s. Collocated pixels can be neighbors in terms of space (they sit adjacent to each other) or time (they occupy the same space but in different frames). Intraframe compression techniques eliminate spatial redundancy. Interframe techniques eliminate temporal redundancy. Intraframe compression relies on transform coding (DCT, for instance). Interframe compression is largely a subtractive process: pixels contained in a frame of video are subtracted from a previous frame's pixels. The difference is information-the entropy that results from the interframe coding process. Early forms of video compression relied solely on interframe coding. In the 1960s, Bell Labs began experimenting with a variation of PCM called differential pulse-code modulation (DPCM). DPCM uses "differencing" to compare successive video frames. Codecs (one transmitting and one receiving), use identical methods to predict, using a past frame, what the pixel composition of a present frame will be. The transmitting codec, after making its guess, checks its accuracy by comparing its prediction, (stored in memory array), with reality. It subtracts reality from its prediction, encodes the errors and transmits them to the receiving codec. Because the receiving codec is making identical predictions, it knows what to do with errors, which it interprets as corrections. It makes the changes, displays the signal, and then stores the amended frame as the basis for the next prediction. The prediction and correction process is continuous; each new frame updates the previous one. Video Communications: The Whole Picture DPCM is still used in standards-based videoconferencing compression, although it is generally optional (by optional, we mean that a decoder must decode a DPCM compressed image, but a coder is not required to encode it). DPCM works because, on average, only about 9% of the pixels in a moving sequence change between frames. In 1969, a process known as conditional frame replenishment (CFR or CR) was developed. CR recognizes that information (what is not