hich provides good vertical resolution, but does not allow for smooth motion restitution in the case of fast-changing scenes. As seen earlier, in the sound compression section, MPEG-1 remains the underlying standard for the most popular MP3 (short for MPEG-1/2 layer 3). MPEG-2 This comes as an enhancement of MPEG-1 in both audio and video. 43 To recall from the previous section on sound, the MPEG-2 audio layer 3 is also an underlying standard for MP3, which allows us here to encode more than just one stereo track. As for the video, just like in MPEG-1, the I-frames are processed according to the JPEG image- compression algorithm (see previous description in the image section). The P-frame offers more compression potential than the I-frame, because it only encodes succes- sive image differences. The comparison algorithm considers macroblocks of 16 X 16 pixels. The differences are then described by the translation of similar groups of pix- els within these macroblocks, characterized by a "motion vector." If the algorithm fails satisfactorily to code the transition with the appropriate motion vector and addi- tional predictive corrections, then the output frame is an unaltered I-frame. The B- frame encoding works according to the same principle as the P-frame, except that it uses the information of the two surrounding images. A supplemental feature is that (unless otherwise specified) frames are interlaced on display or scanning (unlike in MPEG-1), which allows smooth transitions in the case of fast-moving pictures. Here, we shall overlook the supplemental notions of "transport stream" and "program stream," which are more application focused. In terms of image resolution, frame rate and bit rate, MPEG-2 offers a broad variety of possible configurations. One first refers to an MPEG-2: Profile, through (a) the type of frames in the GOP [I-P or I-P-B], (b) the JPEG luma or chroma sampling [4:2:0 or 4:2:2] used for the images, and (c) the stream range [1, 1-2, or 1-3]. The simplest profile, called SP, is thus defined by