Method and device to regulate the output bit rate in variable bit rate video coders

Abstract: Applicable in compressed video transmission where the bandwidth availability of the transmission system varies with time; and which has the object of continuously matching the video compressor output rate to the occupancy conditions of the transmission system. The method has a first phase in which the level of filling of the output memory (7) of the video compressor (6) is analized, obtaining the number of reading and writing commands made on said output memory and calculating its variation in filling. In a second phase, the information concerning the output memory filling is processed, determining from the latter data and another data, the new statistical parameters that condition the operation of the video compressor, and which are sent to the latter in a third phase. The device carries out the different phases of the method.

Description: FIELD OF THE INVENTION This invention, as stated in the title of this document, comprises a method and a device for regulating the data output rate for video compressors with variable output rate, whose data output rate is a function of the image quality required by the receiver, of the type of image being processed at each instant, and of certain statistical parameters of the output rate (mean and variance), and which include an output memory where the data continuously generated are stored. The invention is particularly applicable in compressed video transmission, in which the availability of transmission system bandwidth varies with time, its object being to match the output rate of the video compressor to the occupancy conditions of the transmission system at each moment; achieving this by varying the statistical parameters that condition the output rate of the video compressor as a function of the level of the transmission system occupancy. Background to the invention In the article "Universal ATM Video Coding" by S. D'Agostino et al. published in the Electrical Communication, vol. 64. no. 2/3 1990, pages 186-193 it is disclosed a scheme for variable bit rate coding wherein two separate controls functions -quality control and bandwidth control- are included to regulate the output bit rate. These functions are developed by a encoder (which is also known as video compressor) that detects the irrelevant information from the video signal and removes it. In this way, the encoder provides an image quality that can be selected over a given range corresponding to an output bit rate behaviour. This output bit rate is characterised by a probability density function which is parametrised by its average and variance. The bandwidth control ensures that the output bit rate of the video compressor fits within the bandwidth requirements sets by the network. However, this kind of devices have the inconvenience that they cannot automatically adapt to the conditions of bandwidth availability, since it does receive any information on the state of occupancy of the transmission systems. In this way, if a certain image requires a large bandwidth in order to be transmitted with the quality required, and the transmission system has a relatively low activity, the video compressor does not use all the bandwidth that it could employ since the parameters preset by the video compressor do not permit it. TECHNICAL PROBLEM TO BE OVERCOME Consequently the technical problem to be overcome consists on matching, in real time, the output rate of a variable output rate video compressor to the availability of bandwidth of the transmission system employed. SUMMARY OF THE INVENTION To overcome the inconveniences mentioned above, the invention is defined in the appended claims 1 and 2, with further embodiments defined in dependent claims. Said invention comprises a method and a device for regulating the data output rate of variable output rate video compressors, the object being to match the output rate of the video compressor to the conditions of occupancy of the transmission system at each moment; doing this by varying the statistical parameters that condition the output rate of the video compressor as a function of the level of occupancy of the transmission system. The method is characterised in that it has a first phase in which the output memory of the video compressor is analized, obtaining the number of writing commands that are made each time that an information cell, generated by the video compressor, is stored in said output memory, this number depending on the relation between the length of the cell and the length of the information unit that can be stored in the output memory with a single writing command; obtaining besides, the number of reading commands made when extracting information from the video compressor output memory, for sending it to the transmission system, and calculating the variation in the filling of the compressor output memory as a function of the number of writing and reading commands generated and of the amount of information transferred with each one of these commands. The method is also characterised in that it has a second phase that adds algebraically the value of the variation of the filling of the video compressor output memory and the absolute value of filling of the output memory stored in a register since the preceding operational cycle; calculating the new statistical parameters as a function of the parameters with which the video compressor was working up to that moment, of the absolute value of filling of the video compressor output memory in some preceding operational cycles, of the minimum image quality required at the receiving end, and of the current absolute value of filling of the output memory; and in that it has a third phase for sending the new parameters determined to the video compressor according to the corresponding communication protocol. The device is characterised in that the first phase is carried out by means of a reading/writing command reading module and a filling variation calculator module. The reading /writing command reading module is connected to the output memory and has two registers, one for storing the number of writing commands and the other for storing the number of reading commands made on the output memory. The reading/writing command reading module is also connected to the filling variation calculator module, which computes the change in the filling of the output memory as a function of the values contained in said registers and the amount of information that is transferred with each reading and writing command. The filling variation calculator module, in turn, is connected to the processor module and has a register in which the result of the calculation is stored. It also has outputs for resetting to zero the registers held in the reading/writing command reading module, once their content has been used. The device is characterised in that the second phase of the method is done by means of a processor module and an algorithms module. The processor module is also connected to the algorithms module and to the parameters transmitting module. It adds algebraically the value of the variation of filling of the video compressor output memory to the absolute value of filling of this output memory during the previous working cycle, this value being stored in a register located in the processor module. The processor module also determines the new statistical parameters, taking into account: the statistical parameters with which the video compressor was working up to the moment, the absolute value of the filling of the output memory during some preceding working cycles, the minimum image quality required, and the absolute current value of filling of the output memory. The algorithms module has a memory in which the decision algorithm, the current values of the statistical parameters, the value of the minimum quality required at the receiving end, and the absolute value of filling of the output memory in some preceding working cycles are stored. Finally, the device is characterised in that the third phase is done by means of the parameter transmitting module, which is also connected to the video compressor, and which sends the new statistical parameters, determined in the processor module, to the video compressor according to the corresponding communication protocol. In this way, by means of the method and device of the invention, a better use is made of the resources (bandwidth) of the transmission system, a more efficient transmission being achieved without degrading the quality of the compressed video signal that is being transmitted. On the other hand, it is possible to connect a greater number of video compressors to the same transmission system, since the resources are shared among all of them and they have no fixed allocation. That is, the bandwidth that is not used by one video compressor at any moment can be used by any of the others connected to the transmission system. The device of the invention which carries out the method, employs known, economical technologies, whereby its incorporation into existing video compressor systems implies only a minimal increase in cost if compared with the benefit obtained in the effective employment of transmission system resources. Next, to facilitate a better understanding of this document, and forming an integral part thereof, a single figure is attached which, by way of illustration and in no limiting way, shows the object of the invention. BRIEF DESCRIPTION OF THE FIGURE Figure 1 shows a block diagram representing a prefered implementation of the device for data output rate regulation for a video compressor with variable output rate, which functions in accordance with the method of the invention. DESCRIPTION OF THE INVENTION A description of the invention based on the figure mentioned above is given below. The invention comprises a method and a device for regulating the data output rate for video compressors with variable output rate, applicable to image transmission where the available bandwidth of the transmission system varies with time, as is the case with networks using Asynchronous Transfer Mode (ATM) operation, or with local area networks. Regardless of the type of variable output bit rate video compressor being used, the actual mean output rate of data from the video compressor to the transmission system is variable with time. This variation in rate can be produced in two ways: either by regulating the instantaneous output bit rate, or by holding the output bit rate constant and inserting time intervals of variable duration in the course of which no data are generated (as is the case with ATM networks). In the case of ATM cells generated by a video compressor with variable output rate, the ratio between the number of "ATM information cells" (cells containing useful information), and the number of "ATM empty cells" (cells containing no useful information), depends of the kind of image 8 that is being processed at each moment, on the quality of image required at the receiving end, and on the output rate statistical parameters (mean and variance) previously preset. Therefore, the percentage of "ATM information cells" with respect to the total number of cells generated by the video compressor varies with time and has an upper value set by the statistical parameters already mentioned. Consequently, conventional equipments cannot adapt automatically to the bandwidth availability conditions, since they do not receive any type of information regarding the true occupancy state of the transmission system. That is, if a certain image requires a large bandwidth to be transmitted with the required quality, and the transmission system has a low activity, the video compressor does not use all the bandwidth it could if the mean and variance parameters set previously for the compressor output rate do not permit it. The invention overcomes these inconveniences by means of a method and a device that carries out the different phases of the method, and which can be connected to a video compressor with variable output bit rate of the type normally used. It is to be noted that the commercially available video compressors with variable output bit rate 6, have an output memory 7 in which are stored the data that are being continuously generated by the video compressor. The data are sent to the transmission system when an indication is received that indicates they have permission for sending (for example, by the reception of a signal). The method of the invention has a first phase for analysis of the contents of the output memory 7 of the video compressor, which is done by obtaining the number of writing commands that are made each time that an information cell, generated by the video compressor 6, is stored in this output memory 7, this number depending on the ratio existing between the length of the cell and the length of the information unit that can be stored in this output memory 7 with a single writing command. In addition, in the first phase, the number of reading commands 10 that are made to extract information 9 from the video compressor output memory in order to send it to the transmission system is obtained, a calculation being made of the variation in the filling of the output memory 7 of the video compressor as a function of the number of writing and reading commands generated, and on the amount of information that is transferred with each of these reading or writing commands. The method has a second phase where the algebraic sum is made of the value of the variation in filling of the output memory 7 of the video compressor and the absolute value of filling of the output memory 7 stored in a register from the preceding work cycle. In this second phase, the new statistical parameters are also determined as a function of the parameters with which the video compressor 6 was working at that moment, of the absolute value of filling of the output memory 7 in some preceding work cycles, of the minimum image quality required by the receiving end, and of the present absolute value of filling of the output memory 7. Finally the method has a third phase for transmitting the newly determined parameters to the video compressor 6, according to the corresponding communication protocol. Next, a description of the device of the invention is given, by means of which the different phases of the method described above are put into practice. The device has a reading/writing command reading module 1, which is connected to the output memory 7 of the video compressor 6. In addition, the reading/writing command reading module 1 is connected to a filling variation calculator module 2 which, in turn, is connected to a processor module 3. On the other hand, the processor module 3 is connected to an algorithms module 4 and to a parameter transmitting module 5, which, in turn, is connected to the video compressor 6. A description is given below of the working of the device of the invention based on the method and structure described above. So, the first phase of the method is carried out in the reading/writing command reading module 1 and in the filling variation calculator module 2. The reading/writing command reading module 1 has two registers, in one of which is stored the number of writing commands that are made in the output memory 7 each time that an information cell generated by the video compressor is stored in the output memory 7 of the video compressor 6. The number of writing commands depends on the ratio between the length of the cell and the length of the information unit that can be stored in the output memory 7 in a single writing command. In the other register is stored the number of reading commands made on the output memory 7 each time that information 9 is extracted from it to be sent to the transmission system. The filling variation calculator module 2 of the output memory 7 performs the function of calculating the filling of the output memory 7 during a working cycle, as a function of the values stored in the two registers of the reading/writing command reading module 1, and on the amount of information transferred with each reading or writing command on the output memory 7. The filling variation calculator module 2 has a register in which the result of the calculation mentioned is stored. In addition, the filling variation calculator module 2 has outputs for resetting to zero the registers of the reading/writing command reading module 1, once the contents have been used. The second phase is done by means of a processor module 3 which includes a storage register with the absolute value of filling of the output memory 7 corresponding to the preceding working cycle, and which is updated with the information contained in the register of the filling variation calculator module 2. The processor module 3 also determines the new statistical parameters as a function of the statistical parameters with which the video compressor was working up to that moment, of the absolute filling value of the output memory 7 in some preceding working cycles, of the minimum image quality required at the receiving end, and of the present absolute value of filling of the output memory. The algorithms module 4 has a memory in which the decision algorithm (or comparison table), the current values of the statistical parameters, the minimum quality value required at the receiving end, and the absolute filling value of the output memory in some preceding working cycles are stored. The new values calculated by the processor module 3 are sent to the video compressor 6 by the parameter transmitting module 5, which carries out the third phase of the method, in accordance with the corresponding communication protocol. Consequently, by means of the method and the device of the invention, it is possible to change, in real time, the statistical parameters that condition the data output rate of the video compressor 6, as a function of the level of the transmission system occupancy.

Claims: Method for regulating the output bit rate in a Variable Bit Rate video coder, said video coder involving a video compressor (6) with variable output bit rate and an output memory (7) in which the data that are continuously generated by said video compressor (6) are stored, and said variable output bit rate being a function of the image quality required by the receiver, of the type of image (8) being processed at each moment, and of adjustable statistical parameters of the video compressor output rate; said method being applied in compressed video transmission where the bandwidth availability of the transmission system varies with time, and being directed to match the output rate of the video compressor (6) to the occupancy conditions of the transmission system at each moment by varying said adjustable statistical parameters, whereby said adjustable statistical parameters condition the output rate of the video compressor as a function of the level of the transmission system occupancy; and further comprising a first phase in which the content of output memory (7) of the video compressor is analysed by obtaining the number of writing commands that are made each time an information cell generated by the video compressor (6) is stored in said output memory (7), said number of writing commands depending on the relation between the length of said information cell and the length of the information unit that can be stored in the output memory (7) with a single writing command; by obtaining the number of reading commands (10) made when extracting information (9) from the output memory (7) for sending this information to the transmission system; and by calculating the variation in the filling of the video compressor output memory (7) during a working cycle as a function of the number of commands generated and of the amount of information transferred with each one of said writing and reading commands; a second phase of adding algebraically the value of said variation in the filling of the video compressor output memory (7) during a working cycle and the absolute value of filling of the output memory (7) stored in a register since the preceding working cycle; and of calculating the new statistical parameters as a function of the parameters with which the video compressor was working up to that moment, of the absolute value of filling of the video compressor output memory (7) in some preceding working cycles, of the minimum image quality required at the receiving end, and of the current absolute value of filling of the output memory (7); and a third phase for transmitting the new parameters to the video compressor (6) according to the corresponding communication protocol. Device for regulating the output bit rate in a Variable Bit Rate video coder, said video coder involving a video compressor (6) with variable output bit rate and an output memory (7) in which the data that are continuously generated by said video compressor (6) are stored, and said variable output bit rate being a function of the image quality required by the receiver, of the type of image (8) being processed at each moment, and of adjustable statistical parameters of the video compressor output rate; said device being applied in compressed video transmission where the bandwidth availability of the transmission system varies with time, and being directed to match the output rate of the video compressor (6) to the occupancy conditions of the transmission system at each moment by varying said ajustable statistical parameters, whereby said adjustable statistical parameters condition the output rate of the video compressor as a function of the level of the transmission system occupancy; and further comprising: means (1,2) for analysing the content of the output memory (7) of the video compressor comprising a reading/writing command reading module (1) and a filling variation calculator module (2); means (3,4) for calculating the new statistical parameters comprising a processor module (3) and an algorithms module (4); means (5) for transmitting the new statistical parameters to the video compressor (6) comprising a parameter transmitting module (5); whereby the reading/writing command reading module (1) is connected to the output memory (7) and has two registers, one for storing the number of writing commands and the other for storing the number of reading commands (10) made on the output memory (7), said writing commands being made each time an information cell generated by the video compressor (6) is stored in said output memory (7), said number of writing commands depending on the relation between the length of said information cell and the length of the information unit that can be stored in the output memory (7) with a single writing command said reading commands (10) being made when extracting information (9) from the output memory (7) for sending this information to the transmission system; whereby the reading/writing command reading module (1) is also connected to the filling variation calculator module (2), which computes the change in the filling of the output memory (7) as a function of the values contained in said registers and the amount of information that is transferred with each reading or writing command; whereby the filling variation calculator module (2) is connected to the processor module (3), has a register in which is stored the result of the calculation, and has outputs for resetting to zero the registers held in the reading/writing command reading module (1), once their content has been used. Device according to claim 2, further characterised: in that the processor module (3) is also connected to the algorithms module (4) and to the parameters transmitting module (5), adding algebraically the value of the variation in the filling of the output memory (7) to the absolute value of filling of this output memory (7) during the previous working cycle, this value being stored in a register located in the processor module (3); in that the processor module (3) also calculates the new statistical parameters, taking into account the statistical parameters with which the video compressor was working up to the moment, the absolute value of filling of the output memory (7) during some preceding working cycles, the minimum image quality required, and the current absolute value of filling of the output memory (7); and in that the algorithms module (4) has a memory in which the decision algorithm, the current values of the statistical parameters, the value of the minimum quality required at the receiving end, and the absolute value of filling of the output memory (7) in some preceding working cycles are stored. Device according to claim 2, further characterised: in that the parameter transmitting module (5) is also connected to the video compressor (6), and sends the new statistical parameters to the video compressor (6) according to the corresponding communication protocol.

Inventors: ARIAS GUTIERREZ-ACEITUNO JAIME, CARABANTES SANCHEZ SANTIAGO, HAMARD LOIC, LOPEZ VARONA ANTONIO, RUIZ MINGUEZ JOSE MARIA, ARIAS GUTIERREZ-ACEITUNO, JAIME, CARABANTES SANCHEZ, SANTIAGO, HAMARD, LOIC, LOPEZ VARONA, ANTONIO, RUIZ MINGUEZ, JOSE MARIA
Applicants: ALCATEL ESPACIO SA, ALCATEL ESPACIO S.A.
Application year: 1992
Main classification: H04N7
IPC codes: H04N7, H04L12, H04Q11