What does CTCSS really mean?

[AgentCOPP1]

@Darth Vader

That's all true but it seems like that leads to a lot of confusion as to what the two really are. However, it would surprise me if someone who's into radio doesn't know what CTCSS is. Most radio manuals explain it.

 

[rapidcharger]

In 20 words or less,
in ham radios
"tone" = coded squelch encode only
"ctcss" = coded squelch encode and decode

 

[DaveNF2G]

Those designations are brand and model specific.

My Yeasu FT-857D used TEN for Tone ENcode. My FT-7900R uses (I think, it's out in the van) ENC, DEC, and ENC/DEC.

 

[jeatock]

Squelch 101

Two-way radios have a feature called by many confusing names including (all copyrighted) Motorola's Private Line ("PL") and Digital Private Line ("DPL"), GE/Ericsson's "Channel Guard", E. F. Johnson's "Call Guard", and RCA's "Quiet Channel." Others simply call it tone activated squelch. Few call it by the correct generic name of Continuous Tone Coded Squelch System ("CTCSS").

Some manufacturers won't speak plain English. They give their CTCSS codes a proprietary name and hide them with proprietary identifiers to make life difficult and make them seem more special. Some of us have to remember that a "3Z' is the same as 123.0 Hz. Other manufacturers simply use the frequency of the tone; the most simple and fool-proof method.

No matter what name is used, they are all basically the same.

A simple explanation:

First off, SQUELCH (also known as Carrier Squelch or "CS") says that a received RF signal strength (or "RSSI") "must be this tall' before the receiver will un-mute. Squelch can be adjusted by a knob or control, or set in programming.

Next, SQUELCH CODES tell the receiver that unless the modulated RF signal "contains a green ticket" it will be ignored. Squelch code filtering can be turned off making the receiver simple CS by a user control ("MON" or "MONI"), or by programming the receiver to un-mute on any signal when the microphone is "off-hook". Transmitters typically send the programmed CTCSS even in Monitor mode.

Finally, SQUELCH and SQUELCH CODES are used together tell the receiver to only un-mute when the signal is "this tall' AND "contains a green ticket". Tall signals with red, yellow, blue, gold or silver tickets (or no ticket at all) will be ignored.

CTCSS is used to filter unwanted transmissions from a receiver, and to eliminate non-radio noise. CTCSS lets multiple repeaters in the same radio environment listen to the exact same radio frequency, but allows only one selected repeater to go active when the transmitted signal also contains the correct tone.

Be aware that a repeater may not transmit any squelch code at all, or transmit a code different from its receiver code.

Also remember that a repeater may be programmed to only activate when it hears a special prefix signal; DTMF, MDC and EIA 5/6 tones are used. This practice is employed by repeater owners who use selective calling, or who don't want any foreigners on their system.

A CTCSS system normally uses a one of 51 standard low frequency audio tones (67.0~270.4 Hz) applied to the transmitted signal, and a receiver with a filter or logic gate that keeps the receiver muted unless it recognizes the correct tone. When the receiver hears the correct tone (plus or minus an error percentage) it opens the squelch gate and un-mutes the receiver; otherwise it keeps the receiver silent even though it may be hearing a strong, clear radio signal.

Human speech seldom drops below 300Hz. Long ago, engineers realized this and don't transmit the un-necessary audio spectrum; in fact both telephones and 2-way radios only handle audio between 300Hz and 3,000Hz. CTCSS takes advantage of the lower unused bandwidth by mixing one of the low-range tones into the transmitted audio. If you were to listen to this raw tone/voice combination on a good stereo, you would hear the voice along with a annoying low hum. But before the audio gets to the receiver's speaker, the receiver filters out the annoying low frequency tones and only plays the voice portion. 2-way fidelity really stinks compared to what you expect when you play music, but for passing the human voice over a radio system it is perfectly acceptable.

There's one more wrinkle: there aren't enough CTCSS codes, so radio manufacturers offer a variation of CTCSS called Digital Coded Squelch ("DCS"). DCS (aka DPL) performs the same function as CTCSS but uses a pattern of pulses sent at a low audio frequency with the transmission. Just like CTCSS, receivers that do not recognize their code remain silent. The DCS codeword consists of a 23 bit frame of pulses and nulls transmitted at 134.4 Hz, which is why there is no 134.4 CTCSS. Like CTCSS the receiver filters out the code so the user doesn't hear it.

A CTCSS of 156.7 Hz is not the same as as D156N! CTCSS codes are typically a number containing a decimal point, while DCS codes are a 3-digit number, sometime prefixed with a "D" or with an "N" or "I" suffix.

Midian Electronics has an excellent wall chart and downloadable reference for all industry standard (and non-standard) tones. You can get it at http://www.midians.com/pdf/tone_signaling.pdf.

Digital radios do not use either CTCSS or DCS, but instead put a code directly in the string of "1's" and "0's" that make up digital transmissions. APCO P-25 radios use a feature called a Network Access Code ("NAC"); a NAC of $F7E in a receiver tells it to unmute on any NAC. 6.25 bandwidth radios from Icom, Kenwood, Bendix-King and others use a similar cross-platform feature called a Radio Access Number ("RAN"). Digital systems can also perform filtering based on Unit ID or Talkgroup numbers.

A properly programmed complete system will work dandy using any of the squelch code schemes. Just remember that using any code does not keep your conversation out of CS programmed receivers, and that a receiver set to only open on a specific squelch code will not hear you if you transmit anything different.