In a previous post we introduced the term squelch and how it was used in basic operation. As mentioned there modern ham radio work (especially with repeaters) often involves other technologies that fall under a different category named selective calling, tone squelch being the most common form.
Selective calling is different from ordinary carrier or noise squelch. In practice it is more of a security or channel sharing function. Selective calling encompasses several similar technologies which largely do the same thing: prevent a transmitted signal from being received by other radios unless a particular code is entered by the sender.
From the Wikipedia article, “Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct “key”, in this case a specific code.”
The most common form of selective calling in use by hams is a tone squelch system with the awkward name of continuous tone-coded squelch system (CTCSS). Squelch in general is used to keep commercial and amateur radio repeaters from continually transmitting. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false key-ups. Use of CTCSS is especially helpful where nearby repeaters may share the same frequency or in a high electrical noise or RF environment.
As the name implies, CTCSS sends out a continuous tone along with the transmit audio. The tone is termed, “sub-audible”, although it is often a low audible frequency. Most radio systems filter out these low frequencies so it is unlikely that you will hear the tone when listening to another ham unless you have an unusual radio and/or are wearing high fidelity headphones.
The CTCSS tone is selected by a repeater operator to avoid duplication with nearby repeaters on the same frequency. In most cases, the objective is to reduce interference and not restrict legitimate access. There are 100 established CTCSS frequencies but some are more commonly used than others.
CTCSS is often referred to as PL because it’s easier to say. They mean the same thing but PL® (stands for Private Line) is a registered trademark of Motorola’s implementation and was the original employment of CTCSS.
Besides CTCSS, other forms of selective calling in use by hams include:
- Selcall (mostly European)
- Digital-Coded Squelch (DCS)
Why it matters to us
Because various forms of selective calling prevent a signal from being re-transmitted (repeater) or received (simplex) without the proper code or tone, use of CTCSS or DCS is a possible reason other stations cannot receive you. Especially on a repeater, if others cannot hear you it’s quite likely that you have the wrong code or your tone squelch is turned off. How to know what the proper setting is? Consult the repeater directory.
Some Useful Links
CTCSS – RadioReference
How radio “privacy tones” or CTCSS tones work – YouTube video
CTCSS – MDARC
Get the Right Signal Tone – Ham Radio School
Hey, Why Can’t I Access the Repeater? – Ham Radio School
Differences between CTCSS and DCS – Retevis
CTCSS & Tone Burst Ham Radio Repeater Access – electronicsnotes
A Historical and Technical Overview of Tone Squelch Systems – WA6ILQ
Repeater Related Terminology – Repeater Builder group