MPEG-2 has several enhancements over MPEG-1, and it is designed for broadcast use as well as computer use. Some of the enhancements include:

• Higher Resolutions: MPEG-1 is fixed to 320x240. MPEG-2 supports resolutions up to 2000x1000.
• Field and Frame Prediction: We talk about this in more detail below:
• Slices: Again in more detail below

Remember, TV signals are interlaced. That means that the TV first reads and displays all the odd scan lines before reading and displaying the even scan lines for a frame. MPEG-2 understands this and encodes a frame in two fields, top (odd) and bottom (even).

Each field can be of type I, P, or B, with the following restrictions:

1. If the top field of a frame is I, the bottom field can be either I, P, or B.
2. If the top field of a frame is P, the bottom field can be P only.
3. If the top field of a frame is B, the bottom field can be B only.

In MPEG-2, you can identify a slice, a part of a field that is guaranteed to be intracoded. Therefore, if you know that a certain region of all frames is intracoded and that no prediction is required to decode it, you can use this slice to display frames while fast-forwarding and also for reverse-playback.

Next time, we will cover the MPEG system layer.

There are three primary elements in an MPEG-2 stream:

• PES - Program Elementary Streams are streams for a single medium (e.g. video)
• PS - Program Streams are streams that multiplex PES streams together into a single stream. (Similar to the MPEG-1 system layer)
• TS - Transport Streams are streams that can multiplex PES, PS, and even other TS streams into a single stream for transport.

TS packets are fixed to 188 bytes in size and each has a PID (packet ID). Once you have these packets, the decoder must be able to determine which packets contain which programs. There are two tables defined in order to do this in MPEG-2:

• PMT: Program Map Table. A Program map table maps logical program numbers to PIDs of packets that contain the PES or PS of the program
• PAT: Program Association Table. The PAT maps logical program numbers to PIDs of packets that contain the PMT of that program. The PAT always arrives on a unique PID so that it can be found.