*Question G8A01:
How is direct binary FSK modulation generated?*  

*Correct Answer is  B: By changing an oscillator’s frequency directly with a digital control signal*  

### AI Response: (may contain errors)  

To understand how direct binary Frequency Shift Keying (FSK) modulation is generated, let's break down the correct answer and the other options.

### Correct Answer: 
**B: By changing an oscillator’s frequency directly with a digital control signal.**

- **Explanation**: In direct binary FSK, the frequency of the carrier signal is shifted between two discrete frequencies to represent binary data (0s and 1s). This is achieved by directly controlling an oscillator with a digital signal, which allows for precise frequency changes corresponding to the binary data being transmitted. 

### Why the Other Choices are Incorrect:

- **A: By keying an FM transmitter with a sub-audible tone**  
  - **Explanation**: This describes a method of frequency modulation (FM) rather than FSK. In FSK, the frequency shifts are not based on a tone but rather on binary data. A sub-audible tone would not produce the discrete frequency shifts required for FSK.

- **C: By using a transceiver’s computer data interface protocol to change frequencies**  
  - **Explanation**: While a computer interface can control various functions of a transceiver, it does not directly generate the frequency shifts needed for FSK modulation. The modulation itself must be done at the oscillator level, not merely through a data interface.

- **D: By reconfiguring the CW keying input to act as a tone generator**  
  - **Explanation**: This option refers to Continuous Wave (CW) operation, which is not the same as FSK. CW involves turning the carrier on and off to create Morse code, while FSK involves shifting frequencies to represent digital data.

### Summary:
- Direct binary FSK modulation is specifically about changing the frequency of an oscillator directly with a digital control signal, making option B the correct choice. The other options either describe different modulation techniques or methods that do not apply to FSK.

### Useful Concepts:
- **FSK Modulation**: Involves two frequencies, $f_0$ and $f_1$, representing binary 0 and 1, respectively.
- **Frequency Shift**: The difference between the two frequencies can be calculated as:
  
$$\Delta f = f_1 - f_0$$

This difference is crucial for ensuring that the receiver can accurately detect the transmitted data.