*Question T4B12:
What is the result of tuning an FM receiver above or below a signal’s frequency?*  

*Correct Answer is  D: Distortion of the signal’s audio*  

### AI Response: (may contain errors)  

When tuning an FM receiver above or below a signal's frequency, the correct answer is:

**D: Distortion of the signal’s audio.**

### Explanation of the Correct Answer:
- **Distortion of the signal’s audio** occurs because FM (Frequency Modulation) signals are designed to be demodulated at a specific frequency. If you tune away from this frequency, the demodulation process can produce incorrect audio output, leading to distortion. This is due to the way FM signals encode information in frequency variations, and tuning off-frequency can result in the receiver misinterpreting these variations.

### Why the Other Choices are Incorrect:
- **A: Change in audio pitch**  
  This is not correct because while tuning can affect the perceived audio, it does not simply change the pitch. Instead, it distorts the audio signal, which can lead to a garbled or unintelligible output rather than a clear change in pitch.

- **B: Sideband inversion**  
  Sideband inversion refers to a phenomenon that occurs in single sideband (SSB) modulation, not FM. In FM, the sidebands are not inverted when tuning off-frequency; rather, the audio becomes distorted.

- **C: Generation of a heterodyne tone**  
  A heterodyne tone is typically produced when two signals of different frequencies mix, creating a new frequency equal to the difference between them. While this can happen in some cases, it is not the primary effect of tuning an FM receiver off-frequency. The main issue is the distortion of the audio signal rather than the generation of a heterodyne tone.

### Summary:
When tuning an FM receiver away from the intended frequency, the primary effect is distortion of the audio signal due to improper demodulation. Understanding this concept is crucial for effective operation and troubleshooting of FM receivers.