The diagram shows a sinusoidal waveform at four amplitude levels. Four time periods are represented by five equidistant dotted vertical lines. From left to right, the amplitude of the waveform drops in each time period. The four possible amplitude levels are marked as A, B, C and D, with A being the highest (on the left) and D being the lowest (on the right). The amplitude levels A to D correspond to the two-bit binary numbers 1 1, 1 0, 0 1 and 0 0.