Famicom 3D System

The Famicom 3D System is an accessory for the FamiCom that was only released in Japan in 1987. The 3D system consisted of a pair of liquid crystal shutter glasses that received video signals through dual 3.5mm jacks on a special adapter. It gave compatible games the illusion of 3D depth. Most games released would play in conventional 2D until a "3D mode" was activated by use of the select button. — from Wikipedia

Output ($4016 write)

7  bit  0
---- ----
xxxx xxFx
       |
       +--- 0: Next field of video is for left eye (left eye lens is transparent; right eye lens is opaque)
            1: Next field of video is for right eye

Because the Famicom only generated images at 60Hz, each resultant image will flicker at 30Hz.