Originally posted by ogl DC and SDM are micro-motors. Pentax has no ring motors.
DC is the same SDM with more simple and reliable construction.
Not exactly.
Just like USM, SDM is Pentax' generic name for "ultrasonic" piezo-type motors. Canon had both micromotor and ring-type USM, while Pentax made themselves only micromotor type SDM. However, the rebadged Tamron is using an "ultrasonic" ring-type motor so it's branded as SDM.
A ring-type SDM would not require gears like a micromotor-type SDM.
DC is Pentax' generic name for direct current motors; AFAIK they are not piezo motors. There's a Pentax patent about a ring-type DC motor, it was said to be implemented into the new D FA lenses but I don't remember seeing a definite proof for that - perhaps someone can clarify?