Signals and Systems Practice Test

The statement tests the combination of how time is represented and how amplitude is represented. A discrete-time signal is defined only at specific time instants (like t = nT), not for every moment in time. A discrete-amplitude signal means the signal can take only a finite set of values due to quantization. When both properties apply—values exist only at discrete times and the amplitudes are drawn from a discrete set—you have a discrete-time, discrete-amplitude signal. That’s exactly what the description says: defined at discrete times with discrete amplitude levels. The other descriptions mix one discrete aspect with a continuous one, so they don’t capture both features at once. For example, sampling with continuous amplitude would still allow any real value, and a signal defined for all time with discrete amplitudes isn’t truly discrete-time. An everyday example is a digital audio sample: it’s taken at distinct times and each sample is quantized to a finite number of levels.