Servos - Coreless or Digital?
PPM offers a higher degree of resolution than PCM. It's well documented and proven technology...
PCM is MOST definitely not a pre-requisite for the use of Digital servos.
Digital servos offer several advantages over there analog counterparts. Precision, holding power or torque, repeatable performance and more. The only difference between a Digital core-less and analog core-less servo is the digital amplifier, same motors, same pot's same gear train in comparable model's.
There are only two Digital servos known to us that are programmable (by the user) thus far Hitec and Multiplex.