UHD: ensuring quality throughout the chain is crucial
For UHD to truly take off, the TV industry must deliver enhanced services to consumers and offer genuinely enhanced images
One of the key UHD challenges is ensuring that quality of content is preserved throughout the various stages of delivery, says Matthew Goldman, senior vice president technology, TV compression at Ericsson
UHD can be described as an incremental set of user experience improvements. The first is one of simply more pixels. 4K delivers twice the number of horizontal pixels and twice the number of vertical pixels, but what about improving the quality of each pixel itself?
In order for UHD to truly take off, the TV industry must deliver enhanced services to consumers and offer genuinely enhanced images. The reason that UHD offers the potential for a more immersive viewing experience is due not only to the higher image resolution (four times more than HD) but also a range of other factors which include higher frame rates (double that of today’s 1080i HDTV), wider colour gamut and higher sample bit depth (10-bit versus today’s 8-bit).
One key challenge the industry faces in bringing about true UHD is ensuring that quality of content is preserved throughout the various stages of delivery. To ensure the requisite high level of picture quality, acquisition/master 4K UHDTV video compressed signals require 4:2:2 chroma sampling and 10-bit depth for all content. This ensures consistency of colour fidelity through the many encode, decode and re-encode stages, and through multiple editing stages.
Higher frame rates than those used for SD and 1080i HD are required to represent fast motion, such as is common with sports, without excessive motion blur or motion judder. Large 4K UHD screens necessitate 50-60 frame rates at present because there is more spatial movement for the viewer. Higher frame rates help to compensate for the greater angular change caused by fast motion displayed on these larger screens, ensuring that visual artefacts are minimised.
The future may dictate higher frame rates of up to 120fps in later phases of the technology, as this will further enhance resolution on fast motion. However, the challenge facing the industry is to bring this about while managing significant additional costs for production and post-production.
Another possible development could come in the form of HDR where a viewer can distinguish a wider range of detail between the darkest and brightest images. Unlike humans and modern cameras, today’s TVs do not have a huge perceptual dynamic range. For example, this can affect the way in which objects within dark shadowy areas appear on screen during a sunny outside broadcast.
Further exploration into the development of this area is important because HDR may turn out to have the greatest impact on the TV viewing experience.
Unlike 4K spatial resolution, HDR’s perceptual benefit is not limited to 4K spatial resolution or large screen size; it has the necessary versatility to enhance the picture quality on a tablet or mobile phone, as well as the main living room TV. If the TV industry does opt to go down the path of HDR (and this appears to be the trend), 10-bit sample depth (as opposed to today’s use of 8-bit sampling for TV signals) is the minimum necessary to represent HDR.
Just as the human visual system recognises a wider dynamic range than current TV, it can also see a wider colour gamut than the colour space currently used in HDTV, Recommendation ITU-R BT.709. UHD colour space is defined in Recommendation ITU-R BT.2020; it reproduces a greater range of colours than current HD colour space (Rec.709) and this will give more realistic colour perception.
There are still a number of other technical barriers to overcome, including cost and bandwidth efficiency. It is therefore the case that implementations of HEVC encoders need to happen in order to enable UHD to the home and consumer devices. Its arrival has helped to solve a number of compression issues by offering the potential to halve the bitrate of H.264/MPEG-4 AVC, making the delivery of ‘true’ 4K UHD TV a realistic prospect.
Ericsson lab research shows that a high performance encoder that can achieve a halving of existing bandwidth takes about ten times the processing power than that of MPEG-4 AVC. For UHD there is also four times as much spatial resolution and twice the temporal resolution compared to HD, making a total of 80 times the processing power compared to HD MPEG-4 AVC.
It’s clear that the industry needs to address a number of factors and bring them together in an end-to-end ecosystem to enable consumption expectations to be met with dramatically enhanced user experiences, and to allow a large array of new business models to be brought into this next generation of television.
For more on UHD see this month’s TV Technology Europe: http://www.tvtechnologyeurope.com/digital-edition