What is the term used to describe the use of a sub-audible tone transmitted with normal voice audio to open the squelch of a receiver?

The term used to describe the use of a sub-audible tone transmitted with normal voice audio to open the squelch of a receiver is referred to as Continuous Tone-Controlled Squelch System, commonly known as CTCSS. This system works by sending a specific low-frequency tone along with the voice signal. When a receiver is equipped with CTCSS, it can be configured to open its squelch only when it detects this particular tone, allowing the operator to listen to desired transmissions while ignoring unwanted signals that do not contain the correct tone. This makes communication more efficient and reduces interference from other users on the same frequency.

In contrast, carrier squelch simply mutes the receiver when no carrier signal is present, without considering tone signals. Tone burst typically refers to a brief tone signal that may not be sub-audible and is not necessarily used for squelch control. DTMF, or Dual-Tone Multi-Frequency, refers to the tones used in touch-tone dialing on telephones and is not directly related to the squelch function in radio communications. Therefore, CTCSS is the correct choice as it specifically refers to the sub-audible tone that allows receivers to filter and manage incoming signals effectively.