PMR Signalling Processor
CMX881
The CMX881 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 ($B1).
1.5.4.2 CTCSS Tone
The sub-audio CTCSS tone generated is defined in the Tx CTCSS register ($C2). Table 2 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 ($C8); the same DCS Code pattern is used for detection and transmission.
The DCS Code is NRZ encoded at 134.4±0.4 bits/s, low pass filtered and added to the voice band signal,
prior to passing the signal to the modulator output stages. Valid 23-bit DCS codes and the corresponding
settings for the DCS Code Register are shown in Table 4, this does not preclude other codes being
programmed. The least significant bit of the DCS code is transmitted first and the most significant bit is
transmitted last. The CMX881 is able to encode and transmit either of the two DCS modulation modes
defined by TIA/EIA-603 and described in Table 3.
To signal the end of the DCS transmission, the host should set the special sub-audio bits in the Audio &
CTCSS Control register ($C2) to enable the DCS turn off tone for 150ms to 200ms. After this time period
has elapsed the host should then disable DCS in the Mode register ($C1). Do not enable CTCSS in the
Mode Control ($C1) register when transmitting the DCS turn off tone. To summarize, detection of DCS
turn off tone requires the CTCSS decoder to be enabled, whereas generation of the DCS turn off tone
requires the CTCSS encoder to be disabled.
2004 CML Microsystems Plc
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D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
