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

Choices:  
A: A de-emphasis network  
B: A harmonic enhancer  
C: A heterodyne enhancer  
D: A pre-emphasis network    

Correct Answer is  D: A pre-emphasis network  

Correct answer: D — A pre-emphasis network

Why D is correct:
- In FM, the demodulated noise power increases with audio frequency. To improve intelligibility, the transmitter boosts the higher audio frequencies before modulation using a pre-emphasis network (a simple RC high-pass/shelving network with a rising response).
- The receiver applies the complementary de-emphasis to restore a flat overall audio response while reducing high-frequency noise.
- Rule of thumb: pre-emphasis slope is about +6 dB/octave; de-emphasis is −6 dB/octave.

Why the others are incorrect:
- A: De-emphasis network — This is used in the receiver, not added to the transmitted speech channel. It attenuates highs to complement transmitter pre-emphasis.
- B: Harmonic enhancer — An audio effect term, not a standard FM communications practice or specification.
- C: Heterodyne enhancer — Not a recognized circuit/function in FM audio processing; heterodyning refers to frequency mixing, not HF audio boosting.

Useful formulas and notes:
- First-order emphasis is characterized by a time constant τ; the corner frequency is
  $$f_c = \frac{1}{2\pi \tau}.$$
  Examples:
  - FM broadcast (US) pre-emphasis: $τ = 75\,\mu\text{s} \Rightarrow f_c \approx 2122\,\text{Hz}$.
  - Many communications FM systems use a similar 6 dB/octave emphasis with their specified τ.
- Idealized slopes:
  - Transmitter pre-emphasis: +6 dB/octave (boost highs).
  - Receiver de-emphasis: −6 dB/octave (restore flat audio, reduce noise).