Chapter 2: Overview; Digital Modulation Technology; Voice Compression Technology; Transceiver - Motorola ST7000 Basic Service Manual

MN002952A01-AH

Chapter 2: Overview

Chapter 2
Overview
To achieve a high spectrum efficiency, the radio uses digital modulation technology and sophisticated
voice-compression algorithm. The voice of the person speaking into the microphone is converted into a
digital bit stream consisting of zeros (0) and ones (1).
This stream is then modulated into a radio frequency (RF) signal, which is transmitted over-the-air to
another radio. The process is called digital modulation.
2.1

Digital Modulation Technology

The radio operates in dispatch and phone modes.
The radio also operates in Trunked Mode Operation (TMO) and Direct Mode Operation (DMO). It uses
the π/4 DQPSK and Time Division Multiple Access (TDMA) digital technologies.
π/4DQPSK is a modulation technique that transmits information by altering the phase of the signal.
Data is converted into complex symbols, which alter the RF signal and transmit the information. When
the signal is received, the change in phase is converted back into symbols and then into the original
data.
The system can accommodate 4-voice channels in the standard 25 kHz channel as used in the two-
way radio. TDMA is used to allocate portions of the RF signal by dividing time into four slots, one for
each unit.
Time allocation enables each unit to transmit its data information without interference from other
transmitting units. Transmission from a unit or base station is accommodated in time-slot lengths of 15
ms and frame lengths of 60 ms. The TDMA technique requires sophisticated algorithms and a Digital
Signal Processor (DSP) to perform voice compressions/decompressions and RF modulation/
demodulation.
2.2

Voice Compression Technology

Voice is converted into a digital bit stream by sampling the voice at high rate and converting the
samples into numbers, which are represented by bits.
Voice compression reduces the number of bits per second while maintaining the voice at an
acceptable quality level. The radio uses a coding technique called Algebraic Code Excited Linear
Prediction (ACELP). The compressed voice-data bits modulate the RF signal.
2.3

Transceiver

All radio circuitry are contained in the Digital/RF board and the keypad board.
The Digital/RF board is divided into the following sections.
• Digital
• Transmitter
• Receiver
• Frequency Generating
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