GB2268864A

GB2268864A - Teletext system

Info

Publication number: GB2268864A
Application number: GB9314646A
Authority: GB
Inventors: Brian Gill
Original assignee: Plessey Semiconductors Ltd
Current assignee: Plessey Semiconductors Ltd
Priority date: 1992-07-17
Filing date: 1993-07-15
Publication date: 1994-01-19
Also published as: GB9314646D0; GB9215281D0

Abstract

A television type information display system having an accent conversion capacity, is provided with means for checking the existence of data indicative of accented characters in memory, and means for suppressing the over-writing of non-accented characters in the memory in place of accented characters, to avoid the alternating of non-accented and accented characters and consequent "blinking". <IMAGE>

Description

INFORMATION DISPLAY SYSTEM This invention relates to a telelvision type information display system such as that known generally as teletext.

When teletext data is received by a decoder, it is normally written to some form of memory device with a simple mapping system related to the display system. The code structure used is the ASCII standard to identify to the display system which character is required. Each page data is re-received usually once per cycle of the database.

When an accented character is required at a given location on the display, a simple form of the character is usually transmitted at the same location for decoders without the accent conversion capability. The data required to form the accented character is transmitted using non-display data packet X/26 or on a pseudo page.

Control software must then read the accented character conversion data and write the Read-Only-Memory (ROM) address for the required accented character into the required location in display memory.

A display page is usually cleared to all spaces before the teletext data is written to memory the first time it is received.

All teletext data intended for display is received in seven bit plus odd parity format. The seven data bits, which are numbered zero to six, are written to memory if the parity check is correct.

The parity bit numbered seven (the most significant bit) is not set in the memory. Usually all accented characters are addressed in ROM with bit seven set high.

In each database cycle, the page in its simple form without the accented characters is re-received and the accented character is over-written. The control software must then replace the transmitted character with the required accented character. Because the control software can require many display frames to process all the information for a typical page, this causes the display of the accented character to 'blink' each time the page is re-received, which is undesirable.

According to the invention in its broadest aspect a television type information display system having a memory in which data representative of each page of information to be displayed is temporarily stored, and a display unit for converting said stored data into display signals for displaying the information on a television screen, the system being provided with an accent conversion capability, incorporates means for checking the existence of data indicative of an accented character in the memory, means responsive thereto for suppressing the writing of data indicative of a non-accented character into the memory in place of said accented character, and means for re-writing said character into the memory in the accented form.

With such an arrangement as there is no overwriting of accented characters by non-accented characters "blinking" of the characters on the display is avoided.

Thus where the value of a bit (hereinafter called the protection bit) in a byte of the stored data is used to indicate the existence or otherwise of an accented character, the display means in accordance with the invention includes means for checking the presence of a protection bit set high in the data byte at each location, and means responsive thereto for suppressing the writing of non-accented characters in a page of information into the memory when a protection bit set high is detected.

On the other hand if no protection bit set high is detected the information is written into the memory for display.

Thus, for use with a teletext system as above described utilising a seven bit plus odd parity bit format, the display system will include means for checking the existence of bit seven set high in the data byte at each location, and means responsive thereto for suppressing the writing of non-accented characters in a page of information into the memory when a bit seven set high is detected, whereas if no partiy bit seven set high is detected the information will be written into the memory for display.

However any bit of a data word could be selected to form the function of a protection bit, so that the invention can also be used in systems in which the protection bit is other than bit seven of an eight-bit data word, as well as those a greater number of data word bits.

The invention will be further explained by describing, by way of example with reference to the accompanying drawing, the relevant part of one form of teletext display system embodying the invention.

Thus referring to the illustrated flow chart, in step 1 a buffer circuit receives one byte of information in coded form representative of transmitted data which is to be displayed on a television screen. The coded information consists of a series of 8-bit bytes, bits zero to six corresponding to the data to be displayed, and bit seven providing a parity check. The seven-bit data is written into a memory circuit if the parity check is correct, but the parity bit is not set in the memory. The bytes are subsequently fed to a display system in the form of a complete page of information via a further memory circuit for display on the television screen utilising known circuitry. Accented characters are represented by bytes having bit seven set high.

The figure represents a flow chart indicating a decision to write data to memory if the most significant bit (msb), bit seven, at the address location is not set high, or to ignore the data if bit seven at the addressed location is set high.

In step 1 the buffer circuit receives one byte of data in coded form representative of transmitted data which is to be displayed on a television screen. Whilst incoming data is held in the buffer during step 2, one byte of data at the addressed location is read by circuitry from the page Random Access Memory (RAM) and checked in step 3 by decision logic to see if bit seven is set high.

If bit seven at the addressed location is not set high, the incoming data temporarily held in the buffer circuit, is written to the RAM (step 4).

On the other hand if bit seven at the addressed location is set high, (step 2) no further action is taken, so that the data already in RAM is not changed.

It will be seen that the alternate display of non-accented and accented information, as is customary the case in existing systems, leading to-a "blinking" of the accented characters, is avoided.

Claims

1. A television type information display system having a memory in which data representative of each page of information to be displayed is temporarily stored, and a display unit for converting said stored data into display signals for displaying the information on a television screen, the system being provided with an accent conversion capability, incorporating means for checking for the existence of data indicative of an accented character in the memory, means responsive thereto for suppressing the writing of data indicative of a non-accented character into the memory in place of said accented character, and means for re-writing said accented character into the memory in the accented form.

2. A television type information display system in accordance with Claim 1 wherein the value of one bit of each byte of stored data is used to indicate whether the character represented by that byte is to be displayed as an accented character, and the means for checking for the existence of data indicative of an accented character is arranged to check the value of said one bit.

3. A television type information display system in accordance with Claim 2 wherein each data byte is received in seven bit plus parity bit format, the parity bit not being stored in memory, the eighth bit at each storage location in the memory being set to a logic zero for a data byte representing a non-accented character and being set to a logic one for a data byte representing an accented character.

4. A display system substantially as hereinbefore described with reference to the accompanying drawing.