bits per pixel averages to 24 bits for
bits per pixel averages to 24 bits for 4:4:4, 16 for 4:2:2, and 12 for 4:1:1 and 4:2:0. More detail on these formats can be found in Chapter 4. Result of scaling The typical scaled video data rate at a size and frame rate used with analog modems is 1.15 Mbit/s. Compression to a rate suitable for delivery below 56 kbit/s will require a further 30:1 reduction. To reduce the rate even further, some form of image compression has to be employed. Compression Compression removes information that is perceptually redundant; that is, information that does not add to the perception of a scene. Compression is a tradeoff between the level of artifacts that it causes and the saving in bandwidth. These trade-offs sometimes can be seen on satellite television. If too many channels are squeezed into one transponder, fast-moving objects within a scene can become blocky and soft. Like scaling, compression of video splits into spatial compression (called intraframe) and temporal or (interframe) compression. Intraframe compression Single frames can be compressed with spatial, or intraframe, compression. This can be a simple system like run-length encoding, or a lossy system where the original data cannot wholly be reconstructed. A typical example of a lossy system is JPEG, a popular codec for continuous-tone still images. Video encoding 167 Table 8.6 Sampling Formats Format Nomenclature Sampling 601 Packed 4:2:2 AVI Planar IYUV/I420 4:2:0 or 4:1:1 YV12 4:2:0 or 4:1:1 YVU9 16:1:1 Packed YUY2 4:2:2 UYVY 4:2:2 YVYU 4:2:2
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