GB2239764A - Bandwidth reduction encoding of documents for transmission - Google Patents

Abstract

The document to be transmitted is broken down into a sequence of lines of pixels. At the transmission end, each white line is coded 103 by a single bit on level 1, and each non-white line is coded 104 by a plurality of bits which starts with a bit on level 0, is followed by bits relating to the content of the line, and ends with a synchronisation word EOL. Transmission is effected 105-107 in accordance with a method which permits the correction of transmission errors. At the receiving end, any sequence of bits on level 1 which immediately follows a synchronisation word, is decoded by a sequence of as many white lines, and any bit on level 0 which follows a synchronisation word is decoded by a non-white line. <IMAGE>

Description

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-1DESCRIPTION "A METHOD OF CODING, TRANSMITTING AND DECODING FOR TELECOPIES, AND A TRANSMITTING APPARATUS AND A RECEIVING APPARATUS FOR IMPLEMENTING SAID METHOD"

The present invention is concerned with a method of coding, transmitting and decoding for telecopies, of the type in which any document to be transmitted is broken down into a sequence of pixel lines, all said lines being of the same length, and each of said lines is coded and decoded successively.

One such method is described for example in Recommendation T.30 of the Comite Consultatif International pour le Telegraphe et le Telephone, or C. C.I.T.T.

In this method, each line to be transmitted is broken down into a sequence of pixel ranges, the pixels of one range being of the same value, and the content of this line is indicated in the form of the number and value of each of the ranges of the line. In the most usual case, in which it is considered that a pixel is either black or white, and can therefore have only two values, each line is thus broken down into a sequence of ranges which are alternately black and white.

The number of pixels per range and their value in said range is indicated for each line by a plurality A 1 of binary elements or bits. Furthermore, in order to be able to readily separate. in the transmitted signal, the plurality of bits relative to one line from the plurality of bits relative: to the following line, there is provided at the end of each plurality of bits relative to one line a synchronisation word indicating a line end. In accordance with Recommendation T.30 of the C.C.I.T.T.. this synchronisation word. or EOL ("end of line"), is a word of twelve bits, the first eleven of which are at level 0 and the twelfth at level 1.

The method as described above has a compaction density, that is to say a relationship between the quantity of useful data to be coded and the length of the signal after coding, which is satisfactory in a certain number of applications, but which may lead to transmission times which are too long in applications in which the rapidity of transmission is essential.

The present invention aims to overcome this disadvantage by providing a method of coding, transmitting and decoding, whose compaction capability is superior to that of the known method.

To this end, it has as its object a method of the type mentioned initially, characterised by the fact that: - on the transmitting end, each white line is coded 1 c by a single binary element on a first level among its two possible levels, and each non-white line is coded by a plurality of binary elements which commences with a binary element on the second level among its two possible levels, and which is followed by binary elements relative to the content of this non-white line, - a transmission mehod is used having transmission error correction, and - on the receiving end, following correction of the transmission errors, any binary element received which starts a line is decoded, by a white line if it is on said first level and by a non-white line if it is on said second level, the content of this non-white line being indicated by binary elements received after this binary element received on said second level.

In the method of the invention, the compaction is increased as a result of the extremely simple coding of each white line, which coding is carried out with the aid of single binary element, or bit, for example on level 1. This increase is more important the greater the number of white lines in the document to be transmitted, which is generally the case. It will be remembered that the term "line" should not be understood in this case as being of a line of characters of possible text which may be on the document, but a line of pixels, that is to say a strip of the document having the height of a pixel. A white line is thus a line which comprises only white pixels, a non-white line being a line which comprises at least one non-white pixel, that is to say black in the normal case of binary pixels. In the method of the invention, it is possible to code-in an equally simple manner each white line, on the one hand because it is provided for the coding of each non-white line to be commenced by a bit at level 0, in the example chosen, and, on the other hand, because a method having transmission error correction is used. Correction of transmission errors guarantees the correct value of the bits received and intended to be decoded.

It should be noted that, in the frequent case in which the printing head has a limited printing speed which does not allow it to print a sequence of lines in the rapid rhythm in which they are received if the are white lines, it is necessary to store the lines received in a buffer memory in order to return them one by one to the printer in a rhythm which permits it to print them. However, this does not require the implantation of a supplementary memory, because the use of a method of transmission error correction 1 P assumes the use of such a buffer memory in order to safeguard the data received, at least until it is known whether they are to be corrected or not. Thus, the method of the invention makes particularly efficacious use of the buffer memory. which serves simultaneously to correct errors and to improve compaction density. Advantageously: - on the transmitting end, there is provided, at the end of each plurality of binary elements coding a nonwhite line. a synchronisation word indicating a line end, and - on the receiving end. any sequence of binary elements on said first level which are received immediately after a synchronisation word is interpreted as a sequence of as many white lines.

In this case, the allocation of data received to each of the lines is simple and reliable. However, the compaction density of the method of the invention remains excellent, because no synchronisation word is provided at the end of a white line. Thus, and for example, a sequence of five white lines is coded and transmitted in the form of a sequence of five bits only.

Again advantageously, on the transmission end, each non-white line is broken down into a sequence of pixel ranges, the pixels of one range being of the same value, and the content of this non-white line is indicated in the form of the number and value of the pixels of each of said ranges of the line, with the exception of the last region of the line, for which the number of pixels is omitted, and, on the receiving end, the number of pixels of the last range is determined so that the non- white line is of the correct length.

Further compaction is obtained, because the number of pixels of the last range does not have to be transmitted.

As a further advantage, a method of transmission is used which has error correction, in which: - on the transmission end, coded data are transmitted with redundancy, and - on the receiving end, said redundancy is used to detect transmission errors in the data received, and retransmission is requested of data received which are affected by transmission errors.

One such method is the optional method of error correction mentioned in Recommendation T.30 of the C.C.I.T.T.. It consists of transporting the data in i c frames of the HDLC type (11highrate data link controller") with a control sum. For each frame received, the receiver recalculates the value of the control sum and compares it to the received value. When the calculated value and the received value do not agree, retransmission of the data received is requested by the receiver.

The present invention also seeks to provide an apparatus for transmitting telecopies and an apparatus for receiving telecopies intended for implementing the method of the invention.

To this end, the present invention provides an apparatus for transmitting telecopies, comprising means for breaking down any document to be transmitted into a sequence of pixel lines, all the said lines being of the same length, and means for coding successively each of said lines, said transmission apparatus being intended for transmitting telecopies to a receiving apparatus comprising means for decoding successively each of the said lines, said transmission apparatus being characterised by the fact that it comprises: - means for coding each white line by a single binary element on a first level among its two possible levels, and means for coding each non-white line by a plurality of binary elements which starts with a binary element on the second level among its two possible levels, and which is followed by binary elements relative to the content of this non-white line, and - means for transmitting the binary coding elements of said lines in accordance with a transmission method having transmission error correction, said receiving apparatus comprising means for receiving binary elements transmitted in accordance with said transmission method, means for correcting transmission errors, and means for decoding any binary element received which starts a line, by a white line if it is on said first level and by a non-white line if it is on said second level, the content of this non-white line being indicated by the binary elements received after this binary element received on said second level.

In the same way, the present-invention seeks to provide an apparatus for receiving telecopies which is intended to receive telecopies coming from a transmission apparatus, comprising means for breaking down any document to be transmitted into a sequence of pixel lines, all said lines being of the same length, and means for coding successively each of said lines, i i 1 i said receiving apparatus comprising means for decoding successively each of said lines and characterised by the fact that said transmission apparatus comprises means for coding each white line by a single binary element on a first level among its two possible levels, means for coding each non-white line by a plurality of binary elements which starts with a binary element on the second level among its two possible levels, and which is followed by binary elements relative to the content of this nonwhite line, and means for transmitting the binary coding elements of said lines in accordance with a transmission method having transmission error correction, said receiving device comprising: - means for receiving the binary elements transmitted in accordance with said method of transmission, and means for correcting transmission errors, and - means for decoding any binary element received which starts a line, by a white line if it is on said first level and by a non-white line if it is on said second level, the content of this non-white line being indicated by the binary elements received after this binary element received on said second level.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram of an apparatus for transmitting telecopies and an apparatus for receiving telecopies, these two apparatuses being connected by way of a telephone network; Fig. 2 is a simplified sequence chart for the transmission apparatus; and Fig. 3 is a simplified sequence chart for the receiving apparatus.

In Fig. 1, 1 designates an apparatus for transmitting telecopies, 2 designates an apparatus for receiving telecopies, and 3 a telephone network.

As has already been indicated, the invention relates to a method of coding the data carried by a document to be transmitted, transmitting this coded data and decoding the coded data received in order to reconstruct at the receiving end a copy of the document transmitted.

The transmission apparatus 1 comprises in this case a device 11 for analysing the document 4 to be transmitted, a modulator-demodulator, or modem, 13, and a circuit 12 for coding the analysis signal and controlling the modem 13.

i 1 The analysis device 11, of a known type, is provided with an output which delivers the analysis signal A.

The modem 13. of a known type, is provided with a bidirectional access, which is conndcted to a line of the network 3, a signal input, which receives the signal E to be transmitted, and a signal output which delivers a received signal W.

The coding and control circuit 12 comprises, in particular, and in a known manner, a microprocessor and a buffer memory. It is provided with a first input which receives the analysis signal A, a second input which receives the signal R', and an output which delivers the signal E.

The receiving apparatus 2 comprises in this case a modem 23, a printing device 21, and a circuit 22 for correcting transmission errors, decoding, and controlling the printing device 21.

The modem 23, of a known type, is provided with a bidirectional access which is connected to a line of the network 3. a signal input which receives a signal E' to be transmitted, and a signal output which delivers a received signal R.

The printing device 21. of a known type, is provided with an input which receives a printing signal I, and it prints, the received document.

The circuit 22 for correcting transmission errors, decoding and control comprises, in particular, and in a known manner, a microprocessor and a buffer memory. It is provided with an input which receives the signal R, a first output which delivers the signal E', and a second output which delivers the signal I.

Before commencing the description of the function of the apparatuses whose constructions have just been described, it should be noted that, in general, in order to permit the transmission of documents from the apparatus 1 to the apparatus 2 and from the apparatus 2 to the apparatus 1. these two apparatuses are identical. The apparatus 1 thus comprises elements which have been described for the apparatus 2, and the apparatus 2 itself comprises elements which have been described for the apparatus 1. This does not change anything in the present statement, in which, for the purposes of simplicity, and also because it is possible, the apparatus 1 is limited to the transmission of documents such as the document 4, the apparatus 2 being limited to receiving these documents.

The function of the apparatuses 1 and 2 for transmitting documents from apparatus 1 to apparatus 2 -12in response to this signal, 1 -13is as follows: - When the apparatus 1 wishes to transmit to apparatus 2 a document such as the document 4 in Fig. 1, it calls this apparatus 2 by way of the network 3. As soon as communication is established, the apparatus 2 transmits to the apparatus 1 a series of signals which represents, on the one hand, its own telephone number, and, on the other hand, its characteristics, either standardised or personal.

The standardised possibilities of an apparatus are defined for example in Recommendations TA and T.30 of the C.C.I.T.T., and permit apparatuses from different manufacturers to understand one another.

Personal characteristics are those which are peculiar to a particular manufacturer or to a group of manufacturers, which, for example when two telecopiers of the same manufacturer or of the same group of manufacturers are face to face, allow these telecopiers to escape from the standardised protocol to use a private or personal procedure protocol peculiar to the manufacturer, having, in the proposed situation, certain advantages over the standardised normal protocol.

The present invention is concerned with a private or personal.

The transmission apparatus 1, having received from the receiving apparatus 2 signals indicating the number of the latter and its characteristics, responds by a sequence of signals which represents in particular the number of the apparatus 1, the private protocol selected, that is to say in this case that of the invention, and thus the private functions which the apparatus 2 has to use.

In this case, and in a known manner, the analysis device 11 analyses the document 4 pixel line by pixel line, all the lines being of the same length, for example 1728 pixels in A4 format. For each line analysed, the analysis device delivers to the circuit 12, by way of the signal A, a sequence of 1728 bits, on one level or the other, depending on whether the corresponding pixel is black or white.

The document 4 thus being broken down into a sequence of pixel lines, the circuit 12 codes each of the lines successively.

To this end, the circuit 12, once it receives data relative to a line of pixels, determines whether this line comprises black pixels or not.

When this line is completely white, the circuit 12 codes it with a single bit at a given level among its two possible levels, for example the level 1.

i When this line is not completely white, the circuit 12 breaks it down into a sequence of ranges of pixels, the pixels of one range being of the same value, in this case black or white. The circuit 12 thus codes this non-white line with-'a plurality of bits which starts with a bit at level 0, and which is followed by bits relative to the content of the line, indicated in the form of the number and value of the pixels of each range of the line. In this case, however, the circuit 12 omits to indicate the number of pixels of the last range of the line, and inserts, at the end of the plurality of bits coding the nonwhite line, a synchronisation word indicating a line end, for example an EOL word as defined in Recommendation T.30 of the C.C.I.T.T..

The circuit 12, after having thus coded each of the lines of the document, inserts the bits representing the coded lines in the HDLC-type frames, which, as has already been indicated, comprise a redundant control sum which permits the detection and correction of errors.

The circuit 12 controls the modem 13 such that it transmits the HDLC frames to the receiving apparatus 2. At the same time, the circuit 12 temporarily stores the frames in the buffer memory in such a way as to be able to repeat them if it turns out that the -16corresponding frames received by the receiving apparatus 2 have been affected by transmission errors in the network 3.

The circuit 22 thus receives by way of the modem 23 the signal R which represents th6 H= frames received. In a known manner, it calculates the control sum of each frame received and compares it to the one it has recei,ed. If the two are not the same. this means that the frame has been affected by at least one error, and its retransmission by the apparatus 1 is requested. Of course, the circuit 22 stores the frames received in its buffer memory and replaces the frames affected by errors with correct frames as soon as it receives them.

After having thus corrected the transmission errors, the circuit 22 proceeds to the decoding itself of the data received, in such a way as to control the printing device 21 such that it reconstructs a telecopy of the document 4.

The circuit 22 in this case uses the synchronisation words EOL to allocate the bits received to different lines.

As the coding of each non-white line commences in this case with a bit at level 0, it is clear that if the bit which follows a synchronisation word EOL, and j i j 1 i 1 f -17which thus starts a line, is on level 1, then this bit represents a white line.

As no synchronisation word EOL is provided after the single bit on level 1 which c6des a white line, if this bit on level 1 is followed by hnother bit on level 1. then the latter also codes a white line, and so on. In other words, any sequence of bits on level 1 received immediately following a synchronisation word EOL, must be interpreted as a sequence of as many white lines as there are bits in the sequence. This is what circuit 22 does.

In contrast, any bit on level 0 coming immediately after a synchronisation word EOL, or after a sequence of bits on level 1 representing a sequence of white lines, must be decoded as a white line whose content is indicated by the bits which follow this bit on level 0.

As any end of a white line is indicated by a synchronisation word EOL, the circuit 22 allocates without ambiguity to any non-white line. the bits which indicate the content thereof. The circuit 22 is moreover able to determine the number of pixels of the last range, which was omitted from transmission, so that the non-white line is of the correct length, in this case, and for example, 1728 pixels, taking into f -18account the length of each of the other ranges which it knows.

The circuit 22 is thus able to elaborate the signal I in order to control the printing device 21. In the case in which, as is in principle the case, the speed of the printing device 21 is not sufficient to face up to an influx of white lines, whether simple and thus rapid to transmit, the circuit 22 stores the decoded data in its buffer memory and manages the stored data in order to feed the printing device at an appropriate rhythm.

The diagram of Fig. 2 schematises the different stages in the operation of the circuit 12. In this drawing, the block 101 schematises the stage of receipt of the analysis signal A relative to each line coming from the analysis device 11. The block 102 schematises the stage for determining the fact that the processed line is white or not. If the processed line is white, the circuit 12 moves to the stage schematised by the block 103 for coding of this white line using a single bit on level 1. If the processed line is not white, the circuit 12 moves to the stage schematised by the block 104 for coding this non-white line using a plurality of bits, the first of which is on level 0, and the following ones of which code the content of the non-white line. and the last ones of -19which represent a synchronisation word EOL.

Following the stage schematised by the block 1030 or, depending on the case, following the stage schematised by the block 104, the circuit 12 moves to the stage schematised by the block 105 for inserting the coding bits in the redundant frames, in this case H= frames. This stage is followed in a known manner by stages 106 and 107 for storing in the buffer memory and transmission respectively.

The diagram of Fig. 3 schematises the different stages of the operation of the circuit 22. In this drawing, the block 201 schematises the stage for receiving the signal transmitted by way of the network 3. This stage is followed in a known manner by stages 202 and 203 for storing in the buffer memory and correcting errors respectively.

The block 204 represents the stage for searching for synchronisation words EOL, and the block 205 represents the stage for determining the level of the bit which immediately follows the synchronisation word EOL.

When this bit is on level 1. the circuit 22 moves to the stage schematised by the block 206 to determine how many bits on level 1 immediately follow the synchronisation word EOL. When it has determined the number of bits of this -20sequence of bits on level 1, the circuit 22 moves to the stage schematised by the block 207 for storing in the buffer memory a number of white lines equal to this number of bits of the sequence of bits on level 1.. The circuit 22 then moves to the stage schematised by the block 208, which stage is furthermore carried out directly, if in the course of.the stage schematised by the block 205, it turns out that the bit which immediately follows the synchronisation word EOL is at level 0. The stage of the block 208 is relative to decoding of the content of the non-white line which follows the synchronisation word EOL if the stage carried previously was that of the block 205, or which follows the sequence of white lines if the stage carried out previously was that of the block 207. Decoding the content of the line comprises in particular, in this case, the determination of the length and the value of each of these ranges, it being understood in this case that the length of the last range, which is not transmitted. is calculated as the difference between the known length of the line and that of the other ranges. The circuit 22 thus moves to the stage, schematised by the block 209, for storing in the buffer memory, and resumes the stage schematised by the block 205 for decoding the following lines. At the same time, the circuit 22 i i 1 -21manages the data stored in the buffer memory in order to provide the printing device 21 with the signal I at a suitable rhythym. This is schematised by the block 210.

Of course, the present invention is not limited to the above description, which was merely given by way of example.

In this way, in particular, it would be possible to use other methods,of correcting transmission errors than the one described above.

At the same time, if it is convenient to provide synchronisation words in order to be able to allocate the bits received without ambiguity to one line or the other, this is not absolutely obligatory. If the quality of the connection is very good, it may be possible to proceed by counting the pixels received per line.

If, on the other hand, the quality of the connection is mediocre, it may be possible to add a certain redundancy to the signal by transmitting the length of the last range.

In the same way, the above description is relative to the case in which binary pixels are transmitted, that is to say either black or white pixels. It is within the scope of a person skilled in the art to transpose the method of the invention to -22the case of transmitting pixels capable of taking more than two values, either for the transmission of a grey scale or for the transmission of colours.

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Claims (10)

-23CLAIMS

1. A method of coding, transmitting and decoding for telecopies, in which a document to be transmitted is broken down into a sequence of pixel lines, all said lines being of the same length, and each of said lines is coded and decoded successively. the method including the steps that: (a) at the transmission end, each white line is coded by a single binary element on a first level among its two possible levels, and each nonwhite line is coded by a plurality of binary elements which commences with a binary element on the second level among its two possible levels, and which is followed by binary elements relative to the content of this non-white line, (b) a method of transmission is used having correction of transmission errors, and (c) at the receiving end, following correction of the transmission errors, any binary element received which commences a line is decoded, by a white line if it is on said first level and by a non-white line if it is on said second level, the content of this nonwhite line being indicated by binary elements received after this binary element received on said second level.

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2. A method as claimed in claim 1, in which: at the transmission end. at the end of each plurality of binary elements coding a non-white line, there is provided a synchronisation word (EOL) indicating a line end, and - at the receiving end, any sequence of binary elements on said first level received immediately after a synchronisation word (EOL) is interpreted as a sequence of as many white lines.

3. A method as claimed in claim 1 or 2, in which. at the transmission end, each non-white line is broken down into a sequence of pixel ranges, the pixels of one range having the same value, and the content of this nonwhite line is indicated in the form of the number and the value of the pixels of each of said ranges of the line.

4. A method as claimed in claim 2, in which, at the transmission end, each non-white line is broken down into a sequence of pixel ranges, the pixels of one range being of the same value. and the content of this non- white line is indicated in the form of the number and the value of the pixels of each of said ranges of the line, except for the last range of the line, in which case the number of pixels is omitted, and, at the receiving end, the number of pixels of the 1 i i -25last range is determined so that the non-white line has the correct length.

S. A method as claimed in any of claims 1 to 4, in which a transmission method is used having error correction, in which: at the transmission end, coded data are transmitted with redundancy, and - at the receiving end, said redundancy is used for detecting transmission errors in the data received, and retransmission is requested of received data affected by transmission errors.

6. An apparatus for transmitting telecopies, for implementing the method as claimed in any of claims 1 to 5, including means for breaking down any document to be transmitted into a sequence of pixel lines, all the said lines having the same length, and means for coding successively each of said lines, said transmitting apparatus being intended to transmit telecopies to a receiving apparatus comprising means for decoding successively each of said lines, and a transmitting device comprising: means for coding each white line by a single binary element on a first level among its two possible levels, and means for coding each non-white line by a plurality of binary elements which starts with a binary element on the second level among its two -26possible levels, and which is followed by binary elements relative to the content of this non-white line, and - means for transmitting binary coding elements of said lines as in a method of transmission having correction of transmission errors, said receiving apparatus comprising means for receiving the binary elements transmitted in accordance with said transmission method, means for correcting transmission errors, and means for decoding any binary element received which starts a line, by a white line if it is on said first level, and by a non-white line if it is on said second level, the content of said non-white line, being indicated by the binary elements received after this binary element received on said second level.

An apparatus for receiving telecopies, for lementing the method in accordance with any of claims 1 to 5, intended for receiving telecopies coming from a transmitting apparatust the receiving apparatus comprising means for breaking down any document to be transmitted into a sequence of pixel lines, all said lines being of the same length, and means for coding successively each of said lines, receiving apparatus comprising means for decoding successively each of said lines, said transmitting i 1 1 A -27apparatus comprising means for coding each white line by a single binary element on a first level among its two possible levels. means for coding each non-white line by a plurality of binary elements which commences with a binary element on the second level among its two possible levels. and which is followed by binary elements relative to the content of this non-white line, and means for transmitting the binary coding elements of said lines in accordance with a method of transmission having transmission error correction, said receiving device comprising: - means for receiving binary elements transmitted in accordance with said method of transmission, and means for correcting transmission errors, and - means for decoding any binary element received which starts a line, by a white line if it is on said first level and by a non-white line if it is on said second level, the content of this non-white line being indicated by the binary elements received after this binary element received on said second level.

8. A method of coding, transmitting and decoding for telecopier, substantially as hereinbefore described with reference to the accompanying drawings.

9. An apparatus for transmitting telecopies, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. An apparatus for receiving telecopies, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

i i i 1 Published 1991 at The Patent Ofte. State House. 66/71 High Holborn. London WCIR47P. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point. Cwmfelinfach, Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid, St Mary Cray. Kent.