CTCSS

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The CTCSS ( English C ontinuous T one C oded S ubaudio S quelch or English C ontinuous T one C oded S quelch S ystem ) (sub-tone squelch method; interference code ) allows specific radio devices to be selected within a selected radio channel . When sending, the caller sends a pilot tone with analog audio transmission , to which only the receiving devices that are programmed to this ringing respond. When the detection tone arrives, they will then open reception to listen in.

CTCSS allows different user groups to share one and the same transmission frequency (e.g. the same PMR radio channel) without having to listen to conversations from the other user groups. u.

Audio frequencies

CTCSS transmits control tones (between 67 and 255 Hz) simultaneously with the useful signal. The tones are at the lower end of the audible frequency spectrum. Normal radio devices suppress such low tones and usually only output 300 Hz to approx. 3000 Hz through the loudspeaker. The frequencies are in "crooked" relationships to one another so that they cannot be created by mixing other CTCSS tones. CTCSS signals are for the squelch control ( squelch used) of the receiver. The squelch is only opened if the received signal sends the appropriate CTCSS tone with the useful signal.

The frequencies of the individual tones (pitch of the CTCSS tone) are:

All frequency information in Hz
67.0 69.3 71.9 74.4 77.0 79.7 82.5 85.4 88.5 91.5
94.8 97.4 100.0 103.5 107.2 110.9 114.8 118.8 123.0 127.3
131.8 136.5 141.3 146.2 151.4 156.7 159.8 162.2 165.5 167.9
171.3 173.8 177.3 179.9 183.5 186.2 189.9 192.8 196.6 199.5
203.5 206.5 210.7 218.1 225.7 229.1 233.6 241.8 250.3 254.1

38 standard frequencies

The 38 standard tones according to TIA / EIA -603-D are printed in bold in the table above. These standard tones are used by almost all radios that are sold with an indication such as “38 CTCSS codes”, “38 subchannels” or similar (typically analog PMR radios , Freenet, LPD / SRD). In contrast to this, for amateur radios you usually have to enter the CTCSS frequency in Hertz.

Here again the 38 standard tones in a separate overview table:

No. Frequency [Hz] No. Frequency [Hz] No. Frequency [Hz]
1 67.0 14th 107.2 27 167.9
2 71.9 15th 110.9 28 173.8
3 74.4 16 114.8 29 179.9
4th 77.0 17th 118.8 30th 186.2
5 79.7 18th 123.0 31 192.8
6th 82.5 19th 127.3 32 203.5
7th 85.4 20th 131.8 33 210.7
8th 88.5 21st 136.5 34 218.1
9 91.5 22nd 141.3 35 225.7
10 94.8 23 146.2 36 233.6
11 97.4 24 151.4 37 241.8
12 100.0 25th 156.7 38 250.3
13 103.5 26th 162.2

Position 0 (or 0.0 Hz) or position 39 typically means that CTCSS is switched off

Synonyms for CTCSS

  • Channel Guard (CG) - Abbreviation for General Electric
  • Private Line (PL) - abbreviation for Motorola ; the corresponding tone is the PL tone
  • Interference Eliminator Code - Abbreviation of Motorola (2006)
  • Quiet Channel (QC) - Abbreviation for RCA ( Radio Corporation of America )
  • Debug code
  • Pilot tone
  • Subchannel

Radiotelephony

CTCSS is used to minimize mutual interference between multiple users of the same frequency. For example, the PMR radio , which is accessible to everyone, only uses eight channels. For each channel, however, another 38 “sub-channels” are coded by CTCSS. CTCSS thus acts as a selective call system. A signal is continuously broadcast when it is sent. This must be distinguished from Selcal (also a selective call system ), in which a code signal is only sent at the beginning of the transmission in order to switch off the squelch at the receiver. As the name CTCSS (Continuous Tone Coded Squelch System) suggests, it is a continuous tone. In contrast to the normal squelch system, with the CTCSS no sound at all comes out of the loudspeaker if the code signal does not match or is not recognized due to weak signals. The advantage of CTCSS is that the radio receiver is absolutely silent while it is ready to receive, which creates a relaxed atmosphere for radio operations.

The send / receive switchover also happens absolutely quietly. The voice of the other station simply comes out of the receiver's loudspeaker, without the usual crackling when opening and closing the squelch. This is achieved in that the carrier is first keyed and only a short time afterwards (0.1 second) the CTCSS tone is keyed onto the carrier. The receiver's loudspeaker only opens when the transmitter is already at full power. To switch reception, the CTCSS tone is also first taken from the carrier so that the receiver's loudspeaker can switch off. Only then is the transmitter switched off. There is no more noise in the receiver.

With CTCSS, all remote stations must be equipped with CTCSS in order to evaluate the CTCSS pilot tone and to switch on the receiver's loudspeaker if they match. If the transmitter sends no or a different CTCSS sub-tone, the loudspeaker at the receiver remains mute. If another receiver is listening in on this frequency and has not switched on any CTCSS sub-tone at all (or if his device does not have this function), then he can listen in without any problems. However, it cannot answer the CTCSS transmitter, because it only hears radio messages that are highlighted with the correct CTCSS sub-tone.

This function does not offer security against eavesdropping, as the voice is not encrypted or encoded during transmission. Only the (barely audible) pilot tone is added, but the language itself remains clearly understandable.

The undisturbed use of a frequency by many co-users with different CTCSS sub-tone settings only relates to listening. When transmitting on a frequency that is used by a great many people, you can quickly become a jammer if you carelessly radio into someone else's broadcast, which is easier with CTCSS than without CTCSS: In radio traffic without CTCSS you can hear whether someone else is talking on the frequency. With CTCSS, on the other hand, you cannot hear if someone else is speaking on the frequency but has switched on a different sub-tone. CTCSS-capable radios therefore usually have an indicator that lights up when the channel is currently busy. At this moment you shouldn't send because you act as jammers. In professional radio and with better devices, the device can be programmed accordingly so that it cannot transmit if the channel is busy with the wrong CTCSS tone; the corresponding function is usually called "Busy Lock-Out" (BCL). In radio communication without CTCSS, when transmitting at the same time, audible interference occurs at the receiver, so that neither of the two senders can be understood. With CTCSS radios, on the other hand, you won't hear anything if there is a jamming broadcast, as the CTCSS pilot tone is no longer recognized due to the interference and the radio is therefore muted.

See also

Web links

Individual evidence

  1. ↑ Markus Zubler: Tables of the CTCSS, DCS, Common ID. In: freeradionetwork.ch. Retrieved June 17, 2011 .