*Question E2E11:
What is the difference between direct FSK and audio FSK?*  

*Correct Answer is  A: Direct FSK modulates the transmitter VFO*  

### AI Response: (may contain errors)  

To understand the difference between direct FSK (Frequency Shift Keying) and audio FSK, let's break down the correct answer and the other choices.

### Correct Answer: A - Direct FSK modulates the transmitter VFO

- **Explanation**: 
  - In direct FSK, the frequency of the transmitter's VFO (Voltage Controlled Oscillator) is directly modulated by the digital signal. This means that the actual frequency of the RF signal changes according to the data being sent. 
  - This method allows for more precise control over the frequency and can lead to better performance in terms of signal integrity.

### Why the Other Choices are Incorrect:

- **B: Direct FSK occupies less bandwidth**  
  - **Explanation**: This statement is not necessarily true. The bandwidth occupied by a signal depends on various factors, including the modulation technique and the data rate. Direct FSK can occupy more bandwidth compared to audio FSK, especially at higher baud rates.

- **C: Direct FSK can transmit higher baud rates**  
  - **Explanation**: While direct FSK can support higher baud rates, it is not inherently limited to higher rates compared to audio FSK. The baud rate capability depends on the specific implementation and the characteristics of the modulation scheme used. Therefore, this statement is misleading.

- **D: All these choices are correct**  
  - **Explanation**: Since choices B and C are incorrect, this option cannot be correct. 

### Summary

- **Direct FSK**: Modulates the VFO directly, allowing for precise frequency control.
- **Audio FSK**: Modulates an audio signal that is then used to modulate the RF carrier, which can lead to different bandwidth and performance characteristics.

### Useful Rule of Thumb

- When considering modulation types, remember that direct modulation techniques (like direct FSK) often provide better performance in terms of frequency stability and signal integrity compared to audio modulation techniques.