Question E8A03:
Which of the following describes a signal in the time domain?  

Choices:  
A: Power at intervals of phase  
B: Amplitude at different times  
C: Frequency at different times  
D: Discrete impulses in time order    

Correct Answer is  B: Amplitude at different times  

To describe a signal in the time domain, you plot or specify how its amplitude changes with time.

Why B is correct (Amplitude at different times):
- A time-domain signal is written as $x(t)$ and shows the signal’s amplitude versus time.
- Example: a sine wave in time is $x(t) = A \cos(2\pi f_0 t + \phi)$, which explicitly shows amplitude as a function of time.

Why the others are incorrect:
- A: Power at intervals of phase
  - That’s a phase or phasor-related view, not time. Time domain uses $t$ as the independent variable, not phase.
- C: Frequency at different times
  - That’s a time–frequency view (e.g., a spectrogram). The time domain is amplitude vs time; the frequency domain is amplitude vs frequency.
- D: Discrete impulses in time order
  - That describes a specific sampled or impulse-train signal, not the general definition. Time-domain representation means amplitude at every instant of time (continuous) or every sample (discrete), not necessarily impulses.

Useful formulas and rules of thumb:
- Time domain: $x(t)$ (continuous) or $x[n]$ (discrete samples)
- Frequency domain: $X(f) = \mathcal{F}\{x(t)\}$ and $x(t) = \mathcal{F}^{-1}\{X(f)\}$
- Rule of thumb: Oscilloscope → time domain (amplitude vs time). Spectrum analyzer → frequency domain (amplitude vs frequency).