What is Over-Under 3D (or Top and Bottom 3D)?

With 3D TVs and 3D content catching on in a big way in 2010 and leading into 2011, consumers have had to familiarize themselves with a lot of new terminology and jargon specific to 3D display technology and 3D content delivery. One area that is extremely confusing for consumers is that of 3D content formats and how different 3D formats are made compatible with your 3D TV. The 3D content format guide on this site was written to very specifically address this issue and help clarify things for the consumers with a series of clear and concise articles related to this subject. One of the terms that you have probably come across is the Over-Under or Top and Bottom 3D format. So if you are wondering just what is Over-Under or Top and Bottom 3D, then this article is right up your alley and will do a great job at explaining this concept in simple terms.

The HDMI 1.4 3D format specification allows for a number of different formats for 3D video content. One of the mandatory 3D formats (i.e., the format has to be supported by a 3D TV for it to be HDMI 1.4 compliant) is Top and Bottom or Over-Under 3D. On the other hand, as you may well be aware, almost all 3D TVs currently display 3D content on the screen via Frame Sequential 3D techniques. However, this doesn’t mean that the input 3D signal to the TV has to be in a frame-sequential format. In fact, pretty much all 3D HDTVs can process 3D signals in a variety of different formats and perform on-the-fly conversion of the incoming 3D video signal into a frame sequential format. While frame-sequential 3D is part of the blu-ray 3D spec, when it comes to airing 3D content over cable/air, Side by Side 3D and Top and Bottom 3D are proving to be a popular choice as they are backwards compatible with HDMI 1.3 systems (if they are Frame Compatible versions of these formats) and do not require extra bandwidth for delivery of the 3D video (thus helping with backward compatibility).

How Over-Under 3D a.k.a Top and Bottom 3D Works
Over-Under or Top and Bottom 3D Format
The Top and Bottom or Over-Under 3D format consists of a single 1080p (or 720p) frame consisting of vertically stacked sub-frames for the left and right eye, each possessing half the full HD resolution along the vertical axis

In the Over-Under or Top and Bottom 3D format, the video content is encoded in a manner as shown in the adjacent figure. A single frame has a resolution of 1920×1080 (1080p) or 1280×720 (720p) but is divided into two sub-frames, the upper one consisting of the sub-frame meant for the left eye (Labeled L) and the lower sub-frame meant for the right eye (labeled R). As you can tell, the vertical resolution of each sub-frame is halved as a result of the over-under 3D format. In the case of 1080p content, the resolution of each sub-frame is 1920×540 pixels whereas that of 720p content is 1280×360 pixels.

Thus, the downside of this technique is that you lose half the vertical resolution of the image that is seen by each eye and so you are no longer seeing “true HD 3D”. However, the upscaling algorithms in most 3D TVs compensate for this loss in vertical resolution quite well and you can still enjoy good quality 3D content that is transmitted in the top-bottom or Over-Under 3D format.

This version of the Over-Under configuration is commonly referred to as simply by “Top-and-Bottom 3D” or “Over-Under 3D”. However, it should be noted that you can also have an Over-Under configuration where each sub-frame maintains full 1080p or 720p resolution. The Mandatory Full HD 3D (FHD3D) 3D video format that is specified under HDMI 1.4 is such an Over/Under (top/bottom) 3D format where each sub-frame maintains the full 720p or 1080p resolution. Thus FHD3D is the only 3D format that provides true 3D HD video at 24fps. Read our article on the FHD3D format to learn more about it.

The advantage of the Top-Bottom or Over-Under 3D format (with half vertical resolutions) is that each frame still has a total resolution of 1080p or 720p, the same as regular 2D HD content. As a result, the required bandwidth for transmitting the 3D signal, as well as for processing it, remains the same, thus making it much easier to support in current HDMI 1.3 devices. This is why current HD set top boxes can be provided a firmware upgrade by Cable providers so as to enable 3D content without requiring a total upgrade of the box.

How will my 3D TV decode and display Over-Under 3D?
When your 3D ready TV receives this top-bottom or over-under 3D signal, it splits each frame to extract the sub-frame for each eye, and then rescales these individual frames to a full HD resolution using upscaling algorithms. It then displays these upscaled individual frames alternately in a frame-sequential manner (see the second image below) that is in sync with your active shutter 3D glasses.

While Over-Under 3D or Top and Bottom 3D does halve the vertical resolution of the 3D video content, it has the advantage of not requiring extra bandwidth and is helping make 3D content more readily available as cable companies can enable 3D channels without having to spend billions of dollars on expensive set-top box and infrastructure upgrades. This format is primarily of use for providing 3D content over cable/satellite and is also used by some older 3D Ready TVs (before the establishment of the HDMI 1.4 specification). All the new 3D BluRays provide 3D content in the FHD3D format.

Hopefully, you now understand what over-under or top and bottom 3D is. It is just another method to deliver 3D content to your 3D capable flatscreen TV and although it does lead to the halving of the frame resolution along the vertical axis, it still provides an excellent 3D viewing experience. Additionally, it has the advantage of allowing for backwards compatibility with current HD set top boxes.