Abstract:
There is provided a data sending apparatus comprising a detecting unit to detect a transmission channel state and a coding unit for coding input information and obtaining transmission data, wherein the coding unit changes a coding processing operation in accordance with an output of the detecting unit. There is also provided a data receiving apparatus comprising a receiving unit to receive data transmitted from a communication partner, a detecting unit to detect a transmission error rate of the reception data, and a transmitting unit to transmit the transmission error rate detected by the detecting unit to the communication partner.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a data sending/receiving apparatus and, more particularly, to a data sending/receiving apparatus in a system for transmitting image data or the like by a phase modulation method. 
     2. Related Background Art 
     There is known a transmission system for variable-length encoding quantized image data in accordance with a transmission rate and phase modulating and transmitting to a transmission medium. 
     For instance, such a system is used in the image communication through a communication satellite. 
     FIG. 1 shows a constructional block diagram on the transmission side of a conventional apparatus according to a 4-phase phase modulation method. An image signal is supplied to an input terminal 10. An image coding apparatus 12 performs a variable-length coding of the input image signal in accordance with a transmission rate and, further, adds an error correction code to the coded image signal and generates the resultant image data as two phases of P and Q. In a 4-phase phase modulation apparatus 14, an output of the image coding apparatus 12 is first filtered by roll-off filters 16a and 16b and is 4-phase phase modulated by a modulation circuit 18. The 4-phase phase modulated carrier wave is applied from an output terminal 20 to a transmission medium. A passing frequency band and a roll-off coefficient α of each of the roll-off filters 16a and 16b have been predetermined in accordance with a transmission rate, a transmission frequency occupied band, an error correcting capability, and the like. 
     The roll-off filter is a low pass filter having cosine/roll-off characteristics which satisfy Nyquist theory to perfectly eliminate inter-code interference. FIG. 2 shows the roll-off coefficient α and amplitude frequency characteristics. 
     Assuming that the roll-off coefficient α is constant, it is necessary to widen a passing frequency band of the roll-off filter as the transmission rate rises. When a state of a transmission channel is bad, a decoding error rate increases with a decrease in roll-off coefficient α. On the contrary, when the coefficient α increases, the decoding error rate decreases. However, the transmission frequency occupied band is widened. That is, there is the following relation among the roll-off coefficient α, transmission rate F, and transmission frequency occupied band B. 
     
         B=G(α)·F 
    
     where, G(α) is an increasing function. 
     When the transmission line state deteriorates in the above transmission system, in order to suppress the increase in decoding error rate, methods of increasing the roll-off coefficient α in order to raise the error correcting capability by increasing a redundancy and the like are considered. However, those methods are not practical because a change in transmission format or a change in transmission frequency occupied band occur. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a data sending/receiving apparatus which can suppress an increase in decoding error rate for a deterioration of a transmission channel state under such circumstances. 
     To accomplish the above object, according to a preferred embodiment of the invention, there is provided a data sending apparatus comprising detecting means for detecting a transmission channel state and coding means for coding input information and obtaining transmission data, wherein the coding means changes a coding processing operation in accordance with an output of the detecting means. 
     According to another preferred embodiment of the invention, there is provided a data sending/receiving apparatus comprising receiving means for receiving data transmitted from a communication partner, detecting means for detecting a transmission error rate of the reception data, and sending means for sending the transmission error rate detected by the detecting means to the communication partner. 
     Other objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional sending apparatus; 
     FIG. 2 is a diagram for explaining frequency characteristics of a cosine roll-off filter; 
     FIG. 3 is a block diagram of a data sending apparatus and a data receiving apparatus in the first embodiment according to the invention; 
     FIG. 4 is a block diagram showing a main section on the transmission side of the first embodiment; 
     FIG. 5 is a block diagram showing a main section on the reception side of the first embodiment; 
     FIG. 6 is a block diagram of a data sending apparatus and a data receiving apparatus in the second embodiment according to the invention; 
     FIG. 7 is a block diagram showing a main section on the transmission side of the second embodiment; 
     FIG. 8 is a block diagram showing a main section on the reception side of the second embodiment; 
     FIG. 9 is a block diagram of a variable roll-off filter; 
     FIG. 10 is a diagram for explaining amplitude frequency characteristics of the variable roll-off filter; and 
     FIG. 11 is a diagram for explaining the relation between the roll-off coefficient and the transmission rate in the case where a transmission frequency occupied band is set to a constant value. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first embodiment according to the invention will be described hereinbelow. 
     FIG. 3 is a fundamental constructional diagram of the first embodiment of the invention using a satellite communication channel as a transmission medium. Reference numeral 30 denotes an input terminal of an image signal to be transmitted; 32 an image coding circuit to code the image signal supplied from the input terminal 30; 34 a phase modulation circuit to phase modulate an output code of the image coding circuit 32; 36 a send and receive circuit for sending an output of the phase modulation circuit 34 to a communication satellite 38 and for receiving a signal from the communication satellite 38; 40 a phase demodulation circuit to phase demodulate the signal received by the send and receive circuit 36; 42 an image decoding circuit to decode a demodulated signal from the phase demodulation circuit 40; and 44 an output terminal of the received image signal. 
     Reference numerals 36R, 40R, and 42R denote a send and receive circuit, a phase demodulation circuit, and an image decoding circuit of a receiving station, respectively, and have the same circuit constructions as those of the circuits 36, 40, and 42. Reference numeral 44R denotes an output terminal of an image signal decoded by the image decoding circuit 42R. 
     In the embodiment, in order to check a state of the communication channel through the communication satellite 38, the signal transmitted from the send and receive circuit 36 to the communication satellite 38 is returned from the communication satellite 38 to the same send and receive circuit 36. The signal is demodulated by the phase demodulation circuit 40 and is decoded by the image decoding circuit 42. The image decoding circuit 42 supplies information of a transmission error rate to the image coding circuit 32 and phase modulation circuit 34. As will be explained in detail hereinlater, the image coding circuit 32 and the 4-phase phase modulation circuit 34 optimize the quantization characteristics and the coefficient α of the roll-off filter in accordance with the transmission error rate information from the image decoding circuit 42 as will be explained in detail hereinlater. 
     FIG. 4 shows a detailed constructional block diagram of the image coding circuit 32 and phase modulation circuit 34. FIG. 5 is a detailed constructional block diagram of the phase demodulation circuit 40 and the image decoding circuit 42. 
     The operation on the transmission side will be first described in detail with reference to FIG. 4. The image signal is supplied from the input terminal 30 to an input terminal 50 of the image encoding circuit 32. The transmission error rate information is supplied from the image decoding circuit 42 to an input terminal 52. A transmission rate selection circuit 54 determines a proper transmission rate in accordance with the transmission error rate information from the input terminal 52. The information regarding the determined transmission rate is supplied to a quantization circuit 56, a variable-length coding circuit 58, a data synthesization circuit 62, a clock generation circuit 64, and control terminals of variable roll-off filters 66 and 68. It will be obviously understood that when the transmission error rate information is not supplied from the input terminal 52, the transmission rate selection circuit 54 selects a predetermined value as a transmission rate. 
     The quantization circuit 56 quantizes the image signal from the input terminal 50. However, a quantization coefficient in this instance is determined by both of a control signal from the transmission rate selection circuit 56 and a coding information amount from the variable-length coding circuit 58 at the post stage. The variable-length coding circuit 58 variable-length codes an output of the quantization circuit 56. At the output stage of the coding circuit 58, the image signal has been coded to a predetermined information amount. An error correction code generation circuit 60 generates an error correction code for a predetermined amount of codes which are generated from the coding circuit 58. The data synthesization circuit 62 adds the error correction code from the error correction code generation circuit 60 to such a predetermined amount of output codes from the coding circuit 58 and forms a block. The data synthesization circuit 62 supplies such a block of codes to the 4-phase phase modulation circuit 34 by two phases of P and Q at a rate which is 1/2 of the transmission rate determined by the transmission rate selection circuit 54. 
     In the modulation circuit 34, the P signal is supplied to the variable roll-off filter 66 and the Q signal is supplied to the variable roll-off filter 68. Each of the variable roll-off filters 66 and 68 comprises a non-recursive digital filter or a recursive digital filter. The passing frequency band and the roll-off coefficient α can be changed by properly setting a filter coefficient which is obtained from both of the transmission rate information from the transmission rate selection circuit 54 and a predetermined transmission occupied frequency band. The filters 66 and 68 have been adjusted so as to obtain the optimum amplitude frequency characteristics. The P signal and Q signal are waveform shaped by the variable roll-off filters 66 and 68 and applied to a modulation circuit 70, respectively. The modulation circuit 70 modulates a carrier wave by the waveform shaped P and Q signals. The modulated wave is supplied from an output terminal 72 to the send and receive circuit 36. 
     The send and receive circuit 36 frequency converts the modulated wave from the 4-phase phase modulation circuit 34, namely, the modulation circuit 70, and amplifies and transmits to the communication satellite 38. 
     The operation on the reception side will now be described in detail with reference to FIG. 5. The send and receive circuit 36 (or 36R) receives the transmission wave from the communication satellite 38 amplifies, frequency converts and supplies the result to the 4-phase phase demodulation circuit 40 (or 40R). In FIG. 5, a reception wave is supplied from the send and receive circuit 36 (or 36R) to an input terminal 74. A demodulation circuit 76 knows a transmission rate by an internal clock reproduction circuit, 4-phase phase demodulates a signal from the input terminal 74 and also adjusts the amplitude frequency characteristics of variable roll-off filters 78 and 80 to the optimum values. Two outputs of the demodulation circuit 76 are supplied to a decision circuit 82 through the variable roll-off filters 78 and 80 and are demodulated to the P signal and Q signal, respectively. 
     The outputs of two phases of the demodulation circuit 40 (40R) are supplied to an error correction circuit 84 of the image decoding circuit 42 (or 42R). In accordance with transmission rate information from the demodulation circuit 76, the error correction circuit 84 corrects an error of the output of the demodulation circuit 40 and supplies the error corrected reception signal to a variable-length decoding circuit 86 and also supplies the transmission error rate information to an output terminal 92. On the transmitting station side, the transmission error rate information of the output terminal 92 is supplied from the input terminal 52 of the image coding circuit 32 to the transmission rate selection circuit 54 as described before. 
     In accordance with the transmission rate information from the demodulation circuit 76, the variable-length decoding circuit 86 variable-length decodes the reception signal from the error correction circuit 84. An inverse quantization circuit 88 inversely quantizes an output of the decoding circuit 86 by a quantization coefficient according to the transmission rate information from the demodulation circuit 76. That is, the variable-length decoding circuit 86 and the inverse quantization circuit 88 execute processes opposite to the coding of the variable-length coding circuit 58 and the quantization of the quantization circuit 56, respectively. By the above processes, the original image information is reconstructed and applied to the output terminal 44 through an output terminal 90. 
     Summarizing the above operations, in the above embodiment, when the transmission line state deteriorates, the transmission rate is reduced, the passing frequency bands of the roll-off filters of the modulation and demodulation circuits are narrowed, and the roll-off coefficients a are increased. When the transmission channel state is improved, processes opposite to the above processes are executed. In either of the above two cases, the transmission frequency occupied band remains a constant value. 
     The second embodiment according to the invention will now be described hereinbelow. 
     FIG. 6 is a fundamental constructional diagram of the second embodiment of the invention using a satellite communication channel as a transmission medium. In FIG. 6, the same component elements as those shown in FIG. 3 are designated by the same reference numerals. Two send/reception terminals A and B having the same construction are connected by the satellite communication channel. 
     In the terminals A and B. reference numerals 30a and 30b denote input terminals of image signals to be transmitted; 32a and 32b image coding circuits to code the image signals from the input terminals 30a and 30b; 34a and 34b phase modulation circuits to phase modulate output codes of-the image coding circuits 32a and 32b; 36a and 36b send and receive circuits for sending outputs of the phase modulation circuits 34a and 34b to the communication satellite 38 and for receiving the signals from the communication satellite 38; 40a and 40b phase demodulated circuits to phase demodulate the signals received by the send and receive circuits 36a and 36b; 42a and 42b image decoding circuits to decode demodulated signals from the phase demodulation circuits 40a and 40b; and 44a and 44b output terminals for the image signals received. 
     In the second embodiment, a communication channel state of each communicating method, practically speaking, a transmission error rate is checked at a time point of the establishment of the communication channel or at a proper time point after the communication channel was established. For example, the terminal A codes and modulates the image (for instance, dummy image to check the communication channel) to be transmitted and sends the result to the terminal B through the communication satellite 38. In the terminal B, the send and receive circuit 36b receives the signal from the communication satellite 38, the phase demodulation circuit 40b demodulates the reception signal, and the image decoding circuit 42b decodes the demodulated signal. By the decoding in the image decoding circuit 42b, the terminal B can know a transmission error rate information R of the communication channel from the terminal A to the terminal B. The image decoding circuit 42b supplies the transmission error rate information R ab  to the image coding circuit 32b and the phase modulation circuit 34b. 
     The image coding circuit 32b and the 4-phase phase modulation circuit 34b codes and modulates the image to check the communication channel state, in a manner similar to the case of the terminal A, and transmit the coded, modulated image to the terminal A together with the transmission error rate information R ab . The terminal A can know a transmission error rate information R ba  of the communication channel from the terminal B to the terminal A in a manner similar to the case of the terminal B which has already received the image to check the communication channel state. That is, the terminal A can know the transmission error rate information R ab  and R ba  at this time point. The terminal A sends the transmission error rate information R ba  detected as mentioned above to the terminal B. 
     The operation in case of transmitting information from the terminal A to the terminal B will now be described in detail. FIG. 7 shows a detailed constructional block diagram of the image coding circuit 32a and phase modulation circuit 34a shown in FIG. 6. FIG. 8 shows a detailed constructional block diagram of the phase demodulation circuit 40b and image decoding circuit 42b shown in FIG. 6. 
     The transmitting operation in the terminal A will be first described in detail with reference to FIG. 7. In FIG. 7, the same component elements as those shown in FIG. 4 are designated by the same reference numerals. The image signal from the input terminal 30a is supplied to the input terminal 50 of the image coding circuit 32a. The transmission error rate information R ab  and R ba  are supplied from the image decoding circuit 42a to input terminals 52 and 53. The transmission rate selection circuit 54 determines a proper transmission rate in accordance with the transmission error rate information R ab  from the input terminal 52. The information regarding the transmission rate decided is supplied to the quantization circuit 56, variable-length coding circuit 58, data synthesization circuit 62, clock generation circuit 64, and control terminals of the variable roll-off filters 66 and 68. It will be obviously understood that the transmission rate selection circuit 54 selects a predetermined value as a transmission rate when the transmission error rate information R ab  is not supplied from the input terminal 52. 
     The quantization circuit 56 quantizes the image signal from the input terminal 50. However, a quantization coefficient in this instance is determined by both a control signal from the transmission rate selection circuit 54 and a coded information amount from the variable-length coding circuit 58 at the post stage. The coding circuit 58 variable-length codes the output of the quantization circuit 56. The image signal has been coded to a predetermined information amount at the output stage of the coding circuit 58. The error correction code generation circuit 60 generates an error correction code to a predetermined amount of codes which are generated from the coding circuit 58. The data synthesization circuit 62 adds the error correction code from the error correction code generation circuit 60 and the transmission error rate information R ba  from the input terminal 53 to such a predetermined amount of output codes from the variable-length coding circuit 58, and forms a block of codes and send the block to the 4-phase phase modulation circuit 34a by two phases of P and Q at a rate of 1/2 of the transmission rate which has been determined by the transmission rate selection circuit 54. 
     In the 4-phase phase modulation circuit 34a, the P signal and Q signal are supplied to the variable roll-off filters 66 and 68. FIG. 9 shows a circuit example of the variable roll-off filters 66 and 68. 
     Each of the variable roll-off filters 66 and 68 shown in FIG. 9 comprises a non-recursive digital filter. The transmission rate information from the transmission rate selection circuit 54 is supplied to an input terminal 110. A filter coefficient setting circuit 112 selects and determines a filter coefficient from both of the transmission rate information from the input terminal 110 and a predetermined transmission occupied frequency band and supplies the result to multiplying circuits 102-1 to 102-n. The P signal or Q signal is supplied to an input terminal 106 and is sequentially delayed by delay circuits 100-1 to 100-n each having a predetermined delay amount. The P or Q signals delayed by the delay circuits 100-1 to 100-n are supplied to the multiplying circuits 102-1 to 102-n. 
     The multiplying circuits 102-1 to 102-n multiply the filter coefficient decided by the filter coefficient setting circuit 112 by the P or Q signals delayed by the delay circuits 100-1 to 100-n. An adding circuit 104 adds outputs of the multiplying circuits 102-1 to 102-n. An output of the adding circuit 104 is supplied from an output terminal 108 to the modulation circuit 70. 
     By changing the filter coefficient as mentioned above, the passing frequency band and the roll-off coefficient α can be changed, thereby adjusting so as to obtain the optimum amplitude frequency characteristics in accordance with the transmission rate. 
     The P and Q signals are waveform shaped by the variable roll-off filters 66 and 68 and supplied to the modulation circuit 70, respectively. The modulation circuit 70 modulates the carrier wave by the waveform shaped P and Q signals. The modulated wave is supplied from the output terminal 72 to the send and receive circuit 36a. 
     The send and receive circuit 36a frequency converts the modulated wave from the 4-phase phase modulation circuit 34a, namely, from the modulation circuit 70 and amplifies and sends the result to the communication satellite 38. 
     The receiving operation in the terminal B will now be described in detail with reference to FIG. 8. In FIG. 8, the same component elements as those shown in FIG. 5 are designated by the same reference numerals. The send and receive circuit 36b receives the transmission wave from the communication satellite 38, amplifies, frequency converts and supplies the result to the 4-phase phase demodulation circuit 40b. In FIG. 8, the reception wave from the send and receive circuit 36b is supplied to the input terminal 74. The demodulation circuit 76 knows the transmission rate by an internal clock reproduction circuit, 4-phase phase demodulates the signal from the input terminal 74 and adjusts amplitude frequency characteristics of the variable roll-off filters 78 and 80 to the optimum values by substantially the same operation as that on the transmission side. Two outputs of the demodulation circuit 76 are supplied to the decision circuit 82 through the variable roll-off filters 78 and 80 and are demodulated to the P and Q signals, respectively. 
     The demodulated P and Q signals are supplied to the error correction circuit 84 of the image decoding circuit 42b. The error correction circuit 84 corrects an error of the output of the demodulation circuit 40 in accordance with the transmission rate information from the demodulation circuit 76. The error corrected reception signal is supplied to the variable-length decoding circuit 86. The transmission error rate information R ab  of the communication channel from the terminal A to the terminal B is sent to the output terminal 92. On the transmitting station side, as described above, the transmission error rate information R ab  of the output terminal 92 is supplied to the input terminal 53 of the image coding circuit 32b and synthesized to the image information to be transmitted. 
     The variable-length decoding circuit 86 variable-length codes the reception signal from the error correction circuit 84 in accordance with the transmission rate information from the demodulation circuit 76 and supplies the decoded image signal to the inverse quantization circuit 88. The transmission error rate information R ba  sent from the terminal A is supplied to an output terminal 93. The inverse quantization circuit 88 inversely quantizes an output of the variable-length decoding circuit 86 by the quantization coefficient according to the transmission rate information from the demodulation circuit 76. That is, the variable-length decoding circuit 86 and the inverse quantization circuit 88 execute processes opposite to the coding of the variable-length coding circuit 58 and the quantization of the quantization circuit 56. The original image information is reconstructed by those processes and supplied to the output terminal 44b through the output terminal 90. 
     The operation when the transmission channel state deteriorates will now be described. FIG. 10 shows amplitude frequency characteristics of the roll-off filter in the case where the roll-off coefficient α is increased and the passing frequency band is narrowed. When the transmission frequency occupied band B is set to a constant value and G(α) is approximated by a linear function, there is the following relation between the roll-off coefficient α and the transmission rate F. 
     
         F=g/α 
    
     where, g is a positive constant. 
     FIG. 11 shows the relation between the roll-off coefficient α and the transmission rate F by a logarithm graph. 
     When the transmission channel state deteriorates, in order to reduce the decoding error rate, the roll-off coefficient α of the roll-off filters of the modulation and demodulation circuits is increased from α 1  to α 2  (&gt;α 1 ) as shown in FIG. 10. As described above, when the roll-off coefficient α is increased, the transmission frequency occupied band B is widened. Therefore, to keep the transmission frequency occupied band B constant, the highest passing frequency of the roll-off filter is reduced from f c  to kf c  (where, k&lt;1) as shown in FIG. 10, thereby decreasing the information amount. In FIG. 11, such a process corresponds to reducing the transmission rate from F 1  to F 2 . The transmission rate selection circuit 54 and the filter coefficient setting circuit 112 select and set the transmission rate and filter coefficient upon deterioration of the transmission channel state under a condition such that the transmission frequency occupied band, B is held constant. 
     When the transmission channel state is improved, operations opposite to those mentioned above are executed. 
     In the above embodiment, the transmission error rate in each direction of the communication has been examined and the transmission rate has been controlled in accordance with the value of each transmission error rate. However, in the case where the state of the transmission channels can be regarded to be identical, in each direction even if the transmission rate is controlled by another direction of the basis of the measurement value of the transmission error rate in one direction, a similar effect can be obtained without causing any large error. That is, the measurement value of either one of the transmission error rate information R ab  of the communication channel from the terminal A to the terminal B and the transmission error rate information R ba  of the communication channel from the terminal B to the terminal A is commonly used. It is necessary that the transmission error rate information measured on the reception side is sent to the transmission side. 
     In each of the above embodiments, the 4-phase phase modulation has been applied. However, other phase amplitude modulating methods can be also obviously applied. A recursive digital filter can be also used as a variable roll-off filter. Further, a circuit construction such that a plurality of filters having peculiar characteristics are switched and used can be also used. 
     As will be easily understood from the above description, according to each of the embodiments, the transmission channel state is accurately recognized and the transmission state is changed and adjusted. Therefore, even in a transmission medium such as a satellite communication channel whose channel state is influenced by weather or the like, the stable data transmission can be always easily performed.