*Question E2E04:
Which of the following is characteristic of the FST4 mode?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

The FST4 mode is a digital communication mode used in amateur radio, and it has several distinctive characteristics. Let's break down why the correct answer is "All these choices are correct" and examine each option:

1. **Four-tone Gaussian frequency shift keying (Choice A):**  
   FST4 uses a modulation technique called Gaussian Frequency Shift Keying (GFSK) with four distinct tones. This means that the mode encodes data by shifting the frequency of the carrier wave among four possible frequencies, which are shaped by a Gaussian filter to reduce bandwidth and improve performance in noisy conditions.

2. **Variable transmit/receive periods (Choice B):**  
   FST4 allows for variable transmit and receive periods, meaning that the duration of time spent transmitting and receiving can be adjusted. This flexibility helps optimize communication under different propagation conditions and can improve the efficiency of data exchange.

3. **Seven different tone spacings (Choice C):**  
   FST4 supports seven different tone spacings, which refers to the frequency separation between the tones used in the modulation. This feature allows the mode to adapt to various bandwidth requirements and interference levels, making it versatile for different operating environments.

Since each of these characteristics is true for the FST4 mode, the correct answer is "All these choices are correct" (Choice D). Each option describes a valid aspect of the FST4 mode, and together they highlight its flexibility and adaptability in digital communication.