Cablelabs have taken the first major step to allow broadcasters to launch 3D channels in the US with a definition of the acceptable formats and resolutions for frame compatible 3D. Frame compatible 3D defines all formats where the left and right images are squeezed into a pre-existing 2D frame size. In the case of side-by-side 3D in a 1080x1920 frame the left and right image are each sampled to 1080x960 pixels and broadcast next to each other with the left image taking up the left hand side of the frame. Cablelabs have also standardised the algorithm used to exclude any complex, resolution restoring variants. 

The decision to limit the use of side-by-side to 1080i transmissions means that this will undoubtably become the dominant format used by broadcast in the US since the vast majority of HD transmissions are in 1080i. Top-bottom is excluded from using interlaced formats since it would compound the resolution removed during interlacing by subsampling vertically (removing rows for the 3D format from an image with half the rows of resolution in a single frame). This will constrain top-bottom to use in the few 720p transmissions such as planned by ESPN.

Upon resizing of either a 720p or 1080i transmission from top-bottom or side-by-side frame compatible 3D the resulting image will be sub-HD resolution per eye and so Cablelabs will need to release a follow-up specification to define the processes and pre-requisite chipset upgrades needed to deliver HD 3D to the home.