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

Choices:  
A: RTTY  
B: PACTOR  
C: MT63  
D: PSK31    

Correct Answer is  D: PSK31  

PSK31 is correct because it uses Varicode, a variable-length character code:
- In Varicode, common characters (like space, E, T) have shorter bit patterns, rare ones are longer.
- No Varicode symbol contains the sequence 00; two zeros (00) are sent between characters as a separator.
- This makes PSK31 efficient at its very low symbol rate (31.25 baud).

Why the other choices are incorrect:
- RTTY: Uses fixed-length 5-bit Baudot (ITA2) with start/stop bits. Each character is always the same total length in bits (e.g., 1 start + 5 data + 1.5 stop).
- PACTOR: Uses fixed-length 7- or 8-bit character coding within ARQ/FEC frames. The protocol adapts speed and robustness, but the character codes themselves are not variable-length.
- MT63: An MFSK mode with strong interleaving/FEC and fixed symbol mapping. Characters are not encoded with variable-length codes.

Useful formulas and rules of thumb:
- PSK31 bit rate: $r = 31.25$ baud (symbols per second).
- With Varicode, the average bits per character varies; an approximate character rate is
  $$\text{CPS} \approx \frac{r}{\text{avg Varicode length} + 2}$$
  where “+2” accounts for the 00 separator.
- RTTY (example): At $r = 45.45$ baud with 1 start + 5 data + 1.5 stop bits,
  $$\text{CPS} \approx \frac{45.45}{7.5} \approx 6 \text{ chars/sec}$$

Key takeaway: PSK31 is the HF digital mode that uses variable-length character coding (Varicode).