Media Compression - Scalable Continuous Media Streaming Systems - page 32

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Figure 2.7 Small time scale bit-rate variations in a constant bit-rate encoded video

the outgoing data traffic will vary substantially. This will certainly make network resource
planning and/or allocation more complex. The varying traffic may even lead to occasional
packet loss due to instantaneous congestion in the network routers or at the client. The extent
of bit-rate variations is determined by the rate control algorithm in the video encoder, which
can be classified into two types - constant bit-rate encoding and constant quality encoding.
In constant bit-rate (CBR) encoding, the encoder will attempt to compress the video stream
to a constant average bit-rate for the whole duration of the video content. However, as discussed
earlier, different frame types inherently consume different amount of bits. Thus, in practice,
the average bit-rate is not computed in a frame-by-frame basis. Instead, the average bit-rate is
computed over a fixed number of frames, such as a GOP, and then the encoder will try to keep
the average bit-rate the same at the GOP level.
Alternatively, an encoder can also operate in a constant quality mode. Unlike the CBR
encoding mode where constant bit-rate is the goal, a constant quality encoder will attempt to
adjust the bit-rate to maintain a consistent visual quality over the whole video content. Recall
that a video encoder achieves some of its compression from removing temporal redundancies,
which is closely correlated to the amount of motions in the video content. Thus, for fast motion
scenes there will be less temporal redundancies and thus more errors in the predicted frame,
which then consumes more bits to encode. In CBR encoder, the available bit budget is fixed
due to the need to keep the rate constant, and so more information will be discarded, leading
to lower visual quality for fast-motion scenes.
By contrast, constant quality encoder will simply use more bits to encode the information
to maintain the same quality level, thus resulting in higher bit-rate for fast-motion scenes.
Consequently, constant quality encoders will produce compressed video stream of varying
bit-rates. Note that the rate variations can be of far longer range (in seconds or even tens of
minutes) than the duration of a GOP (typically less than 1 second). Streaming a variable bit-rate