Question E4E02:
Which of the following types of noise can often be reduced by a digital noise reduction?  

Choices:  
A: Broadband white noise  
B: Ignition noise  
C: Power line noise  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

Correct answer: D — All these choices are correct.

Why D is correct
Modern DSP “digital noise reduction” (DNR) in many HF/VHF radios uses time- and frequency-domain processing (spectral averaging, adaptive filtering, notching, and impulse blanking) to reduce several common noise types:
- Broadband white noise: Random, uncorrelated “hiss” is reduced by spectral averaging and effective bandwidth reduction.
- Ignition noise: Impulsive, short-duration spikes (from engines, switching) are mitigated by impulse blankers, median/thresholding, and time-domain gating.
- Power line noise: Periodic 50/60 Hz buzz and harmonics are reduced by adaptive notch/comb filtering and spectral subtraction.

Why A, B, and C are not the best choices
- A, B, and C each name a noise type that DNR can often reduce, but none is uniquely correct. Since DNR can help with all three, the inclusive answer D is the best choice.

Useful formulas and rules of thumb
- Thermal noise power is proportional to bandwidth:
  $$N = k T B$$
  Halving bandwidth reduces noise power by 3 dB. Many DNR methods effectively reduce the “useful” bandwidth or average across it.
- Spectral averaging improves SNR. Averaging $M$ independent estimates gives:
  $$\Delta \text{SNR} \approx 10 \log_{10} M \text{ dB}$$
  This is why averaging reduces white noise and stabilizes periodic-noise estimates for notching.
- Impulsive noise reduction works because impulses occupy a very small time fraction; blanking or clipping those instants removes much of the crackle with limited impact on desired audio.

Exam tip
- “Often” is key: effectiveness varies with severity and settings, but DNR techniques apply to all three listed noise types, so choose D.