73 What are constant and variable bitrate streams? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
73 What are constant and variable bitrate streams? (MPEG-2)
Constant bitrate streams are buffer regulated to allow continuos
transfer of coded data across a constant rate channel without causing
an overflow or underflow to a buffer on the receiving end. It is the
responsibility of the Encoders Rate Control stage to generate
bitstreams which prevent buffer overflow and underflow. The constant
bit rate encoding can be modeled as a reservoir: variable sized coded
pictures flow into the bit reservoir, but the reservoir is drained at a
constant rate into the communications channel. The most challenging
aspect of a constant rate encoder is, yes, to maintain constant channel
rate (without overflowing or underflow a buffer of a fixed depth) while
maintaining constant perceptual picture quality.
In the simplest form, variable rate bitstreams do not obey any buffer
rules, but will maintain constant picture quality. Constant picture
quality is easiest to achieve by holding the macroblock quantizer step
size constant (e.g. level 16 of 31). In its most advanced form, a
variable bitrate stream may be more difficult to generate than
constant bitrate streams. In advanced variable bitrate streams, the
instantaneous bit rate (piece-wise bit rate) may be controlled by
factors such as: 1. local activity measured against activity over
large time intervals (e.g. the full span of a movie), or 2.
instantaneous bandwidth availability of a communications channel.
Summary of bitstream types
Bitrate type
Applications
constant-rate
fixed-rate communications channels like the original Compact Disc,
digital video tape, single channel-per-carrier broadcast signal, hard
disk storage
simple variable-rate
software decoders where the bitstream buffer (VBV) is the storage
medium itself (very large). macroblock quantization scale is typically
held constant over large number of macroblocks.
complex variable-rate
Statistical muliplexing (multiple-channel-per-carrier broadcast
signals), compact discs and hard disks where the servo mechanisms can
be controlled to increase or decrease the channel delivery rate,
networked video where overall channel rate is constant but demand is
variably share by multiple users, bitstreams which achieve average
rates over very long time averages
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