*Question E7E06:
Why is de-emphasis used in FM communications receivers?*  

*Correct Answer is  A: For compatibility with transmitters using phase modulation*  

### AI Response: (may contain errors)  

De-emphasis is used in FM communications receivers primarily for compatibility with transmitters using phase modulation. Here's why:

1. **Correct Answer (A): For compatibility with transmitters using phase modulation**  
   - FM transmitters often use pre-emphasis to boost higher audio frequencies before transmission. This is done to improve the signal-to-noise ratio for those frequencies.  
   - De-emphasis is applied in the receiver to reverse this process, restoring the original audio frequency response. This ensures that the audio sounds natural and is not overly bright or harsh.  
   - This process is crucial for maintaining compatibility with transmitters using phase modulation, as phase modulation can be affected by the frequency response of the audio signal.

2. **Why the other choices are incorrect:**
   - **B: To reduce impulse noise reception**  
     - De-emphasis is not specifically used to reduce impulse noise. Impulse noise reduction is typically handled by other means, such as noise blankers or limiters.
   - **C: For higher efficiency**  
     - De-emphasis does not directly relate to efficiency. It is a process to correct the frequency response of the received audio signal.
   - **D: To remove third-order distortion products**  
     - Third-order distortion products are typically addressed through linearity improvements in the transmitter and receiver design, not through de-emphasis.

In summary, de-emphasis is a technique used to ensure that the audio signal received is a faithful reproduction of the original signal, particularly when pre-emphasis has been applied at the transmitter. This is essential for compatibility with phase modulation systems.