System Feature Overview 13
68007024085 April 2011
2.1.3.4 Audio Balancing
Transmitting voice over a digital air interface requires a voice coder, or vocoder for short. The
vocoder used by MOTOTRBO is the Digital Voice Systems Inc. (DVSI) AMBE+2
TM
. This vocoder
delivers excellent voice quality with robustness to both background noise and RF channel bit
errors in a 6.25 kHz equivalent channel bandwidth. In order to produce optimal voice quality, the
input level into the vocoder must fall within a specific amplitude range.
The diverse nature of users with respect to mouth-to-microphone distance as well as voice level
and directivity can make this a bit problematic. In an effort to produce optimal voice quality over
these diverse input conditions, MOTOTRBO digital always employs Automatic Gain Control (AGC)
in the audio transmit path. The primary function of the transmit AGC is to produce the best voice
quality possible under real life conditions. Since voice is still the main application of a two-way
radio, this is a primary goal.
A secondary result of the AGC is to produce flat received speech loudness level over a range of
input levels at the microphone. The usage of IMPRES Accessories extends this input range so
optimal voice quality occurs over an even greater input range. Section Figure 2-7 “Transmit Audio
Sensitivity”illustrates this extended range flat response in the curve titled MOTOTRBO with
IMPRES RSM (Digital). This same response curve can also be produced in analog mode by using
a IMPRES Accessory and enabling Analog Mic AGC in the CPS General Settings. Figure 2-7
illustrates this type of response in the curve titled MOTOTRBO with IMPRES RSM (AGC on,
Analog). An advantage of this type of response is that soft talkers and users that turn away from
the microphone while speaking will still come through loud and clear.
Figure 2-7 Transmit Audio Sensitivity
75
80
85
90
95
100
80 85 90 95 100 105 110
Transmitter Input [dB SPL]
Receiver Output Speech Loudness
Prof essional Series
MOTOTRBO with IMPRES RSM (AGC off, Analog)
MOTOTRBO with IMPRES RSM (AGC on, Analog)
MOTOTRBO with IMPRES RSM (Digital)
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