115 What are Constrained Parameters Bitstreams? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
115 What are Constrained Parameters Bitstreams? (MPEG-2)
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
corresponding bandwidth figure can be computed as:
352 samples/line x 240 lines/picture x 30 pictures/sec x 1.5
samples/pixel
or 3.8 Ms/s (million samples/sec) including chroma, but not including
blanking intervals. Since most decoders are capable of sustaining VLC
decoding at a faster rate than 1.8 Mbit/sec, the coded video bitrate
has become the most often waived parameter of CPB. An encoder which
intelligently employs the syntax tools should achieve SIF quality
saturation at about 2 Mbit/sec, whereas an encoder producing streams
containing only I (Intra) pictures might require as much as 8 Mbit/sec
to achieve the same video quality.
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