3DTV Explained

3D television is the latest evolution in the world of HDTV displays, adding the perception of depth – perhaps at the expense of resolution and clarity as a whole – to traditional 2D pictures that we have been watching for some time. 3DTVs generally work by delivering slightly offset images to each of the viewer’s eye – the brain is then responsible for fusing these separate images to create an illusion of 3-dimensional depth.

Offset images from Monsters Vs Aliens 3D Blu-ray, as seen without glasses

At this time of writing, there are three types of consumer-grade 3D TV displays, namely the active-shutter, passive/ polarized, and glasses-free (also known technically as autostereoscopic) variants. The first two require the viewer to wear compatible 3-D glasses, whereas the last one doesn’t – this article will attempt to explain the pros and cons of each.

Active-Shutter Glasses (ASG)

… is also known as alternate frame sequencing (ASF), or just active 3D in short. In this method, the aforementioned slightly offset images are produced by the right and left shutters on the active-shutter 3D eyewear opening and closing in alternate sequence. The rate and sequence of shutter opening and closing needs to be in sync with the images shown on screen, which is why the active-shutter glasses from one brand typically cannot be used on 3D TVs from other TV makers. However, lately several manufacturers have tried to establish a common, unified standard to improve cross-compatibility among brands, with an ultimate aim of boosting active 3D adoption among consumers.

The earliest active-shutter 3D glasses communicate with the corresponding tri-dimensional TV sets via infrared signals, but some manufacturers (pioneered by Samsung) have started using Bluetooth technology which does not require direct line of sight. All such eyewear need power to operate: some run on CR2013 lithium batteries; while others are rechargeable over USB or even wireless connections.

Pros

• Active 3D is the only method capable of delivering full high-def resolution to each eye (Full HD 3D) at this time of writing

Cons

• Active-shutter glasses are generally heavier, less comfortable, less convenient (needs synchronisation and recharging from time to time), and more expensive than their passive/ polarized counterparts;
• Some viewers may see flicker through the glasses, which may lead to eye fatigue or headache.

Passive 3D

This is also known as polarized 3D, or “Cinema 3D” – a moniker brilliantly coined by LG Electronics in view of the similarity between this extra-dimensional technology and that employed in cinemas. Under this method, a polarizing filter is placed in front of the TV screen, which sends alternate lines to each correspondingly polarized lens on the 3D glasses (for example, odd lines to the left eye, and even lines to the right eye). The brain will then attempt to fuse these slightly offset images, resulting in the perception of tri-dimensional effect.

3-D image on LG passive 3D TV, seen through left lens of polarized glasses

Pros

• Polarized 3D glasses are typically lighter (and therefore more comfortable) and cheaper than active-shutter ones. They also do not require power (i.e. no battery/ charging needed) nor synchronisation with a 3-D transmitter.
• Flicker-free

Cons

• By sending alternate lines to each eye, the vertical resolution is effectively halved, which means that the resultant 3-dimensional image is not “Full HD 3D”. Some viewers may notice visible scan lines or jagged edges depending on how far they sit from the passive 3DTV.

Glasses-Free 3D

In theory, this is the most appealing technology, as it does not require users to put on 3-D eyewear (which has repeatedly been cited as a major stumbling block for 3D TV uptake) at all. To achieve this, a lenticular sheet is overlaid upon the television screen, which delivers parallax images from each 2D frame to the viewers’ eyes (again in a slightly offset fashion). Because the viewer needs to sit/ stand in the specific position for the correct images to be sent to each eye or else the 3D effect will be lost, the sweet spot  (viewing angle and distance from screen) is usually very limited.

At this time of writing, the only manufacturer to have released a glasses-free 3D television on the UK consumer market is Toshiba with its 55ZL2 which carries a retail price tag of £7,000.

Pros

• No need to wear 3D glasses
• Extra-dimensional images are brighter, as there’s no darkening effect from 3-D eyewear

Cons

• Narrow sweet spot
• Expensive

Crosstalk

Probably the most commonly seen artefact on the current crop of 3DTVs is crosstalk. Caused by leakage of the right-eye view to the left eye (or vice versa), this manifests as double “ghost” images which are particularly noticeable around high-contrast edges (for example, bright objects on dark background, or the other way round). Generally speaking, displays with better motion response (such as 3D plasma televisions) tend to exhibit less crosstalk.

Since the release of Spears & Munsil HD Benchmark 2nd Edition test disc, we’ve started using one of its patterns (screenshot above) to objectively evaluate the amount of crosstalk on all 3D-capable HDTVs and projectors we review. For the assessment to be accurate, the gamma on the display device in 3-D mode needs to be known – this is a value we can obtain during the calibration process.