Databases Reference
In-Depth Information
The ATM network, if it is not congested, will accommodate the variable rate generated
by the compression algorithm. But if the network is congested, the compression algorithm
will have to operate at a reduced rate. If the network is well designed, the latter situation
will not happen too often, and the video coder can function in a manner that provides uniform
quality. However, when the network is congested, it may remain so for a relatively long period.
Therefore, the compression scheme should have the ability to operate for significant periods
of time at a reduced rate. Furthermore, congestion might cause such long delays that some
packets arrive after they can be of any use; that is, the frame they were supposed to be a part
of might have already been reconstructed.
In order to deal with these problems, it is useful if the video compression algorithm pro-
vides information in a layered fashion, with a low-rate high-priority layer that can be used
to reconstruct the video, even though the reconstruction may be poor, and low-priority en-
hancement layers that enhance the quality of the reconstruction. This is similar to the idea
of progressive transmission, in which we first send a crude but low-rate representation of the
image, followed by higher-rate enhancements. It is also useful if the bit rate required for the
high-priority layer does not vary too much.
19.13.3 Compression Algorithms for Packet Video
Almost any compression algorithm can be modified to perform in the ATM environment, but
some approaches seem more suited to this environment. We briefly present two approaches
(see the original papers for more details).
One compression scheme that functions in an inherently layered manner is subband coding.
In subband coding, the lower-frequency bands can be used to provide the basic reconstruction,
with the higher-frequency bands providing the enhancement. As an example, consider the
compression scheme proposed for packet video byKarlsson andVetterli [
263
]. In their scheme,
the video is divided into 11 bands. First, the video signal is divided into two temporal bands.
Each band is then split into four spatial bands. The low-lowband of the temporal low-frequency
band is then split into four spatial bands. A graphical representation of this splitting is shown
in Figure
19.19
. The subband denoted 1 in the figure contains the basic information about the
video sequence. Therefore, it is transmitted with the highest priority. If the data in all the other
subbands are lost, it will still be possible to reconstruct the video using only the information
in this subband. We can also prioritize the output of the other bands, and if the network starts
getting congested and we are required to reduce our rate, we can do so by not transmitting the
information in the lower-priority subbands. Subband 1 also generates the least variable data
rate. This is very helpful when negotiating with the network for the amount of priority traffic.
Given the similarity of the ideas behind progressive transmission and subband coding, it
should be possible to use progressive transmission algorithms as a starting point in the design
of layered compression schemes for packet video. Chen, Sayood, and Nelson [
264
]usea
DCT-based progressive transmission scheme [
265
] to develop a compression algorithm for
packet video. In their scheme, they first encode the difference between the current frame and
the prediction for the current frame using a 16
16 DCT. They only transmit the DC coefficient
and the three lowest-order AC coefficients to the receiver. The coded coefficients make up the
highest-priority layer.
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