Rohde & Schwarz CMU-K20, Operating Manual

Page: 283 / 646

CMU-K20/K21/K22/K23/K24
Adaptive Multi-Rate (AMR) Speech Codec
1115.6088.12 4.205 E-12
AMR
Rate Set
The
AMR Rate Set
softkey opens a popup menu to select four codec modes and
define the decision thresholds for changing the codec mode.
AMR Full Rate
The first line of the
AMR Rate Set Editor
indicates the AMR codec
type (Full Rate or Half Rate) selected by means of the
Traffic Mode
softkey.
Codec – Mode
Selects the data rate for modes 4 to 1. For full rate codecs, the full
set of 8 different rates (4.75 kbit/s, 5.15 kbit/s, 5.9 kbit/s, 6.7 kbit/s,
7.4 kbit/s, 7.95 kbit/s, 10.2 kbit/s, 12.2 kbit/s) is available. The last
two rates are not provided for half rate codecs.
The selected data rates must be different from each other. The
entered values are automatically sorted in descending order so that
Rate (Mode 1) < Rate (Mode 2) < Rate (Mode 3) < Rate (Mode 4).
To restrict the test model to 1, 2, or 3 modes, the codec modes can
be switched off using the
ON/OFF
key.
Threshold
Sets the lower decision thresholds for switching between modes j
and j – 1
(Down j
where j = 2, 3, 4) and the upper decision
thresholds for switching between modes j and j + 1
(Up j
where j 0
1, 2, 3). Both transition thresholds are given in terms of a
normalized carrier to interferer (C/I) ratio and must be entered in 0.5
dB steps (see standard 3GPP TS 05.09).
MODE 4
MODE 3
MODE 2
MODE 1
C/I
Up 1
Down 2
Down 3
Up 2
Down 4
Up 3
To ensure stable operation near the thresholds, switching down to a
lower codec mode is usually initiated at lower C/I threshold values
than switching up to a higher codec mode. The difference between
the upper and lower thresholds is generally termed hysteresis:
Hyst (j) = Up (j) – Down (j + 1).
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