What term describes the use of a sub-audible tone transmitted along with normal voice audio to open the squelch of a receiver?
Why CTCSS stands for Continuous Tone Coded Squelch System, a low frequency tone (roughly 67 to 254 Hz) sent continuously underneath your voice audio. The receiver, usually a repeater, only opens its squelch when it hears the matching tone, which keeps it from being keyed up by noise or by distant stations on the same frequency. The tone is below the normal voice passband, so listeners do not hear it.
Watch out Carrier squelch is the opposite arrangement: any signal strong enough opens the receiver, with no tone required. DTMF is the audible touch-tone keypad signaling used for control functions, not for holding squelch open.
CTCSS = Continuous Tone Coded Squelch System. Continuous and sub-audible; DTMF is the beepy keypad tones you can hear.