Introduction - Digital Speech: Coding for Low Bit Rate Communication Systems - page 2

rapid improvement in digital hardware which has enabled the use of sophis-
ticated digital signal processing techniques that were not feasible before. In
response to the requirement for speech compression, feverish research activ-
ity has been pursued in all of the main research centres and, as a result, many
different strategies have been developed for suitably compressing speech for
bandwidth-restricted applications. During the last two decades, these efforts
have begun to bear fruit. The use of low bit-rate speech coders has been
standardized in many international, continental and national communication
systems. In addition, there are a number of private network operators who
use low bit-rate speech coders for specific applications.
The speech coding technology has gone through a number of phases starting
with the development and deployment of PCM and ADPCM systems. This
was followed by the development of good quality medium to low bit-rate
coders covering the range from 16 kb/s to 8 kb/s. At the same time, very
low bit-rate coders operating at around 2.4 kb/s produced better quality
synthetic speech at the expense of higher complexity. The latest trend in
speech coding is targeting the range from about 6 kb/s down to 2 kb/s by
using speech-specific coders, which rely heavily on the extraction of speech-
specific information from the input source. However, as the main applications
of the low to very low bit-rate coders are in the area of mobile communication
systems, where there may be significant levels of background noise, the
accurate determination of the speech parameters becomes more difficult.
Therefore the use of active noise suppression as a preprocessor to low bit-rate
speech coding is becoming popular.
In addition to the required low bit-rate for spectral efficiency, the cost
and power requirements of speech encoder/decoder hardware are very
important. In wireless personal communication systems, where hand-held
telephones are used, the battery consumption, cost and size of the portable
equipment have to be reasonable in order to make the product widely
acceptable.
In this topic an attempt is made to cover many important aspects of low bit-
rate speech coding. In Chapter 2, the background to speech coding, including
the existing standards, is discussed. In Chapter 3, after briefly reviewing the
sampling theorem, scalar and vector quantization schemes are discussed and
formulated. In addition, various quantization types which are used in the
remainder of this topic are described.
In Chapter 4, speech analysis and modelling tools are described. After
discussing the effects of windowing on the short-time Fourier transform
of speech, extensive treatment of short-term linear prediction of speech is
given. This is then followed by long-term prediction of speech. Finally,
pitch detection methods, which are very important in speech vocoders, are
discussed.