High Definition TV - Everything you need to know

HD TV, where the HD is for high definition television (or high def tvs). This is almost certainly the most significant revolution in broadcasting since TV changed to colour from black & white. HDTV uses widescreen digital flat panel tvs such as a Plasma televisions and LCD tvs where the resolution is at least 1280 pixels x 720 pixels or HD Ready. There are two types of HD broadcasting, 720p/50 and 1080i/25.

What makes a TV 'HD Ready' ?

For a television to be HD ready and display a high def. signal it should have a minimum screen resolution of 1280 x pixels 720 pixels, where 720 is the amount of vertical lines. It should also be capable to receive a signal of the specification of 720p/50 or 1080i/25; the quantity of vertical lines is represented by the 720 and the 1080, Progressive or Interlaced signals are signified by the 'p' or 'i', and the number of frames per second is the 50 or 25. Connection for the high definition signal is by either DVI or HDMI and Component inputs.

Any TV that has a screen resolution of-66 x 768 pixels and is HD Ready will make use of internal scalers to alter a 1080i signal down to the 768 lines; if it receives a 720p signal the internal circuits will upscale the image to 768 lines. The method of converting down or up is completed by advanced software to fill the screen.

If a HD Ready tv receives a full HD signal of resolution'20 x 1080 pixels the picture signal has to be interpolated or converted down to show the image. One to one mapping of the pixels is not achievable.

What does 'HD ready 1080P' mean ?

If a plasma television or lcd tv has a 'HD ready 1080p' badge it has enough pixels to show the full 1080p signal with pixel for pixel mapping without interpolation. A 1080p flat screen lcd tv or plasma television is'20 x pixels 1080 pixels where the 1080 is the vertical resolution and the 'p' is for progressive scan. A HD Ready 1080p widescreen lcd television or plasma television is the highest picture resolution in the UK for high definition and therefore the term 'full'. Any plasma television or lcd television with this resolution can show 1080i and 1080p signals with no up or down scaling, and including one to one pixel mapping. Connectivity ought to be by either HDMI or DVI inputs. HD Ready 1080p full hd tv's must be able to display a 1080p/24 or 1080p/50 signal where the 24 and the 50 represent the number of frames per second.

If a flat panel 1080P TV receives a 720P image the signal is 'oversampled' to suit the resolution of the 1080P widescreen TV. This is completed using very complex algorithm sequences.

Full HD television - What is it ?

Older full HD flat panel tv's might not fulfill all 'HD Ready 1080P' requirements.

Interlaced or Progressive

On an Interlaced Picture the scan lines of a frame are arranged in two fields each. One of the two fields contains the odd lines and the other field contains the even lines. Interlacing is when these two fields are shown in succession at double the rate of the actual frame and at half the resolution.

Interlaced images on video have more fluid motion due to each field being shot at a different time. Interlacing initially benefited CRT (cathode ray tube) tv's by improving the picture quality and using the same amount of broadcast bandwidth.

In the UK PAL televisions operates at 25 frames a second with 50 fields a second. A broadcasted signal that is Interlaced requires half the signal bandwidth of a Progressive signal. A progressive signal has a scan rate of 50 full frames per second in contrast with an interlaced signal which has half the frame speed.

Standard definition LCD tvs and Plasma televisions displays aren't able to operate on an interlaced mode for footage shot with a TV or video camera. Because LCD tv displays and Plasma tv displays do not have an electron scan to create an image they cannot benefit from interlacing. So internal processing in the flat panel tv creates a progressive scan image from the interlaced signal.

Progressive Scanning

Each frame has all of the lines from the image on a progressive scanned image instead of either the even or odd lines as with an interlaced image. Progressive scanning is a means of transmitting, storing and displaying the image.

The vertical resolution when the frame rate is the same is greater for a progressive image than it is for an interlaced image and the image is free of interlace artifacts, blurring, and a good deal lower eye strain. With a progressive image it can be scaled to a higher resolution than an interlaced image giving a higher quality image. This is due to the deinterlacing that is required on an interlaced image preceding any scaling resulting in combing artifacts that are obvious.

What is the difference between the 720p/50 and 1080i/25 formats?

A 1080i/25 (1,920×1080 pixel resolution) interlaced signal has slightly better horizontal resolution on still pictures than a progressive scanned 720p/50 (1,280×720 pixel resolution) picture. But on interlaced moving pictures there are inter line twitters which reduce the subjective vertical resolution. The twitter is caused by the frames being slightly different. Both 720p/50 and 1080i/25 are used by broadcasters depending on their inclination and bandwidth availability.

Progressive scanning 720p gives more fluid motion, especially on slow-motion, than an interlaced 1080i signal. Whereas interlaced 1080i signals give better static resolution. If the internal processing is good enough a 1080i signal on a 1080 display will still look better than the 720p material. The best one depends on whether you are to display more static images or more moving pictures and what is more important to you.

It is easier to convert a progressive signal (i.e. 1080p/50) into an interlaced format, such as 1080i/25, than it is to convert an interlaced format into a progressive format.

What are the advantages of 1080p/24?

Since the films that are shown in cinemas are produced at 24 frames per second a signal is in its purest form at this frame rate. Films are obtainable on Blu ray disc at 24 frames per second that give the definitive picture quality when displayed on widescreen full high def televisions. When a full hd television receives a 1080p/24 signal it creates supplementary middle frames, which are inserted between the original ones to multiply the frame rate to 48 or 72 frames per second giving more fluid motion.

Sources of high def.

HDTV broadcasts are presently at 720p/50 or 1080i/25 and are obtainable on SKY Digital HD, Freesat, BT Vision, and Virgin media Cable. All of these require a HD Ready TV. Currently Full HD 1080P signals are only obtainable on Blu-Ray Disc, Playstation 3 and by download films on the web. The Xbox 360 games machine operates at 720p.

The conclusion

High definition television produces an enhanced subjective picture quality that makes the argument for HD compelling. If you have possession of, or plan to own, a Playstation 3, a Blu ray player, or download full HD films from the internet then a Full HD 1080p television is the best choice. If not a HD Ready television will be suitable. Whilst these recommendations deal with today's available options they don't allow for the chance of forthcoming broadcasts in Full 1080p HD. If you would like to be completely certain that you protect your purchase against this scenario then buy a Full 1080p HD flat screen TV.

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