Abstract:
Data is compressed through implementing the inserting of a regressive reference string scheme, each such reference replacing a particular data string through referring to an associated earlier data string of identical content. In particular, the regressive string reference item to an encoded object is inserted as being based on pointing and delimiting a pointee string, and identifying the reference item through an associated signalling element.

Description:
[0001]    A data compression method and system that use a regressive string reference, pointing to and delimiting an encoded pointee string, and identify the reference through a signalling element, an encoding device and a decoding device arranged for implementing the method, and a storage medium provided with information produced by such encoding device and/or arranged for decoding by such decoding device  
         BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a method as recited in the preamble of claim  1 . The art of data storage and transmission has recognized the advantages of data compression. A particularly advantageous compression scheme has been to conceptually replace a particular data string by an earlier occurrence of an unencoded, or rather decoded, data string that has exactly the same content. The signalling of such replacing is formulated as a relative pointer combined with an indication of the length of the replacing string. Now, a pointer-length pair will often have a greater length than a standard length character. At a subsequent decoding operation, this scheme will therefore necessitate a buffer for storing all earlier decoded information within the range of the pointer to allow the resolving of all back-references. Practical embedded systems will however often feature a scarcity in available RAM storage space that would serve as such buffer. This may imply an unacceptable cost level, or may even defeat the applying of this technology.  
         SUMMARY TO THE INVENTION  
         [0003]    In consequence, amongst other things, it is an object of the present invention to allow diminishing the requirements for RAM storage space. Now, the present inventor has recognized a procedure that would realize an appreciable advantage by having the pointers referring back to earlier encoded objects or strings. One argument in case is that ROM storage will generally be amply available for storing encoded data, such as would be the case in many embedded systems. However, the above storage-related argument is not a prerequisite for attaining the various advantages of implementing the invention. The improved scheme will by and large obviate the need for buffering decoded data, and furthermore will allow for a certain amount of random accessibility. The invention especially envisages its application in relatively inexpensive appliances. In various situations, the invention will also allow for a higher compression factor. Furthermore, a pointer will often span multiple objects, that may include both original versions and pointer-length pairs, rather than only characters or other standard-length items. With the same pointer size, this feature allows using a larger pointing range and therefore, an improved compression rate. On the other hand, through keeping the same pointing range as before, the present invention will allow to use shorter pointers, and in consequence, will realize an improved compression rate as well. Trade-offs will likewise represent fortuitous implementations of the present invention.  
           [0004]    Now therefore, according to one of its aspects the invention is characterized according to the characterizing part of claim 1.  
           [0005]    The invention also relates to a system for implementing a method as claimed in claim 1, to an encoding device and to a decoding device arranged for implementing a method as claimed in claim 1, and to a storage medium provided with information produced by such encoding device and/or arranged for decoding by such decoding device. Further advantageous aspects of the invention are recited in dependent claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]    These and further aspects and advantages of the invention will be discussed more in detail hereinafter with reference to the disclosure of preferred embodiments, and in particular with reference to the appended Figures that show:  
         [0007]    [0007]FIG. 1, a decoded data sequence provided with a pointer-length pair;  
         [0008]    [0008]FIG. 2, a pointer format;  
         [0009]    [0009]FIG. 3, an encoded data sequence according to the present invention;  
         [0010]    [0010]FIG. 4, a pointer format provided with a skip field;  
         [0011]    [0011]FIG. 5, the storage configuration in one embodiment;  
         [0012]    [0012]FIG. 6, an encoding arrangement;  
         [0013]    [0013]FIG. 7, a decoding arrangement;  
         [0014]    [0014]FIG. 8, an encoding flowchart;  
         [0015]    [0015]FIG. 9, a decoding flow chart. 
     
    
     FURTHER CONCEPTUAL CONSIDERATIONS ON THE PRESENT INVENTION  
       [0016]    Now, the present invention allows to discriminate between standard length characters and pointer-length pairs through a specific signalling element. However, the invention also has a non-unifornity in length between standard-length characters on the one hand, and pointer-length pairs on the other hand. This aspect would render it less easy to refer backwards or in a regressive manner to find the pointee string, inter alia, because the pointer may refer back across both standard-length characters and also across further pointer-length pairs. However, the inventor has recognized various ways around this problem.  
         [0017]    A first remedy therefor is to use pointers on the level of single bits, which in the case of eight bit characters will enlarge the length of a pointer by three bits. This approach is straightforward and fully feasible, but the greater length of a pointer will somewhat reduce the compression rate.  
         [0018]    A second remedy therefor is to set the size of a pointer-length combination to a multiple of the length of standard characters. Also this approach is fully feasible, but it will limit the possible choices for the format. Furthermore, the use of the above signalling element means that for eight-bit characters the overall length of the pair will usually be  17  bits (two times eight plus one), which size may be larger than optimum.  
         [0019]    A third solution therefor is to have pointer-length pairs alternating in sequence with standard length characters. This scheme presents an immediate solution to the problem of discriminating between data and pointer-length pairs, without a need for a discriminatory information to this effect. In case of two adjacent uncoded characters, a pointer indicating a distance of zero will then be interposed. This scheme allows immediate bidirectional decoding, whilst still maintaining the advantage of effecting the pointing in the encoded compressed space. However, the overall compression factor will be diminished through the necessity for frequently inserting the zero-distance pointers.  
         [0020]    A fourth and presently preferred remedy therefor is however to provide the above signalling element or discriminatory information, such as a single bit, for all objects. Furthermore, the objects are divided into two parts, of which one part has a uniform length and contains at least the discriminatory information, whereas the second part of the object has the rest. The first part may then have the length of a standard character plus one bit, or rather, amount to just the signalling bit. Furthermore, the location of these two parts in storage is indicated by maintaining separate storage pointers, both for the standard length parts, and also for the remainder parts. The storage pointers relate to the storage configuration and are distinguished from the pointers contained in the pointer-length pairs themselves that relate to the data sequence configuration. The latter, in fact, relate to the data sequencew configuration.  
         [0021]    The present inventor has therefore recognized that the dividing of the objects into at least two parts, and in particular, putting at least the signalization informations in a standard-length separate part and keeping a second part for a remainder, whilst maintaining for each part a secondary storage pointer, will allow to execute regressive decoding in an expeditious manner without the need for a relatively clumsy delimiting procedure for finding the lengths of all the various objects. This will be the presently preferred embodiment disclosed in particular hereinafter.  
         [0022]    Regarding the length indication in the pointer-length pair, any particular embodiment must choose between having the length indicating the number of encoded objects, or rather the number of characters after decoding. The first approach will need fewer bits to indicate the length, but will less easily accommodate the encoding of substrings. The second approach favours the opposite.  
         [0023]    A further extension of the preferred embodiment is the following. In first instance, the pointers should refer back to whole objects, to wit pointer-length pairs and characters, respectively. This will however not allow to refer back to an earlier string that comprises only part of an earlier reference, rather than the whole of such earlier reference. This problem can however be solved by replacing a pointer-length pair by a pointer-length-skip triple, wherein the skip indication will show which elements in the referenced item should be ignored, either before starting the copying of the item, or alternatively, which final elements of the item should be skipped for copying at the end.  
         [0024]    Another solution is to specify the pointer in character units. In the case of a pointer-length pair, the length must then be checked.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]    By way of background introduction, FIG. 1 illustrates a decoded data sequence, that consists of standard length characters, such as eight bits, shown as blocks. As shown, the sequence A, made up of six characters in this example, is followed by a three-character sequence B, after which the sequence A occurs a second time. Through inserting a pointer-length pair (P,L) instead of the second sequence A, a data compression is realized, provided that the pointer-length pair has on the average a shorter length than the replaced sequence. In consequence, the pointer here points to the pointee in the decoded data sequence. This means that the pointee must still be available in intermediate storage space when a decoder would detect the pointer-length pair.  
         [0026]    [0026]FIG. 2 illustrates an exemplary pointer-length pair format, to be used with the above earleir art, as well as with the present invention. The pointer-length pair consists of a signalling bit S that discriminates a pointer-length pair from other data, a ten-bit pointer P that indicates the offset to the sequence that must be copied (the pointee), and a four-bit length indication L that indicates the length of the string that must be copied. In consequence, in this embodiment the span of the pointer is  1024  characters, and the copied sequence may have up to  16  characters. Generally, standard-length characters should also have the signalling bit S.  
         [0027]    [0027]FIG. 3 illustrates an encoded data sequence according to the preferred embodiment of the present invention, wherein the pointers will point to the correct location in the encoded data sequence. For simplicity, the above discriminatory element S for the standard characters as well as for the pointer-length pairs, has not been shown separately in the Figure. Now, the encoded sequence starts with objects C, D, followed by the pointer-length pair P/L that points to the start of C. In FIG. 3, the pointer-length format has not been made equal to that used in FIG. 2. The next-following pointer P′/L′ may generally point across both one or more encoded sequences such as E, but may also point across one or more further pointer-length pairs such as P/L. Through the effect of the compression, this means that either the span of the pointer will have increased, so that generally a particular sequence may be copied more often, or that the pointer may be kept shorter, such as through having only 8 bits instead of ten.  
         [0028]    [0028]FIG. 4 illustrates a pointer format provided with a skip field Sk, the remainder of the pointer-length combination being equal to that of FIG. 4. Here, the indication Sk defines a part of the pointee that should be skipped before copying the remainder of the pointee. Another solution would be that the indication Sk would define a part of the pointee after copying the preceeding part of the pointee. It has been found that such skipping may well increase the degree of flexibility of the inventive method.  
         [0029]    [0029]FIG. 5 illustrates the storage configuration for the encoded data in one embodiment. In this case, there are seven objects, to wit, four standard length characters and three pointer-length pairs, each of these objects occupying its own conceptual column  22  in the Figure. Furthermore, each object has a signalling bit  20 , that for the pointer-length pairs has a different value than for the standard length characters. Furthermore, for both storage parts, a separate storage pointer  24 ,  26  is being updated. For the signalling bits ( 24 ) only the number of pointer-length pairs is considered, so its value is “three” (binary . . . 011). For the other parts ( 26 ) all entries count, so the storage pointer&#39;s value is “seven” (binary . . . . 111). Alternatively, the part  26  could only be counting the number of pointer-length pairs, so that its count would then be “four” (binary . . . 100). It should be clear that the parts of the objects that comprise the signalling element, could also comprise part of the remainder of the object in question, such as to give the part the length of one character.  
         [0030]    [0030]FIG. 6 illustrates an encoding arrangement. Standard-size characters arrive on input  30  and are stored in storage array  32 , that operates in the way of a FIFO. Concurrently with receiving a next character, processor  34  accesses storage array  32  to detect any character sequence within the pointer span of the embodiment that should be replaced by a pointer-length pair pointing to an earlier sequence that has exactly the same content. If found, the items in question in the storage array are replaced by the pointer-length pair, either physically or virtually, the latter by amending the address sequence. The pointer value is measured either in a number of encoded objects, or as a number of characters after encoding, as discussed earlier. Furthermore, register  36  stores the actual span between the first and last actual (physical or virtual, as the case may be), entries in array  32 . In the case of overflow, those items are forwarded through device  38  for transmission on output  40 , mass storage, or otherwise. Each object has its associated signalization bit, and furthermore, the stream is interspersed with the two types of storage pointers that have been discussed with reference to FIG. 5. The repetition rate of the interspersing of such storage pointers is generally a matter of design. Such would determine the random accessibility at an arbitrary point in the data sequence.  
         [0031]    The algorithm used herein is of the greedy type, that undertakes to be applied at the execution. An extension of the above is however to have a first run for detecting all strings that should be replaced by a pointer-length pair, followed by a second run for actually executing the replacing.  
         [0032]    [0032]FIG. 7 illustrates a decoding arrangement. Input  50  receives the stream of objects (characters and pointer-length pairs) and interspersed storage pointers, respectively. In consequence, an input detection facility not separately shown will know where to look for the bits that discriminate between standard characters and pointer-length pairs. Any standard-length character will be stored in storage array  52  and furthermore be read-out by processor  56  and forwarded to output device  58  for outputting on output  62  for further application. A pointer-length pair will activate binary detector  60 , and processor  56  will thereupon read out the pointer-length pair and access the correct data sequence for outputting on output  62  as discussed earlier for the case of standard characters received. After finding a pointer-length pair, the address counter for the actual storage address is incremented with the appropriate value, so that the next pointer length pair will implement the correct address offset. Again, when coming outside the applicable address span for the pointer, the associated information is deleted or otherwise ignored.  
         [0033]    [0033]FIG. 8 illustrates an encoding flowchart. In block  70 , the process is started, and the appropriate amount of hardware and software is assigned. For simplicity, the terminating of the process has not been included in the Figure. In block  72 , the next character to be encoded is received. In block  74 , the character in question, plus any earlier character in the string that has not been rejected for correspondence with an earlier string in the span of the pointer is checked for such earlier correspondence. In block  76 , the test for such correspondence is made. Of course, with the increasing of the string length, the pointer may have to go back still further, because the probability for such correspondence will generally decrease with the stringth&#39;s length. If Yes, in block  78 , the length is incremented by one and the new pointer is saved, and the system reverts to block  72  in order to try for a still longer string. If No, in block  80  the pre-existing pointer-length pair is stored. Processing for taking into account the difference between uncompacted and compacted data may be effected either during the second pass as discussed above, or rather, through immediate reprocessing the pointer value. Subsequently, the process reverts again to block  72  for undertaking to find the start of a new string.  
         [0034]    [0034]FIG. 9 illustrates a decoding flow chart. In block  100 , the process is started, and the appropriate amount of hardware and software is assigned. For simplicity, the terminating of the process has not been included in this Figure, either. In block  102 , the appropriate storage pointer is read out and updated. In block  104 , the associated object is read out. In block  106  the system detects whether the object in question is indeed a character. If Yes, in block  108  the character in question is decoded, outputted and also stored. If No, in block  110  the string pointed at is accessed. Subsequently, the string in question is decoded in block  112 . Finally in block  114 , the decoded data are output, whereas the encoded string is stored. Next, the system again reverts to block  72 .