*Question T2B02:
What term describes the use of a sub-audible tone transmitted along with normal voice audio to open the squelch of a receiver?*  

*Correct Answer is  D: CTCSS*  

### AI Response: (may contain errors)  

The correct answer to the question is **CTCSS**, which stands for Continuous Tone-Coded Squelch System. Here’s a breakdown of why this is the correct answer and why the other options are not:

### Why CTCSS is Correct:
- **CTCSS** is a method used in radio communications where a sub-audible tone (typically between 67 Hz and 254 Hz) is transmitted along with the voice audio. 
- This tone allows the receiver to open its squelch (the noise gate that mutes the audio when no signal is present) only when it detects the specific tone, thus filtering out unwanted transmissions from other users on the same frequency.

### Why the Other Choices are Incorrect:
- **A: Carrier squelch**
  - This method opens the squelch based solely on the presence of a carrier signal. It does not use any sub-audible tone, so it is less selective than CTCSS.
  
- **B: Tone burst**
  - A tone burst is a short burst of audio tone used to signal or identify a transmission, but it does not continuously transmit a tone alongside the voice audio like CTCSS does.
  
- **C: DTMF**
  - Dual-Tone Multi-Frequency (DTMF) is used for signaling and dialing (like touch-tone phones) and involves sending two tones simultaneously for each key pressed. It is not used for squelch control in the same way as CTCSS.

### Summary:
- CTCSS is specifically designed for squelch control using a continuous sub-audible tone.
- Carrier squelch, tone burst, and DTMF do not fulfill the same function as CTCSS in the context of opening a receiver's squelch.

This understanding of CTCSS and its comparison to other methods will help you in the Extra Class exam and in practical amateur radio operations.