*Question E2D09:
What type of modulation is used by JT65?*  

*Correct Answer is  A: Multitone AFSK*  

### AI Response: (may contain errors)  

JT65 is a digital mode used primarily for weak signal communication on HF bands. Let's break down the modulation types to understand why Multitone AFSK is the correct answer.

### Correct Answer: Multitone AFSK

- **Multitone AFSK (Audio Frequency Shift Keying):**  
  JT65 uses a form of AFSK where multiple tones are transmitted simultaneously. Specifically, JT65 uses 65 different tones spaced 2.7 Hz apart. This multitone approach allows for very efficient and reliable communication even under poor conditions, such as low signal-to-noise ratios.

### Why Other Choices Are Incorrect:

- **PSK (Phase Shift Keying):**  
  PSK involves changing the phase of the carrier signal to convey information. Modes like PSK31 use this type of modulation, but JT65 does not. PSK is not multitone and is not used in JT65.

- **RTTY (Radio Teletype):**  
  RTTY typically uses frequency shift keying (FSK) with two tones, often referred to as "mark" and "space." It is not multitone and does not use the specific tone structure of JT65.

- **QAM (Quadrature Amplitude Modulation):**  
  QAM combines amplitude and phase modulation to encode data. It is commonly used in digital television and broadband internet but is not used in JT65.

### Summary

JT65's use of multitone AFSK allows it to operate effectively in weak signal conditions, making it distinct from other modulation types like PSK, RTTY, and QAM. Understanding these differences helps in identifying the correct modulation type used by JT65.