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FURTHERREADING 319CONCLUSION
This chapter has explained the basics of speech and audio
coding and its applications to Internet telephony and In-
ternet streaming. It should be clear that especially for the
lower bit rates, there is no single coder that is good for all
applications, and that careful tailoring toward the appli-
cation is important. Speech and audio coding is a mature
field, with many available solutions. Based on our current
knowledge and the various constraints that exist, it is ex-
pected that future developments in this field will focus
less on compression efficiency and more on application-
specific issues, such as scalability, error robustness, delay,
and complexity.GLOSSARY
Audio compression Reducing the amount of digital in-
formation to describe audio and speech signals, while
maintaining audio quality as much as possible.
Digitization Creating a digital version of an analog sig-
nal by sampling and quantization.
Error mitigation Techniques used to reduce the per-
ceivable impact of transmission errors of a compressed
audio signal.
ITU International Telecommunications Union, which
sets standards for global communications.
Linear prediction A technique commonly used to re-
move redundancies in a speech signal.
Lossless compression A compression technique that
uses statistical properties of a digital representation to
create a compressed version. After decompression the
output signal will be equivalent to the input signal.
Lossy compression A compression technique that cre-
ates a compressed version of the input signal by taking
into account that the decoded audio signal may sound
different.
MPEG Motion Picture Experts Group, which sets
worldwide standards for media compression.
Perceptually lossless compression A particular subset
of lossy compression techniques, in which the differ-
ences between input and out signals are not audible.
Quantization Converting an analog value into a discrete
digital value.Quality of service A term commonly used in the con-
text of Internet communications and broadcasting to
describe consistency in delivering a particular signal
quality.
Sampling frequency The rate at which analog signal
samples are digitized. The value of the sampling fre-
quency will be twice the highest possible frequency that
can be contained in the signal.
Streaming Delivering audio signals over a packet net-
work for continued broadcasting.
Transcoding Converting from one compressed format
to another by decoding and encoding. If the coders
used are lossy coders, the transcoding process typically
introduces additional degradation.CROSS REFERENCES
SeeData Compression; Video Compression.FURTHER READING
Bosi, M., Brandenburg, K., Quackenbush, S., Fielder, L.,
Akagiri, K., Fuchs, H., Dietz, M., Herre, J., Davidson,
G., & Oikawa, Y. (1997). ISO/IEC MPEG-2 advanced
audio coding.Journal of the Audio Engineering Society,
45 (10), 789–814.
Faller, C., Juang, B.-H., Kroon, P., Lou, H.-L., Ram-
prashad, S. A., & Sundberg, C.-E. W., (2002). Technical
advances in digital audio radio broadcasting.Proceed-
ings of the IEEE, 90(8), 1303–1335.
ITU-T. Retrieved May 23, 2003, from http://www.itu.int
Kleijn, W. B., & Paliwal, K. K. (Eds.) (1995). Speech coding
and synthesis. Amsterdam, The Netherlands: Elsevier.
Madisetti, V. K., & Williams, D. B. (Eds.) (1998). The dig-
ital signal processing handbook. Boca Raton, FL: CRC
Press.
MPEG Audio Page. Retrieved May 23, 2003, from
http://www.tnt-uni-hannover.de/project/mpeg/audio
Painter, T., & Spanias, A. (2000, April). Perceptual coding
of digital audio.Proceedings of the IEEE, 88,451–513.
Pohlman, K. C. (2000).Principles of digital audio(4th ed.).
New York: McGraw–Hill.
Quatieri, T. F. (2001).Principles of discrete-time speech pro-
cessing. New York: Prentice Hall.