GB2245461A

GB2245461A - Accented character generator

Info

Publication number: GB2245461A
Application number: GB9013785A
Authority: GB
Inventors: John Ritchie Kinghorn
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1990-06-20
Filing date: 1990-06-20
Publication date: 1992-01-02
Also published as: GB9013785D0

Abstract

A character generator which may be used with a decoder for videotex or teletext data transmissions receives a composition coded character signal CC and has a character read only memory C-ROM for storing a plurality of unaccented alphabet characters and a plurality of character accents (Fig 2), and processing means including a multiplexer MUX, latches L1 and L2 and parallel-to-serial converter/multiplexer C/M for selecting an unaccented alphabet character and character accent and for effectively combining these to afford a required accented character. Thus the character generator requires reduced ROM size because accented characters are not stored in ROM. <IMAGE>

Description

DESCRIPTION CHARACTER GENERATORS This invention relates to character generators and in particular though not specifically to such character generators that may be used in decoders for teletext and videotex data transmission systems.

Teletext is a public service which makes use of broadcast television signals in order to transmit alpha-numeric and graphic information to a domestic or other television receiver. In the United Kingdom there are two systems, one operated by the British Broadcasting Corporation (CEEFAX) and the other operated by the Independent Broadcasting Authority (ORACLE).

Videotex, which has previously been called viewdata, is a similar system for transmitting alpha-numeric and graphic information, but makes use of telephone lines instead of broadcast television signals. One advantage of videotex is that it is possible for data to be transmitted in both directions i.e. both to and from a user, thereby providing an interactive system. Existing videotex systems are Prestel in the U.K. and Teletex in France.

A specification for a videotex system has been agreed by many of the post and telecommunications authorities in Europe under the auspices of CEPT (the European Post and Telegraph Committee) which specification sets out the basic requirements of the system.

Philips Components produce a micro-processor controlled cathode ray tube controller integrated circuit type SAA 5351, commonly referred to as EUROM (European Read Only Memory), for use primarily in so-called level 3 CEPT videotex decoders, but also can be used in higher level teletext decoders. EUROM is a single-chip VLSI NMOS controller which requires only minimal additional hardware in order to produce an inexpensive terminal decoder, details of this integrated circuit being present in the Philips Component Data Handbook - Integrated Circuits, "Video and associated systems, Bipolar, MOS" Volume ICO2a.

One of the functions carried out by EUROM is character generation, this being based on the full ISO repertoire of 335 characters, including mathematical symbols plus 151 "smooth" mosaics and other line drawing characters. These "fixed" characters are stored in four character tables which are in on-chip ROM (read only memory), and which are depicted in Figure 1 of the accompanying drawings in which the four tables are referenced A, B, C and D respectively and are reproductions of Figures 15(a), 15(b), 16(a) and 16(b) from the above Data Handbook.

In the above Tables: i) Table A contains the 128 most commonly used characters: standard upper- and lower-case Roman alphabet, numerals, punctuation, and the more common accented characters. In normal text transmission, Table A is used most of the time.

ii) Table B contains further accented characters.

iii) Table C contains a number of miscellaneous characters, mathematical symbols, the line drawing character set, and accents without associated characters.

iv) Table D contains the block mosaics for the basic alphamosaic service, together with the new "smooth" mosaics, the two sets of which are complimentary.

Although not relevant to the present invention, EUROM also includes a Dynamically Redefinable Character Set (DRCS), the characters of which are transmitted to the text decoder and stored in a random access memory (RAM) thereof. The use of DRCS enormously extends the display repertoire, and additional characters can be defined by the information provider and used for display purposes.

It is an object of the present invention to provide a videotex or teletext decoder which includes a character generator similar to that provided in EUROM but which requires reduced ROM size and simplified display processing.

In accordance with one aspect of the present invention there is provided a character generator for receiving a composition coded character signal, the character generator including memory means for storing a plurality of alphabet characters and a plurality of character accents, characterised in that said character generator additionally comprises processing means responsive to the composition coded signal for separately selecting an alphabet character and a character accent, and for effectively combining these to afford a required accented character.

In the "fixed" character set depicted in Figure 1, part of Table A and all of Table C is taken up with accented characters.

In videotex transmissions, use is made of so-called composition coding in which, when an accented character is transmitted, it takes the form of a single shift or escape code followed by the particular accent and then the particular character required. In the microprocessor controlling the character generator of the videotex decoder, the accent and character information is applied to a look-up table in order to locate within the character set of Figure 1 the particular accented character required for display.

It has now been appreciated that when composition coding is used it is not necessary to store all the required accented characters individually, since substantially all of them can be provided by providing the normal upper- and lower-case unaccented alphabet set together with a set of the required accents, the required accent character being formed by adding the selected accent to the selected alphabet character. By so doing, it has been found, for example, that the four "fixed" character tables of Figure 1 can be reduced to three, and the associated processing is simplified as compared to existing EUROM processing.

In carrying out the invention according to the aforesaid first aspect, it may be arranged that the memory means takes the form of a read only memory, and that the character generator comprises multiplexer means to which the composition coded character signal is applied, the output from said multiplexer means being applied to said memory means which is accessed twice during a character period to produce said required accented character.

In a preferred arrangement it will be arranged that the composition coded character signal includes a character accent code and a character code which are successively applied to said memory means by said multiplexer means, and that first and second latch means are provided operable on the output of said memory means for respectively latching the characters corresponding to said character accent code and said character code.

Conveniently, it may be arranged that the character generator includes a parallel-to-serial converter and multiplexer means operable on the outputs from said first and second latch means and for affording dot data information corresponding to the latched characters.

Preferably the memory means may additionally store unaccented upper-case characters of reduced height for combining with accents which appear above an upper-case alphabet character.

In accordance with another aspect of the present invention, there is provided a method of generating accented characters comprising the step of receiving a composition coded character signal, storing a plurality of unaccented alphabet characters and a plurality of character accents, characterised by the further steps of selecting an unaccented alphabet character and a character accent in dependence upon said composition coded character signal, and combining these to afford a required accented character.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 depict the four character tables containing the "fixed" characters as used in the character generator of a decoder based on EUROM; Figure 2 depicts a modified set of three character tables containing "fixed" characters for use in the character generator according to the present invention; and Figure 3 is a block schematic diagram of a level 3 character generator according to the present invention.

In Figure 1 of the drawings there are depicted the four character tables Table A, B, C and D respectively, already referred to and which contain the "fixed" characters which are stored in on-chip ROM in EUROM. As has already been mentioned, Table A and B contain a very large number of accented characters in addition to the normal Roman upper- and lower-case characters.

In Figure 2 of the drawings, there is depicted a modified "fixed" character set in which Table A corresponds essentially to Tables A and B of Figure 1; Table B corresponds essentially to Table C of Figure 1; and Table C corresponds essentially to Table D of Figure 1.

In Table A of Figure 2, vertical columns numbered 1 and 2 contain reduced height unaccented upper-case characters, and vertical columns numbered 3 to 8 contain basic, full-size, characters again with no accents.

In Table B of Figure 2, vertical columns numbered 1 and 2 contain line drawing characters; vertical columns numbered 3 and 3 contain supplementary characters; vertical column numbered 5 contains various accent symbols; and vertical columns numbered 6 to 8 contain further supplementary characters.

In Table C of Figure 2, vertical columns numbered 1 and 2 contain 32 "smoothed" mosaic characters; vertical columns numbered 3 and 4 contain various block mosaic characters; vertical columns numbered 5 and 6 contain 32 "smoothed" mosaic characters which are substantially the inverse of those contained in vertical columns numbered 1 and 2; and vertical columns numbered 7 and 8 contain further block mosaic characters.

As has already been mentioned, EUROM, which is based on the "fixed" characters contained in Tables A to D of Figure 1, makes use of a transmitted composition coded signal in order to generate a required accented character. The composition coded signal comprises a single shift or escape code followed by the accent required and the character required. The required accent and character codes are then applied to a look-up table in order to generate the required accented character.

Using the modified "fixed" character set of Figure 2, an accented character is generated by using the composition coded signal to locate the required character accent from Table B and the required unaccented character from Table A of Figure 2. If an upper-case accented character is required where the accent is of the type placed above the character, the reduced height unaccented upper-case character in vertical columns 1 and 2 of Table A of Figure 2 are used otherwise, on display, the accent would protrude into the character positioned immediately above.

By doing so, Tables A and B of Figure 1 as presently used in EUROM have been reduced to a single Table A carrying the characters in Figure 2 by using the accents of Table B thereby reducing the size of the on-chip character ROM by 25% and simplifying the required processing, e.g. by obviating the need for the look-up table presently incorporated in the microprocessor which drives the EUROM.

In Figure 3 of the drawings there is depicted a block schematic diagram of a character generator based on the "fixed" character tables of Figure 2 and for use in a videobox or teletext decoder according to the present invention. The character generator of Figure 3 is a modified form of the character generator presently incorporated in EUROM and only those parts of the character generator which are considered essential for an understanding of the invention will be described in detail.

The input to the character generator of Figure 3 is a 32 bit character signal which is stored in the page format memory (not shown) of the decoder and which is turn is loaded from a processing engine or external microprocessor in the complete videotex or teletext chip.

In EUROM, the 32 bit character data is arranged as follows: 7 bits Character Code - CC 3 bits Character Table - CT 5 bits Foreground Colour - FC 5 bits Background Colour - BC 5 bits Flash - F 3 bits Size - S 1 bit Lining - L 1 bit Conceal - C 1 bit Invert - I 1 bit Window/Box -W/B The 3 bit Character Table code CT is for selecting between the Tables A to D of Figure 1 together with an additional table for DRCS. The 3 bit Size code S is for selecting between double height, top or bottom, or double width.

In the character generator of Figure 3, size rules logic SRL has been incorporated which obviates the need to identify the top and bottom of a double height character by means of a top/bottom bit in the page memory so that the 3 bit Size code S in EUROM is reduced to a 2 bit Size code S in the character generator of Figure 3.

The additional bit saved thereby is used in Figure 3 in conjunction with the 3 bit Character Table code CT to provide a 4 bit Accent/Table code A/T to select between the Tables A, B and C of Figure 2 and also to select between the various character accents provided.

Thus, in the character generator of Figure 3, the 32 bit character data is arranged as follows/ 7 bits Character Code - CC 4 bits Accent/Table Code - A/T 5 bits Foreground Colour - F/C 5 bits Background Colour - B/C 5 bits Flash - F 2 bits Size - S 1 bit Lining - L 1 bit Conceal - C 1 bit Invert - I 1 bit Window/Box - W/B Of the 16 combinations of the 4 bit Accent/Table code A/T, 12 combinations are used to add accents to the basic character set and 4 combinations are used to select between the various Tables including an additional table for DRCS. The various parts of the character data are shown as separate inputs on the left hand side of Figure 3 using the codes given above.

In the character generator of Figure 3, the 7 bit Character Code CC and the 4 bit Accent/Table code A/T are applied to a multiplexer MUX which is operated by a clock signal Q at the character data clock rate. This causes the multiplexer MUK to select each of its inputs alternately during the character period. The output from the multiplexer MIX is applied to a character read only memory C-ROM in which the character Tables A, B and C of Figure 2 are stored.The output from the character read only memory C-ROM is fed to a first latch L1 under the control of the clock signal , and to a second latch L2 under the control of the inverted clock signal 0. Thus, when a composition character code is presented to the multiplexer MUX, the accent shape from Table B is latched into one of the latches (L1 or L2), and the unaccented character shape from Table A is latched into the other of these latches. The multiplexer MUX also contains decoding means to ensure that, when an upper-case accented character is required where the accent is to be placed above the character, the reduced height character area of the character read only memory C-ROM is chosen instead of the normal height area (one bit of the input code is reversed). The output of each latch L1 and L2 is fed to a parallel-to-serial converter/ multiplexer C/M in which the outputs of the latches L1 and L2 are combined to afford the required accented character. Data from size rules logic SRL is applied to an address counter AD to address the character read only memory C-ROM, the converter multiplexer C/M, a mosaic generator MG and a DRCS random access memory DRCS-RAM.

The parallel-to-serial converter/multiplexer C/M also receives inputs from the mosaic generator MG and the DRCS random access memory DRCS-RAM, and affords a dot data output DD to attribute logic AL in conventional manner. The character and attribute data are then applied to a colour look-up table CLUT which together with its foreground colour F-C and background colour B/C inputs produces the relevant red, green and blue outputs RGB in analogue form for display.

As has already been mentioned, the character generator of Figure 3 requires a character read only memory C-ROM which is 25% less than that of EUROM and in addition has simplified processing in that, for example, the lookup table required in the microprocessor for driving EUROM is eliminated.

However, it is contemplate that the storage capacity of the character read only memory C-ROM may be reduced even further. It will be appreciated from Table C in Figure 2, that the "smoothed" mosaic characters in vertical columns 5 and 6 are the inverse to those in vertical columns 1 and 2. Thus, it is only necessary to store the "smoothed" mosaic characters of vertical columns 1 and 2 and provide an inverter when the shapes of vertical columns 5 and 6 are required. It is also envisaged that the block mosaic characters of vertical rows 3, 4, 7 and 8 of Table C of Figure 2 may be generated using suitable hardware such as the mosaic generator MG.

The combined effect of these two measures would be to further reduce the storage capacity of the character read only memory C-ROM of Figure 3 to 288 (12 x 10) characters, i.e. 2 x 128 characters corresponding to Tables A and B in Figure 2 and 32 characters corresponding to the "smoothed" mosaic characters of vertical rows 1 and 2 of Table C of Figure 2.

Although in the foregoing embodiment, the invention has been described as applied to character generators for decoders for videotex data transmissions, it should be appreciated that the invention is equally applicable to decoders for teletext data transmissions.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

CLAIM(S)

1. A character generator for receiving a composition coded character signal, the character generator including memory means for storing a plurality of unaccented alphabet characters and a plurality of character accents, characterised in that said character generator additionally comprises processing means responsive to the composition coded signal for separately selecting an unaccented alphabet character and a character accent and for effectively combining these to afford a required accented character.

2. A character generator as claimed in Claim 1, characterised in that the memory means takes the form of a read only memory.

3. A character generator as claimed in Claim 1 or 2, characterised in that the character generator comprises multiplexer means to which the composition coded character signal is applied, and in that the output from said multiplexer means is applied to said memory means which is accessed twice during a character period to produce said required accented character.

4. A character generator as claimed in Claim 3, characterised in that the composition coded character signal includes a character accent code and a character code which are successively applied to said memory means by said multiplexer means, and in that first and second latch means are provided operable on the output of said memory means for respectively latching the characters corresponding to said character accent code and said character code.

5. A character generator as claimed in Claim 4, characterised in that the character generator includes a parallel-to-serial converter and multiplexer operable on the outputs from said first and second latch means and for affording dot data information corresponding to the latched characters.

6. A character generator as claimed in any of the preceding claims, characterised in that said memory means additionally stores unaccented upper-case characters of reduced height for combining with accents which appear above an upper-case alphabet character.

7. A character generator substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings

8. A decoder for videotex or teletext data transmissions incorporating a character generator as claimed in any of the preceding claims.

9. A method of generating accented characters comprising the step of receiving a composition coded character signal, storing a plurality of unaccented alphabet characters and a plurality of character accents, characterised by the further steps of selecting an unaccented alphabet character and a character accent in dependence upon said composition coded character signal, and combining these to afford a required accented character.

10. A method of generating accented characters substantially as hereinbefore described with reference to Figure 2 or Figures 2 and 3 of the accompanying drawings.