Coding Strategies and Standards - Digital Speech: Coding for Low Bit Rate Communication Systems - page 12

Table 2.1 ITU-T narrowband speech coding standards
Bit rate Noise Delay

Speech coder (kb/s) VAD reduction (ms) Quality Year

G.711 (A/ µ -Law PCM) 64 No No 0 Toll 1972

G.726 (ADPCM) 40/32/24/16 No No 0.25 Toll 1990

G.728 (LD-CELP) 16 No No 1.25 Toll 1992

G.729 (CSA-CELP) 8 Yes No 25 Toll 1996

G.723.1 6.3/5.3 Yes No 67.5 Toll/ 1995

(MP-MLQ/ACELP) Near-toll

∼

G.4k (to be determined) 4 - Yes 55 Toll 2001

is a hybrid model of CELP and sinusoidal speech coding principles [27, 28].
A summary of the narrowband speech coding standards recommended by
ITU-T is given in Table 2.1.
In addition to the narrowband standards, ITU-T has released twowideband
speech coders, G.722 [29] and G.722.1 [30], targeting mainly multimedia
communications with higher voice quality. G.722 [29] supports three bit rates,
64, 56, and 48 kb/s based on subband ADPCM (SB-ADPCM). It decomposes
the input signals into low and high subbands using the quadrature mirror
filters, and then quantizes the band-pass filtered signals using ADPCM with
variable step sizes depending on the subband. G.722.1 [30] operates at the
rates of 32 and 24 kb/s and is based on the transform coding technique.
Currently, a new wideband speech coder operating at 13/16/20/24 kb/s is
undergoing standardization.

2.4.2 EuropeanDigitalCellular TelephonyStandards
With the advent of digital cellular telephony there have been many speech
coding standardization activities by the European Telecommunications Stan-
dards Institute (ETSI). The first release by ETSI was the GSM full rate (FR)
speech coder operating at 13 kb/s [31]. Since then, ETSI has standardized
5.6 kb/s GSM half rate (HR) and 12.2 kb/s GSM enhanced full rate (EFR)
speech coders [32, 33]. Following these, another ETSI standardization activity
resulted in a new speech coder, called the adaptive multi-rate (AMR) coder
[34], operating at eight bit rates from 12.2 to 4.75 kb/s (four rates for the
full-rate and four for the half-rate channels). The AMR coder aims to provide
enhanced speech quality based on optimal selection between the source and
channel coding schemes (and rates). Under high radio interference, AMR is
capable of allocating more bits for channel coding at the expense of reduced
source coding rate and vice versa.
The ETSI speech coder standards are also capable of silence compres-
sion by way of voice activity detection [35-38], which facilitates channel