The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 52

group and the US ATSC Project group have defined additional information
for the transmission of digital video and audio programs which is intended
to simplify the operation of set-top boxes and make it much more user-
friendly.

3.3.7 NonPrivate and Private Sections and Tables

To cope with any extensions, the MPEG Group has incorporated an “open
door” in the MPEG-2 Standard. In addition to the “program specific in-
formation” (PSI), the “program map table” (PMT) and the “conditional ac-
cess table” (CAT), it created the possibility to incorporate so-called “pri-
vate sections and private tables” (Fig. 3.20.) in the transport stream. The
group has defined mechanisms which specify what a section or table has to
look like, what its structure has to be and by what rules it is to be linked
into the transport stream.
According to MPEG-2 Systems (ISO/IEC 13818-1), the following was
specified for each type of table:

• A table is transmitted in the payload part of one or more trans-
port stream packets with a special PID which is reserved for
only this table (DVB) or some types of tables (ATSC).
• Each table begins with a table ID which is a special byte which
identifies only this table alone. The table ID is the first payload
byte of a table.

• Each table is subdivided into sections which are allowed to have

a maximum size of 4 bytes. Each section of a table is termi-
nated with a 32-bit-long CRC checksum over the entire section.

The “Program Specific Information” (PSI) has exactly the same struc-
ture. The PAT has a PID of zero and begins with a table ID of zero. The
PMT has the PIDs defined in the PAT as PID and has a table ID of 2. The
CAT has a PID and a table ID of one in each case. The PSI can be com-
posed of one or more transport stream packets for PAT, PMT and CAT
depending on content.
Apart from the PSI tables PAT, PMT and CAT mentioned above, an-
other table, the so-called “network information table” (NIT) was provided
in principle but not standardized in detail. It was actually implemented as
part of the DVB (Digital Video Broadcasting) project.
All tables are implemented through the mechanism of sections. There
are non-private and private sections (Fig. 3.21.). Non-private sections are
defined in the original MPEG-2 Systems Standard. All others are corre-