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CTCSS/DCS Tones and encryption

Many radios on PMR446 utilise tones, the most common two in usage are CTCSS and DCS.

CTCSS

CTCSS stands for Continuous Tone-Coded Squelch Systems or CTCSS for short.

CTCSS is used to allow two or more groups of people to use the same channel as well as covering static noise/interference.

The way CTCSS works is that a code is chosen from the radio, when the radio then transmits alongside the voice it also emits a sub-audible audio tone that can be heard by other CTCSS equipped radios.

If CTCSS is enabled on the receiving radio and it is set to the same code, it will allow the transmission to be heard, if the tone is different or no tone at all exists, it will remain silent.

This allows multiple groups to use the same channel despite being the same frequency, for example if group A were using channel 2 (446.001875) CTCSS tone 1 (67Hz) and group B were using the same channel but CTCSS tone 2(69.3 Hz) the two groups would not hear each others transmissions at all.


CTCSS number CTCSS Frequency
1 67
2 69.3
3 71.9
4 74.4
5 77
6 79.7
7 82.5
8 85.4
9 88.5
10 91.5
11 94.8
12 97.4
13 100
14 103.5
15 107.2
16 110.9
17 114.8
18 118.8
19 123
20 127.3
21 131.8
22 136.5
23 141.3
24 146.2
25 150
26 151.4
27 156.7
28 159.8
29 162.2
30 162.2
31 165.5
32 167.9
33 171.3
34 173.8
35 177.3
36 179.9
37 183.5
38 186.2
39 189.9
40 192.8
41 196.6
42 199.5
43 203.5
44 206.5
45 210.7
45 213.8
46 218.1
47 221.3
48 225.7
49 229.1
50 233.6
51 233.6
52 237.1
53 241.8
54 245.5
55 250.3
56 254.1


DCS

DCS or Digital coded squelch is a system that is identical in function to CTCSS however is achieved not by a sub audible tone but instead using digital data.

Instead of the sub-audible tone, a stream of data is passed alongside the voice containing the DCS ID, radios with the DCS id set will only receive transmissions from other radios with the same DCS ID in the same way as CTCSS.

Disadvantages of tones

Tones such as CTCSS and DCS have common misconceptions.


Interference

One of the biggest misconceptions about CTCSS is that it eliminates interference, it does not eliminate interference but only covers it up.

What this means is that if a group is using a CTCSS tone to communicate, and another group is on the same channel without the same CTCSS tone or none, if one of those other people are transmitting and in a closer vicinity to the person with the correct tone who is transmitting to the group, their message would be blocked due to the transmission being stronger, despite not being heard, the group would only hear silence.

Privacy claims

Many also believe that CTCSS and DCS tones offer privacy this is not helped by the fact several radio manufacturers will often brand CTCSS/DCS as privacy tones.

This is incorrect. Any scanner or radio that is equipped without any receive CTCSS settings will be able to hear the conversations, CTCSS and DCS simply stop YOU from hearing others, not for THEM not to hear you.

Encryption

Encryption is sometimes utilised when a group wants their communications to remain private from any listeners, the most common form of analog encryption is known as voice-inversion scrambling.

The voice itself is scrambled by a specific frequency tone in which the higher tones of the voice are inverted (flipped) to be the lower tones and vice versa. To any receiving radio without the voice-scrambler the speech will be unintelligible and sound alien-like. An example of a radio with a voice-scrambler is the Baofeng BF-888S.