Teletext packets
A teletext transmission consists of a stream of 45 byte packets.
The first three bytes of every packet contain a synchronising sequence of clock run-in and framing code.[1]
Following the run-in and framing code are a pair of bytes used for packet addressing, which contain Hamming 8/4 encoded nibbles, giving an 8-bit address space.
The 8-bit packet address is divided into 3 magazine bits and 5 packet number bits. [2] These two bytes are often referred to as the Magazine Row Address Group (MRAG).
In the teletext specification, packet addresses are written in a slash delimited form.[3] The magazine number may be represented by X for page related packets and M for magazine related packets. For example X/24 indicates packet 24 of any magazine. Y represents a packet number.
A 3-bit magazine number gives a range of 0 to 7, however the value 0 is interpreted as magazine 8.[4] This magazine number is the 'hundreds' digit in the three digit teletext page number.
| The following information relates to an old specification which is deprecated or superseded: |
| In the original 1976 specification packets were referred to as rows and only packets Y = 0–23 were defined.[5] |
Packet types
A packet's type is determined by its packet number Y.[6]
Packet X/0 is a page header and contains the page address and control bits, along with 32 bytes usually intended for display by the decoder on row 0. All subsequent packets X/1–28 with the same magazine number relate to the page.
In a basic Level 1 page, packets X/1–24 correspond to rows 1 to 24 of a decoder's display, and each contain 40 bytes of character data. Packet X/25 is not used in pages intended for direct display.
Packets X/26 and X/28 contain page related enhancement data and are not directly displayed, but provide instructions to advanced decoders to enhance the content of the displayed page using higher level features such as additional colours and alternative characters.
Packet X/27 relates to page linking and navigation.
Packet M/29, like X/28, contains enhancement data for higher level decoders, but where packet X/28 is page related, M/29 is magazine related. The enhancements apply to all basic Level 1 pages in the magazine unless overridden by a page related enhancement packet. Packet M/29 can occur at any point in the packet stream and is not related to any page header.
Packets 30 and 31 carry data independent of the rest of the teletext service and can occur at any point in the packet stream. Packet 8/30 contains the Broadcast Service Data Packet. In databroadcast applications the sixteen possible addresses using these packets are also referred to as data channels.[7]
Designation codes
For packets X/26 and above, the packet addressing is extended by using the byte following the MRAG for a Hamming 8/4 encoded designation code. This gives a 4-bit extension to the address allowing up to 16 packets of each type.
For X/26 packets the designation code enables multiple packets of enhancement data to be transmitted. For other packets the designation code indicates the format of the data within.
The designation code can be included in the slash-delimited packet address notation, for example X/28/0 refers to any packet X/28 with designation code 0.
- ↑ ETS 300 706 chapter 6
- ↑ ETS 300 706 chapter 7.1
- ↑ ETS 300 706 chapter 3.2
- ↑ ETS 300 706 chapter 3.1
- ↑ Broadcast Teletext Specification chapter 2.1.1
- ↑ ETS 300 706 chapter 9.1
- ↑ ETS 300 708 chapter 6.4.2