What is CTCSS and DCS?

CTCSS and DCS are systems used to ignore other users who you may be sharing a radio channel with. These two systems are also referred to by other names, including CDCSS, PL and DPL, but the official terms are CTCSS and DCS.  They both have the same function and work in similar ways.

CTCSS stands for Continuous Tone Coded Squelch System. Basically what it does is this: it adds a tone into your transmission at a certain frequency. Other radios must have the same CTCSS tone or code set to hear the transmission. It will also be heard if the radio has CTCSS and DCS off. Different CTCSS codes have different frequencies, and this is how it filters out other people - as long as they have a different CTCSS, or no CTCSS, then your radio will not pass any audio to the speaker.

DCS works the same, apart from it being digital instead. It stands for Digitally Coded Squelch. It sends a number repeatedly encoded in digital as you speak, in the low frequencies so you do not hear it. If someone else transmits on the channel with a different or no DCS, the radio will not unmute. If they transmit with the same DCS as you, then your radio will unmute. DCS is newer than CTCSS, and has more combinations.

Now that you understand how these work, I also want to clear something up. Some manufacturers call there "Privacy tones" or Privacy codes" or even act like they are different channels, and advertise that the radio has "968 channel combinations."  All of this is completely untrue.  CTCSS and DCS don't stop anyone from listening to you, they just stop you from hearing other people. If someone has a radio with no CTCSS and DCS on, they will be able to hear everyone on the channel, including people using CTCSS and DCS.
Also, CTCSS and DCS do not make additional channels. If someone is on the same channel with a different code, then its not a different channel. If you stand next to them and both transmit, your radios will interfere because they will be trying to use the same channel at the same time. On PMR446, there are only 8 channels, and no CTCSS or DCS can change that.