in summary , a block - coding method and a corresponding device for block coding of data are provided , which envisage detecting in the coding step whether a block currently being coded is liable to generate a mismatch error , and , if so , modifying the current block to eliminate the mismatch error . to take into account the type of implementation of the coding device , i . e ., whether the coding device will be implemented in a purely software way or in a mixed hardware / software way , two different embodiments are proposed , which envisage , respectively : resetting the block coefficients to zero , to enables a simple low - power implementation to be obtained , as will be illustrated more clearly hereinafter with reference to fig2 ; and incrementing the value of the quantizer to prevent the mismatch , which is illustrated hereinafter with reference to fig3 , and appears to be the better approach in terms of quality and of psnr ( peak signal - to - noise ratio ) values , even though it requires all the blocks of one and the same macroblock to be coded with a higher quantizer value . the first embodiment , which contemplates , that is , zero - ing the coefficients of the block before coding , substantially envisages verifying , after the idct at the encoder end , whether a zero block is reconstructed from a block that contains some non - zero dct coefficients . if this occurs , the coding device is configured for forcing to zero a block in terms of huffman coding . fig2 shows a coding device that implements the first embodiment . represented , then , in fig2 is a coding device , designated as a whole by the reference number 200 , which receives at input the noncompressed - data flow i . the noncompressed - data flow i is sent at input to a motion - estimator block 210 , which generates at output motion vectors vm . the noncompressed - data flow i is sent in parallel to one of the inputs of an adder 212 to be added to the output of a frame memory 280 . the frame memory 280 has the function of storing the preceding frame to be used for the operations of temporal prediction together with the motion vectors vm . the sum obtained from the adder 212 is sent at input to a transformation module 220 , which carries out the dct . the output of the transformation module 220 is supplied to a quantization module 230 , which applies a quantization according to a quantization parameter qp , which is supplied by a rate - control module 270 . the rate - control module 270 operates on the basis of the input signal , i . e ., the noncompressed - data flow i , and of the output signal , i . e ., a compressed - data flow o , of the coding device 200 , which the module 270 receives at two appropriate inputs of its own . downstream of the quantization module 230 the signal , constituted by the compressed blocks db , is sent to an inverse - quantization module 290 and to an idct module 295 , to obtain decompressed frames o ′ to be supplied to the frame memory 280 , after being added to the output of the frame memory 280 in an appropriate adder 214 . the output of the quantization module 230 is likewise sent to the output of the coding device 200 through an ac / dc prediction module 235 and a variable - length - coding ( vlc ) module 240 , which adopts vlc techniques and huffman coding to produce a coded data flow with a good statistical efficiency of compression . the output of the vlc module 240 is sent to a multiplexer 250 , together with the motion vectors vm . set downstream of the multiplexer 250 is a buffer 260 , which supplies at output the compressed - data flow o . the modules so far described and their arrangements are in themselves known to the state of the art regarding coding devices of an mpeg type . however , according to the invention , in the coding device described in fig2 , provided downstream of the quantization module 230 and upstream of the ac / dc prediction module 235 is a mismatch - control module 1000 . the mismatch - control module 1000 detects the blocks rb reconstructed by the idct module 295 and modifies the compressed blocks db upstream of the coding performed in the ac / dc prediction module 235 and in the vlc module 240 , via the operation of zero - ing the dct coefficients of the compressed blocks db to be coded , should a zero block be reconstructed from a block containing non - zero dct coefficients . the use of the procedure in the mismatch - control module 1000 imposes some constraints upon implementation of the h / w coding device . in fact , after the quantization performed in the quantization module 230 , the data must wait for the results of the inverse quantization and of the idct in the blocks 290 and 295 to ensure that there will not be any mismatch . upstream of the ac / dc prediction module 235 there must hence be inserted an additional buffer contained in the mismatch - control module 1000 , which leads to an increase in the area dedicated to the chip , an increase in the latency of the data , and a reduction in the parallelism between the computing procedures involved . a second embodiment of the invention overcomes the drawback of having to wait for the results of the inverse quantizations and of the idct before coding via the huffman table , as well as the deterioration in the psnr values . the second embodiment prevents the mismatch error without requiring any checks on the reconstructed block downstream of the idct , since it is based upon the analysis of the sad ( sum of absolute differences ) function and the quantizer values of the current block at the coding - device end . the information is available at the coding - device end because the motion - estimation procedure calculates it to identify the motion vectors . the ensuing description of the second embodiment of the method proposed comprises two parts . in a first part a general approach to the mismatch error is provided , which is irrespective of the implementation of particular proprietary algorithmic procedures , for example , of a forward dct , forward quantization , and inverse quantization type . described in the second part is , instead , the treatment of the specific case of the implementation of the mpeg - 4 - coding procedure . in the treatment that follows the errors due to the finite - precision arithmetic will not be taken into consideration in a first - order approximation . in a second step , described below , the errors will be considered and a final approach to the problem will be provided . regarding infinite arithmetic approximation , by virtue of parseval &# 39 ; s theorem , there is a connection between the energy in the dct domain and the energy in the space domain . ∑ i = 1 n ⁢  x i  2 = ∑ i = 1 n ⁢  x i  2 ( 1 ) where x i indicate coefficients in the space domain corresponding to a block in the input data flow i and x i indicate coefficients corresponding to the dct domain . it is possible to set in relation the sad function with the energy : the above equation ( 2 ) enables an upper limit to be set to the mismatch error . in fact , if one finds the block that presents the maximum value for the summation ∑ i = 1 n ⁢  x i  2 producing the mismatch error , it is possible to establish the minimum value for the sad function that can be used for detecting the occurrence of the mismatch . assuming that the coefficients in the space domain x i represent the data on the mismatch error , we can write sad 2 = ( ∑ i = 1 n ⁢  x i  ) 2 = ∑ i = 1 n ⁢  x i  · ∑ j = 1 n ⁢  x j  = ∑ i = 1 n ⁢ ∑ j = 1 n ⁢  x i  ⁢  x j  ( 3 ) x · y ≤ 1 2 ⁢ ( x 2 + y 2 ) , ∀ x , y ∈ ℜ ( 4 ) sad 2 = ∑ i = 1 n ⁢ ∑ j = 1 n ⁢  x i  ⁢  x j  ≤ 1 2 ⁢ ∑ i = 1 n ⁢ ∑ j = 1 n ⁢ ( x i 2 + x j 2 ) = 1 2 ⁢ ( n ⁢ ∑ i = 1 n ⁢ x i 2 + n ⁢ ∑ j = 1 n ⁢ x j 2 ) = n ⁢ ∑ i = 1 n ⁢ x i 2 ( 5 ) sad ≤ n ⁢ ∑ i = 1 n ⁢ x i 2 ( 6 ) applying parseval &# 39 ; s theorem , it is possible to set in relation the energy in the space domain with the energy in the dct domain , so that sad ≤ n ⁢ ∑ i = 1 n ⁢ x i 2 = n ⁢ ∑ i = 1 n ⁢ x i 2 ( 7 ) where x i are the coefficients in the space domain , while x i are the coefficients in the dct domain . finally , on the basis of the equations given above , it is possible to attempt calculation of the minimum value of the sad function that enables the mismatch error to be prevented . the problem can be solved by finding , from among the blocks that produce the mismatch error , the one presenting the maximum value of energy . the impact of the finite - precision arithmetic must be considered in the light of the fact that the previous approach , in particular with reference to equation ( 7 ) has been obtained considering the validity of parseval &# 39 ; s theorem , which is , however , only proven when an infinite precision of calculation is assumed . in an implementation of a real type , the dct is calculated using finite - precision arithmetic , so that it is necessary to take into account that the parseval relation becomes e = 1 n ⁢ ∑ i = 1 n ⁢  x i  2 = 1 n ⁢ ∑ i = 1 n ⁢  x i  2 ( 8 ) where e is the block energy . in the above calculation the sad function in the space domain and the energy in the dct domain are calculated , so that the fdct ( finite discrete cosine transform ) error must be appropriately studied . assuming the finite - precision calculations as an error ε that affects each dct coefficient , we can write the relation where x i indicates the ideal value of the dct coefficient , x i is the dct coefficient obtained with the finite - precision arithmetic and ε i is the error . assuming that x i , ε i ∈ ∀ i , we have given a maximum error ε x = max ( ε i , i = 1 , . . . , 64 ), we obtain in the real case , to be able to exploit the approach described above , it is necessary to take into account the particular implementation of the idct . once the real implementation of the encoder has been fixed , it is thus sufficient to identify the block having maximum sad that causes mismatch error and to use the value as upper limit th for the equation 12 . it is possible to identify also a minimum value qpmin of the quantization parameter qp above which the mismatch does not occur . this makes it possible to avoid that blocks presenting a value assumed by the sad function ( designated in what follows as value s ) lower than the threshold identified will be considered as being affected by mismatch errors when in actual fact they are not . consequently , the pseudocode that describes the operation of detection of the mismatch , on the basis of the value calculated in this example of embodiment , would be : the function mismatchwarning ( ) corresponding to the modification operations can have two possible embodiments that envisage the operations either of : resetting the block to be coded ; or else of incrementing the value of the quantization step qp = qpmin + 1 . the first approach is simple to implement and , moreover , the blocks that follow can be turned off , with a consequent lower energy consumption . the second approach is simple to implement and ensures a higher quality in so far as some of the blocks detected as being affected by mismatch , with qp = qpmin + 1 , will certainly not in fact be affected . the pseudocode corresponding to the function mismatchwarning ( ) for the first approach that envisages block resetting is : the index b indicates the block b that is each time examined as regards sad and qp values , i . e ., the current block cb . dct_b , iq_b , recon_b and vlc represent variables corresponding to application of dcts , quantizations , inverse quantizations and vlc coding to the blocks b at input , in_b , or at output , out_b . an example of procedure in pseudocode comprising the function mismatchwarning ( ) for the second approach , with the strategy of incrementation of the quantization step qp , is : fig3 shows a possible implementation , in a coding device 300 , of the second approach for controlling mismatch described above , with the strategy of incrementation of the quantization step qp . in fig3 , the modules having functions similar to the ones already described with reference to fig2 are designated by the same reference numbers . in fig3 , a mismatch - control block 1100 is inserted so as to receive from the motion estimator 210 at one input thereof macroblocks mb derived from the noncompressed - data flow i , and at another input thereof values s of the corresponding sad function ; at a third input , the mismatch - control block 1100 receives the value of the quantization step qp applied in the compression of the current macroblock from the rate - control module 270 . at output , the mismatch - control module 1100 supplies a macroblock to be coded mb ′ to the adder 212 , and a minimum quantizer value qmin to the rate - control module 270 . the function mismatchwarning ( ) fixes the minimum value of the quantization step qp to increase the psnr . consequently , the procedure can , for example , be represented , in terms of program instructions in pseudocode , also as : it should be noted that , in this case , the direct dct , the quantization , the inverse quantization , and the inverse dct can be avoided , with a lower energy consumption . the major advantages of this type of approach include : the loop - coding device does not require any modifications ; the block is coded just once ; multitasking is allowed , so that each block can process different macroblocks ( i . e ., the quantization block can process a macroblock , whilst the dct block can process a subsequent macroblock at the same time ); a lower - limit constraint is set on the quantizer value only for critical macroblocks ; and the embodiment is very simple to implement and does not require an excessive use of resources . in actual fact , not all the blocks that present a value of the sad function lower than the value established will give rise to mismatch . using this procedure , a lower limit is set on the quantizer value also for these macroblocks . consequently , without prejudice to the principle of the invention , the details of implementation and the embodiments may vary , even significantly , with respect to what is described and illustrated herein , purely by way of non - limiting example , without thereby departing from the scope of the invention , as defined in the ensuing claims . for example , regarding the procedure of identification of the values of sad function and of minimum quantization parameter , it is possible to obtain the values even in another way , for example , via an exhaustive procedure that tries all the combinations of values , according to a so - called ‘ brute force ’ approach .