Signal Detection Theory (SDT) separates sensitivity (d′ = signal discriminability) from response bias (criterion c). For Morse code detection, a dot/dash exists (signal+noise) or not (noise only) — both produce Gaussian energy distributions. The ROC curve traces all operating points as the criterion shifts. d′ = z(Hit Rate) − z(False Alarm Rate) measures how well the signal stands above noise, independent of decision bias.