Coding Strategies and Standards - Digital Speech: Coding for Low Bit Rate Communication Systems - page 11

Other solutions are often used. A very common one is to detect the voiceband
data and use an interface which bypasses the speech encoder/decoder.
2.4 Standard Speech Coders
Standardization is essential in removing the compatibility and conforma-
bility problems of implementations by various manufacturers. It allows for
one manufacturer's speech coding equipment to work with that of others.
In the following, standard speech coders, mostly developed for specific
communication systems, are listed and briefly reviewed.
2.4.1 ITU-TSpeechCodingStandard
Traditionally the International Telecommunication Union Telecommunica-
tion Standardization Sector (ITU-T, formerly CCITT) has standardized speech
coding methods mainly for PSTN telephony with 3.4 kHz input speech band-
width and 8 kHz sampling frequency, aiming to improve telecommunication
network capacity by means of digital circuit multiplexing. Additionally,
ITU-T has been conducting standardization for wideband speech coders to
support 7 kHz input speech bandwidth with 16 kHz sampling frequency,
mainly for ISDN applications.
In 1972, ITU-T released G.711 [19], an A/ µ -Law PCM standard for 64 kb/s
speech coding, which is designed on the basis of logarithmic scaling of
each sampled pulse amplitude before digitization into eight bits. As the
first digital telephony system, G.711 has been deployed in various PSTNs
throughout the world. Since then, ITU-T has been actively involved in
standardizing more complex speech coders, referenced as the G.72x series.
ITU-T released G.721, the 32 kb/s adaptive differential pulse codemodulation
(ADPCM) coder, followed by the extended version (40/32/24/16 kb/s),
G.726 [20]. The latest ADPCM version, G.726, superseded the former one.
Each ITU-T speech coder except G.723.1 [21] was developed with a view
to halving the bit rate of its predecessor. For example, the G.728 [22] and
G.729 [23] speech coders, finalized in 1992 and 1996, were recommended at
the rates of 16 kb/s and 8 kb/s, respectively. Additionally, ITU-T released
G.723.1 [21], the 5.3/6.3 kb/s dual-rate speech coder, for video telephony
systems. G.728, G.729, and G.723.1 principles are based on code excited linear
prediction (CELP) technologies. For discontinuous transmission (DTX), ITU-T
released the extended versions of G.729 and G.723.1, called G.729B [24] and
G.723.1A [25], respectively. They are widely used in packet-based voice
communications [26] due to their silence compression schemes. In the past
few years there has been standardization activities at 4 kb/s. Currently there
two coders competing for this standard but the process has been put on
hold at the moment. One coder is based on the CELP model and the other