Putting It All Together

Now that we’ve seen most of the major parts of ATSC service information, there is still one part that is missing. In order to increase efficiency in the receiver, another table – the Master Guide Table (MGT) – contains references to all of the various tables that we have already seen.

The MGT provides a detailed description of all of the other tables in the system – which tables are present, the PIDs on which they are transmitted and the size in bytes of each table. This lets the receiver allocate memory appropriately before it actually starts parsing the tables (and also means that it knows which tables to look for). Why is this useful? As any experienced software developer will tell you, programming an embedded system can sometimes be a complicated matter of juggling memory – not allocating too much (which wastes memory that the rest of the system may need), but not allocating too little either (which then forces you to allocate more, and copy data around and generally waste time that could be spent doing more productive things).

By providing this information, the MGT gives the receiver a helping hand to parse the SI data as efficiently as possible. And as any software developer will tell you, that is always appreciated.

An example of the MGT looks like this:

As with the VCT and the RRT, the MGT is carried on PID number 0x1FFB – this lets the receiver decode these common tables in an efficient way, since only one PID needs to be monitored. The sections containing the MGT have the following structure:

Telling the time correctly

Having all of this event information can be very useful, but it’s only useful under the right circumstances: if the receiver doesn’t know the correct time, having all of the information about when events start and finish is a bit pointless. To make sure that all of the receivers have the correct time, the network operator broadcasts a time reference. This is contained in the System Time Table (STT), and tells the receiver a number of things. The first, and most obvious one is the time – that’s the whole point of the table, after all. This is broadcast as the number of GPS seconds since midnight on January 6th, 1980, and is measured as UTC (GMT) time. Since this is not always useful, the GPS_UTC_offset field tells the receiver what time zone it’s currently in.

The STT also contains a structure telling the receiver whether daylight savings time should be applied. This tells the receiver three things: whether it is currently in daylight savings time or not, the day at which it should change its daylight savings setting, and the time at which it should do so. In the month that daylight settings time will start or end, the table contains this information to tell the receiver exactly when to change: at other times, these two fields are set to zero.

The full structure of the system time table is as follows: