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

*Correct Answer is  D: PSK31*  

### AI Response: (may contain errors)  

PSK31 is the correct answer because it uses variable-length character coding, specifically Huffman coding, to efficiently transmit data. This means that more frequently used characters are represented with shorter codes, while less common characters use longer codes. This approach helps to reduce the overall transmission time and bandwidth usage.

### Explanation of Choices:

- **A: RTTY**  
  RTTY (Radio Teletype) uses fixed-length character coding, typically 5-bit Baudot code, which does not vary in length based on character frequency.

- **B: PACTOR**  
  PACTOR is a protocol used for error-correcting data transmission, often in HF bands, but it does not use variable-length character coding. It focuses more on robust data transfer and error correction.

- **C: MT63**  
  MT63 is a digital mode designed for high noise environments and uses fixed-length coding. It is known for its robustness rather than variable-length coding.

- **D: PSK31**  
  PSK31 uses variable-length character coding, making it efficient for keyboard-to-keyboard communication. This is why it is the correct answer.

### Key Points:

- **Variable-Length Coding**: Efficient for reducing transmission time by using shorter codes for common characters.
- **PSK31**: Known for its efficiency and low bandwidth usage, making it popular for HF digital communications.