A 3D display system has a timing control device for synchronizing the opening and closing of shutters on a viewing apparatus. The display system comprises a display unit configured to alternately show a right image and a left image in a sequence. The display unit has a backlight unit to illuminate a display panel. The light output from the backlight unit is frequency-modulated with one frequency when a right image is displayed and another frequency when the left image is displayed. The viewing apparatus has a photo-detector and a frequency discrimination circuit to detect the modulation frequencies in the light output. Based on the detected modulation frequencies, the shutters on the viewing apparatus can be opened and closed in a coordinated way.