Device for transmitting data words representing a digitized analog signal and device for receiving the transmitted data words

Abstract: A transmission system is disclosed for transmitting digitized analog information. At the receiving side, for the purpose of transmitting an additional information signal, data words representing the digitized information are modified depending on the bits of an additional information signal. The modifying means (21, 22) are constructed in such a way that the modification depends both on the additional information signal and on other bits (b7) which are not correlated with the bits (d) of the additional information signal. This results in the intercorrelation between the errors (Δ; e) in the data words introduced by the modification being reduced, so that a flatter power density error spectrum is obtained and the errors introduced in the data words by the modification are less conspicuous. At the receiving side the transmission system comprises means (28, 29) which recover the additional information signal from the data words received.

Description: The invention relates to a device for transmitting a series of data words representing a digitized analog signal, which device comprises modifying means constructed to modify the series of data words, prior to transmission, dependent on an additional information signal in such a way that after modification the additional information signal is represented by less significant bits of selected data words, the values represented by the modified data words being slightly different from the values represented by the relevant data word prior to modification. The invention further relates to a device for receiving data words for use in conjunction with such a transmitting device, which receiving device comprises means for recovering the additional information signal from said less significant bits of selected received data words. Such devices are known from European Patent Application EP-A-0,205,200. Said Application describes a transmission system in which for the transmission of the additional information signal one or more of the least significant bits of every n-th signal sample of a series of digitized audio signal samples is or are replaced by bits of the additional information signal. After the series of signal samples thus modified has been transmitted the additional information signal is recovered from the signal samples, the original audio signal being recovered from the signal samples by digital-to-analog conversion. The substitution of the least significant bits in every n th signal sample introduces a minor error in the signal values. If the logic value of the additional information signal remains constant for a longer time repetition patterns may arise in the errors, resulting in audible whistle tones in the recovered audio signal. This is caused by the fact that the errors are intercorrelated if the additional information signals do not vary significantly. In the known method the occurence of annoying whistle tones is precluded in that, after recovery of the additional information signal, the signal sample bits used for the transmission of the additional information signal are replaced by a bit of an arbitrary logic value, which substantially reduces the correlation in the consecutive errors. However, this known method has the drawback that additional circuits are needed in the signal processing path where the received signal samples are converted into the original audio signal. This is undesirable, in particular if the number of receiving devices is substantially larger than the number of transmitting devices, as for example in the case that the information is transmitted via a record carrier, such as for example a magnetic tape or an optically readable disc. Further GB-A-1203768 discloses a device for transmitting a digital representation of an analog signal (speech) and an additional information signal (telegraph) over a single telecommunication channel. The device comprises modifying means constructed to modify the series of bits representing the analog signal prior to the transmission dependent on the additional information signal. Insertion of additional information bits is carried out by replacing bits representing the analog signal (bit-stealing) and is carried out intermittently so that the transmitted analog signal is not substantially affected. For the reduction of the perceptibility of the effects of additional information signal insertion the modifying means are adapted to modify the series of bits representing the analog signal depending both on the additional information signal and on the logic values of other bits than the replaced bits. The modification depends on the values of bits adjacent to the replaced bit, which adjacent bits are not correlated with the additional information signal. Furthermore, transmission of digitized analog signals arranged in a series of data words together with additional information signals by using "bit-stealing" is known from GB-A-2063018. It is the object of the invention to provide means which enable information to be transmitted by means of a device as defined in the opening paragraph, in such a way that the correlation in the consecutive errors is reduced without additional circuits being needed in the signal processing path of the signal samples at the reception side, and which can be achieved by means of simple circuits. This object is achieved by a device which is characterized in that the modifying means are adapted to modify the selected data words depending both on the additional information signal and on the logic values of other bits of the data words than said less significant bits, which other bits are non-correlated with the additional information signal, and in that the modifying means are adapted to subject bits to the additional information signal and said other bits of the data words to an Exclusive-OR operation in order to determine the logic values of said less significant bits. A receiving device for using conjunction with the transmitting device according to the invention is characterized in that the signal recovery means comprise means which are constructed to perform an exclusive OR operation for said least significant bits and the specific other bits of the data words in order to determine the logic values of the additional information signal. This combination of a transmitting and receiving device advantageously uses the fact that a logic "exclusive-OR" operation is reversible. Further embodiments of the transmitting device and the receiving device and their advantages will now be described in more detail, by way of example, with reference to Figs. 1 to 11, of which Figs. 1 and 2 serve to illustrate prior-art methods of transmitting an additional information signal, Figs. 3 and 6 show transmission systems utilizing different embodiments of the transmitting device and of the receiving device in accordance with the invention, Figs. 4 and 7 illustrate the data-words modification dependent upon the additional information signal for different embodiments of the transmitting device in accordance with the invention, and Fig. 5 illustrates the relationship between the prevailing error and the logic values of bits of the data word and the additional information word for different embodiments of a transmitting device in accordance with the invention. Fig. 1 shows a series 1 of 8-bit data words, representing a series of digitized signal samples of an analog-signal, for example an audio or a video signal. The reference numeral 2 indicates a series of data words derived from the series 1 by replacing the least significant bit b0 of every third data word (in Fig. 1 these are the least significant bits of the 2 nd , 5 th , 8 th , 11 th , 14 th and 17 th data word) by a bit of an additional information signal comprising a series 3 of bits d5, ..., d0. The series 2 of modified data words can subsequently be transmitted via a customary data transmission channel or a record carrier. After transmission the additional information signal can be recovered by detecting the least significant bits of every n th data word received. The analog signal represented by the data word can be recovered from the series of data words by customary digital-to-analog conversion techniques. This method of transmitting additional information along with the digitized analog information is frequently referred to as "bit stealing". Transmitting additional information by means of bit stealing has the advantage that the data format used for the transmission need not be modified. Since for the transmission only the least significant bits of the data words are used the error introduced in the recovered analog signal as a result of bit stealing is minimal. Fig. 2 by way of illustration gives the magnitude of the error Δ as a function of the logic value of the least significant bit b0 of the original data word and the logic value of the bit d of the additional information. If the additional information varies only to a small extent repetition patterns may arise in the errors Δ, which in the case that the analog signal is an audio signal result in audible whistle tones and in the case that the analog signal is a video signal result in a visible interference pattern in the video picture. This is caused by the fact that the errors Δ are intercorrelated and the spectral power density distribution of the error signal consequently exhibits peaks. In the embodiments to be described hereinafter steps have been taken to reduce the intercorrelation between the errors, which results in a flatter spectral power density distribution and hence in less conspicuous errors in the audio or the video signal. Fig. 3 shows an information transmission system employing a first embodiment of a transmitting device and a receiving device in accordance with the invention. The reference numeral 10 denotes an analog-to-digital converter by means of which an analog signal, for example an audio or a video signal, applied via an input 11 is converted into data words, for example 8-bit data words. The individual bits of the data words are referred to as b7, ..., b0, where b7 is the most significant bit and b0 is the least significant bit. The bits b7, ..., b0 are fed out via the respective outputs 12, ...., 19. The output signals on the outputs 12, ...., 18, which represent the seven most significant bits, are applied directly to the parallel inputs of a signal processing circuit 20. The output signal on the output 19, which signal represents the least significant bit b0, is applied to the signal processing circuit 20 via a two-channel multiplex circuit 21. The processing circuit 20 in a customary manner converts the data words which are applied in parallel form into a serial bit stream, synchronization signals being inserted into the bit stream for the purpose of synchronizing the information transmission. The processing circuit 20 may comprise for example a known circuits as used for recording standard CD signals or standard DAT signals. The bits d of the additional information signal are applied to an input of an Exclusive-OR gate 22. The bits b7 of the data words generated by the analog-to-digital converter 10 are applied to the other input of the Exclusive-OR gate 22. The output signal of the Exclusive-OR gate is applied to the signal processing circuit 20 via the multiplex circuit 21. By means of a clock signal generator 23 the signal processing circuit 20, the analog-to-digital converter 10 and the multiplex circuit 2 are controlled in such a way that upon every n th data word generated by the analog-to-digital converter the output signal of the Exclusive-OR gate 22 is applied to the signal processing circuit 20, the least significant bit b0 of the other data words being applied to the signal processing circuit 20. Thus, the least significant bit of every n th data word is replaced by a bit having a logic value derived from a bit d of the additional data word and one of the bits of the data word, in the present case the most significant bit b7, by means of an Exclusive-OR operation. By way of illustration Fig. 4 shows a series 31 of data words in which the least significant bit of every 3 rd data word is replaced by a bit b0* depending upon the most significant bit b7 and one bit d of the data word by means of the circuit described above. Fig. 5 gives the error Δ, introduced in the data words as a result of the bit stealing method illustrated in Fig. 4, as a function of the most significant bit b7, of the least significant bit b0 and of the data bit d. The error Δ is a function of three substantially uncorrelated bits b0, b7 and d. The intercorrelation between the errors Δ will therefore be reduced in comparison with the known bit stealing method as illustrated in Figs. 1 and 2, in which the error Δ depends only on the bits b0 and d. After transmission the bits d of the additional information signal can be recovered simply by means of subjecting the least significant bits b0* and the most significant bits b7 of every n th dat word received to an Exclusive OR operation. Fig. 3 also shows the circuit for recovering the additional information signal. In the circuit the bit series transmitted via a transmission channel or transmission medium 24, for example a DAT cassette tape or an optical storage medium, is applied to a second processing circuit 25 and a synchronizing circuit 26. The synchronizing circuit, in a manner known per se, detects the synchronizing signals in the received bit stream to derive clock signals for controlling the processing circuit 25, which recovers the data words from the serial bit stream. The bits b7, ..., b1 and b0* are applied to a digital-to-analog converter 27, which reconverts the data words into an analog signal. The bits b7 and b0* are not only applied to the digital-to-analog converter 27 but also to an Exclusive-OR gate 28. The output signal of the Exclusive-OR gate 28 is applied to the input of a flip-flop 29, which is controlled by the synchronizing circuit 26 in such a way that upon reception of every n th data word the flip-flop 29 is loaded with the result of the logic operation performed by the Exclusive-OR gate 28, so that the additional information signal becomes available on the output of the flip-flop 29. For the transmission of the additional information signal the circuit shown in Fgi. 3 the most significant bit b7 and the data bits b7 to the subjects logic exclusive-OR operation. However, this logic operation may also be applied to one of the bits b6, ..., b1 or a logic combination thereof and the bits d. It is essential only that the bits d of the data word used in the logic operation are uncorrelated with the bits of the additional information signal. Neither is it necessary that the logic operation is performed with a bit of the data words in which the least significant bits are modified. It is also possible to use bits of other data words for this purpose. In the circuit shown in Fig. 3 the data words are converted into a series of serial bits. However, it will be evident that, although this is common practice in information transmission, it is not necessary to convert the data words into serial form but that the information may also be transmitted in parallel form. In the circuit shown in Fig. 3 only one bit of every n th data word is replaced. However, it is equally possible to replace more than one bit in every n th data word. Fig. 6 shows an embodiment in which instead of one bit two bits of every selected data word are replaced. In Fig. 6 elements corresponding to those in the embodiment shown in Fig. 3 bear the same reference numerals. In addition to the Exclusive-OR gate 22 and the multiplexer 21 for replacing the least significant bit b0, the embodiment of Fig. 6 also comprises an Exclusive-or gate 40 and a multiplex circuit 41 by means of which the least significant bit but one b1 of every n th data word is replaced in a similar way. At te receiving side an additional Exclusive-OR gate 42 and an additional flip-flop 43 are provided for recovering the bits d of the information signal. Upon reception of every n th data word two bits of the additional information signal are recovered by means of the Exclusive-OR gates 28 and 42 and the associated flip-flops 29 and 43. By way of illustration Fig. 7 shows the series 1 of data words which, depending upon the series 2 of bits of the information signal, can be converted by means of the circuit shown in Fig. 6 into a series 45 in which the two least significant bits of every 6 th data word have been replaced.

Claims: A device for transmitting a series of data words representing a digitized analog signal, which device comprises modifying means constructed to modify the series of data words prior to the transmission dependent on an additional information signal in such a way that after modification the additional information signal (d) is represented by less significant bits (bo*) of selected data words (2), the values represented by the modified data words (2) being slightly different from the values represented by the relevant data words (1) prior to modification, characterized in that the modifying means (21,22) are adapted to modify the selected data words (2) depending both on the additional information signal (d) and on the logic values of other bits of the data words than said less significant bits (bo*), which other bits (b7) are non-correlated with the additional information signal, and in that the modifying means (20,21) are adapted to subject bits to the additional information signal and said other bits of the data words to an Exclusive-OR operation in order to determine the logic values of said less significant bits. A transmitting device as claimed in Claim 1, characterized in that the modifying means are constructed to assign the logic value which depends on the additional information signal and on the other bit exclusively to the least significant bit (bo*) of each of the selected data words (2). A receiving device for use in conjunction with a transmitting device as claimed in Claim 1 or 2, which device comprises means (28,29) for recovering the additional information signal from the bits (bo*) of low significance of the selected received data words, characterized in that the signal recovery means comprise means (28,29) which are constructed to subject said less significant bits and the specific other bits of the data words on an Exclusive-OR operation in order to determine the logic values of the additional information signal (d). A receiving device as claimed in Claim 3, characterized in that the signal recovery means (28,29) are adapted to derive the logic values of the bits of the additional information exclusively from the least significant bits (bo*) of said selected data words (2) and specific other bits (b7) of the data words.

Inventors: KEESMAN GERRIT JOHAN, VELDHUIS RAYMOND NICOLAAS J, KEESMAN, GERRIT JOHAN, VELDHUIS, RAYMOND NICOLAAS JOHAN
Applicants: KONINKL PHILIPS ELECTRONICS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1989
Main classification: H04J3
IPC codes: G11B20, H04N7, H04B14, H04J3