A scroll-type refrigeration compressor is disclosed which incorporates an efficient, reliable, low cost modulation system employing a single actuator to effect switching between full and reduced capacity operation. The modulation system of the present invention includes an annular valving ring rotationally supported on the non-orbiting scroll or main bearing housing which operates to ensure simultaneous opening and closing of one or more unloading passages thus avoiding the possibility of even transient pressure imbalances between opposed compression pockets during operation of the compressor or in one of the alternative embodiments, providing a controlled imbalance to provide a noise reducing torsional loading on the Oldham coupling. Further, the modulation system of the present invention provides for reduced capacity at both start up and shut down thus enabling the use of more efficient lower starting torque motors and reducing the potential for noise generating reverse rotation on shut down. The valving ring may be actuated by either a single or double acting pressure actuated piston or by an electrically operated drive.