Digital squelch code (DCS)
A slow digital code sent under FM voice, doing the same job as a sub-audible tone.
What does it sound like?
Usually inaudible; sometimes a faint low rumble.
Who uses it?
Repeaters, businesses and family radios.
Where and when can I hear it?
Under FM voice on VHF/UHF.
The science
Instead of a tone, the radio sends a slow stream of bits, 134.4 per second, underneath the voice. Each code word is 23 bits long and uses a Golay (23,12) error-correcting code, which can correct up to three wrong bits; one word takes about 171 milliseconds. Codes are written as three-digit octal numbers such as 023. Each code also has an inverted twin, because some receivers flip the data's polarity. At the end of a transmission, radios send about a fifth of a second of a 134.4 Hz on-off pattern that tells receivers to mute at once.
Why use it?
Digital codes give more distinct choices than the list of standard tones and are harder for random noise to imitate, so crowded channels can be shared by more groups. Like tone squelch, it only controls when a speaker opens and does not hide the conversation.
Lore
Each maker gave it its own name: Motorola calls it Digital Private Line, General Electric called it Digital Channel Guard, and Icom calls it DTCS.
What do the decoded parts mean?
- Code: A three-digit code such as 023 that radios must match to hear each other.
Easily confused with
- Sub-audible tone (CTCSS / PL), from the same family