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CDMA 120 – cdmaOne and CDMA2000 Concepts and Terminology phần 6 doc
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CDMA 120 – cdmaOne and CDMA2000 Concepts and Terminology
Section 5: Attributes and Techniques
80-13321-1 X11
© 2002 QUALCOMM Incorporated 5-13
CDMA 120
cdma university Section 5-13
Variable Rate Vocoder
Pulse Coded
Modulation
(PCM)
64 kbps 14.4 kbps (max)
101001
about 200 milliseconds
CDMA takes advantage of periods of reduced speech activity
Codec Vocoder
MMT98010674Ac.emf
Codec
A codec is an analog-to-digital and digital-to-analog converter.
The figure depicts the codec as an analog-to-digital converter whose output is a wide band PCM
signal (bit rate = 64 kbps).
CDMA 120 – cdmaOne and CDMA2000 Concepts and Terminology
Section 5: Attributes and Techniques
80-13321-1 X11
© 2002 QUALCOMM Incorporated 5-14
CDMA 120
cdma university Section 5-14 Variable Rate Vocoder (continued)
20 msec
24 bits
20 msec
48 bits
20 msec
96 bits
20 msec
192 bits
Eighth
Rate
Quarter
Rate
Half
Rate
20 msec
96 bits
Half
Rate
Full
Rate
MMT98010675Ag.emf
Variable Rate Vocoder
The vocoder compresses the output of the codec to a lower bit rate to reduce bandwidth.
The variable rate vocoder takes advantage of low speech activity and transmits at lower rates, thus
reducing the average transmission to about 4 kbps.
The figure illustrates how the vocoder outputs frames at Full, Half, Quarter, and Eighth rates.