Just about all radios used for 2-way communications come with a device called a squelch. A squelch serves as a sort of check-valve to turn on the receiver in the presence of a signal; without it, you would be listening to non-stop static between transmissions, it works as a way to keep the radio quiet unless it receives a signal.
A basic squelch looks at only signal strength to determine when to turn on the speaker; however this makes it susceptible to interference, as not every signal a radio picks up is necessarily a signal you want to listen to. In places with lots of interference generating sources, the user can often still be bothered by the squelch opening up and causing a bunch of unwanted noise.
To address this issue; manufacturers came up of a few ways to signal to a receiving radio that the signal source is in fact another radio, by injecting an audio signal that is not likely to occur from external interference. This tone or data pulse is known as a coded squelch, and is most commonly based on the CTCSS and DCS standards. These at times are also marketed under the names Private Line (PL), Privacy Code, Sub-Channel, and a few other variants, but they’re essentially all the same thing using the same tone or pulse standards.
Because CTCSS and DCS can set unique squelch codes, using a code squelch essentially gives the following benefits:
- Eliminates random interference from coming through the squelch and being heard on a receiver. This is the primary reason repeaters utilize coded squelches; it prevents them from keying up and re-transmitting a bunch of noise.
- Lets groups of users use the same channel/frequency, while only being able to hear others in their party using the same squelch code. Users who transmit at the same time in different groups may still cause interference to one another, but it prevents others from being bothered by transmissions unrelated to their party.