Teletext, Subtitles and VPS for DVB - Digital Video and Audio Broadcasting Technology - page 161

The analog TV teletext line shown in Fig. 9.2. begins with the 16-bit-
long run-in (1010 sequence) followed by the 1-byte-long framing code
with a value of 0xE4. This marks the beginning of the active teletext. It is
followed by the magazine and line number of 1 byte each. After that, 40
payload characters consisting of 7 bits payload and 1 (even) parity bit are
transmitted. The total amount of data per line is 360 bits (= 45 bytes) and
the data rate is 6.9375 Mbit/s. In DVB teletext (ETS 300472), the teletext
data are inserted into the PES packets after the framing code (Fig. 9.3.).
The 6-byte PES header starts with a 3-byte start code (0x00 0x00 0x01)
which is followed by the stream ID 0xBD which corresponds to a "Pri-
vate_Stream_1". Next comes a 16-bit (= 2-byte) length indicator which in
the case of teletext is always set so that the total PES length corresponds
to an integral multiple of 184 bytes.

44 byte
TTXT data fields

Structure
according to
EBU teletext
(= 1 TTXT line)

“0x2C“

Framing
code
„0xE4“

Magazine
& packet
address

Field
parity

Line
offset

40 byte (characters)
teletext data

reserved

8 8 2 1 5 8 16 320 Bit

“0x02“=EBU teletext without subtitling
“0x03“=EBU teletext with subtitling

Fig. 9.4. Teletext data block in a PES packet

Then comes a 39 byte optional PES header so that the overall PES
header length for teletext is 45 bytes. The actual teletext information is di-
vided into blocks of 44 bytes. The last 43 bytes are identical to the struc-
ture of a teletext line of an EBU TTXT after the run-in-code. These bytes
include the magazine and line information as well as the actual 40 bytes of
teletext characters per line. A teletext page consists of 24 lines of 40 char-
acters and the coding is identical to that of EBU or British teletext.
The teletext, processed to form long PES packets, is divided into short
transport stream packets comprising the 184 byte payload and a 4 byte