At present however relatively few HD services transmit in "full" 1080p HD. Most, such as Sky's HD service in the UK, transmit at either 1080i or 720p. 1080i displays at 1920 x 1080 pixels like 1080p, but with interlacing so that only alternate lines are refreshed each cycle. This means that each line is only refreshed every two cycles and is out of phase with the line immediately above and below it. The effect is that 1080i is therefore almost as good as 1080p for video content such as documentaries and wildlife programs where the action is generally slow, unless it features say a fast running animal. But it is less good for fast moving content as in movies and especially many sports, because the speed of movement can cause distortion as some of the pixels will be in slightly the wrong place by the time the alternate lines above and below them have been refreshed.
720p on the other hand is progressive, displaying every line simultaneously, but only at 1280 x 720 pixel resolution. This reduces the quality of image display but works better for fast moving action. For this reason some broadcasters transmit documentaries in 1080i but sports and movies in 720p.
Over the next few years interlaced scan is likely to fade away as it is no longer needed to serve old CT displays which were incapable of displaying every line at once without suffering from fading, and as available bandwidth makes it possible to step up to full 1080p HD.
But super HD, or ultra HD, will require a massive 16-fold further increase in bandwidth if it does indeed arrive in the format currently being evaluated, which is 7680 x 4320 pixels progressively scanned. That is four times full HD in both the horizontal and vertical directions, making 16 times more pixels altogether, which will require 24Gb/s to transmit uncompressed. It is possible that frame rates may also have to rise, consuming even more bandwidth, since that may become the "quality bottleneck" as resolutions increase, especially for very large screens. Current frame rates may then lead to some jerkiness in the picture.
Currently no commercially available TV set can display super HD, which is confined to a few trials, having first been demonstrated in 2008 beaming images from City Hall in London to a conference centre in Amsterdam. The technology was developed by Japanese public broadcaster NHK, which is now cooperating with the BBC in the UK to use super HD to display pictures of the 2012 London Olympics on public viewing screens.
This may provide some momentum for development of super HD TV sets. By 2016 with 40Gb/s and even 100Gb/s speeds more widely available both for local and wide area transmission over fiber-optic networks, super HD programming will then be more feasible for production and contribution. But it will exert huge pressure on distribution networks and hence perhaps hasten SD switchover. However there are some alternatives to a complete SD switch off that will bring relief. In the case of terrestrial, one option is to switch the SD to the latest digital standard in order to gain bandwidth from various technical improvements. In Europe, this means moving from DVB-T to DVB-T2, yielding an immediate capacity gain of around 50 percent. Changing the video from MPEG-2 to H.264 encoding, which is often done at the same time as a conversion to DVB-T2, will also free up space for super HD.