t degradation (Sayood, 2002). The compression efficiency or gain is very limited, and the high bitrates make lossless codecs unsuitable for the majority of purposes except high-resolution images. A lossy codec acknowledges that higher compression gains are required and achieves this at the expense of image quality (Ghanbari, 1999). A lossy codec discards information, resulting in an imper- fect reconstruction or facsimile of the original image; however, it is generally suitable for entertainment purposes. Many years of research have culminated in the modern generation of hybrid codecs that compromise bitrate and fidelity (Ghanbari, 1999). These codecs include: MPEG-1, MPEG-2, H.261, H.263, H.26L, and MPEG-4. Chapter 3 Digital Video Transmission These codecs exploit the following: Spatial redundancy, recognizing that pixels inside a picture are similar Temporal redundancy, recognizing that successive frames are similar Statistical redundancy, recognizing that more frequently occurring events should be assigned shorter codewords It should be noted that image coding exploits spatial and statistical redundancy, whereas video coding uses exploit spatial, statistical, and temporal redundancy. Compression Ratios A compression ratio is the ratio of the raw bitrate compared to the bitrate after compression, as seen in Figure 3.2. For any given technique, the compression ratio, K, is given by K = B1/B2 for still images = R1/R2 for video The use of compression can have profound effects for the application in question. Figure 3.3 shows the possible ranges for typical video applications both with and without the use of compression. The upper limit of each block corresponds to the raw or uncom- pressed bitrate, R1, and the lower limit to the compressed rate, R2. Because the scale is logarithmic, the height of the block, log K = logR1 logli R2. Compression of two orders of magnitude is achievable for some applications. Good overall performance is obtained by a combination of efficient source coding and efficient channel coding 