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High Definition Television FAQ for the UK channels on Sky, Telewest and BBC

What resolution can the eye see?

Assuming you have good 20/20 vision the highest resolution you can appreciate depends on the size of the screen and how close you view it from. Research by the BBC confirmed that the correct figure for 20/20 visual acuity is a sixtieth of a degree. Smaller detail blends together and is not appreciated by the eye. This means that for someone with 20/20 vision the smallest pixel distance they should need is the viewing distance divided by 3400. If you sit 2.7 meters from the TV screen (this is the UK average according to the BBC) then you want a pixel size of 0.8 mm. By this logic you should be content with the standard definition 576 lines up to screen diagonal sizes of 37", 720 lines up to 45" and 1080 lines up to 70". This assumes 20/20 vision which most people can manage with glasses if necessary. Some people may have only 20/30 or 20/40 vision in which case they may not see the benefits of HDTV at all. Many people have 20/15 vision or even 20/10, in which case they would appreciate HDTV on smaller screens. The following table shows the optimal screen sizes for different formats (number of lines and aspect ratio) depending on vision and viewing distances.

For example, if you sit about 8 feet (2.5 meters) from your screen and plan to get a 720p HDTV set, you should go for a screen size in the range 34 to 42 inches (this is diagonal size). If it is smaller than 34 inches you will be no better off than with standard definition TV at that distance, and if it is larger than 42 inches you will be too close and will see the pixels (assuming 20/20 vision). This may lead to eye-strain.
As another example, if you are lucky enough to have acquired a 50 inch 1920 x 1080 screen to watch 1080i broadcasts you should sit no closer than 2 meters for 20/20 vision (2.7 m for 20/15 vision.) If you set further away than 3 meters you would not be benefiting from the higher resolution and would have been better off with 720 lines. This demonstrates that getting the right screen size and viewing distance is quite critical.

Another way to express the optimal viewing position is in terms of the ideal viewing angle. The viewing angle is the angle subtended by the width of the screen from your viewing position. For 720p the ideal viewing angle is about 20 degrees and for 1080i it is 30 degrees. If you have better than 20/20 vision, the angle should be a bit less than that.

What does "HD Ready" mean?

For the European market the EICTA (European Industry Association for Information Systems, Communication Technologies and Consumer Electronics) has defined a basic set of requirements for an "HD Ready" TV. Displays that meet the standard can be labeled with the HD Ready logo shown. The requirements demanded to meet the standard are:
(1) A native resolution of at least 720 lines in widescreen format
(2) The ability to accept HD input in YPbPr and via DVI or HDMI
(3) It must accept 720p50, 720p60, 1080i25 and 1080i30 input signals
(4) The DVI or HDMI input supports HDCP content protection

Is the HD Ready logo all I need to look for?

No. The HD Ready logo only guarantees a minimum standard. For example,
- It only requires a 720 line native screen resolution, not the higher 1080 resolution.
- It does not even require 1280x720 native screen resolution, stretched 1024x720 will pass.
- It does not require that the display can render the full 50 or 60 frames per second of the 720p standards.
- It only requires DVI or HDMI, not both.
- It does not specify how well the 720p and 1080i inputs have to map to the screens native display format
- It says nothing about other picture quality elements such as physical size and contrast ratio.
- It says nothing about the audio standards supported, or background noise levels.

Outside Europe the term "HD Ready" is also used but it may not imply the same conditions

Who checks the HD Ready status of a TV?

The TV manufacturer is meant to be responsible for adding the HD Ready Logo when it applies. In practice it may be the seller who sticks on the logo labels. It is not sure that all vendors know what they are doing and there are not obvious indications that anyone is checking. However, we are not aware of any reports of mislabeling.

So what should else should I look for?

Make sure the HDTV set you buy at least has the native resolution you want and HDMI input. If you want to be more discerning, compare all the other features, especially things like the contrast ratio. But don't worry, with the HD Ready logo you will be able to view great HD pictures. In case you want more detail we have drawn up some sample comparison tables.

What are DVI, HDMI and HDCP?

DVI is Digital Visual Interface. This is an old standard for digital video input used initially for PC screen connections but also suitable for HDTV. PC owners may be familiar with the DVI cables used to connect their monitor.

HDMI is the High Definition Multimedia Interface. It was introduced more recently with Digital TV in mind and has been called the SCART socket of HDTV. Unlike DVI it includes audio content as well as video.

HDCP stands for High-bandwidth Digital-Content Protection. It is a standard used to encrypt digital signals to protect integrity. In practice it is used to prevent illegal pirating of HDTV while allowing recording and transmission for personal use. It is essential that your HDTV supports this standard because most HDTV broadcasts and recordings will be using it. If you connect your HDTV to a set-top box or HDTV DVD player without using a digital connection that supports HDTV, then the signal will be downgraded to standard definition. If you use HDMI you are safe because support for HDCP is built into the HDMI standard. A DVI connection may or may not support it. This is why you should ensure that your HDTV has HDMI. The HDCP standard was introduced quite late in the development of HDTV, so people who bought HDTV sets early may not have it. Sky will allow HDTV to pass through component video cables from their first set-top boxes so if you have an HDTV set without HDTV it will be worth subscribing to Sky HD early on while this capability is available.

What input standards will the TV need?

A digital input will be needed. Some older sets have analogue input only but to avoid piracy it is likely that set-top boxes for HDTV will reduce resolution to standard definition for analogue connections to the HDTV. That is certainly not what you want to happen! DVI and HDMI are the digital standards in use. HDMI is the newer standard. It can take a higher bit-rate and includes audio. It has been called the digital scart. Sky have announced that their first batches of set-top boxes will permit HDTV to be run through component video cable so if you have an older HDTV you probably need to get one of the early set-top boxes.

If my HDTV has only one HDMI will I be able to connect all my video sources to it?

The Sky set top box will have just one HDMI connector so it will connect to the HDTV leaving no sockets for further inputs. However, the XBOX 360 has just component video and a scart adapter so you will be able to plug that into a separate scart socket if the TV has one. When you get a DVD you either need to make sure it has two HDMI sockets that allow daisy chaining, or get an HDMI switch box. The first versions of the Sky set-top box will allow HDTV to go through ordinary component video connections so that will provide another option. It is always possible that they will use a software upgrade to disable this feature and impose HDCP protection at some later point in which case the component video connection will be no good.

What is the specification of the Sky HD Box?

The Sky HD Box made by Thompson uses DVB-S2 modulation and MPEG-4/H.264 AVC compression codec with videogarde encryption. 1080i/25 and 720p/50 will be the supported display standards. It has video output via a single HDMI socket and analogue component video. The component video will allow HD output which is not copy protected, so the older non HD Ready HDTVs sets should be able to view HD video. The box has a 160 GB Hard Drive available for about 30 hours of HD recording if you have the Sky+ subscription. It will also be possible to run a second TV from the box in standard definition. The Sky HD box comes with a new remote control.

How can different standards be displayed on the same display?

Upscaling, deinterlacing and motion compensation can be used to convert from one standard to another.

Upscaling is used to increase the number of lines, e.g. from 576i to 1080i. This can be done crudely by interpolating or it can be done better using image processing techniques. Some DVD players and HDTV sets can do this but don't expect great results. It currently requires very expensive hardware to do this well. Some of the content broadcast on HD channels is actually standard definition pictures upscaled. The broadcaster will do this with much better technology than you can find in a home DVD player or HDTV but no upscaling technology can create detail that is not in the original. Sky have said that their HD channels will include some upscaled content, but they will label clearly so that we know when it is true HD.

Deinterlacing is used to convert from interlaced standards such as 1080i to progressive standards such as 720p. This is more difficult to do well than upscaling. Some TV sets will do it very badly by throwing away half the fields and upscaling instead. This means that 1080i transmissions may not be very good on 720p sets, so this could be a common problem. Again, the broadcasters can use much better technology to do the deinterlacing well.

There are several techniques to convert from one frame rate to another. Mostly this will be done by skipping or duplicating frames. It is possible to transform slower frame rates such as those used in films to the higher frame rates of 720p by using motion compensation to work out what intermediate frames should look like. Unless this is done very well there are likely to be visible artifacts on screen such as shadowing around moving objects.

Will better HD standards come later?

It is possible that 1080p/50 or 1080p/60 will be available with some media such as DVD, game consoles and online programs. 1080p combines the high resolution of 1080i with the fast frame rate of 720p. It is a natural next step because HDTV sets can be made to display 1080p without too much extra cost over 1080i. Sadly, most HDTV sets do not yet accept 1080p even if they have sufficient native resolution. This is because better electronics are needed to run at the bit rate of 1080p which is twice that of 1080i. It is unlikely 1080p will be available as direct TV broadcasts in the foreseeable future because it would not be effective use of the limited bandwidth. However, over the next few years different systesm such as on-demand-video from the internet could make higher resolution formats more viable.

Higher resolutions than 1080p become less worthwhile because to appreciate them you need to be sitting close to the screen unless it is very large. With 1080p the optimum viewing distance already gives a subtended viewing angle of about 30 degrees. The Japanese are reported to be experimenting with a standard with 16 times as many pixels, dubbed "Super Hi Vision". The ideal viewing angle is then 90 degrees so such resolution only becomes meaningful for large cinema screens or other special displays. However, another way to view such a high resolution image would be through special goggles or head sets. With the speed that video technology and internet bandwidth is advancing, there could be enthusiasts watching films or playing video games on something like this in the not too distant future.

What HD DVD standards will be available?

There are two competing standards for High Definition DVD: HD-DVD owned by Toshiba, and Blu Ray (BD) owned by Sony. They will become available for PC, games consoles and films gradually over the next two years. It is not yet clear which if either will win the biggest market share. Current release dates in the US are April for HD-DVD and May for Blu-Ray. The UK launches will be a few months later.

What HD games consoles will be available?

The new Xbox 360 has a native format of 720p. It will be sold with HD DVD player later in the year. The Playstation 3 expected towards the end of the year will also be HD. It will come with a Blu Ray drive from its launch towards the end of 2006.

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