Question E2E02:
Which of the following synchronizes WSJT-X digital mode transmit/receive timing?  

Choices:  
A: Alignment of frequency shifts  
B: Synchronization of computer clocks  
C: Sync-field transmission  
D: Sync-pulse timing    

Correct Answer is  B: Synchronization of computer clocks  

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WSJT-X modes (FT8, JT65, WSPR, etc.) use fixed time slots. Stations decide when to transmit or receive based on their computer’s clock. Therefore, the T/R timing is synchronized by the computers being on the same time.

Why B (Synchronization of computer clocks) is correct:
- WSJT-X uses fixed slot boundaries:
  - FT8: $T_{slot} = 15\ \text{s}$; starts at $t = 0, 15, 30, 45$ s.
  - JT65: $T_{slot} = 60\ \text{s}$.
  - WSPR: $T_{slot} = 120\ \text{s}$.
- Each station transmits or receives according to $t_{start} = n \cdot T_{slot}$, using the PC clock.
- NTP or GPS keeps clocks within the needed accuracy so everyone switches T/R together.

Why the other choices are incorrect:
- A: Alignment of frequency shifts
  - Frequency tuning or AFC helps decode tones but does not determine when to switch between transmit and receive.
- C: Sync-field transmission
  - Modes like FT8 include an on-air sync pattern (e.g., Costas array) for symbol/frame and frequency/phase alignment within a slot, not for scheduling T/R slot changes across stations.
- D: Sync-pulse timing
  - There is no external “sync pulse” sent over the air to coordinate T/R timing; timing is local from the computer clock.

Useful rules of thumb:
- Keep clock error small: $|\Delta t| \lesssim 1\ \text{s}$ (preferably $< 0.5\ \text{s}$) for reliable operation.
- Use NTP or a GPS-disciplined clock to maintain time.
- Even/odd slot convention (e.g., FT8): one station transmits on even seconds, the other on odd; this relies on accurate computer clocks.

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