Digital Signal Processing Reference
In-Depth Information
Table 2.3
TIA/EIA speech coding standards for North American CDMA/TDMA
mobile communications
Bit rate
Noise
Delay
Speech coder
(kb/s)
VAD
reduction
(ms)
Quality
Year
IS-96-A (QCELP)
8.5/4/2/0.8
Yes
No
45
Near-toll
1993
IS-127 (EVRC)
8.5/4/2/0.8
Yes
Yes
45
Toll
1995
IS-733 (QCELP)
14.4/7.2/3.6/1.8
Yes
No
45
Toll
1998
IS-54 (VSELP)
7.95
Yes
No
45
Near-toll
1989
IS-641-A (ACELP)
7.4
Yes
No
45
Toll
1996
Table 2.4
DoD speech coding standards
Bit rate
Noise
Delay
Speech coder
(kb/s)
VAD
reduction
(ms)
Quality
Year
FS-1015 (LPC-10e)
2.4
No
No
115
Intelligible
1984
FS-1016 (CELP)
4.8
No
No
67.5
Communication
1991
DoD 2.4 (MELP)
2.4
No
No
67.5
Communication
1996
STANAG (NATO)
2.4/1.2
No
Yes
>
67.5
Communication
2001
2.4/1.2 (MELP)
on the mixed excitation linear prediction (MELP) vocoder [48] which is based
on the sinusoidal speech coding model. The 2.4 kb/s DoD MELP speech
coder gives better speech quality than the 4.8 kb/s FS-1016 coder at half the
capacity. A modified and improved version of this coder, operating at dual
rates of 2.4/1.2 kb/s and employing a noise preprocessor, has been selected
as the new NATO standard. Parametric coders, such as MELP, have been
widely used in secure communications due to their intelligible speech quality
at very low bit rates. The DoD standard speech coders are summarized in
Table 2.4.
2.4.5 SatelliteTelephony
The international maritime satellite corporation (INMARSAT) has adopted
two speech coders for satellite communications. INMARSAT has selected
4.15 kb/s improved multiband excitation (IMBE) [9] for INMARSAT M sys-
tems and 3.6 kb/s advanced multiband excitation (AMBE) vocoders for
INMARSAT Mini-M systems (see Table 2.5).
2.4.6 SelectionofaSpeechCoder
Selecting the best speech coder for a given application may involve extensive
testing under conditions representative of the target application. In general,
lowering the bit rate results in a reduction in the quality of coded speech.
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