The channel's output must be analog, and the receiver's filter - Hr(f) - can't turn an analog signal into digital, so I believe the converter is indeed analog to digital.
This link explains in some detail ASK (Amplitude Shift Keying) and will likely make your head hurt. The last stage is an A/D converter that outputs the binary sequence. A Google of Amplitude Shift Keying should bring up more information.
I read it, then read it again. Then took two aspirin and opened a beer. I think in the original drawing it is a little misleading as the final signal looks to be the output of a D/A right down to the step increments. That was my take anyway, then came Google.