Page 230 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.4. E"ects of Errors 207

9.3.1 RandomBit Errors
Random bit errors can occur in the form of bit inversion, bit insertion, and=or
bit deletion. They are usually quanti ed using a parameter called the bit error
rate (BER), which is the average probability that a bit is in error. Random
bit errors are usually caused by physical e ects like thermal noise.

9.3.2 Erasure (or Burst) Errors

Erasure errors occur in the form of a loss of (or damage to) contiguous seg-
ments of bits. They are usually quanti ed using parameters like the number
of bursts, the length of a burst, and the BER within a burst. Burst errors in a
mobile channel can be caused by a number of mechanisms, such as short-term
(multipath) fading, long-term (shadowing) fading, and co-channel interference.
In a packet-based network, burst errors occur in the form of packet losses due
to di erent reasons, such as congestion, misrouting, and delivery with unac-
ceptably long delays.
It should be pointed out, however, that this classi cation does not take
into account the impact of errors, which is highly dependent on the coding
method. For example, it will be shown later that due to the use of predictive
and VLC coding, random bit errors in a video bitstream can cause severe
error propagation. Thus, random bit errors in a video bitstream are e ectively
equivalent to burst errors. In what follows, no distinction will be made between
the two types of errors, and the generic term transmission errors will be used
to refer to both types.

9.4 E(ects of Errors

Errors occurring in a video bitstream can cause isolated e"ects, spatial error
propagation, and=or temporal error propagation.

9.4.1 Isolated E(ects

In this case the e ect of an error is limited and does not propagate either
spatially or temporally. An example is an error in a FLC codeword. An-
other example is an error that converts a VLC codeword into another valid
codeword of the same length. Note, however, that for both cases to have an
isolated e ect, it is assumed that the damaged codeword is not a prediction for
another codeword and that no temporal error propagation occurs due to motion-
compensated prediction. Clearly, such isolated e ects are rare occurrences in
video bitstreams, and when they do occur their damage is usually acceptable