les and coefficients. At each layer, coding and compression algorithms are defined. Methods are also described for synchronizing, accessing, buffering and error recovery. Part 3-AUDIO (IS 11172-3). This part describes the compression of audio signals. It includes syntax and semantics for three compression methods (known as I, II, and III). These methods, or classes, define algorithms that vary in complexity and coding efficiency to produce streams of audio at different bit rates. Layer I (audio at 384 Kbps) is used for compact disc video. Layer II (audio at 224 Kbps) is used in satellite broadcasting and compact disc video. Layer III (128 Kbps audio) is used in ISDN, satellite and Internet audio applications. Part 4-CONFORMANCE (IS 11172-4). This part describes the procedures for determining the characteristics of coded bit streams and how they are decoded. It also defines the meaning of MPEG-1 conformance for the first three parts of IS 11172 (Systems, Video, and Audio), and provides two sets of test guidelines for determining compliance in bit streams and decoders. Part 5-SOFTWARE SIMULATION (IS 11172-5). This part contains a sample ANSI C language software encoder and compliant decoder for video and audio. It also provides an example of a system's codec, that can multiplex and demultiplex the separate video and audio streams that are used in computer files. BRIEF EXPLANATION OF How MPEG-1 WORKS MPEG-1 compresses relatively low-resolution source input format (SIF) video sequences (approximately 352-by-240 pixels) that are synchronized with relatively high-quality audio. The color images have been converted to the YUV space with the chrominance channels (U and V) further decimated to 176-by-120 pixels. MPEG-1 uses motion prediction (temporal compression) first and then follows it up with Video Communications: The Whole Picture DCT (spatial compression). DCT acts on 8-by-8 pixel blocks while motion prediction operates on 16-by-16 pixel blocks. After temporal and spatial compression are complete MPEG-1 uses fixed-