For each bit duration $m$ (I also call it a `signaling interval’ due to having a possibility of sending multiple bits on one waveform, as we will shortly find out), the modulated cosine waveform is written as \end{equation*} Since\begin{equation*}\cos 0^\circ = 1 \qquad \text{and} \qquad \cos 180^\circ = -1,\end{equation*} such a demodulation process has transformed the phases into amplitudes!