Last Updated:
Friday, 27 February 2009 By: Steve Warner

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Teletext Information

Packet 31 - Independent Data Line


This packet is used to implement independent data services. Recently, the potential for using teletext as a means for distributing data nationally and at speed. All the other packets relate to a particular page, and thus cannot carry data which is not associated with a specific page. Packet 31 is defined to be page independent and can be transmitted at anytime without affecting the correct operation of the basic teletext service.

Packet 31 consists of

Up to 15 different data channels

Optional Data format
Repeat Facility
Continuity Indicator
Data Length Indicator

Service Packet Address

Allows individual users or groups to be identified

User Data

Between 28 and 36 data Bytes
Cyclic Redundancy Check word

 

This is the system used by DBI to transmit vast amounts of data via there licence on ITV

 

A Packet 31 line is different is structure from the ordinary teletext line. It also differs from ordinary teletext in that the information in a IDS line can be interpreted without reference to other lines. The ordinary teletext structure is page based. The structure of a IDS line is shown in the diagram below. The initial section of the line is similar to the ordinary teletext line. The subsequent structure is different from that of the ordinary teletext line.

Bytes in Packet 31 - Independent Data Service Line

2

1 1 1 1 upto 6 upto 6 1 1 28 to 36 2

CRI

Framing Code Data Channel Design
Code
Format
Code
IAL Service
Packet Address
Repeat
Indicator
Continuity
Indicator
User Data CRC












The Packet 31 line structure is different from the ordinary teletext line structure in that it is not page dependent. This means that only changing data needs to be sent to the receiving decoder. The background image could be generated by a computer program resident in the receiving decoder. This improves the overall security of the system.

The clock run in and framing code are identical to ordinary teletext line parameters. This is to enable the use of standard teletext circuitry in the IDS decoders. The fourth and fifth byte, used for magazine and row address in ordinary teletext (MRAG), are used for data channel group identification and independent line identification. The fifth byte identifies the line as an independent line by carrying the data 1111. The data bits are Hamming system coded to form the byte.

Data Channel ( Byte 4 )

The fourth byte carries the data channel information. Again only four of the bits are data bits, the rest are Hamming System coding bits. This code specifies which of the 16 available channels the following data for.

Designation Code ( Byte 5 )

Set to 1111, this tells the decoder that the following data is an Independent Data Service Packet.

Format Type ( Byte 6 )

Specifies format of data following.

Interpretation and Address Length ( Byte 7 )

SAA5250 circuit example

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