The Vertical Blanking Interval (VBI) is part of every standard video signal. For many programs that are broadcast, the VBI contains auxiliary data signals that provide special data services. Examples include closed captioning, electronic program guide data, teletext, and V-chip program ratings. These signals were discussed in more detail in Chapter 3.

As we have discussed in detail, MPEG and other video compression systems focus on eliminating redundant and unnecessary information from the incoming video signal. For normal analog video, the horizontal lines that make up the VBI fit both descriptions. The VBI is redundant, because it is in every video frame, and it is unnecessary, because the location of the active video pixels on every line is already part of the MPEG (or other) data stream.

However, when the VBI contains data, the situation changes. In this case, the encoder needs to handle the data and send it to the decoder. There are at least two ways of doing this. First, normal video compression techniques (you guessed it DCT) are capable of accepting extra lines of video from the VBI and compressing them along with the other image data. This method works for low-speed data but may not be appropriate for higher speed data, where the patterns encoded in the VBI are very detailed. Second, the data in the VBI can be extracted and sent as a separate data stream embedded inside the compressed video stream. This method has the benefit of being very efficient, because the...