*Question E7E05:
What is added to an FM speech channel to boost the higher audio frequencies?*  

*Correct Answer is  D: A pre-emphasis network*  

### AI Response: (may contain errors)  

To boost the higher audio frequencies in an FM speech channel, a **pre-emphasis network** is used. 

### Explanation of the Correct Answer:
- **D: A pre-emphasis network**  
  - This network increases the amplitude of higher audio frequencies before modulation. The purpose is to improve the signal-to-noise ratio for those frequencies during transmission. The pre-emphasis typically follows a specific time constant, often defined as a ratio of 75 microseconds for FM broadcasting. This helps to counteract the effects of noise that can affect higher frequencies more than lower ones.

### Explanation of Incorrect Answers:
- **A: A de-emphasis network**  
  - This is the opposite of pre-emphasis. It reduces the amplitude of higher frequencies after demodulation to restore the original audio signal. It is used to counteract the pre-emphasis applied during transmission, not to boost frequencies.

- **B: A harmonic enhancer**  
  - This term generally refers to devices that add harmonic content to a signal, which is not specifically related to boosting higher audio frequencies in FM speech channels. It does not serve the purpose of improving the clarity of speech signals.

- **C: A heterodyne enhancer**  
  - This term is not standard in the context of audio processing. Heterodyning is a technique used in mixing signals, typically in radio frequency applications, and does not pertain to boosting audio frequencies in FM speech.

### Summary:
- The correct answer is **D: A pre-emphasis network** because it specifically boosts higher frequencies before transmission to improve the overall quality of the FM signal.
- The other options do not serve this purpose and are either oppositional or unrelated to the context of FM speech channels. 

### Useful Rule of Thumb:
- Remember that pre-emphasis is applied before transmission, while de-emphasis is applied after reception. This distinction is crucial for understanding audio processing in FM systems.