Pulse amplitude modulation in general is often explained using plots like this one from wikipedia:
There is a sinusoid analog signal (red) to be "imprinted" on a train of pulses. The result is a train of pulses with varying amplitudes. (the blue signal)
In the first case (the wikipedia plot), we have an anolog information that is put onto a pulse train.
In the second case case we have a quartenary discrete signal that is encoded in an analog signal that does not seem to have any pulses. We can see in the eye diagram, that the signal does not abruptly reach an amplitude and return to some "baseline" like a pulse, instead it's almost just a direct line between different amplitudes levels. Is the waveform that actually goes over the wire the equivalence of the red signal of wikipedia's plot rather than the blue? Does the pulse train enter the stage when the signal is decoded? The only connection I can see is that the amplitude of the signal represents the information. Otherwise how do these concepts relate at all?