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.