GMRS tone codes (CTCSS/DCS) work by adding an inaudible, low-frequency tone to your radio’s transmission, acting as a “key” that only radios set to the matching tone will “unlock” and let audio through, filtering out unwanted chatter on busy channels while not truly encrypting your conversation, and also enabling repeater access by triggering their activation. When you transmit with a tone, your radio sends the tone; receiving radios with the same tone open their speaker, while others remain silent, essentially creating private “sub-channels” on shared frequencies
How they function (CTCSS/DCS)
- Transmitting: When you press transmit (PTT), your radio sends your voice and a continuous, low-frequency tone (CTCSS) or a digital code (DCS).
- Receiving (with matching tone): If a nearby radio is set to the same tone, its squelch (speaker) opens, and you hear the transmission.
- Receiving (without matching tone): If a radio isn’t set to your specific tone, or has no tone set, its squelch stays closed (muted), and it won’t hear your transmission, even if it’s on the same frequency.
Key concepts
- CTCSS (Continuous Tone-Coded Squelch System): Uses specific, standardized audio frequencies (e.g., 100.0 Hz).
- DCS (Digital-Coded Squelch): Uses digital codes (e.g., 023N).
- “Privacy” Codes: A misnomer; they offer selective squelch, not true privacy. Anyone on the frequency can still hear you if they disable their tone or use a tone scanner, but it helps filter out noise.
- Repeaters: Repeaters require a specific tone (input tone) to activate. Once activated, they retransmit your signal, often with an output tone that other users must use to hear the repeater’s transmission.
Using tone codes
- To Reduce Interference: Set the same tone as your group to ignore other users on a busy GMRS channel, notes K0TFU.
- To Access Repeaters: Use the required tone (often found via community lists or scanning) to key up a repeater and communicate further.
- Tone Scanning: Some radios can scan for and identify tones being used on a frequency, helpful for finding repeater access tones or matching group tones