On almost all communication systems, transmitted signals should have certain characteristics. AC signals (ie biphase) have zero offset. Offsets merely serve to waste energy over the transmission line, hence signals are usually ac-coupled on the transmission side. Also, offsets could lead to signal drop-off over extended distances.
Most digital demodulation techniques involve correlation. Correlation means you integrate the product of the received signal r(t) and a particular signal pattern s(t), wrt to t, to find the "similarity" between them. So if there are, for eg, 4 different possible signal patterns, then you find the correlation of all 4 patterns, the the one with the highest correlation will be the best estimate of the transmitted signal from the received signal, regardless of whether they are DC or AC.
Suppose there are n possible transmitted signals. So using correlators, you need n integrators and n mixers. Mixers are very "expensive" components. Hence, a substitute to the correlator is a filter, with impulse response h(t) = s(T-t), where s(t) is the possible transmitted signal. It is also very important that the output of the filter is sampled at exactly the right moment, and that s(t) is synchronized exactly with r(t), using PLLs or some other means.
Over at my university here, the entire demodulation process is taught over an entire module on Digital Communications. I'd suggest you grab any text on digital comms if you want to know more.