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MRG Systems - Teletext glossary
Category: Electronics and Engineering > Teletext
Date & country: 13/11/2007, UK
Words: 44


Clock Run-In
A sequence of alternating bits at the start of a data-line to allow a receiver to achiever bit synchronisation.

Closed Subtitling
Unlike open subtitling, where the text for the subtitling is inserter directly into the displayed picture prior to on-air transmission, closed subtitling transmits the subtitles as encoded data carried in a channel such as teletext which accompanies the picture signal.

CRC
See Cyclic Redundancy Check: One of the methods used within teletext for providing indication of error-free reception.

Cyclic Redundancy Check
One of the methods used within teletext for providing indication of error-free reception.

Data Broadcasting
The transmission of data to receivers which is not necessarily intended for display as teletext pages, using teletext coding techniques.

Data-Line Tv
transmission line containing binary coded information, as opposed to picture. One VBI line period allows transmission of a single teletext packet.

Decoder
The unit in a teletext receiver which extracts the coded data from the TV signal and converts it into usable information.

Encryption
Coding function carried out on transmitted data to ensure reception only by selected users.

Extension Packets Or Ghost Packets
Teletext rows or packets which do not correspond to the normal text rows visible on a page but instead contain additional or supervisory information, e.g. display modifications, page linking, etc. Packets of this kind are numbered in the range 25 to 31.

Fastext
Fast Teletext. Allows faster and easier access to teletext pages by the transmission of linked pages'. A suitable equipped teletext receiver can store linked pages while the user is viewing the first page. The linked pages can be accessed almost instantly by a single key request.

Field
A single scan of the TV raster from top to the bottom of the screen. This may be interlaced with subsequent fields to produce a frame, the unit of TV screen display.

Flof
Originally intended to mean Full Level One Features. Now obsolete - see Fastext.

Frame
Comprises two interlaced fields producing a complete screen image.

Full Field Teletext
Use of entire video capacity of TV signal for Teletext purposes facilitating either very rapid access times or a large number of pages. Ordinary teletext typically uses up to 16 lines within the VBI.

Ghost Rows Or Ghost Packets
See Extension Packets.

Hamming Code
Forward error correction system used within teletext, particularly on addressing and control information. Allows recovery of single bit errors within the data byte if facility is implemented in receiver as well as detecting other forms of error.

Header
See Page Header:A packet with row address 'o' separating the pages of a magazine in the transmission sequence.

In-Vision Subtitling
See Open Subtitling

Information Provider
Organisation providing information for transmission and sale particularly using data broadcast.

Inserter
A device which adds teletext data to a television signal.

Interlace
The interleaving of two video fields to produce a frame. Interlacing is adopted to reduce screen flicker for a given frame rate.

Line 21
The VBI line used by the USA National Captioning Institute for subtitling in North America. Also known as 'closed captions'.

Magazine
A group of up to a hundred teletext pages, each carrying a common magazine number in the range 1-8 (Magazine 1 comprises pages 100-199). Up to eight magazines may be transmitted in sequence or independently.

Ntsc
Defines the encoding of colour video signals used mainly in the USA, Canada, Japan and Mexico. The NTSC composites video signal is composed of luminance and chrominance signals. An NTSC video frame comprises two interlaced fields transmitted at 60 cycles per second. One frame comprises 525 scan lines. (See also PAL).

Open Subtitling Or In-Vision Subtitling
This involves the insertion of subtitles directly into the displayed picture prior to on-air transmission; as opposed to closed subtitling where the subtitles are transmitted as encoded data carried in a channel (e.g. teletext) which accompanies the picture channel.

Packet
In the context of teletext, a group of associated bytes transmitted within one TV line period. Packets with address 0-24 correspond to displayed text tows. (See also Extension Packets).

Page
An associated group of packets intended to be seen or utilised all at once. In conventional teletext, a page consists of packets 0-24.

Page-Header
A packet with row address 'o' separating the pages of a magazine in the transmission sequence.

PAL
Defines the encoding of colour video signals used mainly in Europe e.g. Britain, Germany, Italy, Spain, Scandinavia, etc. The PAL composite video signal is composed of luminance and chrominance signals. A PAL video frame comprises two interlaced fields transmitted at 50 cycles per second. One frame comprises 625 scan lines. (See also NTSC).

Programme Delivery Control
This is a system which permits simple programming and recording control of VCRs using teletext technology. The teletext service is used to carry TV programme schedule information and VCR control commands.

Repeats
The repetition of each data broadcast packet for a specific service up to 15 times.

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 or 25 rows of characters. Each row corresponds to 1 data packet and is generated from the information on one television data-line.

Row-Adaptive Transmission
Teletext transmission in which rows containing no information are not transmitted. This reduces the access time of the system. The non-transmitted rows are displayed as rows of black spaces.

SECAM
Defines the encoding of colour video signals used mainly in France and Eastern Europe e.g. Russia, Hungary etc. Very similar in structure to the PAL system but uses different chroma modulation technique.

Service
In data broadcasting this defines a single service data channel for an IP including address, continuity, repeats, etc.

Service Address
Defines the 'address' of the databroadcast packet. It is constructed from two parts. The Data channel group (DCG) which has values in the range 8 to 11 decimal, and the packet address which can be omitted or have a value in the range 0 to FFFFFF hexadecimal.

Short Packet
If the user data area in a packet is not filled with user data then the packet is referred to as a short packet. The amount of user data in the packet is signalled via the data length parameter in the packet header.

Stuffing Byte
Within data broadcast packets, long sequences of 00 or FF bytes have to be avoided for old decoder designs. The stuffing byte is inserted to interrupt these long sequences.

Teletext
Data service transmitted on TV lines not used for picture information, either utilising spare capacity or instead of video information.

VBI
Vertical Blanking Interval. Field blanking period. Period between active picture fields.

VIDEOTEX
Describes TV equipment used to display computer-based data, whether this comes to the set vie a telephone (sometimes known as viewdata) or via a broadcasting channel (otherwise known as Teletext).

WST
World System Teletext. International teletext specification.

WSTTS
World System Teletext Technical Specification.