Coding Strategies and Standards - Digital Speech: Coding for Low Bit Rate Communication Systems - page 8

coding principles to encode different types of speech segments have been
introduced [11, 12, 13].
A hybrid coder can switch between a set of predefined coding modes.
Hence they are also referred to as multimode coders. A hybrid coder is an
adaptive coder, which can change the coding technique or mode according
to the source, selecting the best mode for the local character of the speech
signal. Network or channel dependent mode decision [14] allows a coder to
adapt to the network load or the channel error performance, by varying the
modes and the bit rate, and changing the relative bit allocation of the source
and channel coding [15].
In source dependent mode decision, the speech classification can be based
on fixed or variable length frames. The number of bits allocated for frames of
different modes can be the same or different. The overall bit rate of a hybrid
coder can be fixed or variable. In fact variable rate coding can be seen as an
extension of hybrid coding.
2.3 Algorithm Objectives and Requirements
The design of a particular algorithm is often dictated by the target application.
Therefore, during the design of an algorithm the relative weighting of
the influencing factors requires careful consideration in order to obtain a
balanced compromise between the often conflicting objectives. Some of the
factors which influence the choice of algorithm for the foreseeable network
applications are listed below.
2.3.1 QualityandCapacity
Speech quality and bit rate are two factors that directly conflict with each
other. Lowering the bit rate of the speech coder, i.e. using higher signal
compression, causes degradation of quality to a certain extent (simple para-
metric vocoders). For systems that connect to the Public Switched Telephone
Network (PSTN) and associated systems, the quality requirements are strict
and must conform to constraints and guidelines imposed by the relevant
regulatory bodies, e.g. ITU (previously CCITT). Such systems demand high
quality (toll quality) coding. However, closed systems such as private com-
mercial networks and military systems may compromise the quality to lower
the capacity requirements. Although absolute quality is often specified, it is
often compromised if other factors are allocated a higher overall rating. For
instance, in a mobile radio system it is the overall average quality that is often
the deciding factor. This average quality takes into account both good and
bad transmission conditions.