Complex sentence: Unipolar encoding is literally the simplest form of "amplitude modulation" (AM) comprising only two amplitudes; known generically as "on" and "off" where the "off" state is simply a no-signal "silent period".
Split into simpler sentences: Unipolar encoding is the simplest form of "amplitude modulation" (AM) composed of only two amplitudes; known generically as "on" and "off". The "off" state is simply a no-signal "silent period".