y the mid-1980s, they reduced the bandwidth required for a conference to below T-1 speeds. Visual compression techniques usually involve manipulating pixels-the medium of exchange in video communications. As a codec compresses, it replaces the data of the original signal with complex mathematical models. The goal is to eliminate as much of the signal as possible without destroying its information content. Compression involves two-steps. Anything that is compressed must be decompressed. A trade-off exists between the time it takes to compress an image and how much compression one achieves. Time-consuming compression techniques do not work for video communication applications, which must move quickly to produce the illusion of fluid motion. In video communication, there are two primary methods for categorizing compression. One technique, redundancy elimination, is based on eliminating signal duplication. The other is based on degrading the picture and sound slightly, preferably in areas where the human eye and ear are not particularly sensitive. It is quality reduction. Before we get into the details of these techniques, we must introduce some new terms. One fundamental way to categorize compression is to look at the result. After decompression, are the data identical to those that constitute the original signal? If so, the compression is lossless. If compression is achieved by permanently discarding Video Communications: The Whole Picture parts of the signal (ideally, components not critical to its interpretation), the technique is lossy. When reversed, lossy compression does not produce a signal that is identical to the original but, rather, approximates it. Lossy techniques can produce very high ratios of signal compression, but do SO at a considerable price. That price is reduced image quality or sound fidelity (or a combination of both). Generally, signals degrade as compression increases. Interactive video compression uses both lossy and lossless techniques. However, lossless methods usually occur at the end 