Guardian G25AMK005, Technical Manual

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G U A R D I A N V H F 1 1 0 W M O B I L E 3-7
Figure 3-2: Transmit DSP Chain
3.3.1.4 CVSD DES Module
Audio data from the Audio Processing module is sent to the audio circular buffer. The sample rate is increased from
8 ksps to 12 ksps. The CVSD encodes the data and sends it to the transmit CVSD audio circular buffer. The data is
DES-encrypted and differentially encoded before sent to the physical interface buffer. The CVSD physical layer
converts the CVSD encoded, DES encrypted 12 ksps data stream into a 48 ksps waveform suitable for processing by
the Modulation module. The module contains a finite impulse response (FIR) raised cosine filter that acts as an
interpolation filter. Transmitting an end of message (EOM) indicator, consisting of 160 ms of alternating ones and
zeros, signals the end of a transmission. This allows the receiving radio to squelch the audio output before the radio
stops transmitting.
3.3.1.5 Analog FM Module
Audio data entering the Analog FM module is sent through a linear-phase, FIR, audio-shaping filter. Interpolation
from 8 ksps to 48 ksps is accomplished using a linear-phase, FIR filter. A single-quadrant sine look-up table (LUT),
using fractional addressing and quadrant folding, generates continuous tone controlled squelch system (CTCSS)
tones. If the DCS audio turn-off code is transmitted, the tone is fixed at 134.4 Hz and the codes transmitted at a rate
of 134.4 bps, derived using the CTCSS tone generator. The DCS data stream passes through a raised cosine filter
before added to the speech. The 8 ksps audio stream, with CTCSS/DCS controls, is interpolated to 48 ksps before
sent to the Modulation module.
3.3.1.6 Modulation Module
The Modulation module prepares the signal for transmission. The signal is split into a reference oscillator signal and
a voltage controlled oscillator (VCO) signal. This allows independent scale and offset values for each signal. A
modulation-balance variable scales the reference oscillator voltage, so that the maximum frequency deviation is
constant for all RF channels. A transmit modulation variable does the same for the VCO signal.
CVSD DES
Module
Audio
CODEC
Transceiver
Audio
Processing
Module
Modulation
Module
Project 25 Voice
Module
Analog FM
Module
VOICE
DTMF
Tones