*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)  

In FM (Frequency Modulation) broadcasting, a **pre-emphasis network** is used to boost the higher audio frequencies before transmission. This is done to improve the signal-to-noise ratio for high-frequency sounds, which are more susceptible to noise.

### Explanation:

1. **Correct Answer: D: A pre-emphasis network**
   - **Pre-emphasis** is a technique used in FM transmission to amplify higher audio frequencies. This helps to reduce the impact of noise on these frequencies during transmission. The process involves increasing the amplitude of higher frequency audio signals before they are modulated onto the carrier wave.
   - The corresponding **de-emphasis** network at the receiver end reduces these frequencies back to their original levels, ensuring that the overall audio signal is balanced and clear.

2. **Why the other choices are incorrect:**
   - **A: A de-emphasis network**: This is used at the receiver end to reverse the effect of pre-emphasis, not to boost high frequencies during transmission.
   - **B: A harmonic enhancer**: This is not a standard component in FM transmission. It would imply adding harmonics, which is not the goal in FM speech channels.
   - **C: A heterodyne enhancer**: Heterodyning is a process used in frequency conversion, not in boosting audio frequencies.

### Key Points:
- **Pre-emphasis** boosts high frequencies before transmission.
- **De-emphasis** reduces these frequencies at the receiver to restore the original audio balance.
- The combination of pre-emphasis and de-emphasis helps maintain audio quality by minimizing noise impact on high frequencies.

This understanding is crucial for ensuring high-quality audio transmission in FM systems.