CTCSS

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Continuous Tone-Coded Squelch System

Initially the purpose of this system - also known as tone squelch was to be able to "tune out" of conversations in a shared communications channel. Only stations sharing the same tone could communicate with each other. It works by the transmitting station transmitting a continuous tone wich the receiving station locks on to and hence lets the voice signal in to be decoded.

In ham radio the system is fequently used in repeater locations that are shared with other spectrum users such as pagers. Close physical proximity and in some cases poor filtering can cause interference to repeater operations.

CTCSS is used to "open" the repeater for use. Any signal that does not contain the required CTCSS signal is effectively filtered out because it is rejected by the repeater.

CTCSS tones and codes

#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Freq Hz
67.0
71.9
74.4
77
79.7
82.5
85.4
88.5
91.5
94.8
97.4
100.0
103.5
107.2
110.9
114.8
EIA code
XZ
XA
WA
XB
SP
YZ
YA
YB
ZZ
ZA
ZB
1Z
1A
1B
2Z
2A
#
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Freq Hz
118.8
123.0
127.3
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
EIA code
2B
3Z
3A
3B
4Z
4A
4B
5Z
5A
5B
6Z
6A
6B
7Z
7A
M1
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