6 NEWS & VIEWS

April 2015

The optimum viewing distance for HDTV is 3H ( where H is height of the picture ), whereas the optimum viewing distance for 4k is 1.5H . by philip laven

Delivery of 4K services is now technically feasible : for example , any of DVB ’ s second-generation standards for satellite , cable or terrestrial services can deliver 4K by using HEVC video compression .

However , it is important to recognise that there are many unresolved issues about the business case for 4K . Rather than just reading this article ( and agreeing or disagreeing with my opinions ), I would like each of you to undertake a simple experiment at home involving nothing more complicated than a tape measure .

For each TV set in your home , you should first measure the vertical height ( H ) of the screen and then measure the distance from the TV screen to your normal viewing position ( sitting in your favourite chair , for example ).

Let us say you have a 46-inch TV set with an aspect ratio of 16:9 , you will find that H is about 57cm . If your viewing distance is 3m ( 300cm ), the viewing distance ( expressed in picture heights ) is 300 / 57H = 5.3H . Similarly , a viewing distance of 200cm would correspond to 200 / 57H = 3.5H .

What do such figures tell us ?

Based on the acuity of the human eye , HDTV was designed for a viewing distance of 3H . Hence , anybody viewing a TV set at 5.3H is unlikely to see much difference in the sharpness of HDTV and SDTV . Obviously , HDTV pictures look much sharper than SDTV when you are very close to the screen — but the extra resolution offered by HDTV becomes irrelevant at large viewing distances . I often hear people saying that the trend towards very large screens will mean that broadcasters will be forced to move to 4K .

Let us examine that claim on a scientific basis .

If you have an 80-inch screen

( which means that H = 100cm ), a viewing distance of 3m would correspond to 3H — which is just perfect for HDTV .

Noting that the ideal viewing distance for 4K is 1.5H , your 80-inch screen does not need the enhanced resolution of 4K unless your viewing distance approaches 1.5m . On the other hand , if you keep your viewing distance fixed at 3m , you would need a 160-inch screen to achieve a viewing distance of 1.5H .

By the way , the ideal viewing distance for 8K services is 0.75H which , at a viewing distance of 3m , would require a 320-inch screen !

In my home , all of the viewing distances from various TV sets were between 5.2H and 10.1H . Obviously , HDTV is not needed at 10H !

When I made similar measurements for viewing of a tablet with a 9-inch screen , I found that my normal viewing distance was about 3.3H , implying that my tablet needs HDTV — as well as being the most-demanding screen in terms of required picture resolution .

Although many readers of APB

are “ early adopters ” of new technologies ( and , hence , not representative of typical consumers ), I would be surprised if many of you are watching TV sets at distances of less than 3H . The reality is that the layout of our homes has not generally been optimised for TV viewing : chairs and sofas are laid out for comfort and to facilitate interaction with other people .

However , if you are rich enough to have a 100-inch TV set , it is likely to be mounted on a distant wall in your large and spacious home — perhaps at a viewing distance of 3H .

The enhanced resolution provided by 4K is probably “ overkill ”, unless you have a huge TV screen in a small room or habitually stand very close to your TV set !

Nevertheless , 4K screens are already on sale — and manufacturers are understandably keen to persuade consumers that they really do need 4K . While I am sceptical about the real benefits of enhanced resolution , we should not underestimate the power of advertising and / or fashion .

For example , digital still cameras now seem to be judged solely by the number of megapixels — whereas expert photographers know that the performance of the lens is far more important . Similarly , young people seem ashamed if they do not have the

latest version of a smartphone .

Such pressures might play a significant role in ensuring long-term success of 4K — but we need to be honest with ourselves ( and with the public ) about the limited benefits of improving static resolution .

In my opinion , we should take the opportunity of the transition to 4K to add other enhancements such as higher dynamic range , a wider colour gamut and higher frame rates ( 100Hz or 120Hz , for example ).

In my personal opinion , higher dynamic range is likely to be more attractive to consumers than increased static resolution — especially as the improved picture quality is visible at considerable distances ( from the far side of a room , for example ), unlike improved resolution which , as we have seen above , is critically dependent on viewing distance .

The transition to 4K will inevitably be expensive for broadcasters . In addition to paying for new delivery systems , they will need to invest in the infrastructure to allow them to produce 4K content .

Compared with “ full ” HDTV ( 1920 x 1080i ), production at the basic 4K standard ( 3840 x 2160p ) implies increasing the uncompressed data rate by a factor of 10 ( a factor of 4 due to the increased resolution , a factor of 2 due to the change from interlaced to progressive scanning and a further factor of 1.25 due to the change from 8-bit to 10-bit video ).

All content producers face the same dilemma : should they become pioneers by launching “ basic ” 4K services as soon as possible or should they wait a little longer for “ enhanced ” 4K services ?

If they opt for the former , their content will not remain competitive in the longer term , resulting in the need for a second transition ( from basic 4K to enhanced 4K ). If they opt for the latter , they will miss any short-term benefits of an early launch of basic 4K services , but they will need only a single transition ( HDTV ➝ enhanced 4K ).

Phil Laven is chairman of The DVB Project .