Question E2E07:
How does Q65 differ from JT65?  

Choices:  
A: Keyboard-to keyboard operation is supported  
B: Quadrature modulation is used  
C: Multiple receive cycles are averaged  
D: All these choices are correct    

Correct Answer is  C: Multiple receive cycles are averaged  

Correct answer: Multiple receive cycles are averaged.

1) Why this is correct
- Q65 includes an explicit decoder feature to combine information from multiple consecutive receive periods (multi-period averaging). By coherently accumulating likelihood metrics across $N$ copies of the same message, it can decode signals well below the single-period threshold.
- Rule of thumb for SNR improvement from averaging:
  $$G_{\text{SNR}} \approx 10 \log_{10}(N)\ \text{dB}$$
  where $N$ is the number of periods averaged.

2) Why the other choices are incorrect
- Keyboard-to-keyboard operation is supported:
  - Incorrect. Q65 (like JT65) is a structured, sequence-based weak-signal mode, not a free-form keyboard chat mode.
- Quadrature modulation is used:
  - Incorrect. Both JT65 and Q65 are multi-tone FSK modes (65-tone MFSK), not quadrature (QPSK/QAM) schemes.
- All these choices are correct:
  - Incorrect because the two statements above are not true.

3) Useful notes and rules of thumb
- Q65’s multi-period averaging is especially helpful for extremely weak paths (e.g., EME, ionoscatter, troposcatter).
- SNR gain from averaging:
  $$G_{\text{SNR}} \approx 10 \log_{10}(N)\ \text{dB}$$
- JT65 generally decodes each T/R period independently and does not provide Q65’s explicit multi-period averaging feature.