PMR Signalling Processor
CMX881
CTCSS Tones
Table 2 lists the CTCSS tones available. The tone numbers are decimal equivalents of the numbers
written to the Audio & CTCSS Control register ($C2) and reported in the Tone Status register ($CC).
Table 2 CTCSS Tones
Tone
Number
001
01
Freq.
(Hz)
Tone
Number
20
Freq.
(Hz)
Tone
Number
40-54
552
Freq.
(Hz)
No Tone
67.0
131.8
136.5
141.3
146.2
151.4
156.7
162.2
167.9
173.8
179.9
186.2
192.8
203.5
210.7
218.1
225.7
233.6
241.8
250.3
69.3
Reserved
Invalid
tone
21
02
71.9
22
03
74.4
23
>=56
Reserved
04
77.0
24
05
79.7
25
06
82.5
26
07
85.4
27
08
88.5
28
09
91.5
29
10
94.8
30
11
97.4
31
12
100.0
103.5
107.2
110.9
114.8
118.8
123.0
127.3
32
13
33
14
34
15
35
16
36
17
37
18
38
19
39
Notes:
1
Tone number 00 in the Tone Status register ($CC) indicates that none of the above subaudio
tones is being detected - see also section 1.6.19. If tone number 00 is programmed into the
Audio & CTCSS Control register ($C2) no tone will be scanned for. If CTCSS transmit is
selected this tone setting will cause the CTCSS generator to output no signal.
2
Tone number 55 is reported in the Tone Status register ($CC), when CTCSS receive is enabled
and a subaudio tone is detected that does not correspond to the selected tone. This could be a
tone in the subaudio band which is not in the table or a tone in the table which is not the
selected tone.
1.5.3.3
Receiving and Decoding DCS Codes
DCS Code is in NRZ format and transmitted at 134.4 ±0.4bps. The CMX881 is able to decode any 23 or
24 bit pattern in either of the two DCS modulation modes defined by TIA/EIA-603 and described in
Table 3. The CMX881 can detect a valid DCS Code quickly enough to avoid losing the beginning of
voice transmissions.
Table 3 DCS Modulation Modes
Modulation Type:
A
Data Bit:
FM Frequency Change:
Minus frequency shift
Plus frequency shift
Plus frequency shift
Minus frequency shift
0
1
0
1
B
The CMX881 detects the DCS code that matches the programmed code defined in the ‘DCS Code’ words
(P2.2-3) in the Programming register ($C8).
To detect the pre-programmed DCS code the signal is low pass filtered to suppress all but the sub-audio
band using the filter shown in Figure 7. Further equalisation filtering, signal slicing and level detection
are done to extract the code being received. The extracted code is then matched with the programmed
23 or 24-bit DCS code to be recognised, in the order least significant first through to most significant DCS
2004 CML Microsystems Plc
18
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
