Note: Descriptions are shown in the official language in which they were submitted.
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DECODER FOR DIGITAL INFOR~A~ION IN ~.V. SIGNAL
The present invention relates to television
systems and more particularly to a method of apparatus
for decoding digital information processed for inclusion
in a wide band T.V. video signal.
In any normal television system, the transmission
of the wide band video signals which are to produce the
actual picture elements on the screen of the receiver is
interrupted between the scanning periods for line and
field synchronization purposes. Consequently, there are
periods during which no video signals are being trans-
mitted. It is now possible to use these periods for the
transmission of data which is not necessarily concerned
with the video transmission itself.
Basically, data representable by standard
symbols such as alpha-numeric symbols can be transmitted
via a restricted channel provided that the rate of
transmission is restricted. It is now possible to use
periods as aforesaid especially the line times of the
field blanking intervals (i.e. the times of the individual
lines occurring between fields which correspond with the
times occupied by video signals on active picture lines),
for the transmission of pages of data. Typically, using
8-bit digital signals representing alpha-numeric characters
(7 bits of data plus 1 bit for protection) at a bit rate
of 2.5~ bit per second, 50 pages of data each consisting of
22 strips of 40 characters can be transmitted repeatedly
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in a total cycle time of 90 seconds using only a single
line of the field blanking period per field of the 625
lines system as operated in the United ~ingdom.
Data transmission as described above is already
commercially available in the United I~ingdom under the
name "Teletext"9 and transmitters and receivers are
described in more detail in our U.X. Patents Nos.
1,486,771; 1,486,772; 1,486,773 and 1,486,774.
~xisting teletext displays consist of 40 characters
per row and 24 rows per page. The U.K. teletext transmission
standard specifies a data rate of 6.9375 Mbits per second
(which has proven to be at the upper reasonable limit of
transmission rate for system I, B/G system) so as just to
achieve transmission of a complete row of text on ore
15 video line of the field blanking time.
The advantage of conveyins one row of text on one
video line is to achieve maximum economy in requirements
for transmission of addressing information needed t~
correctly position the text information on the displayed
page. Since whole rows of text are transmitted on each
line1 only a row number need be transmitted Witil each
data line of text. Row zero which acts as the page
demarcation signal requires additional page numberirg
information and also incorporates various display ;.nd
interpretation codes appropriate to the particular page.
In order to facilitate parallel magazine working every row
of text also incorporates a 3-bit magazine number~ being
the most signifi~ant digit of tha page number.
The above structure incorporating as it does one
text row on every data line thus results in a very el`ficient
utilization of the transmission facility. However, the
existing Teletext transmissions do have limitations in
so far as they arc less satisfactory when il1 a "graphics"
modc as comparcd with an "alp71a-numcric"mode.
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An object of an aspect of the present invention
is to provide a system of digital transmission which
will produce improved pictures when in the "graphics"
mode while being compatible with existing U.K. Teletext
transmissions. The type of graphics in which we are
interested is usually termed alpha-geometrics but in
this specification is called high resolution graphics
(HRG).
An aspect of this invention is as follows:
In a television system including input means
for receiving a demodulated television signal carrying,
during picture-free periods thereof, a set of pages
of information represented by serial binary signals
provided in a repeated cycle and at least some of the
binary signals representing coded instructions, the
form of the cycle being such that the binary signals
for the set of pages received are in groups, which
groups are received each in a different one of said
periods and the whole of the information for the set
of pages is received during each occurrence of the
cycle; a parallel binary memory device connected with
the input means for storing binary signals representing
a page of information processing means for controlling
the system and connected with the input means and the
parallel binary memory device; selector means connected
with the processing means for generating a desired
page address code and activating the processing means
to effect storage of the binary signals representing
the page; further storage means connected with the
processing means and the parallel binary memory device
for storing a plurality of routines for use by the
processing means; the processing means being arranged
to access a received coded instruction in the parallel
binary memory device and accessing the further storage
means in order to carry out the required instruction;
signal generator means connected with the parallel
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binary memory device for processing the signals stored
in the parallel binary memory device to produce output
signals Eor visual display; and display means connected
with the signal generator for visual display of the
signals produced by the signal generator, the improvement
comprising:
(a) a timer means connected with said process-
ing means for enabling the same to control the timing
of the processing of the incoming coded instructions;
(b) inhibiting means connected with said
signal generator means and said processing means and
operable by said processing means to prevent operation
of the said signal generator; and
(c) auxiliary storage means connected with
said signal generator means, said parallel binary memory
device, said further storage means and the said processing
means for receiving signals representing a visual image
from at least one of the processing means, signal gener-
ator and the parallel binary memory device, said processing
means and timer means being arranged to effect and
control the input of signals from said at least one
of the processing means, signal generator and parallel
binary memory device to the auxiliary storage means,
said auxiliary storage means having an output connected
with said display means, thereby to display the contents
of said auxiliary storage means.
In a preferred form, the picture to be displayed
is notionally divided into a number of vertical and
horizontal locations hereinafter termed "pixels" and
these pixels are utilized rather than the character
locations as in existing Teletext. Thus the coded
instructions relate to pixels rather than to character
locations. The
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pixels are preferably square.
In order that the present invention be more
readily understood, an embodiment thereof will now be
described by way of example with reference to the accom-
panying drawings, in which:-
Figure l shows the construction of data as itwould be assembled if it were displayed;
Figure 2 shows the construction of a typical
data word to be transmitted using the new system;
Figure 3 shows a block diagram of a receiver
including a decoder of decoding data of the form shown
in Figure l;
Figure 4 shows a flow chart for the micropro-
cessor shown in Figure 3;
Figure 5 shows in diagrammatic form a display
before and after rounding;
Figure 6 shows apparatus for carrying out the
rounding shown in Figure 6;
Figure 7 shows typical control signals for
part of the circuit shown in Figure 6;
Figs. 8A and 8B constitute a Table showing the
commands and the bits required for defining the useful
number of options.
As was mentioned above, a conventional Teletext
data transmission will transmit the data required for
forty characters in the line period of the broad band
television signal. Thus, with 8-bit words being trans-
mitted, it is possible to transmit 8 x 40 bits of inform-
ation in each line time of the field blanking interval.
This is done in the present system for improved graphics.
It is conventional to send a page header as
the first row of information. With the present system
it is necessary to send a code identifying the fact that
the new system is being used so that the decoder can
take the appropriate action. The code can be a range of
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page numbers not included in or recognizable to
conventional Teletext or an additional code. If an
additional code is used, it can be included in the
data for the header or it can, as shown in Figure 1,
S occupy the first two eight bit locations of the second
row of data transmitted. The additional code can be
any code sequence which will identify the new system
and the codes used in conventional Teletext for the
letters q and z have been chosen because it is
considered unlikely that this particular combination
will ever appear in practice. Following this additional
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code, a further 8-bits of info-rmation are provided
for telling the new system how much information to
expect in terms of pages of transmitted data. Thus,
the first 4-bits will indicate the actual page number
while the second 4-bits indicate the total number of
pages to expect. If necessary an additional lead-in
code can be provided before the page data in order
to provide an additional check.
The system of the present invention provides
data words of variable length rather than of fixed
length of 8-bits and each of the new data words consists
of a lead-in section, a command section and an argument
section as shown in Figure 2. These new data words will
be called HRG words. However, the lead-in section
always consists of the same sequence and number of bits,
in this case a 5-bit sequence 10101. The command section
is a further sequence of bits whose number determines
the total number of commands available. In this case,
5-bits are used to give up to 32 commands. The length
of the argument is variable and is either implicit in
the command or ends with a known delimiter.
Although the length of the HRG words is variable
it is convenient to continue to utilize the 8-bit word
recognition facility of the conventional Teletext decoder
to provide timing of the start of each HRG word. Thus,
it is convenient to have each HRG word commence at the
beginning of an 8-bit location in a row of data.
The last 8-bits on each row of transmitted
data for rows 1 to 23 inclusive contain a CRC code. It
is convenient to provide an additional CRC code as the
penultimate 8-bits on row 23 so as to check the CRC
codes on the preceding rows. This arrangement means
that execution of the commands has to wait until a page-
of information has been received by the decoder. This,
however, is not necessary but it does provide the presently
preferred mode of operating the system.
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Turning now to the decoder and its mode of
operation, rather than transmitting the data required to
enable the display to produce the desired display for
each character location on each row, the present system
is arranged to have a processor for producing the required
signals for display under the control of the HRG words
transmitted. Further, the system enables the selection
of any point i.e. any pixel of the desired display to be
the starting point because each starting point is defined
in the argument of each HRG word. Thus, the HRG words
are akin to a program being transmitted to the processor
of the decoder. There are preferably 320 pixels across
the display (horizontally) and 240 pixels vertically.
Alternatively, for even better definition there could be
lS 640 horizontally and 430 vertically. ~he exact number
of pixels is not important as long as the numbers involved
are multiples of 40 horizontally and 24 vertically if it
is desired to maintain compatibility with existing
Teletext.
Referring now to Figure 3, the preferred
receiver/decoder arrangement consists basically of a
display device 30, provided with a switching arrangement
31 so as to be able to display either a normal video
transmission or a Teletext transmission which is received
from the usual RF demodulator, a decoder section 32 for
removing the Teletext data from the video signal, an
enhanced graphics section 33, and, if desired~ a user
section 34 to enable a user to directly make use of the
enhanced graphics section and making use of a transmitted
programme.
The decoder section 32 is basically the same as a
conventional Teletext decoder in that it has a data
acquisition circuit 40, a parallel binary storage device
41 for storing a page of Teletext data, and a control
circuit 42 operated by a remote handset 43. It will
therefore not be described in detail since it is largely
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con~entional. I-lowever, the control circuit 42 does
not directly control the page store 411 rather it controls
a microprocessor 45 which in turn controls the storage
device 41.
The arrangement is such that if normal alph~-numeric
characters are to be display, then the mirco-processor 45
will simply enable the storage device 41 to cause it to
store the desired page and, using the conventional character
generator 44 and an isolating switch 44a~ cause the
signals indicati~e of a desired display to be fed to an image
store 48. The signals in the store 48 are read out by
conventional scan and display apparatus 46. Thus9 as far
as the user is concerned, when in the alpha-numeric
mode the receiver is functioning as before. Also, when in
the normal graphics mode the apparatus will function as before.
Howe~er, if the HRG identification code i9 recei~ed, the
microprocessor 45 will alter the operation of the image
production to enable HRG display. Thus, it will firstly
store the incoming HRG words. This is done using either
the storage device 41 alone if there is less than one page
of data or using the device 41 in conjunction with one or
more additional s-torage devices. The number of devices
required is determined by the maximum number of pages
of data which can be identified by the paging code. Since
four bits are used to indicate the total number of pages,
up to 16 storage devices may be required, each capable of
storing one pase of data.
Once a complete page of data is correctly received
the processor commences to execute the con~ands contained
in the HI~G words, using the data contained in the ar~uments
of the HRG words. The routines appropriate for the commnnds
are stored in a prosram and telesoftware store 47.
Table 1 sives a table of the commnnds which are
considered at present to be useful as well as the number
3~ of bits required to define what are considered to be
useful numbers of options~
Of these commands, two are worthy of special attention.
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The first is the "chain" command number 9. This command
is used to draw irregular shapes and provides an important
additional facility to that provided by the "polygon"
command number 4 because it reduces the number o~ bits
that are required to be transmitted in ordcr to draw
the shape.
If one looks at the "polygon" command, the argument
consists of a number of words. The first is identified by
the letter C which indicates the colour of the line to be
drawn. The letters X1 and Y1 indicate the location of the
start of the line, the letters X2 and Y2 in~icate the
position ~o which the line is to be drawn etc. and -1
is a delimiter~ It will be seen that each X word is
,
an 11-bit word and each Y word is a 10-bit word. ~lUS,
to move a short distance, say two pixels, would require
42-bits to be transmitted.
If one now considers the "chain" command, number 9,
again it hac the C, X1 and Y1 words to identify the line
colour and starting position. Howe~er, the next word is
"mo~e 1". This is 4-bit word, the first three bits of
which indicate in which one of eight directions the line
should move and the other bit indicates whether the move
should be one or two pixels. There is thus a saving in
data for a large number of short moves in different
directions such as are required by a map. Chain commands
are terminated by sending as the last piece of data the
argument to cause a backtracking of the line drawn on the
screen. This is necessary because all possible codes are
used in this command.
To add extra protection to HRG words endin$ in a
delimiter e.~. the "chain" command, a special code is
tranSInitted AS an HRG word and this code is called the
continuation code in Table 1. If this code is present
after a predetermined num~er of steps the previous command
continues to be executed, if it i9 not the ~revious command
is terminatcd.
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The "chain" command is produced by means of
digitizing apparatus for producing digital signals
representing points on, say, a curve. One way in which
this can be done is to position a cursor at the start of
a curve to be digitized and put into a "chain" command
using the conventional cursor control provided by computers.
Thereafter, the cursor is moved by one or two pixels in
one of the eight directions under the control of a nine
key keyboard, whose key depressions are digitized and
stored as the data associated with the "chain" command.
Alternatively, other more conventional digitizing tech-
niques may be used such as moving a stylus over a line
drawing of the required display and noting the direction
and number of pixels moved. Colour information is added
to this data using the computer keyboard and the whole
argument is assembled. The "chain" command can then be
assembled in the program it is wished to transmit to a
receiver and which is decoded as described hereafter.
Details of the remainder of the transmission
side of the system are not given because they are largely
conventional to the presently used Teletext system in
this country.
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The other important command is the "Delay~
command number 6. Thls requires that the mlcroprocessor
be provided with a timlng clrcuit or clock 45a. The
~delay~ command completely releases the display from the
timing of the incomlng data. Thus, lt is possible with
conventional teletext to provide a delay, but this is
determined by the timing of the lncomlng data. With the
"delay" command all the incoming data, both before and
after the delay period, is received and the delay is
achieved using the microprocessor. Thls saves on the
number of pages required to be transmltted and breaks
the exlsting r~latlonshlp between transmlsslon tlme and
display time.
The result is that complex shapes can be drawn
on the screen and yet the amount of data required to be
transmitted in order to produce those shapes is less than
with conventional teletext even if such shapes were
possible with conventional teletext.
With this arrangement, it is possible to construct
two or more display images and store each in a separate
storage device e.g. store 48 or store 49 and switch
between the t~o stores under the control of the micro-
processor 45 using a switch 49a controlled by the processor
45. This would enable animation or another way to overlay
one display with another to form a composite display as
far as the viewer was concerned.
With this facility and the user section 34, it
would be possible for a viewer to receive all the informa-
tion required for a T.V. game and to play it, e.g. chess.
For the sake of completeness, a flow chart for
the main program of the microprocessor is shown in Figure 4.
This flow chart assumes that there are five possible
variations or levels all based on the conventional ~.K.
teletext format. Level 1 is the ~J.K. system as it operates
at present. Level 2 adds compatibility to level 1 using the
ISO range of characters and full parallel attributes so that
the editor at the transmission end can change the colour or
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display characteristics of any character position etc~
Le~el 3 adds dynamically redefinable cl~aracter sets ~DnCS)
capable of producing inter alia characters which can be
altered and which could be text characters or s~lbols
as chess pieces as well as high resolution line drawings.
Level 4 is the HRG system which is the subject of the
present specification and level 5 adds full resolution
colour pictures.
Each of the levels is indicated by an appropriate
code or, in the case of level 1 by the absence of such
a code. After data acquisition, the first page of data
is stored in the store 41 and is monitored for the
appropriate code in order to determine the level of
teletext which has been transmitted. If the user has selected
level 4, the first page is checked for errors by the
processor summing the CRC codes present in +he transmission.
If there are no errors the microprocessor then chec~s to
see if more pages are required and this, as ~ll be recalled
from above is also indicated on the first page~ If no
further pages are required the processor then starts to
execute the commands contained in the storad page. If
other pages are required, the processor continues to cause
pages to be stored and checked whereupon the processor then
begins to execute the commands.
Execution of the commands entails the processor
accessing the program and telesoftware store 47 for the
routine instructions required in order to carry out the
instructed command using the data which is also contained
in that HRG word. The store 47 may contain things li~se
look-up tables for decoding various of the commands.
The result of the exeuction of the command is fed to one
of the image display stores 48 or 49. The microprocessor
then checks to see if the command l~hich it has ju~t
executed is the last command nnd if it is not it returns
to the store ~ll for the next commanc] and executcs it and
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so on ~mtil either it receives the "end of program" command
or until it has execu-ted the last command stored whereupon
the processor resets and is available for the user
to chose the next desired Teletext display.
Tl~e image display store 4O or 49 is scanned
by the image store scan and display apparatus Ll6 to
produce the signals for driving the display. Associated
with the apparatus ~6 are a rounding circuit 46a whose
operation is described in detail with reference to Figure
6 and a colour pallet circuit 46b which contains a look-
up table for decoding the colour numbers stored in the
image display store 48 or 49. The look-up table produces
signals indicative of the colour as well as attribute
signals i.e~ signals indicative of whether the colour is
a fore$round or a background colour. These sisnals are
then fed back to the rounding circuit 46a. After rounding
if this is done, the colour signals which may be modified
as a result of the rounding are then re-applied to the
colour pallet circuit 46b whereby to produce the signals
which are ultimately D/A converted and encoded for
application to the display. Preferably the D/A conversion
and encoding is carried out by circuitry on the colour
pallet board.
The rounding operation will now be described in more
detail.
The matrix of pi~els adopted uses a line pair
of each row of the display. The same information is
output on both the odd and even field when a 625 line
interlaced scan display is used. The resolution which is
possible could , however, be tw-ice this giving a less
Jasged appearance to any curved or angled linesO
The improved resolution could be achieved by
quadrupling the size of memory which iS presently used.
Every one of the present sized pi~els would be sub-di~ided
into four small ones each ~ith its own four bit colour number.
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An alternative method of increasing the resolution
without greatly increasing the memory size is to interpolate
between diagonally adjacent pixels and to position extra
quarter si~e do-ts where required. The techniques have been
developed for charac-ter rounding on an alpha-numeric display
where the comparisons required are made upon pixel data
from two character ROMs. One is configured to give the basic
'full dot' character pattern without rounding whilst the
second ROM is addressed to give the same informatlon
as the display RO~I but from the previous or the following
line of dot information depending upon which field is
being displayed.
When rounding from an image store as in the present
application the reference data may be provided by using
a line buffer arranged such that pixel dot information
is always available from two successive lines.
Another major difference between rounding upon an
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image store and rounding with a character based system
is the absence of a known foreground and background on
the image plane. Any pixel can be independently coloured
and there is no limitation of one foreground and one
background colour in each character cell.
This problem has been overcome by the addition
of attribute information assigned to each colour number
along with its colour information and stored in the
store 47. Four levels of foreground/background have
been defined. When the terminal displays the contents
of the image store 46, the hardware interprets the
attribute data and rounds foreground colours over back-
ground colours.
It will be noted from Table 1 that it is
possible to write characters with this system (see commands
Ai) and A ii)). The character cell of the basic image
is a 10 x 8 matrix which appears jagged compared with
the quality of commercial Teletext character displays.
By image plane rounding the characters are resolved on
to a 20 x 16 matrix with a considerable improvement in
appearance.
Figure 5 shows a curve produced by the "chain"
code mentioned previously. Figure 5a represents the
curve as it is produced with the blocks representing
pixels and the designations "odd" and "even" indicating
raster scan lines of the TV display. It will be noted
that where two diagonally related pixels are illuminated
a very jagged result is produced.
- Consider now the four pixels identified as A,
B, C and D in Figure 5a. Pixels B and D are illuminated
and shown in full lines whereas pixels A and C are not
illuminated and are therefore shown in broken lines.
Two different types of rounding logic may be
applied to these four pixels. Firstly diagonally adjacent
pixels are tested in pairs to see if they are the same colour
and to see if they are both foreground or background
colours. Thus, the colour numbers for pixels A and C
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are compared as are thc colour number for pixels B and D.
~urther, the attributes i.e. whether they are background
or foreground colours, of the pixels A and B are compared
and the following logic executed:-
i) If B = D and ~B~ ~ LA~ THEN pre-round B by adding B'
to pixel A
ii) If A = C and [A] ~ ~B~ THEN post-round A by adding A'
to pixel B.
Where ~A~ is the attribute of the colour A etc.
Further, if logic test i) is executed and pixel B is pre-
rounded ~s shown in Figure 5b it may follow automatically that
D is post-roundad by adding D' to pixel C on the next
field. In practice, the effect of carrying out this
logic is to delay the first line o~ each pixel by two raster
scan lines and place it after the second scan line.
The other type of roundillg prohibits the automatic post-
rounding'D' above in the case wl~ere B = D D=C
and A ~ G. This prevents rounding of the inner
corners of a display e.~. a square or a character "E".
The advantage of keeping corners square is apparent
on text but a filled block of colour e.g. a filled circle
will have a number of pixel corner groups at its
peri.neter which will not be fully rounded. If rounding
occurs for any diagonal then rounding o~ filled shapes
is automatic.
Figure 6 shows apparatus for carrying out the rounding
described above in relation to Figs. 5a and 5b. In this
diagram it is assumed that each colour is identified by a
6-bit word, four of the bits identifyins the colour itself
an~ to remaining two bits being the attribute bits allowing
two levels of background and two levels of foreground.
Words from the image store 4~ are fed sequentially
to a 6-bit latch 50, controlled by a cloclc 51 rurming at
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the horizontal pixel rate and from there to a 6-bit data
selector 52 either directly or via a line store 53 capable
of storing the pixels for one complete lineN The store
53 is controlled by a counter 55 driven by the clock 51 and
a read and then a write cycle is performed for each address
location. The selector 52 is operated by a flip flop 56
and its output is fed to the combination of an 8~bit latch
57a and a 2-bit latch 57b. Thus, the output of the selector
52 comprises 4-bits representing the colour of a pixel
which will be used f`or comparison purposes and 6-bits
which form the complete word of that pixel but from one line
either before or after the'ref row" pixel. Stored data
is used as the "ref row" for odd fields of the interlaced
pi~ture while data coming directly from the scan and
display circuit ~6 is used as the "ref row" or even fields
o~ the interlaced PiCture. Hence a one line shift is made
upon data cominS direct from the image store and the
pairing for each pi~el with tne odd raster scan line
above the even raster scan line becomes reversed. The
6-bit word from the data selector 52 is separated into
its 4-bit colour number which is latched into the latch
57a and its 2-bit attribute number which is latched into
the latch 57b. A further combination of latches 58a and
58b are provided so that the words representing colours
of adjacent pixels in successively displayed rows are
available for comparison. Operation of the selector 52
is such that information from t~o successive rows is
input and can be crossed over so that the stored line
can be displayed on the scree~ (during the odd field)
or the line direct from the image plane can be displayed
(during the even field).
The odd field requires the line ahead to be used
for reference cluring the roundin~ tests whilst the
even field requires the line behind thc displayed line
to be used. The outputs from the latclles 57b and 58b
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~Z~577~
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are compared in a 2-bit comparator 60 and if the output
from latch 57b i9 greater than the output from latch 58b
an outpu-t signal is fed to one input of an AND-gate 61
whereas if the con~erse applied an output signal is fed
to one input of a further AND-gate 62. Outputs of the gates
61 and 62 are fed as inputs to a data selector 63 controlled
by the clock 51 and the output of the data selector 63 is
.5~7~7
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delayed for timing purposes by a delay circuit 6~ also
controlled by the clock 51.
It will be noted that the letters A, B~ C and D
appear on Figures and .these letters represent the pixels
shown in Figures 5a and 5b. It is thus apparent that
the comparator 60 compares the attributes of the pixels
A and B and produces a control signal to indicate whether
"rounding" is appropriate. The output of the latches 57a
and 58a are the colour numbers for pixels A and D and B and
C respectively. Three comparisons are undertakan. Firstly,
the outputs from the latches 57a and 58a representing
pixels B and D are compared in a 4-bit comparator 70.
ikewise the colour numbers for pixels A and C and C and D
are compared in 4-bit comparators 7~ and 72 respectively.
The output from the comparator 70 is fed as a second
input to ~ND~gate 62 whereby if the attribute.of B is
greater than the attribute of A and the colour o~ B is
equal to the colour of D pre-rounding of pixel B is
appropriate and an output is produced from the AND-gate
62 and fed via the delay circuit 64 as a control signal
to a ~-bit output data selector 75~ Likewise the output
of comparator 71 is fed as a second input to AND-gate 61
whereby if the attribute of A is greater than the attribute
of B and the colour of A is the same as the colour of C
then "post-rounding" of pixel A is appropriate~
The provision of the comparator 72 for comparing
pi~els C and D is to enable circuit to retain "not rounded"
edges in case such as are described above. This selection
of "rounding" or not is achieved by operation of a switch
76 which can be used to switch in the "not rounded" facility
for certain cases determined by -the type of display required
under the control of the processor ~5.
The output from the latch 5~a representing pixel B
is passed through two further latches 7~ and 79 in series
3~ with the ~-bit data selec-tor 75. Thc output of the
latches 7~ and 79 are fcd as inputs to the sclector 75.
' ', . ' ,: ' ' '. . ' ' ' ' '' '. ' . , ',
. .;
~:lS~2
- 16 -
Figure 7 shows the control sequences for the
latches 78 and 79 and each pixel is indicated as being
constituted by portions for ease of understanding the
operation of the control signal produced by the data
selector 63 and delay 64 because the delay 64 introduces
a half pixel delay into the control signal timing whereby
to enable pre- and/or post-rounding. The latches 78 and
79 are clocked at pixel frequency and it is not thought
that further explanation is required. The output from
the data selector 75 is used to provide the display.
Rounding greatly improves the appearance of
text on the display by ensuring that all lines are
approximately of the same intensity independent of the
direction in which they are drawn and by producing
smoother changes for boundaries and map outlines.
Finally, although it has been considered that
the digital data will be received as part of a TV trans-
mission it will be appreciated that alternatively the
data could be received via a separate data channel such
as a telephone line or other cable network with the
microprocessor being used to operate a channel selector
60 to switch between data sources. In this case the
data may be in a slightly different form to that described
above but by using certain conventional escape sequences
the data can be decoded to the same format as the codes
in a page of teletext before being processed by the
microprocessor to produce the picture.
