The UK’s BBC R&D recently conducted a survey of the size of televisions and kinds of environments in which people watch TV at home. The results are especially important as the broadcast industry moves towards UHD images.
The research concluded that the median viewing distance, that is half of those interviewed, watched at a viewing distance of 2.63m (8’ 7”), and in terms of the ratio of the height of the screen this converted to approximately 5.5H. Let us examine why these numbers are important in light of the coming UHD tidal wave.
The BBC provided an online survey advertised on social media and its own website, receiving 2416 respondents. The authors Katy Noland and Louise Truong used statistical methods to remove out of scope data that was clearly wrong and analysed the data to create the BBC White Paper WHP287.
Comparisons from a previous UK study by Tanton in 2004 showed that the screen sizes have increased, but the viewing distances have stayed the same. And there was little correlation between the size of the screen and the real viewing distance from it. Instead the study suggested that the viewing distance was limited by the size of the room, this is intuitively correct as UK homes have not significantly increased in size over the past twelve years.
From the data gathered in the survey it is possible to determine how many people are able to experience the full benefit of HD, and how many might benefit from the increased spatial resolution provided by UHD.
The full UHD experience must take into consideration the viewing conditions and sound reproduction system to determine whether the viewer would benefit from upgrading to a UHD television or not, and this can be considered in three areas; viewing environment, distance from the screen and the audio system used.
The aim of high dynamic range is to expand the contrast ratio between dark and light areas of the screen as well as extending the colour gamut available by delivering 12bit video to the home and increasing the available peak brightness of the screen to more than 1000 NIT’s (one NIT is the unit of luminance equivalent to one candela per square metre). A typical HD television is capable of delivering peak luminance levels of 230 NIT’s so the contrast ratio is significantly greater on UHD monitors.
Viewing conditions affect the UHD immersive experience as a light room, or room with bright light sources such as a window or table lamp, will illuminate the front of the screen. Although the effects will be less noticeable in highlights, the low lights and dark areas will be affected causing a flare effect on the screen. To truly gain UHD experience the home viewer should watch in a light controlled environment, preferably in a dim or dark room.
Perceived resolution is not totally dependent on the pixel count, but is also reliant on the contrast ratio and colour spectrum employed. Opposing primary or secondary colours can give a greater perception of contrast and don’t rely on simply increasing the pixel or line count. Increased colour gamut and dynamic bandwidth greatly influence the perceived contrast and hence resolution of a screen.
In this survey, 10% of the respondents watched their televisions in preferred dim or dark rooms.
The need to upgrade audio
Surround sound systems enhance the viewing experience further by adding multi-direction audio, the effect being most noticed on films, drama and high energy sports.
A further challenge with UHD surround sound is that the AV amplifier will have to be upgraded or replaced with the HDMI-2 standard to decode the higher frequency progressive 4K data streams, potentially adding another cost to the system.
The survey showed that 17.9% of respondents had a surround system, with over 60% still using the built in television stereo loudspeaker system.
Except for a few test broadcasts UHD is mainly confined to internet delivery for the UK home viewer. The survey showed that 91% of respondents watched broadcast TV, with 61% also watching internet streams and 68% watching packaged media such as DVD’s.
The viewing distance is influenced by two factors, the angle of adjacent lines subtended at the eye to gain the optimum resolution, and the comfort of how close we want to be to the screen. There are several schools of thought on determining the resolution of the human eye, a popular example being that of Dr Hermann Snellen, the Dutch 19th century ophthalmologist who invented the visual acuity chart, giving rise to 20/20 or 6/6 vision. From his research we have derived that the optimum viewing distance of HD televisions is approximately 3 x height of the screen, and 1.5 times for UHD.
Noland concludes that 10.2% of respondents watched their televisions at 3H or less, and 1.5% at 1.5H. This suggests those viewing a 42inch television at a distance of 3H, that is a distance of 60 inches or 1.5m, could potentially benefit from UHD, especially if they combined this with improved environments such as dim or dark rooms and surround sound audio. And those viewing the same 42inch television at 1.5H, that is 30 inches or 0.75m would almost certainly benefit.
Sitting 30 inches from a television might be uncomfortable for some. However, the general trend has been shown to keep the viewing distance the same, as the size of viewers houses hasn’t changed that much in the past twelve years, and make the screens much bigger.
If we assume the scenario where somebody buys a bigger television, they will almost certainly have a better immersive UHD experience but the UHD screen would have to increase to a size of 80 inches to maintain a minimum viewing distance of 60 inches (1.5m) and achieve the maximum resolution.
Another factor is the material being broadcast. HD material viewed on a UHD screen will almost certainly have up-res artefacts and these may well be noticeable at a viewing distance of 60 inches. The viewer should consider whether they will be watching mostly UHD or up-res’ed HD. An alternative might be to watch any HD material in its native format on the UHD screen, the 3H relationship would be restored at the expense of a smaller picture in a bigger screen.
The full report can be found on the BBC’s website at White Paper 287.
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