System Feature Overview 3
SECTION 2 SYSTEM FEATURE OVERVIEW
2.1 MOTOTRBO Digital Radio Technology
This section provides a brief overview of MOTOTRBO digital radio technology. It addresses two of
the primary benefits delivered by this technology: spectral efficiency and improved audio
performance.
2.1.1 Digital Radio Technology Overview
The digital radio technologies employed by MOTOTRBO can be summarized as follows:
Figure 2-1 “MOTOTRBO Digital Radio Technology” is broken down into four parts which are
described in the following subsections.
2.1.1.1 Part One: The Analog to Digital Conversion
When a radio user presses the Push-To-Talk (PTT) button and begins speaking, his voice is
received by the radio microphone and converted from an acoustic waveform to an analog
electrical waveform. This voice waveform is then sampled by an analog to digital converter. In
typical radio applications, a 16-bit sample is taken every 8 kHz, this produces a 128,000bps (bits
per second) digital bitstream, which contains far too much information to send over a 12.5 kHz or
25 kHz radio channel. Therefore some form of compression is required.
2.1.1.2 Part Two: The Vocoder and Forward Error Correction (FEC)
Vocoding (Voice encoding) compresses speech by breaking it into its most important parts and
encoding them with a small number of bits, while greatly reducing background noise. Vocoding
compresses the voice bitstream to fit the narrow (for MOTOTRBO) 6.25 kHz equivalent radio
channel. The MOTOTRBO vocoder is AMBE+2
TM
which was developed by Digital Voice System,
Inc. (DVSI), a leader in the vocoding industry. This particular vocoder works by dividing speech
into short segments, typically 20 to 30 milliseconds in length. Each segment of speech is analyzed,
and the important parameters such as pitch, level, and frequency response are extracted. These
parameters are then encoded using a small number of digital bits. The AMBE+2
TM
vocoder is the
Figure 2-1 MOTOTRBO Digital Radio Technology
1 2 3 4
data input
or
microphone input
digital
bitstream
compressed
digital voice
digital
packets
Slot 1:
Radio Transmit
Transmission,
Encoding &
RF Amplication
Framing
header payload
Vocoder &
Forward Error
Correction
IP Data Interface
Analog to Digital
Slot 2:
Radio waits;
spectrum available
to another radio
Slot 1:
Radio Transmit
next burst
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