FRS Signalling Processor
CMX882
The CMX882 provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz,
matching the template shown in Figure 10.
16
12
+6dB/octave
8
+1dB
4
0dB
0
dB
-4
-3dB
-8
-12
-16
-20
+12dB/octave
100
1000
10000
Frequency (Hz)
Figure 10 Audio Frequency Pre-emphasis Template
Modulator Output Routing
The sub-audio component can be combined with the voice band signal and this composite signal routed
to both MOD_1 and MOD_2 outputs, or the sub-audio and voice band signal can be output separately
(sub-audio to MOD_2 and voice band to MOD_1), in accordance with the settings of the Signal Routing
register ($B1).
Tx Companding (Compressing)
The CMX882 incorporates an optional syllabic compandor in both transmit and receive mode. This
compresses voice band signals before transmission to enhance dynamic range. The compandor attack,
decay and 0dB point are defined in section 1.8.1. See section 1.6.9 for details of how to control this
function.
Audio Scrambling
The CMX882 incorporates an optional frequency inversion scrambler in transmit mode. This scrambles
voice band signals to be de-scrambled in the receiver. See section 1.6.9 for details of how to control this
function.
Voice Processing Combinations
Table 1 shows the valid voice processing combinations (see section 1.5)
1.5.4.2 CTCSS Tone
The sub-audio CTCSS tone generated is defined in the Tx CTCSS register ($C2). Table 3 lists the
CTCSS tones and the corresponding value for programming the TX TONE bits.
1.5.4.3
DCS Code
A 23 or 24-bit sub-audio DCS Code can be generated, as defined by the ‘DCS Code’ words (P2.4-5) of
the Programming register ($C8); the same DCS Code pattern is used for detection and transmission.
The DCS Code is NRZ encoded at 134.4±0.4 bits/s, low pass filtered and added to the voice band signal,
prior to passing the signal to the modulator output stages. Valid 23-bit DCS codes and the corresponding
settings for the DCS Code Register are shown in Table 5, this does not preclude other codes being
programmed. The least significant bit of the DCS code is transmitted first and the most significant bit is
2004 CML Microsystems Plc
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D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
