NOTE: This is the second article in a multi-part series on MPEG-2.

Transmitting the video stream quickly and completely to obtain a quality picture without wasting bandwidth is obviously a crucial part of the MPEG-2 standards. MPEG-2 is the leading digital video compression technique for many of the newest video and broadcast applications. This article, the second in a series on MPEG-2, will examine the various elements that make up the MPEG-2 video stream and regulate its transmission.

The Transport Stream

The transmission of programs and services via packets of data is called the Transport Stream (TS). The Transport Stream is the result of the multiplexing of elementary data packets into Packetized Elementary Streams (PES) that contain the video, audio and data information for a single program. In addition to the actual program data, the Packetized Elementary Streams include a great deal of other information, such as:
· All elements necessary for synchronizing the audio and video
· Conditional access data
· Conditional access control information
· Electronic Program Guide (EPG) data
· Service Identification (SI)

(Graphic 1 - Transport Stream)

The Transport Packet
All transmitted data is multiplexed and subdivided into Data Packets, also called Transport Packets. Each Transport Packet has a fixed length of 188 bytes or 1,504 bits. An additional four bytes contain packet header information and 16 bytes are used for the Reed Solomon error correction system, making the total length of each transport packet 204 bytes.

(Graphic 2 - Transport Packet)

The Packet Header

Each Transport Packet dedicates four bytes for packet header information. The four bytes are assigned as follows:
· The first section (8 bits) identifies the beginning of each packet.
· The second section (one bit) is the Transport Error Indicator.
· The third section (one bit) is the Payload Unit Start Indicator.
· The fourth section (one bit) is the Transport Priority Indicator.
· The fifth section (13 bits) is the Program Identifier.
· The sixth section (2 bits) is the Transport Scrambling Control.
· The seventh section (2 bits) is the Adaptation Field Control.
· The eighth section (4 bits) is the Continuity Counter between each PES.

(Graphic 3 - Packet Header)

System Information

System Information (SI) is necessary for the receiver to function, and permits such services as the Electronic Program Guide (EPG). The EPG is a user interface that informs the user of the program schedule and provides descriptions of the available programs' content for the viewer.

The receiver uses SI to group services into one of two categories: Program-Specific Information or Service Information. Each category of information is then divided into tables, as illustrated below.

(Graphic 4 - Program Specific Information Tables)

· Program Allocation Table (PAT) - indicates the association between the program number and the packet identification (PID) of every packet relative to the Program Map Table (PMT).
· Program Map Table (PMT) - contains all PID information, including encryption/scrambling information relative to Entitlement Control Message (ECM) data.
· Conditional Access Table (CAT) - indicates the PIDs relative to each packet that transports Entitlement Management Message (EMM) data for one or more conditional access systems.

(Graphic 5 - Service Information Tables)

Main Tables

· Network Information Table (NIT) - contains all information for all programs, as well as the frequencies, symbol rate and FEC relative to the bouquet.
· Service Description Table (SDT) - contains a list of all program names and other parameters associated with each service or program.
· Event Information Table (DIT) - contains all event information relative to each service or program that is present in the current transmission. Optionally, data for other services on a separate transmission can also be included.
· Time and Date Table (TDT) - used to update the receiver's internal clock for synchronization of EPG.

Optional Tables

· Bouquet Association Table (BAT) - includes information relative to programs of the same category, such as News, Sports or Movies.
· Running Status Table (RST) - identifies one or more specific events and is periodically repeated throughout the transmission.
· Stuffing Table (ST) - used to dispose of outdated tables.

In the next installment of this article, we will cover the compression and transmission techniques of MPEG-2.