3D Television - How does it work?

One of the main reasons that we can perceive images of the real world in three dimensions is the fact that we have two eyes, arranged side by side. Each eye sees slightly different pictures (stereoscopic effect) that are then processed by the brain to render depth and distance information (there are other important depth clues such as head and object movements, light, texture, focus, etc).

In consumer 3D television applications, it is this stereoscopic effect that is currently used to give the impression of three dimensional perception from a 2 dimensional TV screen (other types of systems have been proposed and may be developed in future).

But how can you make sure that each eye sees a different picture as it would when looking at a scene in the real world?

Implementation of 3D TV

The first challenge is to record the different image streams of a scene that each eye would see simultaneously. Luckily, the required basic technology has been around for quite some time. A stereoscopic pair of cameras is normally used that consists of two cameras arranged side by side, separated by the average distance between human eyes.

Each camera records therefore a slightly different picture stream, just as it would be seen by the eyes of a person looking at the scene from the same view point. Variations in camera separation distance and relative angle (toeing-in) make it possible to create additional effects.

The next challenge is to make sure that the right eye can only see the view of the right camera and the left eye only the view of the left camera. Tricky? - there are two main main solutions currently adopted for the domestic market: Passive 3D television and active 3D.

Passive 3D Television

In passive 3D, the TV screen is usually fitted with a filter that polarizes every even numbered line in one direction and every odd line in another direction. The picture frames of one camera (e.g. right camera) are then only displayed on the even numbered lines and the picture frames from the other camera are displayed on the odd lines.

When the screen is viewed through glasses that have right and left lenses with different polarization so that one lens blocks all the even numbered lines and the other lens blocks all the odd lines, each eye can only see the view from one of the two cameras (right or left).

The main advantage of this system is that the glasses are very cheap and uncomplicated (on average less than £5). On the other hand, each 3D video stream can only use half of the available native screen resolution i.e. each right/left picture of 3D material cannot be displayed in full HD. This system is used, for instance, in pubs (mainly because of the cheaper glasses), but LG is now also going to offer a passive 3D TV set for the domestic market.

Active 3D television

Active 3D is the technology employed by the majority of manufacturers for domestic applications. In active 3D television, the screen is viewed through active LCD shutter glasses and alternate-frame sequencing is used to display the right/left camera views on the TV screen. This allows it to display each picture using the full native screen resolution (full HD).

In other words, the picture streams recorded by the right and left cameras are displayed alternately, frame by frame in fast succession on the screen. When a picture from the right camera is displayed, the left lens of the LCD shutter glasses is darkened electronically and vice versa - in synchronization with the television pictures. This synchronization is achieved via timing signals sent from the TV set via Bluetooth, infrared, radio link, etc. to the glasses in sequence with the alternating TV images.

The active LC shutter glasses

The LC shutter glasses used for viewing 3D TV pictures are electronic devices in their own right and may be sold separately form the 3D TV set. Their lenses employ liquid crystal technology and are normally transparent but can be darkened temporarily very rapidly by applying a voltage to them (i.e. the synchronization signal from the TV set).

To produce a picture that is perceived as smooth without flicker effect, a sufficiently high picture refresh rate is required. A rate of 100 Hz (120 Hz for NTSC) is regarded as the minimum. As the refresh rate is split between the alternating right/left camera views, this results in only 50 Hz picture refresh rate for the video stream (right/left) seen by each eye. At this rate, people do usually not notice the fast changes and a smooth picture is perceived. For more on picture refresh rate ...

Unfortunately, manufacturers have not yet been able to agree on a common standard for LCD shutter glasses and their interfaces, and you have to make sure you get the suitable glasses for the TV system you are using.

For a large family the purchase of extra LC glasses will be a considerable cost factor, as everyone would need their own pair to watch in 3D.

3D ready television sets

There are still only a few 3D-ready TV sets on the market but interest from manufacturers and consumers is growing. The sets on offer or announced tend to be high end models with full HD and may offer LED dynamic backlight, very high picture refresh rates on large screens at a price that reflects these high specifications.

Samsung has already launched its version of 3D TV sets, the SAMSUNG C7000 with edge LED and Sony is lined up with two models, the BRAVIA HX903 3D capable TV and BRAVIA LX903 3D TV . More models have been announced by Panasonic, LG, Toshiba, etc. Most of the manufacturers also offer their own versions of 3D Blu-ray players, which will probably be the main source for 3D material for some time to come (although sky is talking about 3D broadcasts already). To allow transfer of the much higher data requirements (bandwidth) the 3D TV sets and Blu-ray players usually have HDMI 1.4 connections.

Author: HDK