Question E2E03:
To what does the "4" in FT4 refer?  

Choices:  
A: Multiples of 4 bits of user information  
B: Four-tone continuous-phase frequency shift keying  
C: Four transmit/receive cycles per minute  
D: All these choices are correct    

Correct Answer is  B: Four-tone continuous-phase frequency shift keying  

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Correct answer: Four-tone continuous-phase frequency shift keying

Why this is correct:
- In FT4, the “4” denotes 4-level FSK: the signal uses four distinct frequency tones.
- The modulation is continuous-phase FSK (CPFSK), meaning the phase changes smoothly between tones, producing a constant-envelope signal that is amplifier-friendly.

Why the other choices are incorrect:
- Multiples of 4 bits of user information:
  - Naming is based on the number of tones, not bit groupings.
  - While 4-FSK carries $b = \log_2(4) = 2$ bits per symbol, FT4’s name does not refer to bits or message length multiples.
- Four transmit/receive cycles per minute:
  - FT4 uses 7.5-second T/R periods, so cycles per minute are about $60 / 7.5 = 8$, not 4.
- All these choices are correct:
  - Only the 4-tone CPFSK statement is correct.

Useful formulas and rules of thumb:
- For $M$-ary FSK, bits per symbol:
  $$b = \log_2 M$$
  For FT4, $M=4 \Rightarrow b=2$ bits/symbol.
- T/R cycle rate estimate:
  $$\text{cycles per minute} \approx \frac{60}{\text{T/R period (s)}}$$
  For FT4: $60 / 7.5 \approx 8$.
- Memory aid: FT8 uses 8-FSK; FT4 uses 4-FSK. The number in the name matches the number of tones.