Multiple PES (Program Elementary Stream)  streams get multiplexed together into a faster stream and the System Information or SI stream is added, resulting in the complete MPEG-2/DVB multiplex that gets up linked to a transponder on the satellite:

 

 

The SI is responsible for telling the receiver all kinds of useful information about the data stream, so that the receiver can write the appropriate data into its program guide.

 

The first part of the SI is called the Program Association Table or PAT. The PAT is always transmitted on PID 0000 and contains a list of Program Map Tables or PMT's that are part of the data stream.

 

For example:

 

PAT (PID 0000) = 0100, 0200, 0300, 0400

PMT 1 (PID 0100) = Video PID 0101, Audio PID 0102, Audio PID 0103, PCR 01FF

PMT 2 (PID 0200) = Video PID 0201, Audio PID 0202, PCR 01FF

PMT 3 (PID 0300) = Video PID 0301, Audio PID 0302, PCR 02FF

PMT 4 (PID 0400) = Video PID 0401, Audio PID 0302, PCR 0401

 

Given this information, the receiver knows that the DVB transport on the current frequency contains four programs. The first channel contains two audio services (perhaps for multiple languages) and all of them except for the fourth program contain separate timing information - the fourth has the PCR timing embedded into its video stream.

 

The reason that the PCR might be transmitted separately from the video stream is in the case of multiplexed channels, which were encoded with a common clock reference. In this case, it would be redundant to send the PCR again, since the receiver would always use the same clock reference for all the signals within the multiplex. In the above example, one might assume that the first three video channels came from a common encoder and the fourth stream was multiplexed in, perhaps after being received from an SCPC feed or line-line and not re-encoded prior to multiplexing.

 

Obviously, the PMT contains other information, such as pointers to the name of the channel in the SDT table and things like information about data services that might be multiplexed in as part of the PES. But in addition to the PAT and PMT, there are a few more interesting ones. The Network Information Table (NIT) on PID 0010 contains a list of associated transponders that make up the package along with their SR and FEC values, which can be different.

 

When doing a search on a single channel on the Echostar DBS service, most smart MPEG-2 receivers (like the d-box) will automatically go off and search the other frequencies used by Echostar since the NIT on each transponder points to all the other transponders. The NIT can also point to transponders on other satellites, so that in the case of Echostar, the receiver would know to switch to another dish to receive programming from the Echostar 3 DBS satellite at 61.5 degrees when you tune the receiver to a channel carried on this satellite. In its own strange and totally non-standard way, Digicipher 2, uses a similar technique to allow the 4DTV receiver to know where other satellites are and turn to them when a particular channel is chosen.

 

Now the receiver knows all the frequencies associated with a package, there are few other PIDs that make a DVB receiver work the way it does. The optional BAT or Bouquet Association Table tells the receiver about programs of the same type (such as sporting events, movies, news etc.) that are part of the package. Echostar uses this part of DVB for their "Themes" menu. The EIT or Event Information Table on PID 0012 contains a list of the programs (or events) that when interpreted by the receiver's firmware, make the program guide. The EIT allows for up to two weeks worth of programming to be sent ahead of time.

 

The NIT dialog window shown below contains three transponders, which are together a package. This table was shown while viewing Canal+ Rood of the Dutch Canal+ package. As can be seen from the details, it also contains information about the position of the satellite and the details about Symbol rate, FEC, etc. A smart channel-scanning program could use the information to group the program details in the channel list.