What are Constrained Parameters Bitstreams?
MPEG-1 CPB are a limited set of sampling and bitrate parameters designed to normalize decoder computational complexity, buffer size, and memory bandwidth while still addressing the widest possible range of applications. The parameter limits were intentionally designed to permit decoder implementations integrated with 4 Megabits (512 Kbytes) of DRAM. Bitstream Parameter Limit pixels/line 704 lines/frame 480 or 576 pixels/frame 101,376 pixels pixels/second 2,534,400 frames/sec 30 Hz bit rate 1.86 Mbit/sec buffer size 40 Kbytes The sampling limits of CPB are bounded at the ever popular SIF rate: 396 macroblocks (101,376 pixels) per picture if the picture rate is less than or equal to 25 Hz, and 330 macroblocks (84,480 pixels) per picture if the picture rate is 30 Hz. The MPEG nomenclature loosely defines a pixel or “pel” as a unit vector containing a complete luminance sample and one fractional (0.25 in 4:2:0 format) sample from each of the two chrominance (Cb and Cr) channels. Thus, the
CPB are a limited set of sampling and bitrate parameters designed to normalize computational complexity, buffer size, and memory bandwidth while still addressing the widest possible range of applications. CPB limits video to 396 macroblocks (101,376 pixels) per frame if the frame rate is less than or equal to 25 fps (frames per second), and 330 macroblocks (84,480 pixels) per frame if the frame rate is less or equal to 30 fps. Therefore, MPEG video is typically coded at SIF dimensions (352 x 240 x 30fps or 352 x 288 x 25 fps). The total maximum sampling rate is 3.8 Ms/s (million samples/sec) including chroma. The coded video rate is limited to 1.862 Mbit/sec. In industrial practice, the bitrate is the most often waived parameter of CPB, with rates as high as 6 Mbit/sec in use.
