CA1219334A

CA1219334A - Teletext decoder using a common memory

Info

Publication number: CA1219334A
Application number: CA000468168A
Authority: CA
Inventors: Paul D. Filliman
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1983-11-29
Filing date: 1984-11-19
Publication date: 1987-03-17
Also published as: DE3443630A1; HK26493A; JPS60134685A; JP2608398B2; GB8430039D0; FR2555849A1; FR2555849B1; DE3443630C2; KR850003650A; GB2150798B; GB2150798A; US4595951A

Abstract

ABSTRACT
TELETEXT DECODER USING A COMMON MEMORY
A teletext decoder extracts digital information from a video signal for displaying graphics and textual information embedded in the video signal. The decoder includes a prefix processor (24) responding to commands supplied by a keyboard (26) operated by a user for selecting and storing the pertinent embedded information.
The decoder includes a common memory (28) for storing the digital words provided by the prefix processor, and a microcomputer (25) capable of reading from and writing to the common memory. The microcomputer (25) reads the data provided by the prefix processor, converts it to digital words representing the picture elements and stores the converted digital words in the common memory (28). The decoder includes a display processor (43) which reads the converted digital words from the common memory to drive an image display device. The times of access by the microcomputer (25), prefix processor (24) and display processor (43) to the common memory (28) are controlled by a timing unit (29) and a switch (32) which give the display processor (43) highest priority of access, and give the prefix processor (24) priority over the microcomputer (25).

Description

~2~933~
-1- RCA 80,003 This lnvention relates to a decoder for teletext-like signals.
Teletext or videotex is a means of transmitting textual and graphical lnformation by digitally encoding the information for transmission. The specific manner of encoding may vary somewhat depending on the system or standard used. In a teletext digital transmission, the digital code is incorporated into a television signal.
1~ In a videotex transmission the digital code is incorporated into a signal transmitted via the public switched telephone network. In this specification and in the claims "teletext-like" is used as a generic term for teletext and videotex.
In telete~t, a TV scan line is utilized for 15 broadcasting textual and graphical information encoded in a digital binary representation. Teletext may be sent during the vertical blanking interval, when no other picture in-formation is sent. The teletext binary information includes control and display digital information serially organized 20 in data blocks. The organization of the binary information in the broadcast signal is determined by the standard employed by the broadcaster. In the following description, reference is made by way of example to the proposed NABTS
(North American Broadcast Teletext Specification) which has 25 been described in ~he article 'Tele text Standards in North America" by B Astle in RCA Engineer Sept/Oct 1983.
In the NABTS each horizontal line containing teietext data is referred to as a dataline and contains data packets. The binary data in the packet is divided into 30 bytes, each byte includes eight binary units (bits). The first eight bytes of each packet are collectively known as the packet header. Three bytes of the packet header define the channel number and each channel is organized into pages.
Each page is made up of a number of packets.
After its reception by the television receiver, the digital data included in the video signal is processed by the teletext decoder. Then the digital data is extracted from the video signal by a data slicer providing a stream ~ . ' j., 121~33~

-2- RCA 80,003 1 of bits to a data processor (sometimes referred to as the prefix processor). The data processor receives user-initiated commands specifying the information to be displayed. The data processor stores, in a buffer memory, the data contained in the teletext channel selected for display.
The huffered data is processed and provided to a display processor which outputs the displaying signals. When a television picture tube (CRT) is used as an image displaying device, the display processor has to output the displaying signals periodically for maintaining the image on the television screen.
According to the present invention, there is provided ~ decoder cf teletext-like signals containing lS binary data representing control information and display-able information for display by a display device cor,lprising:

a common memoryarrangement for storing binary data;
means for deriving from the teletex-like signals preselected data for storage in the memory;
processing means for processing the stored preselected data to produce processed data, for storage in the memory;
a display processor responsive to the stored processed data to produce signals for causing the display to display the said disDlaycble informa~ion;
switching means for selectively coupling the deriving means the processing means and the display processor to the common memory arrangement and timing means for controlling the switching means so that the deriving means, th~ processing means and the display prccessor are coupled to the memory arrangement during the time slots of a predetermined sequence of time slots.

~21933~

-3- RCA 80,003 1 In an embodiment, there is provided a decoder of teletext-like signals containing picture information comprising a data processor for obtaining a digital message received from the teletext-like signals.
A common memory is used for storing the digital message for further processing by a microcomputer. The microcomputer reads out the stored data, processes it and stores it in the common memory. A display processor reads the processed data from thecommon memory and generates driving signals for a displaying device. A switch directs data between the common memory and each of the microcomputer, data processor and display processor. The switch operates under the control of a timing unit. The timing unit makes the memory available for access as required by the microcomputer, the data processor and the display processor.
For a better understanding of the invention, reference will now be made, by way of example, to the drawings, in which:
FIGURE 1 is a schematic block diagram of an illustrative teletext decoder embodying the invention, FIGURE 2 is a schemati~ i'lustration of memory storage allocation for a common memory of the decoder FIGURE l;
FIGURES 3a, 3b are diagrams of the utilization of the common memory in various time slots, and FIGURE 4 is a bl~ck diagram of an illustrative embodiment of a switch used in the decoder of FIGURE 1.
The teletext decoder in FIGURE 1, receives a video-modulated signal at input 20 of a television processor 21. Processor 21 includes such well known television receiver stages as the tuner, the intermediate frequency amplifier and the video detector. Data slicer 22 receives the detected video from television processor 21 for detection a~d separation of the teletext binary data.
Data slicer 22 generates ~Z~933~
_4_ RCA 80,003 horizontal and vertical sync signals along signal lines H
and V respectively synchronized to the incoming composite video signal. ~ata slicer 22 provides a serial data stream and a reconstituted clock on lines 70 to a data processor 24 such as the conventional prefix processor of a teletext decoder. The reconstituted clock is used to synchronize the teletext system clock developed by data processor 24 and distributed to various stages within the decoder.
User-initiated commands are coupled to data processor 24 by a microcomputer 25. By operating a keyboard 26, the user selects the magazine and page number to be displayed. Microcomputer 25 receives the user selected data from keyboard 26 along a signal line 42 and issues a 12 bit word to data processor 24 on select lines 47. This word signifies the required NABTS defined packet address.
After the occurrence of horizontal sync, the data processor begins searching for the presence of the NABTS-defined framing code in the serial data stream received from data slicer 22. If a valid framing code occurs, data processor 24 begins pac~ing the serial data stream into 8 bit units called bytes. Data processor 24 processes the next 3 bytes to obtain the packet address.
Microcomputer 25 provides a 12 bit word to data processor 24 on lines 47 for specifying the required packet address.
When a match accurs between the required packet address and the packet address of the incoming teletext data, data processor 24 begins the transfer of all the subsequent bytes included in the NABTS-defined data packets to a common memory 28 of the teletext decoder at time slots controlled by a timing unit 29.
Data words are transferred to memory 28 from data processor 24 using a two-step process. In the first step, an address word is transferred from an address port 30 of data processor 24 on lines 31 to a port E of a switch 32. ~e timing unit 29 provides timing signals 54 to control switch 32 toconnect switch lines S to contact port E

~2~933~
_5_ RCA 80,003 for transferring the address words to a buss 33 by way of a port G. Buss 33 may be made of 16 lines to define a 16 bit buss. From buss 33 the address word is transferred on lines 36 to an input port 34 of an address latch 35. The address word is stored in address latch 35, and an output port 37 transfers the stored address word to a memory address port 38 for selecting the location in memory 28 to which the transfer of the teletext word is directed.
In the second step, a data word is transferred on lines 40 from a port 39 of data processor 24 to a port F of switch 32. Timing unit 29 controls switch lines S of switch 32 for transferring the data to the same buss 33.
Buss 33 directs the data to memory data port 41. The data word is then stored in memory 28 in the location selected by the stored address word of address latch 3S.
A data word transferred to a memory location while applying a certain memory address, may be transferred from the same location by applying the same - address at a later time.
Common memory 28 is time-shared by microcomputer 25, data processor 24 and a display processor 43. Time sharing of common memory 28 is controlled by the timing unit 29. Timing unit 29 assigns a time slot for each access to common memory 28. FIGURES 3a and 3b illustrate the assignment of time slots for the decoder illustrated in FIGURE 1. Each time slot for the decoder described in FIGURE 1 has a duration of 349 nanoseconds.
The two-step process in which a data word is transferred through switch 32 is accomplished in one time slot. An access to common memory 28 by data processor 24 defines the data processor access time slot. Likewise, such an access by display data processor 43 defines the display processor access time slot, and such an access by microcomputer 25 defines the microcomputer access time slot. The time slots are implemented in a nonoverlapping manner, such that only one of microcomputer 25, display processor 43 and data processor 24 may have access to common memory 28 during any one time slot. Access to co~mon lZ~933~
-6- RCA 80,003 memory 28 is accomplished by transferring digital words through switch 32. When switch 32 provides access to commom memory 28 for one of data processor 24, microcomputer 25 and display processor 43, it excludes the other two from access to common memory 28.
Timing unit 29 provides timing signals 54 to control switch 32, timing signals 55 to control the timing in microcomputer 25 and timing signals 56 to control the timing in display process 43. By means of these timing signals timing unit 29 assigns every other time slot for access to memory 28 by display processor 43. ~e inte~ening time slots not assigned for such access, are assigned by timing unit 29 to either microcomputer 25 or data processor 24. The decision to assign a time slot to either microcomputer 25 or data processor 24 depends on the status of both at a time determined by timing unit 29.
If data processor 24 is ready to perform a data transfer to common memory 28 in a time slot not assigned to display processor 43, timing unit 29 may provide this time slot to data processor 24 for obtaining access to common memory 28. If data processor 24 is not ready to perform a data transfer to common memory 28 in a time slot not assigned to display processor 43, timing unit 29 may provide this time slot to microcomputer 25 for obtaining access to memory 28, provided that microcomputer 25 is ready to perform a transfer in such time slot. Request lines 59 and 58 respectively indicate to timing unit 29 that data processor 24 and microcomputer 25 require an access to common memory 28.
By using predetermined time slots , it is possible to provide access to common memory 28 in an efficient manner. Each time slot lasts a sufficient period of time to obtain access to common memory 28 using the two-step process. ~ecause display processor 43 obtains an access to memory 28 once every two time slots, it is guaranteed that it receives the required display information at a sufficiently rapid rate to display each picture element at the appropriate place on the scan line.

9~3~
_7_ RCA 80,003 The two-step process for the teletext data access is illustrated in FIGURE 3b as taking place in an access time slot between time Tn~l and time Tn+2 and also in an access time slot between time Tn+g and time Tn+lo.
Data processor ~4 collects 2 by~es of incoming teletext data for storing it in memory 28. As illustrated in FIGURE 3b, this occurs once every eight access time slots so that 2 bytes of data may be loaded to memory 28 every 2.8 microseconds, which is the rate for data received in the NABTS system.
From the time data processor 24 collects 2 bytes of teletext data for storing one data word in common memory 28, until the next byte is obtained, a data processor access time slot is guaranteed to occur, as may be deduced from FIGURE 3b. Therefore, data processor 24 is not required to buffer more than one data word. This aspect of data processor 24 simplifies the design of data processor 24.
Data processor 24 stores each subsequent data word in a consecutive memory address. In doing so, it creates a data processor buffer 201 as illustrated in the schematic arrangement in FIGURE 2 of common memory 28 of FIGUXE 1. This data buffer may be read by microcomputer 25 for further processing, as explained later on. By reading lines 47, microcomputer 25 may ascertain the number of data words transferred by data processor 24 to common memory 28. Line 44 is used to select a data transfer on lines 47 to or from data processor 24.
As illustrated in FIGURE 3a and FIGURE 3b, microcomputer 25 access to memory 28 occurs at time slots occupied by neither data processor 24 nor display processor 43. Microcomputer 25 reads data processor buffer 201, located in memory 28, and transfers its contents to a different group of locations, a page storage buffer 202 of FIGURE 2 in common memory 28 of FIGURE 1.
Page storage buffer 202 is used for storing the teletext data corresponding to the most likely pages the user may request. ~or example, the preceding page is ~ ~933~
-8- RCA 80,003 likely to be requested by the user. By storing it in buffer 202, the decoder may provide quick response to a user for the preceding page because the preceding page is already stored in buffer 202 at the time the user initiates such request.
After transferring the teletext data of data processor buffer 201 to page storage buffer 202, microcomputer 25 processes page storage buffer 202 and stores the results in a different set of locations in common memory 28 called a display bit map 203, illustrated in FIGURE 2.

The actual data word transfer between microcomputer 25 and memory 28 is also perfo~med by a two-step process. In the ~irst step, an address word is transferred from an address port 45 of microcomputer 25 on lines 46 to a port A of switch 32. Timing unit 29 controls switch 32 for transferring the address word to buss 33. From buss 33 the address word is transferre~ on lines 36 to input port 34 of address latch 35. The address word is stored in address latch 35. Output port 37 transfers the stored address word to memory address port 38 for selecting the location in memory 28 to which the transfer of the teletext word is directed.
In the second step, microcomputer 25 performs either a transfer to or a transfer from memory 28. If a transfer to memory is required, a data word is transferred on lines 47 from a data port 48 of microcomputer 25 to a port B of switch 32. Timing unit 29 controls switch 32 for txansferring the data word to buss 33. Buss 33 directs the data to memory data port 41. The data word is ~Z~9334 -~- RCA 80,003 then stored in memory 28 in the location selected by the stored address word of address latch 35.
On the other hand, if a transfer from memory 28 to microcomputer 25 is required, a data woxd is transferred from memory data port 41 to buss 33 and from there to port B of switch 32 under the control of timing unit 29. From port B of switch 32, the data word is transferred on lines 47 to microcomputer data port 48.
As may be inferred from the previous discussion, FIGURE 3a illustrates the access time slots of transfers from microcomputer 25 to memory 28 in a situation where teletext data is not transferred by data processor 24 to data pro~essing buffer 201. In this case, alternate time slots are allocated to microcomputer 25. However, it may happen that microcomputer 25 will attempt to address memory 28 at a time not assigned for microcomputer 25 data access. When this happens, microcomputer 25 is held at a wait state until the next available microcomputer 25 data access time slot. Microcomputer MC68000, made by Motorola Inc., Phoenix, Arizona, for example, has a built-in capability to enter such a wait state in response to an appropriate input signal.
FIGURE 3b illustrates microprocessor access time slots to memory 28 in a situation where teletext data is being transferred by data processor 24. In this case, microcomputer 25 is assigned only those time slots that are assigned neither to display data access nor to tèletext data access. A display data access time slot is assigned every alternate access time slot and a teletext data access time slot is assigned one time slot in every eight access time slots.
In a situation when data processor 24 is performing an access to common memory 2B, microcomputer 25 has to wait its turn for access when a data processor access time slot is given priority. Except for such a waiting time caused by the priority given to data processor 24, microcomputer 25 continues to operate without waiting delays.

~2~9~3~

-10- RCA 80,003 The concept of preassigning alternate time slots for display processor 43 and allocating the remaining time slots for data processor 24 and microcomputer 25, as carried out by timing unit 29, results in an efficient sharing of common memory 28 and a teletext decoder that is capable of fast processing of full field teletext data.
The transfer of a data word from common memory 28 to display processor 43 is similar to the transfer to microcomputer 25 from common memory 28. In this case, as illustrated in FIGURE 1, an address word is provided from an address port 83 of display processor 43 and the data word is received at a data port 81. The address word is coupled to a port C of switch 32 and the data word is coupled from a port D. Timing signals 56 from timing unit 29 provide timing signals to control operation of display processor 43. Data transfer is performed in a similar way to the two-step process employed for transferring a data word from common memory 28 to microcomputer 25. An access to common memory 28 requires the two-step process for the embodiments of FIGURE 1 because buss 33 is used for transferring both address and data words. It may be understood that the access operation accomplished by ~he two-step process may also be accomplished by a one-step process in other variations where address words and data words are provided to a common memory on separate busses.
Timing unit 29 provides display processor 43 with the highest priority for obtaining access to memory 28 in that it provides a 349 nanosecond display processor access time slot in every period of 698 nanoseconds irrespective of the status of microcomputer 25 and data processor 24. Furthermore, as explained before, a time slot not used by display processor 43 is given to data processor 24 if it has a data word ready for transfer, and to microcomputer 25 if data processor 24 does not require a transfer to memory 28.
Display processor 43 of FIGURE 1 reads 4 pixel data words each time it is provided with an access to common memory 28. Each pixel word includes 4 binary bits.

121~334 ~ RCA 80,003 T~erefore, a 16-bit wide memory ~ord is u6ed to provide the 4 pi~el words over a 16-bit bus 33 in one ~cces~ time clot. Di~play proce~sor 43, in the embodiment of FIGURE
1, is provided with an access to common memory 28 in altcrnate time slots from the 6equence of consecutive time slots provided by timing unit 29.
Display processor 43 ~ay be required to provide display 49 with pixel infor~ation at a ~ufficiently rapid rate for di6play in display 49. In accordance with sne aspect of the invention, the capability of rapid rate display is obtained by having display processor 43 fetch or read a plurality of pixel data words in each access to common memory 28. Illustratively, it may read 4 pixel words included in each memory word.
Display processor 43 translates each 4-~it pixel word to a color code, illustratively comprising 3 groups of 3 bits to a group and a transparency codel illustratively comprising one bit. The groups of the color code determine the value of separate red, green and blue analog ~ignals respectively. These three analog signals are coupled to a port TELETEXT of a switch 50. A
second port TV of switch 50 provides a different set of red, green and blue signals provided by a luma/chroma stage 52, of conventional design, 52 which receives the video signal from television processor 21.
Switch 50 couples the signals from its port TELETE~T. Alternately, it couples the signals from its port TV, according to the digital code of the transparency code translated for the pixel. Therefore, the transparency code associated with a pixel word causes, according to its digital code, that display 49 displays either telete~t information from display processor 43 or, alternatively, other ~ideo information such as the conventional television picture from television processor 21. This capability of the transparency code may be of use, for example, in captioning. An advantageous way of processing pixel color codes and the transparency code is described in copendina P~tent Application 468169 121933~
-12- RCA 80,003 inventor P. D. ~illiman, entitled A TELETEXT DECODER
HAVING A REGISTER ARRAY FOR OPERATING O~ PIXEL ~ORDS, concurrently filed ~erewith.

Switch 32, illustrated in FIGURE 1, ~ay also be implemented using a buss approach a~ illustrated in FlGURE

4. In FIGURE 1 and FIGURE 4, identical numbers identify the ~ame functions. The circuit included within the dashed-line in ~IGURE 4 represents switch 32. A driver 424, 425, 426, 4 7 or 428 may drive buss 33 under the control of timing signals 54 of timing unit 29. Timing unit 29 provides t~at only one driver drives buss 33 at a time to obtain a valid transfer of a digital word.
If a digital word transferred across buss 33 has to stay on it for a shorter period of time than required by the receiving device, a storage element such as a data register 429, illustrated in FI5~E 4, should be introduced to save the transferred digital word until the the device is ready to read the word. Such configuration may be used for reading a data word to microcomputer 25.
Using thi~ approach, it is possible to allocate a shorter ti~e slot for transferring digital words across buss 33 than in a situation where switch 32 has to stay in the same state until microcomputer 25 reads the data word.

~21933~

-13- RCA 80,003-In the illustrative embodiment of the invention descrihed hereinbefore the microcomputer 25 is used to control the data processor 24 for selecting the information to be stored in the memory 28. The microcomputer issues control signals in response to a user-initiated command.
The microcomputer also performs the required data processing of the buffered data by reading the memory 28 to obtain the buffered data, by performing the required operations on it and by storing the processed data in the same memory, but not necessarily in the same locations where the buffered data resis~le. The microcomputer also uses the same memory for storage and retrieval of intermed iate results and of status information.
Because microcomputer 25 is, in effect, a general purpose microcomputer, it may perform tasks unrelated to teletext signal ~lecoding. To perform these tasks, micro-computer 25 may use a scratch-pad 204 storage space of memory 28, as illustrated in FIGURE 2. The microcomputer 25 may additionally perform signal processing on signals from the keyboard.
In the embodiment the time-shared common memory 28 is used for buffering the incoming data, for providing a work space for the microcomputer 25 and for providing access to the display processor 43. Because only one memory is used, a simplified interconnection is achieved. This lends to a cost effective utilization of the storage space required by the teletext decoder.
In the embodiment a timing unit 29 provides the timing sisnals 54 to operate the microcomputer 25, the data processor 24 and the display processor 43 and to operate a switching means 32 which provides access to the common memory for each of the microcomputer, the data processor and the display processor. The timing unit makes the memory available for access, as required by the display processor, the microcomputer and the data processor.
The timing unit defines consecutively recurring time slots.
The time slots occur in a predetermined regular time interval. An access to the common memory is ~IZ19334 -14- RCA 80,003 1 accomplished by providing an address word to the memory and by transferring a data word either to or from the location defined by the address word. The timing unit provides an access to the common memory during the time slot and only one access may occur in each time slot.
The sequence of consecutive time slots is independent of real time operations in the microcomputer, the data processor and the display processor so that if the data processor, for example, requires an access to the common memory, its access timinghas to "fit" the predetermined timings of the time slot.The assignment of each time slot to the data processor, the microcomputer or display processor, is under the control of the timing unit.
The timing unit allocates a predetermined order of time slots for the exclusive usage of the display processor.
The timing unit provides the timing signals to the display processor such that its timings for access coincide with the time slots allocated exclusively for its usage. A
time slot is allocated according to a priority scheme. Sim-ultaneous requests for access to the common memory arehandled by the priority scheme which determines the assignment of each time slot prior to the beginning of that time slot. Therefore, the arbitration in this decoder is accomplished synchronously with those time slots not preassigned to the display processor.
In the embodiment a digital word which is stored in the common memory 28 for the display processor 43 includes more than one pixel word. A pixel word provides information to the display processor for displaying one picture element or pixel. The display processor reads the pixel words included in the digital word during the display processor access time slot.

Claims

-15- RCA 80,003 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A decoder of teletext-like signals containing binary data representing control information and display-able information for display by a display device comprising a common memory arrangement for storing binary data;
means for deriving from the teletex-like signals preselected data for storage in the memory;
processing means for processing the stored preselected data to produce processed data, for storage in the memory;
a display processor responsive to the stored processed data to produce signals for causing the display to display the said displayable information switching means for selectively coupling the deriving means the processing means and the display processor to the common memory arrangement and timing means for controlling the switching means so that the deriving means, the processing means and the display processor are coupled to the memory arrangement during the time slots of a predetermined sequence of time slots.

2. A decoder according to claim 1, wherein the switching means selectively couples each one to the exclusion of the other two, of the deriving means, processing means and display processor to the memory arrangement.

3. A decoder according to claim 2 wherein the timing means provides timing signals to control said switching means, said timing signals defining a recurring first access slot wherein access to said memory arrangement is provided for said display processor, a recurring second time slot wherein access to said memory arrangement is provided for said -16- RCA 80,003 processing means and a recurring third access time slot wherein access to said memory arrangement is provided for said deriving means, with the access time slots for said display processor being provided at predetermined time intervals.

4. A decoder according to claim 2, wherein the timing means is arranged to cause the switching means to couple the deriving means the processing means and the display processor to the memory arrangement according to a predetermined order of priority.

5. A decoder according to claim 4, wherein the display processor has the highest priority.

6. A decoder according to claim 5, wherein the display processor is coupled to the memory arrangement in every other time slot

7. A decoder according to claim 5 wherein the deriving means has higher priority than the processing means.

8. A decoder according to claim 7, wherein the deriving means and processing means are arranged to produce request signals indicative of requiring access to the memory arrangement and the timing means is responsive to the requests according to the order of priority.

-17- RCA 80,003

9. A decoder according to claim 1 or 2 wherein the memory arrangement comprises a common memory having an address port and a data port for transferring data into and out of the memory and a plurality of memory locations; and addressing means having an input port for receiving a memory address word, and an output port for coupling the received memory address word to the memory address port and wherein the switching means selectively transfers data from the processing means to the memory, data from the data processor to the memory, data from the memory to the processing means and data from the memory to the display processor, locations in the memory or to from which data is transferred being determined by memory addresses which are provided at the output port of the addressing means at the times transfers take place, so that data transferred from the processing means or from the data processor to a memory location while applying a certain memory address at any one time can be transferred at a later time to the display processor or to the processing means from the same memory location by applying the same certain memory address.

10. A decoder according to claim 1 or 2 further comprising a data store for storing data during transfer between the memory arrangement and at least one of the deriving means processing means and the display processor via the switching means.

11. A decoder according to claim 1 or 2 wherein said switching means comprises a buss coupled to said memory arrangement having a plurality of signal lines and a plurality of data drivers providing data words to said buss, said data drivers receiving address and data words from said processing means data processor and display processor and being responsive to the timing means for providing the addresses and data words to the buss -18- RCA 80,003 during respective time slots assigned to said data drivers, so that only one data driver is capable of driving said buss at any one time.