What is a sub-audible tone that activates a receiver audio output called?

A sub-audible tone is specifically designed to activate a receiver's audio output when it detects this tone. The CTCSS tone, which stands for Continuous Tone-Coded Squelch System, operates at a frequency below the range of human hearing. This allows users to communicate on the same frequency without hearing unwanted transmissions from other users who are not transmitting the specific tone.

CTCSS tones are used primarily to prevent interference from other users sharing the same communication channel. When a receiver is set to a specific CTCSS tone, it will only activate the audio output when it receives that tone along with a voice transmission. This effectively filters out other communications that are not using the designated tone, leading to clearer and more organized communication.

In contrast, other tones such as DTMF (Dual-Tone Multi-Frequency) are used for signaling and dialing in telephone networks. While audio signals encompass a broad range of sounds and RTTY (Radio Teletype) is a form of digital communication used for text transmission, neither is specifically utilized as a sub-audible activator for a receiver, like the CTCSS tone. Thus, the CTCSS tone is the correct answer for a tone that activates audio output on a receiver.