*Question E2E09:
Which of the following HF digital modes uses variable-length character coding?*  

*Correct Answer is  D: PSK31*  

### AI Response: (may contain errors)  

The correct answer to the question is **D: PSK31**. Here's why:

1. **Why PSK31 is correct:**
   - PSK31 (Phase Shift Keying at 31 baud) uses variable-length character coding. This means that the length of the transmitted characters can vary, allowing for more efficient use of bandwidth. PSK31 is designed for keyboard-to-keyboard communication and optimizes the transmission of text by using a more compact representation of characters.

2. **Why the other choices are incorrect:**
   - **A: RTTY (Radio Teletype)**: RTTY uses a fixed-length character coding scheme, typically 5 bits for each character, which corresponds to the Baudot code. This means every character transmitted takes the same amount of time.
   - **B: PACTOR**: PACTOR is a digital mode that also uses fixed-length coding for its characters. It is designed for efficient data transmission over radio links but does not employ variable-length character coding.
   - **C: MT63**: MT63 is a digital mode that uses fixed-length coding as well. It is designed for robust data transmission and typically uses a fixed number of bits per character.

3. **Useful rules of thumb:**
   - When studying digital modes, remember that variable-length coding allows for more efficient data transmission, especially in modes designed for text communication.
   - Fixed-length coding is often used in traditional modes like RTTY, where each character has a predetermined length, making it simpler but less efficient in terms of bandwidth.

In summary, PSK31 is the only mode listed that utilizes variable-length character coding, making it the correct answer.