MPEG2 Transport Stream information for beginners

[BGonaSTICK]

Firstly, I should reiterate that this brief guide only considers MPG2 transmissions - though this covers the vast majority of DTH satellite TV over Europe. Slightly different details apply to MPEG4 for example.

The MPEG2 Transport Stream is in fact a multiplex (combination) of many elemental Program Streams. Each satellite transponder can send only a single MPEG2 Transport Stream, but many TV or radio channels.

Each Program Stream (be it video, audio or data) is given a specific ID so that the receiver can select it out of the overall multiplex.

This ID is called the Packet IDentifier or Program ID: the PID.

The multiplexed Transport Stream contains additional information in a layer of data that sits logically above that of the elemental streams, and carries (amongst other things) four PSI (Program Specific Information) tables:

Program Association Table
Program Map Table
Conditional Access Table
Network Information Table

These tables contain the information necessary to demultiplex (strip out) and present individual programs.

The Program Association Table is a boring program reference lookup table.

The Program Map Table specifies, as well as other information, which PIDs, and therefore which elementary streams are associated with each program. This table also indicates the PID of the Transport Stream packets carrying the PCR (Program Clock Reference - a timestamp from which the decoder can derive the time) for each program. The PCR is critical, as it provides the timing element which enables audio and video to be re-synchronised when the two are brought back together at the receiver.

The Conditional Access Table will be present if scrambling is employed. The CA Table provides the association between one or more CA systems, their EMM (Entitlement Management Message) streams and any special parameters associated with them.

The Network Information Table is optional (though it's usually present), and carries the network data. This data is not 100% reliable (especially with feed data) where the correct IDs are just not setup correctly or left to default.

The embedded Network ID should indicate which network the stream is being received from. Even the humble digibox can extract and display this information (by pressing Services, 4, 6) as well as the Transponder ID (though the digibox refers to this as 'Transport Stream'.)

From the NID (which is often an indication of the 'provider', though it can also be that of the company leasing the transponder) you can usually work out the satellite.

For example, Transport Steams on Astra 1 19.2E often have a Network ID of 0001, Astra 2 28.2E is currently always 0002, Hotbird 13E is primarily 013E etc.

These figures are in hexadecimal, so 013E hexadecimal is 318 decimal.

There are a few transponders on Astra 1 though that have provider-specific NIDs like Premiere (0085) and Canal Digitaal Satelliet (0035). Hotbird has plenty of NIDs that aren't 013E. 00B0 is used for TPS, 0006 for American Forces Network, 0071 for Polsat and FBFF for Sky Italia to name a few.

Just looking at the translated provider name will usually tell you which satellite you're on. Most PC applications and many STBs will provide the NID and TID, which you can then look up online. Lyngsat for one shows them on the main transponder listing pages.

STICK

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