*Question G8A09:
What type of modulation is used by FT8?*  

*Correct Answer is  A: 8-tone frequency shift keying*  

### AI Response: (may contain errors)  

FT8 is a digital mode used in amateur radio that is known for its ability to decode weak signals. Let's break down the modulation type and why the correct answer is A: 8-tone frequency shift keying.

### Explanation:

1. **Correct Answer: 8-tone frequency shift keying (FSK)**
   - FT8 uses a form of frequency shift keying where 8 distinct tones are used. Each tone represents a different symbol, and the transmission consists of a sequence of these tones. This is why it is called 8-tone FSK.
   - FT8 is designed for weak signal communication, and the use of multiple tones allows for efficient and reliable data transmission even under poor conditions.

2. **Why the Other Choices are Incorrect:**
   - **B: Vestigial sideband**
     - Vestigial sideband (VSB) is a type of amplitude modulation used primarily in television broadcasting, not in digital modes like FT8.
   - **C: Amplitude compressed AM**
     - Amplitude compressed AM refers to a form of amplitude modulation where the amplitude is compressed to reduce bandwidth or power. FT8 does not use amplitude modulation; it uses frequency shift keying.
   - **D: 8-bit direct sequence spread spectrum**
     - Direct sequence spread spectrum (DSSS) is a modulation technique used in some wireless communication systems, like Wi-Fi, to spread the signal over a wider bandwidth. FT8 does not use DSSS; it uses frequency shift keying with 8 tones.

### Summary:
FT8's use of 8-tone frequency shift keying allows it to effectively transmit data in challenging conditions, making it a popular choice for weak signal communication in amateur radio. Understanding the modulation type helps in recognizing the characteristics and applications of FT8.