There is not much different between digital servo and analog servo except the power supply and I should how it supplies the power to the motor inside the servo. Digital equips a more advance esc and chip inside which use higher frequency in the giving out the power. It translate the signal from the receiver and turn it into a series of pulse to drive the motor. You may goto Futaba's web site for more details.
There is no receiver cannot use a digital servo. Instead there are analog servo cannot match with HRS system. Because HRS receiver send out signal differ from conventional PPM receiver