High Definition Television FAQ for the UK channels on Sky, Telewest and BBC
What are the HD standards available?
The main HDTV standards which are on offer in the UK are called 720p and 1080i. 720p will show 50 frames per second (fps) each with 1280 by 720 pixels (When specifying the frame rate the standard may be written as 720p50 or 720/50p). The "p" stands for progressive to distinguish it from the interlaced standards with an "i". The interlaced standards show only every alternate horizontal line of the image for each frame so it takes two fields to cover all the lines. 1080i is an interlaced standard with 1920 by 1080 pixels and will be transmitted at 50 half frames per second in the UK (i.e 25 frames split over 50 fields). When specifying the frame rate these should be written as 1080i25 or 1080/25i (Often this is incorrectly written as 1080i/50 or 1080/50i.) Both standards use an image aspect ratio of 16/9. In the US a faster frame rate of 60 fields/sec is in use but the EBU (European Broadcasting Union) have chosen the slower rate of 50 fields/sec for Europe. It is possible that DVD, computers, games consoles and other HD devices may use 60 fields/sec even in the UK. Of the two, the 1080i standard looks set to be more common than 720p for TV broadcasts in the UK.
Why is interlacing used?
To produce the illusion of smooth motion to the human eye the image rate must be at least 24 fps. However at 24 fps the image flickers badly. To remove the flicker 48 fps are needed. The minimal way to do this is to show each image frame twice 24 times a second. That is what you see when you watch a film at the cinema. An interlaced standard such as 1080i25 shows half the lines of the picture on each frame so that the illusion of motion is achieved without flicker for the most efficient use of bandwidth. When you watch a film on TV, each frame is scanned into a pair of interlaced frames which are shown successively. TV cameras capture the half frames at staggered intervals so the motion should be smoother when watching a sequence from a TV camera with an interlaced standard. However the 1080i standard is most appreciated over 720p because of its higher resolution for still images.
What is 720p good for?
720p50 shows twice as many frames as are necessary to achieve the illusion of motion. But 24 fps is only the minimum rate needed to avoid obvious judder. The eye can appreciate the improved effect of motion up to about 80 fps, so the faster frame rate of 720p is very worthwhile for action sequences and especially sports. Even for slow movement the higher frame rate is not wasted. Video image moves most of the time and the eye follows the moving objects on the screen so the pixels are not necessarily seen in the same place for each frame. The perceived visual effect can be an increase of detail resolution. Some people still see flicker at 50 fps especially in peripheral vision so 60 fps is preferred. If your main interest is watching films rather than sports, remember that cinema movies are filmed at 24 fps. When this is converted to 720p they simply show each frame twice so the higher frame rate is wasted. It may be possible in theory to use "Motion Compensation" to interpolate between film frames and achieve a better effect. Some HDTV sets will do that in real time but it can create distracting artifacts such as halos around moving objects. It is possible that the studios will use better techniques when transferring films to DVD in HD, but don't count on it.Which HD standard is best?
If you look at a still picture the 1080i image is going to look better. With an image that does not move it does not make any difference if it is interlaced or not. If an image is moving, the progressive image gains an advantage, especially if the image is scrolling vertically up or down. So 720p might be potentially better than 1080i for fast action because of the higher effective frame rate. The image compression methods used must also be taken into account. Some detail is lost in compression especially with moving images. The result also depends critically on the standard the broadcast was originally captured or recorded in.
How do the new HD standards compare with previous standards for TV in the UK?
Current PAL transmissions are 576i25. In widescreen this means 720 by 576 resolution. The 720p standard has just a 56% better spatial resolution than widescreen 576i but the 1080i standard is 3.5 times better than 576i. When viewing a HD TV and comparing with what you are used to, it is worth bearing in mind that you are likely to have been watching PAL on an old CRT TV and you will probably view HD on an LCD or similar screen. Images on LCD tend to be sharper than CRT even at the same resolution. The standard used in the US before HDTV was 480i. Many programs seen on UK TV are recorded at 480i and converted up to 576i. Obviously that is not as good as starting with a 576i recording.
Why is a 50 Hz frame rate being adopted in the Europe when 60 Hz is being used in the US?
The standards to be used in Europe were specified by the Technical Committee of the EBU (European Broadcasting Union.) One reason given is that 50 Hz will mean a lower bit rate so the channel uses less bandwidth. This is of course silly since the same effect can be achieved at 60 Hz by turning up the compression ratio. The incompatibility with the US standards of 60 Hz will of course be a problem. It will mean a reduction of quality in conversion of US programs to Europe because they will effectively have to repeat every 5th frame. No doubt this will show as a judder to the discerning eye. There may be further problems with incompatibility between DVD, computer video and games consoles. Another reason suggested for using 50 Hz in Europe is that the mains frequency is 50Hz. If you film at 60 fps under artificial lighting from a 50 Hz AC supply you may see a 10 Hz flicker. The significance of this is probably overstated for professional recordings but there are certainly cases where it will be an important consideration, e.g. floodlit football matches. The most convincing reason for the choice of 50Hz is simply that the existing European TV standards use 50Hz. The standards will co-exist for some time and it will be simpler to produce programs in two standards if they at least have the same frame rate. This is a short term local gain that will leave us to deal with global incompatibility for the long term. More significant will be the extra costs of having all hardware such as cameras produced with multiple frame rate standards and the loss of quality in converting between them. Not to put to fine a point on it, it was an appallingly stupid decision by a European standards committee that we will have to live with until the standards become obsolete and they have another chance to get it right. To be fair, not everyone agrees on this. Some people think that the EBU made the right decision. They point out that films and many other broadcasts are actaully recorded at 24 fps and the process of upscaling to 50 Hz gives a better result than upscaling to 60 Hz.
What standard are HDTV programs being captured and recorded in?
The production standard for TV programs is not necessarily the same as the
broadcast standard. EURO1080 is (as its name suggests) broadcasting in 1080i,
but that does not mean that all its programs were recorded at such high resolution. European
studies (including those done by the BBC) have concluded that the progressive standards should
be preferred for TV transmissions because it is hard to convert well from interlaced to
progressive formats in the display set. They also concluded that most viewers would not
benefit from higher resolution than 720p given the size of their screens. They even concluded
that the extra benefits of 60Hz frame rates were not warranted over the increased bandwidth required.
BBC Research and Development concluded that it would be good to film in 1080p50,
but the cameras are hard to come-by.
They recorded Bleak House using a Sony HDW-750P camera (pictured) which captures in 720p25. I.e. only half the
frame rate of the 720p50 standard. That is probably not so bad for a period drama, but it would
be disappointing for sports. This is sadly typical of the BBC's half-hearted approach to HDTV.
Other programs the BBC have recorded in HD include Rome and Planet Earth.
Films produced for the cinema use just 24 frames per second but the image resolution of celluloid film is very high so they will transfer well to 1080i. The Torino Winter Olympics are being captured with 1080i/25 using Sony HDC-1500 portable cameras. These are a popular choice for broadcasters in Europe because they can switch between 720p/50 and 1080i/25 formats. BSkyB have made a deal with Sony that includes equipping their studios with HDC-1500 cameras and other HDTV equipment.
Which standard is more expensive?
The bit rates for transmission are about the same for either standard so the receiving and processing equipment may not differ much in cost. However, display technology such as LCD with a native resolution of 1920 by 1080 is currently much more expensive than 1280 by 720.
Which HD standard is broadcast in the UK?
Sky have said that they will broadcast in 720p/50 and 1080i/25 allowing the broadcaster to choose.
What compression standards are used?
Sky are using DVB-S2 MPEG-4 for HD broadcasts. MPEG-4 offers a much better compression ratio than MPEG-2 that they use now because it compresses across frames. This will allow them to broadcast high definition video with a bandwidth only about twice what they currently need for standard definition. A single HDTV channel will probably require about 8 Mbps (Mega bits per second) bit rate. The downside of using such a high compression ratio is that more information is lost from the picture. It is likely that a degradation of picture quality will be noticed with fast moving action. US viewers of the Torino Winter Olympics in HD have reported noticeable pixilation effects. When the picture changes slowly MPEG-4 allows more detail to be seen because it can use details from a previous frame to enhance the next, but when the picture changes fast it must lower the amount of detail to keep within the bandwidth limit.
