A dynamic jack reference control system may include a tilt sensor operatively associated with the vehicle so that the tilt sensor senses a tilt angle of the vehicle. The speed change device is capable of changing a reduction ratio thereof and capable of automatically switching between a high speed low torque mode in which a high speed low torque is output and a low speed high torque mode in which a low speed high torque is output, based on an external torque applied to a spindle. The described techniques may improve video quality relative to techniques that use the same explicit weights for explicit bi-directional prediction and explicit uni-directional prediction within a B-unit.