Method of and apparatus for transmitting video data

Abstract: According to the present invention, when video data of, for example, a real moving picture is transmitted, the video data is data-compressed (12), data-encoded (14) and transmitted (15,16) while the combination of the compressing ratio of the video data and the encoding ratio of the video data is varied in response to a condition of the transmission path. Therefore, regardless of the condition of the transmission path, video data of the best quality can be transmitted at a constant transmitting speed.

Description: The present invention relates to a method and apparatus for transmitting video data. In general, when a video signal (a television signal) is transmitted via a communication satellite, a frequency band of higher than 10 GHz is used. Radio waves of such a high frequency band are considerably attenuated by rainfall. In transmitting a video signal through a communication satellite, an FM modulation scheme is generally employed. However, if the signal level of the FM signal is attenuated to become lower than a threshold carrier-to-noise (C/N) ratio, the quality of the reproduced picture deteriorates rapidly. Accordingly, when the video signal is transmitted through the communication satellite, it is necessary to make up some suitable counterplan for bad weather such as rainfall or the like. In this connection, a serviceability of higher that 99.9% is requested in the case of bad weather. In that case, if a direct broadcast by satellite (DBS) is performed between stationary stations, the image quality of the picture can be prevented from being deteriorated by increasing the transmission output. However, when the broadcasting station transmits a video image to its own station from a spot covered via a communication satellite, if the transmission output is increased, then not only the transmitter but also the power supply source apparatus or the like must be enlarged in size and increased in weight. Therefore, an increase of the transmission output is not desirable. When, on the other hand, a video signal is digitized to perform the DBS, 4-phase shift keying (PSK) is frequently used as the modulation system. When the 4-phase PSK is employed, the attenuation of the signal by rainfall or the like can be avoided by switching it to a 2-phase PSK. According to this method, however, the signal band is widened in the 2-phase PSK so that an expensive frequency band of the communication satellite must be increased. This is described in an article by Y. Yasuda et al., "THEORETICAL BIT ERROR RATE PERFORMANCE OF SOFT DECISION VITERBI DECODING," Transaction of the Institute of Electronics Information and Communication Engineers , No. CS80-126, pp. 31 to 36, Sept. 24, 1980. As described above, when the video signal is transmitted via the communications satellite, the image quality of the received picture is deteriorated by rainfall or the like. In order to remove this disadvantage, the prior art apparatus must be made large-sized, heavy in weight or complicated in construction. Alternatively, the signal band is widened. A space communication system incorporating a concatenated Reed Solomon Viterbi coding channel for transmitting compressed and uncompressed data from a spacecraft to a data processing center on earth is known from patent document US 3,988,677. Imaging data is first compressed into source blocks which are then coded by a Reed Solomon coder and interleaved followed by a conventional encoder. The receiver data is decoded by a Viterbi decoder, followed by a Reed Solomon decoder and deinterleaver. The output of the latter is then decompressed, based on the decompressing criteria used in compressing the data in the spacecraft. The decompressed data is processed to reconstruct an approximation of the original data-producing condition or images. It is the object of the present invention to provide a method and an apparatus for transmitting data in which the best transmission quality can be obtained for any given transmission condition, especially in bad weather conditions like rainfall. The object is solved by the features of claim 1 or claim 5 for the method and the apparatus, respectively. Further developments of the invention are given in claims 2 to 4 and 6 to 7 for the method and apparatus, respectively. The invention is described in the following with reference to the drawings, in which: Fig.1 is a diagram of an example of an encoding gain, and to which reference will be made in explaining the present invention; Fig. 2 is a block diagram showing an outline of an embodiment of an apparatus for transmitting video data according to the present invention; and Fig.3 is a table used to explain the operation of the present invention. In order to understand the present invention more clearly, let us first explain a fundamental principle of the present invention. Fig. 1 shows an example of an encoding gain wherein values in the table of Fig. 1 are quoted from the foregoing literature, "THEORETICAL BIT ERROR RATE PERFORMANCE OF SOFT DECISION VITERBI DECODING." Referring to Fig. 1, "(4,3)" convolutional code, for example, indicates that, of the code of 4 bits, 3 bits are assigned to information bits and the remaining 1 bit is assigned to an error correcting code. More specifically, a (n0, k0) convolutional code indicates that, of n0 bits encoded, the information bit is formed of k0 bits and the error correcting bit is formed of remaining (n0 - k0) bits. Further, a value Q is a soft decision value Q. Assuming that an encoding ratio R is expressed as R = k0 / n0 then, this equality demonstrates that, according to Fig. 1, the larger the encoding ratio becomes the encoding gain becomes larger in the convolutional code regardless of the value Q. This is equivalent to the fact that, if the encoding ratio R is increased, the gain of the transmission path is equivalently increased, whereas the attenuation of the signal (radio wave) is reduced correspondingly. From such a standpoint, the gain of the transmission path can be equivalently changed by the encoding ratio of the convolutional encoding and using data compression in the DBS of the digital video signal, so that, according to the present invention, image data is compressed and is convolutionally encoded and is transmitted. Simultaneously, in response to the condition of the transmission path, the data compressing ratio and the convolutional encoding ratio are varied so as not to vary the transmission rate. The embodiment of the present invention will now be described in detail hereinunder. Referring to Fig. 2, there is provided a transmitting apparatus 1 which is located, for example, at the news source place. A receiving apparatus 2 is located, for example, at the broadcasting station and a communication satellite 3 for DBS is provided. In this embodiment, a transmission rate at which image data is transmitted from the transmitting apparatus 1 to the receiving apparatus 2 via the communication satellite 3 is selected to be 24 Mbps (Megabits per second). As shown in Fig. 2, in the transmitting apparatus 1, a digital video signal S₁ input at terminal 11 is supplied to a data compressing circuit 12 which is what might be called a high efficiency encoding circuit. In that case, the digital video signal S₁ is formed according to a signal format which conforms, for example, to the CCIR Recommendation 601. In other words, a luminance signal is converted to a digital luminance signal by a sampling frequency of 13.5 MHz and a quantization bit number of 8 bits, whereas red and blue color difference signals are converted to digital red and blue color difference signals by a sampling frequency of 6.75 MHz and a quantization bit number of 8 bits. Thus, the digital video signal S₁ is a signal of 216 Mbps on the whole. The data compressing circuit 12 data-compresses the signal S₁ to provide data S₂ according, for example, to an adaptive predictive coding system and a variable length coding system. At that time, in accordance with a control signal Sc, a data amount N after the signal S₁ is compressed and the data compressing ratio C are controlled to fall within any one of groups I to IV on the rows 2 and 3 in Fig. 3 where an equality of C = (P-N)/P is established. The value P in this equation represents the data amount when the reference data compression is carried out without the application of the present invention, and is selected to be equal to the transmission rate of 24 Mbps. That is, if the reference data compression is contained, the original data S₁ is compressed to the data S₂ by the compression ratio is expressed as 24 216 x 24 - N 24 [Mbps) Further, a code generating circuit 13 generates, for example, a cyclic redundancy check (CRC) code S₃ as an error checking code, and this CRC code S₃ is added to the data S₂. In this embodiment, the aforementioned data amount N and compressing ratio C is applied to the signal which includes the code S₃. The data S₂ to which is added the code S₃ is supplied to a convolutional encoding circuit 14. The convolutional encoding circuit 14 convolutionally encodes the data S₂ to data S₄ and can also vary freely the code (n0, k0) of the data S₄ in the convolutional encoding and the encoding ratio R to be any of those described in the groups I to IV in the rows 4 and 5, respectively, of the table of Fig. 3 in accordance with a control signal Se. In that case, if the data compression in the data compressing circuit 12 belongs to the group I, then the data compression in the encoding circuit 14 must belong to the group I. In other words, the data compressing ratio C in the data compressing circuit 12 and the encoding ratio R in the encoding circuit 14 are controlled so as to lie in the same group. Accordingly, the transmission rate of the data S₄ becomes constant, for example, 24 Mbps regardless of the groups I to IV. That is, the data compressing ratio C and the encoding ratio R of each of the four groups I to IV are determined such that the transmission speed becomes constant. In this embodiment, the transmission speed is selected to be 24 Mbps so that numerals [2,3,4,8] which are factors of [24] are selected as a denominator of a fraction which indicates the data compressing ratio C and the encoding ratio R. The data S₄ from the encoding circuit 14 is supplied to a transmitting circuit 15 which is comprised of a modulating circuit, a frequency converting circuit, a power amplifier and the like not shown. In the transmitting circuit 15, the data S₄ supplied thereto is converted, for example, to a 4-phase PSK signal S₅, and this signal S₅ is transmitted to the communication satellite 3 through an antenna 16. The signal S₅, relayed by the communication satellite 3, is received by a so-called parabola antenna 21 in the receiving circuit 22 of, for example, a double superheterodyne system, in which it is demodulated to provide the data S₄. The demodulated S₄ data is supplied to a Viterbi decoding circuit 23 and is thereby demodulated as the data S₂ in response to the control signal Se. The data S₂ is supplied to a data expanding circuit 24, in which it is expanded in response to the signal Sc to provide the original video signal S₁. This original video signal S₁ is supplied to an output terminal 25. In the foregoing, the data expanding circuit 24 is what might be called a high efficiency decoding circuit. In that case, the data expanding circuit 24 separates and derives the CRC code S₃, and this CRC code S₃ is supplied to an error deciding circuit 26 which decides the error condition or error rate of the code S₃. The decided result is supplied to a control circuit 27 which derives the above-mentioned control signals Sc and Se in accordance with the decided result of the deciding circuit 26. The signal Sc, which represents a compressing ration C, is supplied to the data compressing circuit 12 and the encoding circuit 14, whereas the signal Se, which represents an encoding ratio R, is supplied to the encoding circuit 14 and the decoding circuit 23. The circuits 12, 24 and 14, 23 are controlled by the signals Sc and Se such that, if the error rate of the CRC code S₃ becomes larger, a group of a lower number is selected from the groups I to IV. More specifically, when the transmission condition is good, the group IV is selected. When the transmission condition becomes worse, the groups III, II and I are selected in that order. Although the method for supplying the control signals Sc and Se to the data compressing circuit 12 and the encoding circuit 14 from the receiving apparatus 2 is not specific, the control signals Sc and Se may be transmitted to the data compressing circuit 12 and the encoding circuit 14 via a communication network line between the television broadcast station and the news source place. Alternatively, the groups I to IV are selected by the control signals Sc and Se so that the groups I to IV selected in accordance with the control signals Sc and Se are transmitted to the transmitting side via a communication telephone to thereby determine the compressing ratio C and the encoding ratio R. According to the above-described arrangement, when the weather condition is good, the video signal S₁ is transmitted by the group IV. At that time, the data compressing ratio C for the video signal S₁ is as small as 1/8 so that a video signal S₁ of good image quality can be transmitted as shown in the row 7 of Fig. 3. When the weather condition is bad, the video signal S₁ is transmitted by the group I. At that time, the encoding ratio R for the video signal S₁ is small, i.e., the encoding gain is large as shown in the row 6 of Fig. 3 so that although the image quality is deteriorated because the data compressing ratio C is large, the video signal S₁ can be transmitted at any rate. Therefore, it is possible to avoid the worst cases where the transmitting apparatus cannot be operated at all. If the weather condition becomes fine, the group is changed from the group I to the group II or the group III or IV is selected, then the image quality becomes better. As described above, according to the present invention, since the video signal is data-compressed and convolutionally encoded and then transmitted with the various combination of the data compressing ratio C and the encoding ratio R in response to the transmission conditions at that time, the best image quality can be obtained under the above-mentioned transmission conditions and the worst cases, where the apparatus cannot be operated by the rainfall or the like, can be avoided. Further, since the encoding ratio C and the data compressing ratio R are combined to form a group and the transmitting speed is selected to be constant, for example, 24 Mbps in any of the groups, the characteristics of the apparatus from the transmitting circuit 15 to the receiving circuit 22 need not be varied in response to the transmitting conditions and can be commonly used under all transmission conditions. Therefore, the scale of the apparatus can be reduced, and particularly in the transmitting apparatus 1, not only the transmitting circuit 15 but also a power supply source apparatus (not shown) can be made compact in size and made light in weight. Thus, the transmitting apparatus 1 and the power supply source can be transported or installed at a news source spot with ease.

Claims: Method for transmitting data comprising the steps of: compressing (12) the data to be transmitted according to a selected compressing ratio, convolutionally encoding (14) the data to be transmitted according to a selected encoding ratio, transmitting (15,16) the convolutionally encoded data (S4) at a predetermined transmitting speed, receiving (21,22) the transmitted, convolutionally encoded data and decoding (23) the received, convolutionally encoded data (S4) using Viterbi decoding, characterized by - expanding (24) the Viterbi decoded data (S2), whereby the combination of the compressing ratio and the encoding ratio of the data is varied while the transmitting speed of the data is maintained constant. The method according to claim 1, further comprising the steps of detecting the error situation in the expanded Viterbi decoded data, and changing the combination of the compressing ratio and the encoding ratio of the data according to the detected error situation. The method according to claim 1 or 2, wherein the transmitting step comprises transmitting the data via a satellite (3). The method according to anyone of claims 1 to 3, wherein the combination of the compressing ratio and the encoding ratio of the data is changed according to the weather. Apparatus for transmitting data comprising: compressing means (12) for compressing the data to be transmitted according to a selected compressing ratio, encoding means (14) for convolutionally encoding the data to be transmitted according to a selected encoding ratio, transmitting means (15,16) for transmitting the convolutionally encoded data (S4) at a predetermined transmitting speed, receiving means (21,22) for receiving the transmitted, convolutionally encoded data (S4), viterbi decoding means (23) for Viterbi decoding the received, convolutionally encoded data (S4) and expanding means (24) for expanding the Viterbi decoded data (S2) characterized by - error detection and control means (26,27) for detecting the error situation in the expanded Viterbi decoded data (S1) and controlling the compressing means (12) and encoding means (14) in order to change the combination of the compressing ratio and the encoding ratio, while the transmitting speed is kept constant. Apparatus according to claim 5 characterized in that the transmitting means comprises a satellite data transmission network (16,3). Apparatus according to claim 5 or 6, characterized in that the error detection and control means (26,27) varies the combination of the compressing ratio and the encoding ratio in order to compensate changes in the weather situation.

Inventors: KOJIMA YUICHI, KOJIMA, YUICHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H03M13
IPC codes: H04N7, H04N1, H03M13, H04L1, H04B7