 Guide to packets (2)
Packet 8/30 format 1
Packet 8/30 commonly known as the broadcast service packet is not sent
as part of any page but is sent approximately once per second, on the seconds
change. The packet contains time, data and general broadcast service information.
The packet is made up of a number of different data fields some of which
are described as follows:
DESIGNATION CODE
Denotes whether the transmission is VBI or full-field.
INITIAL TELETEXT PAGE
Tells the decoder which page should be captured and stored on power-up.
This is normally an index or menu page.
NETWORK IDENTICATION
A two-byte code uniquely identifying the transmitting network. Most national
broadcasters have such a code.
TIME OFFSET, DATA AND TIME
Contains local data and time data.
TV PROGRAMME LABEL
Defines a programme label for the currently transmitted programme.
STATUS LABEL.
Used to display a transmission status message.
Packet 8/30 format 1
Packet 8/30 format 2
Used for PDC recorder control. The packet contains a programme label
identifying the start of each transmitted programme. The label normally
appears a preset time before the start of the programme (typically 30 seconds)
to enable the VCR tuner to detect it and place the machine into record mode.
Some of the more important data fields are described as follows:
COUNTRY & NETWORK IDENTIFIER - CNI
This parameter is split into two data groups. The first part identifies
the country, the second identifies the network channel or programme provider.
LOCAL CHANNEL IDENTIFIER - LCI
Up to four labels may be transmitted simultaneously. For example, a new
programme label can be transmitted with the existing label. The LCI identifies
each of the labels.
PROGRAMME IDENTIFICATION LABEL - PIL
This data field carries the label used to indicate the start of the programme.
The label normally carries the announced broadcast time of the programme
i.e. day, month, hour, minute.
PROGRAMME CONTROL STATUS - PCS
Carries real time status information about the programme e.g. type of sound
transmission.
Packet 8/30 format 2
Packet 31
Packet 31 is used exclusively for carrying private data to closed user
groups. Typically a broadcaster will dedicate one or more VBI lines to packet
31 data. The packet carries address information which allows data to be
sent to specific service users and between 28 and 36 bytes of user data.
Packet 31
As with all teletext packets, the first three bytes consist of a clock
run in followed by framing code. Packet 31 is identified by the message
bits 1111 in the fifth byte which is hammed.
BYTE 4 - DATA CHANNEL GROUPS - DCG
The four message bits of the fourth byte (which is hammed) define part of
the user address. Four settings are defined 0001, 1001, 0101, 1101 (in transmission
order), which are referred to as data channel groups 8, 9 10 and 11. The
DCG address, being short and at the start of the packet, is a useful initial
indicator to decoders that the packet is addressed to them. There is no
need to decode further data in the packet if the DCG does not match the
user's DCG address.
BYTE 6 - FORMAT TYPE BYTE - FT
This Hamming-code byte controls the interpretation of subsequent bytes.
The meaning of the four message bits, in transmission order is:
bit 1 - set to 0 for Data Broadcast format.
bit 2 - set to 1 if packet repeat facility is in use, otherwise 0.
bit 3 - set to 1 if continuity indicator is explicit, otherwise it is implicit.
bit 4 - set to 1 if data length indicator is in use, otherwise 0.
BYTE 7 - INTERPRETATION & ADDRESS LENGTH BYTE - IAL
The first three message bits of this Hamming-coded number indicate how many
immediately following Hamming-coded bytes are allocated to the packet address.
The minimum is none, giving no packet address within the data-line and the
maximum is six, giving a 24-bit address.
ADDRESS BYTES
The service address is defined by this address and the DCG address. The
IAL byte indicates the number of bytes allocated to address information
- the minimum is zero the maximum six using bytes 8 to 13.
PACKET REPEAT INDICATOR BYTE - RI
Optional byte. Each data broadcast packet for a specific service can be
repeated from 0 to 15 times to reduce error rates. The RI byte is incremented
for each repeat.
PACKET CONTINUITY INDICATOR - CI
The number of bytes transmitted for the particular service address is always
transmitted. It is incremented for each new transmitted packet of the same
service data channel. The count can either be placed in this optional CI
byte- which is known as the explicit continuity indicator or it can be encoded
in the final CRC bytes of the packet where it is called the implicit continuity
indicator. The CI byte is only present if set by the FT byte. It does not
change during repeated transmission of the same packet.
If an explicit continuity indicator has not been signalled an implicit
continuity indicator is sent.
DATA LENGTH BYTE - DL
The DL byte is only present if it has been signalled by the fourth message
bit of the FT bytes. It represents a six-bit number defining the number
of eight-bit bytes of the user data byte group intended to be delivered
to the user.
The DL byte is used when it is necessary to send an incompletely filled
packet in order not to delay the transmission of data. Any remaining bytes
of the user data group should be stuffing bytes and are still subject to
the CRC (see below).
USER DATA BYTE GROUP
The remaining bytes in the data-line, excepting the last two, and further
limited by the DL byte when present, constitute the data carried for the
users of the service bearing that packet address. Depending on the address
length, whether the repeat facility is used, and whether the continuity
indicator is explicit or implicit, there are generally between 28 and 36
user data bytes in packet 31.
STUFFING BYTES
An optional stuffing byte (ascii 165 = A5 hex) may be inserted into the
data field to eliminate long strings of 0s or 1s from the transmission.
This idea was conceived to assist old decoder designs which could not handle
long streams of non changing data.
CYCLIC REDUNDANCY CHECK - CRC
The last two bytes are a 16-bit cyclic redundancy check on the CI byte (if
present) together with the user data bytes.
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