Broadcast Teletext Specification

4. DEFINITION OF TELETEXT TERMS

Access Time
The time between selecting a page at a receiver and the first complete reception of that page.
Alphanumerics Character
One of the 96 Display Characters listed in columns 2,3,4,5,6, and 7 of Table 3. The shapes of the characters are not defined but should all be different and recognisable.
Alphanumerics Mode
The Display Mode in which the Display Characters are those in the Alphanuerics Set.
Background Colour
The colour filling the parts of the Character Rectangle not occupied by the character itself (see figure 8). The Background Colour may be black or any of the seven Display Colours. It may be changed within a Row by Control Characters.
Blast-Through Alphanumerics
The use of the 32 Alphanumeric Characters of columns 4 and 5 of table 3 during the Graphics Mode.
Boxed Mode
The Display Mode in which, under the user's control, the characters are intended to be inset or added to a television picture. When a Newsflash or Subtitle is transmitted this operation may be automatic under the control of Control Bits.
Broadcast Teletext
The information broadcastings system defined in this document
Byte
A group of eight consecutive data bits intended to be treated as an entity.
Character Byte
The Byte obtained by appending an odd-parity bit to the Character Code.
Character Code
A seven-bit binary number representing one of a set of Display Characters, or a Control Character.
Character Rectangle
One of the 960 units in the regular matrix of 24 rows by 40 sites in which the characters are displayed on the screen.
Character Row
See Row.
Clock Run-In
A sequence of alternating bits at the start of a Data Line to allow a receiver to achieve bit synchronization.
Conceal
A Display Mode during which all characters, although stored in the receiver, are displayed as spaces until the viewer chooses to reveal them.
Contiguous Graphics Set
The set of 96 Display Characters comprising the 64 Contiguous Graphics Characters listed in columns 2a, 3a, 6a and 7a of table 3, together with the 32 Blast-Through Alphanumerics Characters of columns 4 and 5.
Contiguous Mode
The Display Mode in which the six cells of the Graphics Characters fill the Character Rectangle (see figure 8).
Control Bits
Each Page Header contains 11 Control Bits to control the display of the Page and its header (see 2.3.1).
Control Character
One of the 32 characters listed in columns 0 and 1 of table 3. Five of these are reserved for compatibility with other data codes. The others are used to alter the Display Modes. They are usually displayed as Spaces (but see 3.1.7)
Data Line
One of the otherwise unused lines in the television field blanking interval used to carry information for a Teletext Character Row. A Data Line is identified by the Clock Run-In sequence followed by a Framing Code at the appropriate time on a line in the field interval.
Display Character
One of 222 different shapes which can be generated in a Character Rectangle as part of a Page. These are Alphanumerics Characters to provide text and Graphics Characters to provide elementary pictorial information. There are three sets: the Alphanumerics set, the Contiguous Graphics set and the Separated Graphics Set, each of 96 Display Characters, some of which are common.
Display Colour
One of the seven colours (white, yellow, cyan, green, magenta, red, blue) used to depict a Dispaly Character against the Background Colour in a Character Rectangle. The Display Colour may be changed within a Row by Control Characters.
Display Mode
The way in which the Character Codes corresponding to Display Characters are interpreted and displayed depends on Display Modes established by previous Control Characters (see table 2). These modes may be changed within a Row, and an initial set of modes is defined for the start of a Row.
Flash
A display mode in which all character alternate as they would otherwise be displayed, and as Spaces, under the control of a timing device in the receiver.
Framing Code
A Byte following the Clock Run-In sequence, selected to allow the receiver to achieve Byte synchronization even if one of its bits is wrongly decoded.
Graphics Character
One of the 127 different Display Characters based on the division of the Character Rectangle into six cells, the cells being Contiguous or Separated. The corresponding Character Codes have B6=1; there is a direct correspondence between the other six bits of the code and the states of the six cells of the Character Rectangle. Examples are given in figure 8.
Graphics Mode
The Display Mode in which the Display Characters are those of one or the other Graphics Sets, depending on whether the Contiguous or Separated mode is being used.
Hamming code
In the Teletext system a Hamming Code is a Byte containing four message bits and four protection bits as indicated in table 1. A single-bit error in such a Byte can be corrected. Hamming Codes are used for sending address and control information.
Hold Graphics
A Display Mode in which any Control Character occurring during the Graphics Mode results in the display of a Held Graphics character (see 3.1.7).
Magazine
A group of up to 100 numbered Pages, each carrying a common Magazine number in the range 1 to 8. Up to eight magazines may be transmitted in sequence or independently on a television programme channel.
Newsflash Page
A Page in which all the information for display is Boxed, and Control Bit C5 is set to allow this information to be automatically inset or added to a television picture.
Page
A group of up to 24 Rows of 40 characters intended to be displayed as an entity on the television screen.
Page Header
A Page Header Data Line has Row address '0' and it separates the Pages of a Magazine in the sequence of transmitted Data Lines. In place of the first eight Character Bytes it contains Hamming Coded address and control information relating to that Page. Thus the top Row of the Page has only 32 Character Bytes. These are used for the transmission of general information such as Magazine and Page number, day and date, programme source and clock time.
Release Graphics
The Display Mode in which Control Characters are invariably displayed as Spaces. It is complementary to the Hold Graphics Mode.
Reveal
The Display Mode complementary to the Conceal Mode.
Rolling Headers
The use of the top Row of the page to display all the Page Headers of the selected Magazine as they are transmitted. This gives an indication of the Page transmission sequence while the user is watching, or awaiting, a selected page.
Row
A Page comprises 24 Rows of characters. When displayed on a television screen each row occupies about 20 television display lines. Each Row is generated from the information on one television Data-Line. It is to avoid confusion with television 'lines' that the Teletext Pages are said to contain 'Rows'.
Row-Adaptive Transmission
Teletext transmission in which Rows containing no information are not transmitted. This reduces the access times of the system. The non-transmitted Rows are displayed as Rows of unboxed black spaces.
Separated Graphics Set
The set of 96 Display Characters comprising the 64 Separated Graphics Characters corresponding to the Contiguous Graphics Characters listed in columns 2a, 3a, 6a and 7a of table 3, together with the 32 Blast-Through Alphanumerics Characters of columns 4 and 5.
Separated Mode
The Display mode in which there is Background Colour boundary around and between the six cells of the Graphics Characters within the Character Rectangle (see figure 8).
Space
a Character Rectangle entirely filled by the background colours.
Subtitle Page
A Page in which all the information for the display is Boxed, and Control Bit C6 is set to allow this information to be automatically inset or added to the television picture.
Teletext
An information transmission system using the data and display formats described in sections 2 and 3 of this document.
Television Data Line
See Data Line.
Time-Coded Page
In addition to a Magazine number and Page number a Page may be assigned a 'Time Code' of one of 3200 numbers arranged as two 'Hours' digits and two 'Minutes' digits. This code may be used to selct one of many Pages, bearing the same Magazine and Page number, transmitted in sequence. When the transmission of each version of the page is isolated or infrequent this code may be made literally the Hours' digits and two 'Minutes' of the clock-time at which it is transmitted.
Time Display
the last eight characters of every Page Header are reserved for clock-time. A receiver may be arranged to display these characters from the Rolling Headers to give a clock-time display.