, the comparator 52 produces a NQ pulse. This pulse is applied to a deviation or shift input S in the counter 51 to deflect or shift the phase of this counter divided by M in a one bit period. The pulse NQ is also applied to a cleaning input C of the offset or offset register 53 to clear the record. The arrangement shown in Figure 5 is simple and inexpensive but has the disadvantage that a relatively long period of time must be observed until it can be assumed that the position of the bit of a watermark has been found, and that the message of the mark of Water has not been completely acquired from the beginning. The expected time has been observed until the fix has found the position of the watermark bit depends on the number of bits (M) by which the bits of the watermark are separated and the probability (p) of that the bits in both channels are identical (p = l / 2 for audio signals modulated sigma-delta). The expected period of time until a false block is recognized is: The average time period to detect the bit position of the watermark is: M2p Tbloqueo- 2 (1 - p) The use of a number of parallel detection circuits, each of which processes a shifted sequence of the bits of a potential watermark, can accelerate the detection and extraction of the brand of Water. The detection and extraction of the watermark can also be carried out by a microprocessor which is loaded with appropriate programs and programming systems. Figure 6 shows a flow diagram of the operation steps that are carried out by such a microprocessor in accordance with an additional embodiment of the arrangement. It is assumed again that each Month? Bit of each channel bit stream is the bit of a watermark. The microprocessor uses for each position of a bit m (m = 0..Ml, see Figure 4) an element of the array R (m) to indicate whether a bit of a potential watermark has been detected in such a position of the bit (R (m) = l) or not (R (m) = 0). In a first step 60 of the program, all elements of the array R (0) .. R (M-1) are initially given the value of 1. In step 61, the position of the m bit being tested obtains a initial value of 0. The program then waits for the reception of a new bit of the channel bit streams zi and z2. In a step 62, both bits are separated. If they are identical, an optional step 63 is carried out which will be described later. The element of the arrangement corresponding to R (m) remains unaffected. If the bits of the channel are not identical, the corresponding array element R (m) is given the value of 0 (step 64) to indicate that the position of the current bit m is definitely not the position of the bit of a watermark that is being blocked. In a step 65, the position of the bit is increased. In step 66, it is verified whether the position of the increased bit is still within the range 0..M-1. If this is the case, the program returns to step 62 to test the next bit position. If this is not the case, the program first performs an optional step 67, which will be described later and then returns to step 61 (in which the position number of the m bit is reset) to test the next set of M bits channel. After the first series of M channel bits have been processed in this way, it has already been found that a Number of positions of the bits does not belong to the positions of the bits of a watermark. With reference to the example shown in Figure 4, this applies to the positions of bits 1, 5 and 7. The corresponding array elements R (l), R (5) and R (7) have now obtained the value of 0 and will not be modified anymore, even if the bits in such positions are later the same. When a series of M bits is processed in the channel, an additional number of bit positions will be excluded as a bit position of a candidate watermark. In the example shown in Figure 4, the positions of bits 0, 4, 8 and 9 will be excluded during the second round, the position of bit 6 will be excluded during the third round, and, finally, the position of bit 3 will be Excluded during the fourth round. The array has now identified the position of the watermark bit m = 2 in this simple example. The method shown in Figure 6 has detected the position of the watermark bit m while an element of the arrangement R (m) has the value of 1 and all other elements have the value of 0. To give meaning to this, the program further includes the processing step 67 in which it is tested whether the sum of all the elements of the array is 1. The processing step 67 can be located in several places of the program. In this example, the test is carried out when the position of bit m has assumed the value of, ie when a series of M candidate bit positions has been processed. The acquisition of the watermark data message may be postponed until the position of the watermark bit has been found. However, it is possible to acquire the watermark message while searching for your position. Up to this point, the arrangement includes an additional arrangement in which each message of the candidate watermark received is far from being acquired. Up to this point, the program shown in Figure 6 includes step 63 in which the bit of the received watermark is added to the message currently received. As soon as the position of the bit of the watermark m has been found, the corresponding array element W (m) has already acquired the message of the watermark received remotely. It should be noted that the arrangement for detecting watermark bit positions can be further extended to verify if an accidental deviation or displacement has occurred in one of the channels. For example, if the watermark is included in three or more channels, the arrangement can be arranged to look for the deviation or displacement of the channels. Or the arrangement is arranged to intentionally introduce a deviation or displacement in one of the channels if the watermark has not been identified in a predetermined portion of the signal or within a predetermined period of time. Although the invention has been described with reference to a two-channel audio signal (encoded by unit bits) modulated sigma-delta, they are not limited to audio encoded signals or unit bits. The samples of the signal can be multiple bit samples (for example PCM samples), the watermark is included in one or more less significant bits of the samples. The coding algorithm is relevant (this can, for example, be the modulation of the pulse code), it is not required that the inclusion step of the supplementary data be carried out within the feedback loop of an encoder. The essential feature of this invention is that the same supplementary data samples are included in the corresponding positions of two or more channels of a multi-channel information signal. In summary, a method and arrangement for including supplementary data in an information signal such as a sigma-delta modulated audio signal is described. The encoded signal includes two or more channels, for example, a bitstream of a left channel (zi) and a bitstream of a right channel (z2). The same supplementary data (w) are accommodated in the corresponding predetermined bit positions, for example, each Més? M0 position of a bit, of the bit streams. This allows the detection and extraction of supplementary data in the need to accommodate a synchronization pattern in the signal. At the receiving end, a sequence of Month? S bits of the first channel is compared to a corresponding sequence M? S bits of the second channel. If they are identical, the sequence is a sequence of candidate supplementary data. This is done for different positions of the sequence (m) until supplementary data has been found.

Claims (10)

CHAPTER CLAIMING Having described the invention, it is considered as a novelty and, therefore, what is claimed is contained in the following CLAIMS:

1. A method for including supplementary data (W) in an information signal (z), comprising the step of including supplementary data samples () in predetermined positions of the information signal, characterized in that the information signal comprises at least two channels of signals (zxz2), the method includes the step of including the same supplementary data samples () in corresponding positions of such channels.

2. The method according to claim 1, characterized in that the channels are the components of a left and right audio signal of a stereo audio signal, the method comprises the step of coding each channel (x? X2) in a respective bit stream (y? y2) and include the same bits of the supplementary data (w) at the positions of the corresponding bits of the bit stream.

3. An arrangement for including supplementary data in an information signal comprising means for including supplementary data samples (w) in positions predetermined of an information signal, characterized in that said information signal comprises at least two signal channels (z?, Z2), the inclusion means are arranged to include the same samples of supplemented data (w) in corresponding positions of the channels. The arrangement according to claim 3, characterized in that the channels are the components of the right and left audio signal of a stereo audio signal, the arrangement comprises means for encoding each channel (x? X2) in a stream of respective bits (y? y2) and include the same bits of the supplementary data (w) at the positions of the corresponding bits of the bit streams. 5. A method for extracting supplementary data from an information signal comprising at least one first (zi) and a second (z2) channel, each of which has identical supplementary data samples (w) included in sample positions of the predetermined signal, the method is characterized in that it comprises the steps of: comparing a first sequence of first predetermined samples of the channel signal separated by a number (M) of signal samples with a corresponding second sequence of the second samples of the channel signal, and - moving the first and second sequences one position of the sample, and repeating the comparison step for the first and second displaced sequences as long as the sequences are not identical. 6. The method of compliance with the claim 5, characterized in that it further comprises the step of storing the data samples of the signal that are identical in the sequences being compared. 7. An arrangement for extracting supplementary data from an information signal comprising at least one first (zi) and a second (z2) channel each of which has identical supplementary data samples (w) included in the predetermined corresponding signal sample positions, the array is characterized in that it comprises: - means for comparing a first sequence of samples of first predetermined channel signals with a second corresponding sequence of signal samples of the second channel, and means for moving the first and second sequence one position of the sample, and repeating the comparison step for the first and second sequences displaced as long as the sequences are not identical. 8. An information signal that has supplementary data samples included in predetermined positions of the signal, characterized in that the signal of information comprises at least two channels and the same supplementary data samples have been included in corresponding positions of the channels 9. The signal according to claim 3, characterized in that the channels are components of the left and right audio signal of a audio and stereo signal, each component is encoded in a corresponding bit stream and the same supplementary data bits are included in predetermined bits positions of the bit stream. 10. a storage medium, characterized in that it has registered in it a signal according to claims 8 or 9.