Glossary

access time
The mean time between selecting a page on a receiver and the first complete reception of that page.
alphanumeric
One of the display letters or numbers.
AM
Amplitude modulation. A method of modulating an RF carrier with information, its amplitude being changed by the information.
ARI
Autofahrer Rundfunk Information A system for transmitting and receiving traffic information used in Germany.
ASCII
American Standard Code for Information Interchange.
background colour
The colour filling the parts of the character rectangle not occupied by the character itself. The background colour may be any of the seven display colours in level one.
BBC
British Broadcasting Corporation.
blast-through alphanumerics
Allows upper-class alphanumerics to be displayed while in the graphics mode.
boxed mode
The display mode in which the teletext characters appear inset or added to the television picture.
BREMA
British Radio Equipment Manufacturers Association.
broadcast service packet
The packet which contains information on the broadcast service.
broadcast teletext
Teletext intended to be distributed by a broadcast transmission.
bi-phase
Digital coding in which the logical ones and zeros are represented by signals of opposing phase.
byte
A group of eight consecutive data bits intended to be treated as an entity.
CEEFAX
The name of the BBC teletext service.
CCIR
International Radio Consultative Committee.
CCITT
International Telegraph and Telephone Consultative Committee.
CRC
Cyclic Redundancy Check.

  1. A method for checking for errors by counting the digits in a section of code and comparing that number with the original number inserted into the code when the code was originated.
  2. An additional code added to the transmission (the CRC code) which relates in a mathematical way to a known sector of the code in the message. Repeating the mathematical process at the receiver and comparing the result with the CRC code allows the validity of the message to be confirmed.
character byte
The byte obtained by appending the parity bit to the character code.
character code
A 7-bit binary number representing one of the display characters or control characters.
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.
CTS
Clear-To-Send.
clock run-in
A sequence of bits at the start of a data line to allow a receiver to achieve bit synchronization.
co-channel
Interference from a distant station on the same nominal carrier frequency.
command row
The row usually at the bottom of the screen, which is used to send commands to the teletext system from a terminal. These rows are not displayed as part of the page.
conceal
A display mode during which designated characters, although stored in the decoder, are displayed as spaces until the viewer chooses to reveal them by a command from the handset.
contiguous graphics set
The set of 96 display characters comprising the 64 contiguous characters together with the 32 blast through alphanumerics characters.
contiguous mode
The display mode in which the six cells of the graphics characters fill the character rectangle.
control bits
Each page header contains 11 control bits to control the display of the page.
control character
One of the 32 characters that are not displayed but control the character or page display modes.
current loop
Method of interconnecting terminals and transmission equipment where a ‘1’ is indicated by a current and a ‘0’ is indicated by the absence of current.
data amplitude
The separation between the all Os level and the all 1s level of the data signal. In the World System Teletext intended for use on 625-line television systems, the data amplitude is specified to be 66% of the nominal video level.

data line
One of the otherwise unused lines in the field blanking interval (FBI) of a television signal that may be used to carry teletext information.
DBS
Direct broadcast by satellite.
DCM
Decoding margin meter, an instrument for measuring the eyeheight of a data signal, taking into account the noise level present.
display character
One of the shapes that can be generated for display in character rectangle as part of the page.
display colour
The colour used to depict a character against the background colour in a character rectangle. At level 1 of WST, this may be one of seven colours, but with higher levels many shades of colour can be reproduced.
display mode
The way in which the character codes corresponding to display characters are interpreted and displayed.
DRCS
Dynamically redefinable character sets.
DSR
(Data Set Ready) A modem interface control signal indicating to the attached terminal that the modem is connected to the telephone circuit.
DTE
(Data Terminal Equipment) The equipment acting as the data source.
DTR
(Data Terminal Ready) The modem interface signal indicating to the modem that the terminal is ready to transmit.
EBU
European Broadcasting Union.
eyeheight
In a noise-free signal, the smallest difference between any ‘1’ data pulse and any ‘0’ pulse for sampling positions equally spaced at the data rate and position chosen to maximize the quantity. It is expressed as a percentage of the data amplitude.
FBI
Field blanking interval. In a television signal, the interval between the fields of video information. It is now preferred to ‘VBI’, vertical blanking interval.
flash
A display mode in which the display of a character(s) alternates with the display of a space(s) under the control of a timing device in the decoder.
FLOF
Full level one features, a term used during the development of level 2 WST.
FM
Frequency modulation, a modulation method where the frequency of the carrier signal is changed in response to the modulating signal, the carrier amplitude being kept constant.
foreground colour
The colour of alphanumerics or graphics.
framing code
A byte following the clock run-in sequence which allows the receiver to achieve byte synchronization. In WST it is so protected that it may be currently decoded even if one of its bits is wrongly decoded.
graphics character
One of the 64 different display characters based on the division of the character rectangle into six cells. The cells can be displayed contiguously or separated.
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.
Golay code
A forward error connecting code. In the (23 12) form used in the MAC/packet system, Golay code allows for the correction of up to three errors in the group of 23 bits.
ghost-rows
An early term for an additional row that was not intended to be displayed but which contained additional control information for the page.
Hamming code
A forward error-correcting code, due to Hamming. Several versions are used in WST. At level 1 the Hamming code is a byte containing four message bits and four protection bits. Hamming codes are used to protect address or control information and, in the stated form, are able to correct the error in the byte.
handshake
An exchange of signals used in communication protocols to indicate that the previous part of the signal had been received successfully and that the next part can be sent.
hold graphics
A mode of graphics display in which any control character occurring during the graphics mode results in the displayed graphics character being held to cover the space occupied by the next control character. Its use allows the avoidance of space which would otherwise occur between changes of graphics colour.
IBA
Independent Broadcasting Authority.
IRT
Institut fiir Rundfunktechnik (Germany). linked pages Pages which contain information which ‘link’ them to another page. Packet 27 is designed to carry information of this type.
LSI
Large scale integrated circuit, a complex device in which a large number of semiconductors are deposited on one piece of silicon usually to perform a particular task.
magazine
A group of up to 100 numbered pages, each carrying a common magazine number in the range 1 – 8. Up to eight magazines may be transmitted in series or parallel on a television channel.
network identification
Part of the broadcast service packet 8/30.
newsflash page
A page in which all the information for display is boxed so that it can be seen better against the television picture. Control bit C5 is set to indicate to the decoder that the information should be inset into the television picture or added to the television picture.
NRZ
Non-return-to-zero, a form of coding for data in which the data stays in the ‘one’ position until the next zero data pulse causes it to return to zero.
ORACLE
The name of the UK independent television companies teletext service.
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 a 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 byte, it contains Hamming coded address and control information relating to the page. Thus the top row of the page has only 32 display 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.
page subcodes
An addition to the page numbers to extend the page address range.
PDC
Programme Delivery Control, the control of domestic videocassette recorders by teletext packet 8/30.
RDS
Radio Data System, a method of adding a sub-carrier signal to the normal FM stereo radio signal for the transmission of digital data.
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 to the viewer of the page

transmission sequence, watching or awaiting a selected page.

RS232-C
Electronic Industries Association (EIA) standard applicable to the interconnections between data terminal equipment (DTE) and data communication equipment (DCE) employing serial communications. The ‘C’ indicates the latest revision and the standard is usually referred to as RS232. (See Appendix for application details.)

row
A page comprises 24 rows of characters. When displayed on a television screen each row occupies about 20 television lines. To avoid confusion with ‘television lines’ the lines of text are called rows.
row adaptive transmission
Teletext transmission in which rows containing no information are not transmitted rather than being transmitted as a row of space characters. This reduces the access time of the transmission. The non-transmitted rows are displayed as unboxed black spaces.
separated graphics set
The set of 96 display characters comprising the 64 separated graphics characters (corresponding to the contiguous graphics characters) together with the 32 alphanumeric characters.
separated graphics mode
The display mode in which there is background colour boundary around and between the six cells of the graphic characters within the character rectangle.
smoothed graphics
A graphics set available in the higher levels of WST, giving better resolution graphics.
space
A character rectangle entirely filled by the background colour.
sub-carrier
An additional carrier signal for transmission of supplementary information.
subtitle page
A page in which all the information for the display is boxed and the control bit C6 is set to allow the decoder to automatically inset or add to the television picture.
teletext
A method of transmitting data, usually in the field blanking interval of a television signal.
television data line
See data line.
time coded page
A page containing a time code in the header, and transmitted at a pre-set time.
time display
The last eight characters of every page header are reserved for clock-time. A receiver may be arranged to display these characters to give a clock-time display as an insert into the picture.
VBI
See FBI.
videotext
This is the general name used for an information service that uses a telephone line for the transmission of the data.
WST
World System Teletext.

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