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08.03.2008
Originally featured on BroadcastEngineering.com
Transport streams 101

A transport stream (TS) is where the DTV signal actually comes together; it contains all the audio, video and data in a multiplexed data stream that is sent from the studio to the transmitter and broadcast to viewers. The 8-VSB modulation used in DTV exciters is designed to carry this data stream and deliver it error free, or at least error corrected, to viewers’ homes. ASI, SMPTE 310 and sometimes GigE (1000BASE-T) interfaces are used to interconnect the TS from the multiplexer (mux), where it originates, to a studio transmitter link (STL), but most DTV exciters use SMPTE 310 for their input. The DTV receiver extracts the TS from the received 8-VSB RF signal and supplies it to the decoder, where it is usually converted into an analog signal for display on a TV set or monitor.

The TS is comprised of one or more packetized and multiplexed MPEG-2 compressed video signals and their associated audio in the form of AC-3 (also known as Dolby Digital) or AAC (Advanced Audio Coding), along with program descriptors and other data.

History

During the early 1990s, the Digital Video Broadcasting (DVB) Group was established in Europe as a consortium of companies to develop standards to create a system to deliver digital video to the home viewer. Prior to this, it was thought impractical to do so because of the bandwidth requirements. The goal of this group was to develop a complete suite of digital satellite, cable and terrestrial broadcasting technologies in one “prestandardization” body. The results of this group can be seen everywhere today because their standards have been implemented in almost all areas of broadcasting.

The TS in use today is a result of the efforts of the DVB Group with additional error testing and measurement guidelines from the EBU/ETSI JTC (European Broadcasting Union/European Telecommunications Standards Institute Joint Technical Committee).

The TS is used by broadcasters as well as cable companies and satellites operators worldwide. While there are differences in implementations, this is largely due to the particular difficulties different transport methods present.

Packetized elementary stream

To combine or multiplex the various data streams (compressed audio and video signals) that make up the TS, they need to be packetized. The elementary streams such as MPEG-2 or AAC audio are encapsulated into defined serialized data bytes called packetized elementary streams (PES). Once the elementary streams are packetized, they can be combined and multiplexed into a single data stream known as a TS. Each packet is 188B, or 1504b, long.

Packet identifiers

With several different packets used for each program (e.g. MPEG-2 video, one or more AAC audio channels, metadata, etc.) and one or more programs, the number of packets can increase quickly. Because the individual streams are no longer separate but combined as packets, this requires a system to identify and sort through the packets to extract the correct program.

Packet identifiers (PIDs) are used for this function and are attached to each packet in the TS. PID numbering is arranged to keep associated packets grouped together. For example, the MEPG-2 video would be PID 65, while the associated AC-3 audio is PID 68; another program has its MPEG-2 at PID 81 and AC-3 at PID 84.

Program map table

The program map table (PMT) is a list of the PIDs used for each program and what they are; there is one PMT for each program within a TS. The PMT also has a PID and it is always the first (lowest number) PID for the program. In the above example, the program with PIDs 65 and 68 has a PMT with a PID 64, and the program with PIDs 81 and 84 has a PMT with a PID 80. The information contained within the PMT lists the PIDs for all the packets and a description of what the packet is (e.g. MPEG-2, AAC, AC-3, data, etc.).

Program association table

The program association table (PAT) is a list of all programs contained within the TS. This is where the PIDs for the PMTs are found and is the first step in extracting the desired program from the stream.

Program clock reference

The program clock reference (PCR) is used to lock the local 27MHz clock to the clock used to create the encoded stream. If there is a fault with the PCR, then a number of errors can occur, such as lip-sync errors, picture freeze and dropped frames.

PSIP

Program and system information protocol (PISP) is the last item added to the TS. It provides much of the glue that holds the disparate elements of the stream together. PSIP contains the terrestrial virtual channel table (TVCT), master guide table (MGT), rating region table (RRT), system time table (STT) and event information tables (EIT). All of these provide for an easier user interface as well as tuning and channel branding. The TVCT indicates which DTV channels are associated with which analog TV channels and what frequencies and modulation modes are used. It also provides channel names and tuning information.

The MGT lists all the other tables available in PSIP; RRT is where various types of program ratings are located for all the programs in the TS; STT provides time of day information referenced to UTC; and EIT contains lists of TV programs and their start times contained in the TS. RRT, STT and EIT are basically intended for the viewer.

Next time
The next “Transition to Digital” tutorial will cover more of the contents of the TS and some of the errors that can be expected as well as how to monitor for them.



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