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An MPEG-2 video stream is transmitted as a GOP (group of pictures). A GOP typically contains 12 to 15 pictures, beginning with an I-picture (Intracoded). The I-picture gives as complete information as possible about the particular picture being transmitted. Each picture following the I-picture represents only the differences between the I-picture and the one being shown. These can be either B-pictures (bidirectional predicted) or P-pictures (predicted).

A P-picture displays only the differences between itself and the preceding I-picture; B-pictures can differentiate themselves from the preceding I-picture or the succeeding P-picture. A vector value is included with the B-picture to tell the decoder whether to differentiate forward or reverse.

The number of P and B pictures following an I-picture and how they are ordered determine the bandwidth and the quality of the MPEG-2 stream. Four combinations are used: I, IP, IBP and IBBP. I gives the best picture quality, as each image is encoded on its own and does not have to relate to any others. If one picture is lost, succeeding I-pictures aren't affected, as they all have their own full encoding. IBBP, on the other end of the spectrum, creates the lowest bandwidth and possibly the worst image. If an I-picture is dropped, the succeeding B- and P-pictures will not have the correct frame information to differentiate themselves. The result is the grainy and blocky images you might encounter on your DSS satellite on a rainy night.