Abstract:
Data compression apparatus for compressing an input data unit to produce an output data unit in accordance with a desired data quantity of the output data unit, the degree of compression being determined by a compression control variable having a range of possible values. A trial encoder compresses successive sections of the input data unit in accordance with a trial group of two or more values of the compression control variable. A data quantity detector detects the trial data quantities generated by each trial encoding of the sections of the input data unit and selects a base compression control variable applicable to the input data unit on the basis of the detected trial data quantities, in order to comply with the desired data quantity. An allocator allocates a compression control variable for use in final encoding each section of the input data unit to generate the output data unit, the allocator being comparable to determine whether to use the base compression control variable or another possible value of the compression control variable for the current section.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to data compression. At least preferred embodiments of the invention relate to video data compression. 
     2. Description of the Prior Art 
     It has long been recognised that some video data compression systems, such as systems broadly defined by the MPEG-2 standard, use compression techniques in which the number of compressed data bits generated for a picture, or a part of a picture, depends on the nature of the image represented by that picture. Also, the main compression parameter which can be altered from block to block or picture to picture to change the bit rate, namely the degree of quantisation, has a somewhat non-linear and difficult to predict effect on the resulting bit rate. 
     These characteristics are of particular concern in systems such as video tape recorders, where there is generally a fixed allocation of bits for each picture or group of pictures (GOP) and little or no scope for exceeding that fixed allocation. As a result, techniques for bit rate control in video data compression are very important. 
     The so-called “Test Model 5” of the MPEG 2 system proposes a rate control algorithm that allocates bits between pictures in accordance with a “global complexity estimation” dependent upon the actual number of bits generated in respect of a preceding picture and the quantisation parameters used to achieve this. The actual bit rate achieved during compression of a picture is then monitored and the degree of quantisation varied during compression to try to achieve the desired total bit rate for that picture. This system can, however, be slow to react to changes in image type during the compression of a picture and cannot predict the presence of difficult-to-encode image portions (requiring a higher bit rate) towards the end of a particular picture. Also, it is difficult for such a system to react to a rapidly changing picture content, such as that caused by a scene change. 
     Another previously proposed system described in GB-A-2 306 831 uses a system of trial encoding of at least part of a picture or group of pictures in order to assess the most appropriate degree of compression for use with those pictures. In particular, a “binary search” technique is proposed whereby an multi-stage process is used to iterate towards the required quantisation factor. However, such a multi-stage process to narrow the search down to the required quantisation factor is expensive in terms of the delay it adds to the processing chain as well as the actual processing overhead involved. 
     SUMMARY OF THE INVENTION 
     This invention provides data compression apparatus for compressing an input data unit to produce an output data unit in accordance with a desired data quantity of the output data unit, the degree of compression applied by the apparatus being determined by a compression control variable having a range of possible values; 
     the apparatus comprising: 
     a trial encoder for compressing successive sections of the input data unit in accordance with a trial group of two or more values of the compression control variable, the trial group being a subset of the range of possible values of the compression control variable; 
     a data quantity detector for detecting the trial data quantities generated by each trial encoding of the sections of the input data unit and for selecting a base compression control variable applicable to the input data unit on the basis of the detected trial data quantities, in order to comply with the desired data quantity; and 
     an allocator for allocating a compression control variable for use in final-encoding each section of the input data unit to generate the output data unit, the allocator being operable to determine whether to use the base compression control variable or another possible value of the compression control variable for a current section by a comparison between (a) the increase in data quantity determined for compression of the current section using a next less harsh compression value from the trial group; and (b) the maximum possible increase in data quantity, while still complying with the desired data quantity, over that obtained if the remainder of the input data unit were compressed using the base compression control variable. 
     The invention provides a data compression apparatus which addresses the shortcomings of both the predictive and the “full” trial encoding systems outlined above. 
     The invention operates to trial encode a current input data unit (e.g. an image) and so avoids the lack of responsiveness which can be experienced by the TM5 type of technique, particularly at scene changes. However, by deliberately trial-encoding at only a subset (and preferably a non-adjacent subset) of the possible values of the compression control variable (e.g. the quantisation factor Q) and then subsequently deriving a decision on which value to use based on the trial group of values, the invention alleviates the processing overhead and delay of the system of GB-A-2 306 831. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
     FIG. 1 schematically illustrates a video tape recorder; 
     FIG. 2 schematically illustrates a data compression apparatus; 
     FIG. 3 schematically illustrates a pre-encoder; 
     FIG. 4 schematically illustrates a quantisation factor selector; 
     FIG. 5 schematically illustrates a quantiser and entropy encoder; and 
     FIGS. 6 to  8  are schematic flow charts illustrating features of the operation of the quantiser and entropy encoder of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a video tape recorder (VTR) using data compression. Video data received by the VTR is supplied first to a data compression apparatus  10  in which the data quantity of the video data is reduced by compression techniques to be described below. The compressed video data is then passed to an error correcting code (ECC) processor and formatter which formats the data into an appropriate form for storage on tape and adds various error correcting codes in accordance with conventional techniques. The formatted data is then stored on a tape medium  30 . 
     At replay, data is read from the tape medium  30  and processed by an ECC processor and formatter  40 . This uses the ECC to detect any errors resulting from the data storage process and, hopefully, to correct them. It also re-formats the data into an appropriate form for decompression. Decompression is then carried out by a data decompression apparatus  50  which is arranged to provide a decompression process complimentary to the compression process applied by the data compression apparatus  10 . 
     The key features of the embodiment which will be described below are found in the data compression apparatus  10 . The remaining parts of FIG. 1 may be implemented using known techniques. 
     FIG. 2 is a schematic diagram of the data compression apparatus  10 . 
     The VTR described in connection with this embodiment uses so-called “I” (intra) pictures only. So, unlike some implementations of systems such as MPEG-2, each picture (generally a frame) is compressed without reference to adjacent or nearby pictures. While this means that some of the compression efficiency which is possible with a long-GOP system using P frames and B frames cannot be achieved, it does mean that editing can easily take place at any desired frame boundary in the video signal. So, an I-frame VTR is particularly suited for studio use. 
     The fact that only I-frames are used means that the diagram shown in FIG. 2 is much simpler than a conventional long-GOP encoder. 
     So, FIG. 2 illustrates a DCT encoder  60 , a pre-encoder  70 , a quantizer  80  and an entropy encoder  90 . The DCT encoder  60  operates to decompose the picture into blocks of 8×8 pixels and to apply a discrete cosine transform to generate a corresponding matrix of 8×8 DCT coefficients representing increasing spatial frequency components. (It is noted here that in other embodiments of the invention other transformations could of course be used, such as a wavelet transformation). 
     In parallel with the DCT process, the pre-encoder  70  examines the input images and allocates a proportion of the available number of bits for encoding each image (which is generally a fixed quantity because of storage constraints imposed by the tape medium  30 ) to different areas of the image. In the present example, the allocation is carried out on a macroblock (MB) by macroblock basis. Here, the term macroblock refers to an array of 16×16 pixels, i.e. four DCT blocks. The specific operation of the pre-encoder will be described in much more detail below but, in general terms, as its output it supplies target data quantities for each macro to the quantizer  80 . 
     The quantizer  80  carries out a thresholding and quantization process which involves zeroing coefficients below a certain threshold and quantizing the remaining ones, with the degree of quantization being selected in order to control the resulting output data quantity and also to account (in a conventional way) for image attributes such as so-called image activity which can vary from area to area within the image. 
     Finally, the entropy encoder  90  carries out run length coding and variable length (Huffmann) coding so that more frequently occurring bit patterns within the run length encoded sequence are encoded to form shorter output data words. Again, it is noted that other types of entropy encoding are of course applicable as well or instead. 
     The system described above can operate using:either of two sets of possible quantisation factors, a “linear” set, in which successive values are evenly spaced throughout the available range, or a “non-linear” set, in which the values are more closely spaced towards the lower (less harsh quantisation) end of the range. 
     The complete “linear” and non-linear sets of quantisation factors Q for the MPEG system are as follows: 
     Linear: 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 
     Non-linear: 1 2 3 4 5 6 7 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 53 56 64 72 80 88 96 104 112 
     In the present embodiments, a “reduced” non-linear set of possible quantisation values are used, in order to ease the processing requirements of the system. The reduced non-linear set is as follows: 
     Non-linear: 1 2 3 4 5 6 8 10 12 14 16 18 20 24 28 32 36 40 44 48 53 56 
     In other words, the values 7, 22 and 64-112 have been omitted. The maximum quantisation factor in the non-linear set is now 56 and in the linear set the maximum is 62. 
     FIG. 3 schematically illustrates the operation of the pre-encoder  70 . 
     The pre-encoder  70  comprises four pre-encode units  102 ,  104 ,  106 ,  108  operating in parallel to pre-encode DCT data received from the DCT encoder  60  using respective quantisation factors. So-called image “activity” is taken into account in a conventional way. 
     For both the non-linear and the linear options, the following Q values are used in the four pre-encoders: 
     102 Pre-encode  1 : use Q 1 =2+activity 
     104 Pre-encode  2 : use Q 2 =6+activity 
     106 Pre-encode  3 : use Q 3 =10+activity 
     108 Pre-encode  4 : use Q 4 =20+activity 
     Each of the four pre-encoders operates on an image slice by image slice basis, where a slice is defined as an image area at least one macroblock high and at least one macroblock wide. The slices are preferably all the same size, though this is not essential to the operation of the system. The pre-encoders generate output data representing the number of data bits which would be generated if each slice were encoded with that quantisation value. 
     The sets of “bits per slice” figures from each of the four pre-encoders  102  . . .  108  are summed over a complete image (e.g. a complete frame) by respective summation processors  112 ,  114 ,  116 ,  118 . These generate “bits per frame” figures which are passed to a Q selector  120 . 
     The Q selector  120  is shown schematically in more detail in FIG. 4, but in brief its operation is to select a “base” quantisation factor for the entire image on the basis of the four “bits per frame” figures received from the summation processors. In doing this, it compares the bits per frame obtained by pre-encoding with the required bits per frame of the overall compression system. Its output is a single Q value to be passed to the quantiser  80 . The bits per slice figures generated by each of the pre-encoders  102  . . .  108  are also passed to the quantiser  80 . 
     Referring now to FIG. 4, the Q selector comprises compare logic  130  and an interpolator  140 . 
     The compare logic  130  receives the four bits/frame figures from the summation processors  112  . . .  118  and compares each figure with the required bits per frame for the overall system. The outputs of the compare logic are two pre-encode quantisation values Q U  and Q L  and a high/low flag. Three conditions will now be described, depending on whether the required bits per frame value lies within the range of the four pre-encode results, below that range or above that range. 
     (a) Required Bits/Frame Lies Within the Range of the Pre-encode Results 
     The usual situation is that the required bits per frame lies between two of the bits per frame values obtained through pre-encoding. In fact, the pre-encode quantisation factors are in fact deliberately chosen so that this happens for a majority of images for the particular bit rate (50 Mbps) of the current system. In this circumstance, the compare logic  130  outputs the quantisation factors which led to the generation of the nearest pre-encode bits/frame values to either side of the required number of bits per frame and sets the h/l flag to indicate “no overflow or underflow”. In particular, Q U  is the pre-encode quantisation factor which gave the highest number of bits not to exceed the required bits per frame, and Q L  is the pre-encode quantisation factor which gave the lowest number of bits to exceed the required bits per frame 1 . 
       1 In this embodiment, because each coefficient is divided by the value Q, a numerically smaller Q implies a less harsh quantisation of the input data. So, Q L  represents a less harsh quantisation (generating more bits) than Q U . Of course, this is just a definition by convention and the opposite convention could instead be used.  
     Where valid Q L  and Q U  have been generated, that is, where the required bits per frame lies between two pre-encoder bits/frame values, the frame Q value can be selected by linear interpolation between Q L  and Q U . For each available quantisation value x between Q L  and Q U , an estimate est_bits(x) is calculated of the number of bits which would be generated if x were used:          est_bits        (   x   )       =     pre   -     encode        (     Q   L     )       -       (     x   -     Q   L       )     *     (       pre   -     encode        (     Q   L     )       -   pre   -     encode        (     Q   U     )             Q   U     -     Q   L         )                                
     where pre-encode(Q L ) and pre-encode(Q U ) are the actual number of bits obtained during pre-encoding of the frame using Q L  and Q U  respectively. 
     This equation is applied repeatedly to obtain the lowest value of x for which: 
     
       
         est_bits(x)≦required bits per frame 
       
     
     The value of x meeting this requirement is selected as the frame quantisation factor Q. 
     (b) Required Bits/Frame Lies Below the Range of the Pre-encode Results 
     In this situation, all of the pre-encoders have generated too many bits, so a quantisation factor higher than the highest pre-encode value (Q 4 =20) needs to be selected. The high/low flag is set by the compare logic to “high” to indicate that a higher quantisation value than 20 needs to be used. This flag causes the interpolator to alter its operation, as described below. 
     In empirical trials of a prototype 50 Mbps system this situation has not been observed, but the following algorithm to be followed by the interpolator  140  is included as a precaution. 
     Linear interpolation can no longer be used to select a Q value. So, in order to test values of Q above the highest pre-encode value of Q=20, the following formula is used to provide est_bits(x):          est_bits        (   x   )       =     pre   -     encode        (     Q   =   20     )       -       (     x   -   20     )     *     (         0.5   *   pre     -     encode        (     Q   =   20     )             Q   max     -   20       )                                
     where Q max  is the maximum Q value in the relevant range, i.e. 62 in the linear set and 56 in the non-linear set. This equation assumes that the number of bits will fall by about half from Q=20 to Q=Q max . 
     Again, this equation is applied repeatedly to obtain the lowest value of x for which: 
     
       
         est_bits(x)≦required bits per frame 
       
     
     The value of x meeting this requirement is selected as the frame quantisation factor Q. 
     (c) Required Bits/Frame Lies Above the Range of the Pre-encode Results 
     In this situation, all of the pre-encoders have generated too few bits, so a quantisation factor lower than the lowest pre-encode value (Q 1 =2) may need to be selected. Of course, the option of a still-lower quantisation value is available only in the non-linear Q set. If the linear set is in use then the following description does not apply and Q=2 would be used. 
     The high/low flag is set by the compare logic to “low” to indicate that a lower quantisation value than 2 needs to be used. This flag causes the interpolator to alter its operation, as described below. 
     It is noted here that estimates by extrapolation of the number of bits which would be generated if Q=1 were used are prone to large uncertainties. 
     To assess the number of bits which would be generated if Q=1 were used, a different formula, reflecting the potentially large increase in bits obtained when the quantisation factor is changed from Q=2 to Q=1, is used to estimate the quantity est_bits(Q=1):          est_bits        (   x   )       =     pre   -     encode        (     Q   =   2     )       -     2   *     (       pre   -     encode        (     Q   =   2     )       -   pre   -     encode        (     Q   =   6     )         4     )                                
     where pre-encode(Q=2) and pre-encode(Q=6) are the actual number of bits obtained during pre-encoding of the frame using quantisation values of 2 and 6 respectively. 
     If the results of this test, est_bits(Q=1), meet the following requirement: 
     
       
         est_bits(x)≦required bits per frame 
       
     
     then Q for the frame is set by the interpolator  140  to Q=1. 
     In summary, as a result of the above processing, a value of Q for the whole frame is passed from the Q selector  120  to the quantiser  80 . 
     FIG. 5 schematically illustrates the quantiser  80  and the entropy encoder  90 . The quantiser  80  comprises first and second test logic  200 ,  210 , a slice tester  220 , a last slice detector, a bit accumulator  240  and a quantisation unit  250 . 
     The first and second test logic  200 ,  210  receive the Q value selected by the Q selector  120  and test whether it is a pre-encode Q value (2, 6, 10 or 20) and whether it is the value Q=1. The results of these tests are passed to the slice tester  220  and affect its operation as described below. 
     The slice tester  220  receives the Q value selected by the Q selector, the bits per slice figures from the pre-encoders  102  . . .  108  and test results from the first and second test logic  200 ,  210 , the last slice detector  230  and the bit accumulator  240 . The slice tester operates to select a Q value, Q sl  for each slice of the image. 
     The quantising unit  250  and a last slice detector  230  receive DCT data from the DCT encoder  60 . The last slice detector  230  detects whether the current DCT data represents the last slice of an image, and passes this information as a control signal to the slice tester  220 . The quantising unit quantises data from each image slice in accordance with the Q value, Q sl , selected by the slice tester  220  for that slice and also the image activity of that slice. 
     The entropy encoder  90  performs entropy encoding on the quantised data from the quantising unit  250 . The number of bits generated by this process is detected and accumulated by the bit accumulator  240 , with the result being passed as a control signal to the slice tester  220 . 
     The basic operation of the apparatus shown in FIG. 5 will be described in the context of three situations (as detected and indicated by the first and second test logic  200 ,  210 ). These are: (a) where the Q value selected by the Q selector is one of the pre-encode Q values; (b) where the Q value selected by the Q selector is not one of the pre-encode Q values; and (c) the special case where the Q value selected by the Q selector is Q=1. 
     (a) The Selected Q Value is a Pre-encode Q Value 
     FIG. 6 is a schematic flow chart showing the operations performed by the apparatus of FIG. 5 where the selected Q value is a pre-encode Q value, that is to say Q=2, 6, 10 or 20. 
     At a step  300 , a test is carried out to test whether the selected Q is 2. Q=2 is a special case. No attempt is made to reduce the Q value to the next lowest (Q=1, which is in any event only available in the non-linear set) because estimates of the bit rate for Q=1 have such a large amount of uncertainty attached to them. Accordingly, at a step  310 , Q is maintained at 2 and the frame is encoded using Q=2 at a step  320 . The process ends as far as that frame is concerned. 
     In this case where Q=2, because Q=2 was a pre-encode Q value, it is known that the available bit rate will not be exceeded. So, the frame can be safely encoded at that Q value. 
     Returning to the step  300 , if it is found that Q does not equal 2, control passes to a step  330  where a variable x is set: 
     
       
         x=pre_encode_data(Q L )−pre_encode_data(Q sl )) 
       
     
     where Q L  is the next lower Q value in the linear or non-linear set. 
     At a step  330 , x is compared with a variable “spare”, where spare is initially defined by: 
     
       
         spare=max_per_frm−bits(Q frm ) 
       
     
     and: 
     max_per_per_frm=maximum allowed bits per frame (1.6×10 6  bits for a 50 Mbps, 30 frm/s system) 
     Q frm =selected Q for frame 
     bits (Q frm )=actual bits produced at Q frm  (NB this in fact could apply whether or not Q frm  is a pre-encode value, but if it were not a pre-encode value then bits (Q frm ) would be an estimate) 
     If it is found that x is less that spare, then the next lower Q value is selected at a step  350 . This is the next lower Q value in the available range of Q values, so in the present example the Q value selected at the step  350  will not, by definition, be a pre-encode Q value. The slice is then encoded at a step 360. 
     (As an aside, there is of course nothing to prevent the pre-encode values being chosen as adjacent Q values in the linear and/or non-linear sets in other embodiments. So, for example, a set of pre-encode Q values could be 2. 8, 10 and 20 where 8 and 10 are adjacent in both the linear and non-linear series. Similarly, there could be more or fewer than four pre-encode Q values). 
     On the other hand, if x is not less than spare at the step  340 ; the Q value is kept at the initially selected base Q value at a step  370  and the slice is encoded as the step  360  using the base Q value. 
     At a step  380  the variable spare is updated according to the following calculation: 
     
       
         spare=spare−(actual_slice_bits−slice_bits(Q sl )) 
       
     
     Of course, the expression in parentheses in the above equation will be zero if Q sl  is the originally selected pre-encode Q value. 
     Finally, if it is detected at a step  390  that there are more slices to be processed, control returns to the step  330 . Otherwise, the process ends as far as the current frame is concerned. 
     (b) The Selected Q Value is not a Pre-encode Q Value 
     FIG. 7 (formed of FIGS. 7 a  and  7   b  which combine to show a single flow chart) schematically illustrates the processing operation carried out where the Q value selected for the frame is not a pre-encode value and is not Q=1. 
     At a step  400 , two variables are initialised, slice_error and carry. The initial settings for these variables are as follows: 
     slice_error=0 
     carry=0 
     Two other values are also defined: Q L  is defined as the Q value immnediately below the selected Q value (in either the linear or the non-linear range as appropriate) and Q H  is the Q value above the selected Q value in the appropriate range. 
     At a step  410 , a test is performed to detect whether Q L  is a pre-encode Q value. This test is important because Q L  represents a less harsh degree of quantisation than the selected Q value, implying a greater data quantity will be generated. If Q L  is a pre-encode value, the system has an absolute measure (from the pre-encoding process) of data quantity produced at Q L . If Q L  is not a pre-encode Q value, however, an estimate has to be produced instead. 
     So, if Q L  is a pre-encode value, a variable “safety” is set to 0 at a step  410 . On the other hand, if Q L  is not a pre-encode value, the variable “safety” is set to a value such as 5000 bits at a step  430 . The variable safety represents a safety margin in the calculations that follow, so that an estimate of the data quantity produced at Q L  is compared with the available maximum data quantity minus the safety margin. In other words, it is made harder for Q L  to meet the requirements of not exceeding the available bit rate. 
     After steps  410  or  430  control passes to a step  440  where the variable “carry” is set using the following formula:        carry   =     carry   +     spare       no_of      _slices     -   1                                
     A variable final_error is set at a step  450  according to the following formula:        final_error   =     no_of      _slices   ×     (     slice_error     current_slice      _no       )                              
     At a step  460  Q is reset to the Q value initially selected for the frame by the Q selector. Then, at a step  470 , a detection is made as to whether the current slice is the last slice of the current image. If so, control passes to a step  560  to be described below. In the case where the current slice is not the last slice of the current image, control passes to a step  480  where a detection is made as to whether the current slice is one of the first two slices of the current image. If not, then at a step  490  the variable final_error is compared with the variable spare. If final_error is greater than spare then Q is increased at a step  500  to Q H  and the slice is encoded at a step  540 . The variable slice_error is updated at a step  550  in accordance with the following calculation: 
     
       
         slice_error=slice_error+(actual_slice_bits−slice_estimate(Q frm ) 
       
     
     and control then returns the step  440  for the next slice. 
     If the step  480  detected that this was one of the first two slices, control would pass to a step  520  (see below). Returning to the step  490 , if the variable final_error is not greater than the variable spare then control passes to a step  510 . 
     At the step  510 , the variable slice_error is tested as follows: 
     
       
         slice_error+slice_estimate(Q L )+safety&lt;(slice_estimate(Q frm )+carry) 
       
     
     If the outcome of this test is negative, that is to say the inequality defined by the above equation is not true, then control passes to a step  520  where Q is set to the selected Q for the frame. The slice is encoded  540  and slice_error is updated  550  as before. If the outcome of this test is positive, that is to say the inequality is true, then Q is set to Q L  at a step  530 , the slice is encoded  540  and slice_error is updated  550  as before. 
     So, it will be seen that the first two slices are always encoded at the selected Q for the frame (via the steps  480  and  520 ) to build up an early indication of how good the bit estimates are. There is a small danger that large underestimates in the bits produced for the first two slices would cause an overflow of the maximum bits per frame that could not then be corrected. To avoid this, the “safety” bits are included in the calculations. 
     Returning to the situation where the step  470  detects that the current slice is the last slice, a variable final_spare is set at a step  560 , where: 
     
       
         final_spare=spare−slice_error 
       
     
     A series of tests  570 ,  580 ,  590  and  600  detect whether any of the pre-encode Q values can be used for this final slice, given the spare data quantity established by the variable final_spare. So, the test carried out at the step  570  is as follows: 
     
       
         slice_pre_encode(Q=2)&lt;slice_est(Q frm )+final_spare 
       
     
     If this test is positive, that is to say the inequality defined by the above equation is true, then Q is set to 2 at a step  610 . Similar tests follow for Q=6, 10 and 20, with a positive result meaning that Q is set to the test (pre-encode) value. It is noted that the test  580  only takes place if the test  570  is not passed; the test  590  only takes place if the test  580  is not passed and so on. 
     In the event that none of the four tests is passed, control passes to a step  650  where Q for the last slice is set to the highest available Q value in the appropriate linear or non-linear set. 
     The final slice is then encoded using the selected Q value at a step  660  and the process ends as far as the current frame is concerned. 
     (c) The Selected Q Value is Q=1 
     Finally, FIG. 8 concerns the processing which takes place if the Q selector chooses a value Q=1. 
     It is noted that the value Q=1 is available only in the non-linear set of Q values. Q=1 has a somewhat unpredictable nature, and in many respects does not give a noticeable subjective improvement over Q=2. However, it does have the advantage that in “easy to encode” pictures, moving to Q=1 can appear to make better use of the available bit rate. This is psychologically important for the user, especially in apparatus such as a 50 Mbps VTR with a display readout of the current actual bit rate. The user might feel more comfortable if, having purchased such a VTR, the perceived “best possible use” is seen to be made of the available bit rate of the VTR. 
     So, on FIG. 8, a variable slice_error is initialised to 0 at a step  700 . 
     At a step  720  a test is performed to detect whether the current slice is the last slice of the current image. If so, then at a step  730  Q is set to 2 and the slice is encoded at a step  740 . The process then ends as far as the current frame is concerned. 
     Setting Q to 2 for the last slice is important because there is no longer an opportunity to correct for any error in the estimate of the data quantity produced at the last slice. So, a pre-encode value of Q=2 is used for absolute safety. 
     Returning to step  720 , if the current slice is not the last slice then a test is carried out at a step  750  to detect whether the current slice is the first or second slice of the current image. If it is, then at a step  760  Q is set to 1. This gives the opportunity at the start of the image to test the bit rate produced for Q=1 (not a pre-encode value) the slice is then encoded at a step  810 . 
     If it is detected at the step  750  that the current slice is not the first or second slice, a variable final_error is set at a step  770 . This is an extrapolated rolling prediction of the total deviation from the predicted bit rate (data quantity) for the current frame, which is of course less than the maximum bit rate, which will occur for the whole of the current frame. 
     The equation for the variable final_error to be given below reflects the fact that it is known from the algorithm that the last slice of the image will be compressed at Q=2, and so will not contribute to the error.        final_error   =       slice_error   *     (       no_of      _slices     -   1     )         current_slice      _number                              
     and the actual number of bits for the frame will be: 
      actual bits=frame_est(Q=1)−slice_estimate(last_slice at Q=1)+slice_estimate(last_slice at Q=2) 
     At a step  780  final_error is compared with the variable spare less a safety margin gamma. If final_error is greater than (spare—gamma) then Q is set to 2 at a step  800  and the slice is encoded at a step  810 . On the other hand, if final_error is not greater than (spare—gamma) then Q is set to 1 at a step  790  and the slice is encoded a 10. Following the step  810 , variables slice_error and spare are updated at a step  820  as described above, and control returns to the step  720 . 
     While a preferred embodiment of the invention relates to an II-VTR using intra (I) pictures only, it will be appreciated that the invention is equally applicable to a system employing GOPs also including B pictures, P pictures or both. 
     It will also be clear to the skilled man that the techniques described above may be implemented by software running on a general purpose data processing apparatus. In this case, it will be appreciated that such software, and a data carrier such as a magnetic or optical disk bearing such software, are also envisaged as aspects of the present invention. 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.