High Definition TV Explained

HD TV, where the HD is for high definition tv (or high def tvs). This is perhaps the most significant revolution in broadcasting since TV changed to colour from black & white. HDTV uses widescreen digital flat panel televisions such as a Plasma televisions and LCD televisions where the resolution is at least 1280 pixels x 720 pixels or HD Ready. There are 2 types of high definition broadcasting, 720p/50 and 1080i/25.

What does ‘HD ready’ mean ?

For a tv to be HD ready and show a high definition signal it must have a minimum screen resolution of 1280 x 720 pixels, where 720 is the number of vertical lines. It should also be able to receive a signal of the specification of 720p/50 or 1080i/25; the number of vertical lines is represented by the 720 and the 1080, Progessive 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.

For a 1366 pixels x 768 pixels HD ready tv, if it receives a 1080i signal then scalers within the flat panel tv will down convert the picture to fit the 768 lines flat panel tv. This is done using complex algorithm sequences in the scaler which systematically crops the picture down to the resolution of the screen. If the signal is at 720p it follows that the signal will be slightly upscaled to fill the 768 vertical lines with complicated algorithms.

Most HD ready tvs don’t have a sufficient amount pixels to offer true pixel-for-pixel mapping without interpolation of the higher HD resolution (1920 pixels x 1080 pixels).

What is ‘HD ready 1080P’ ?

A TV that has the ‘HD ready 1080P’ label is able to show a full 1080p signal since it has sufficient pixels for one to one pixel mapping. This can be done with out Interpolation. The resolution of a 1080p flat panel tv is 1920 x 1080 pixels or 1080P. The 1080 signifies the vertical resolution and the P is progressive scan. This is the maximum resolution obtainable in the United Kingdom on high definition televisions so that’s why the term ‘full’. Televisions with this resolution are capable of displaying 1080p and 1080i signals without any distortion and with exact pixel for pixel mapping. They must also be equipped with DVI or HDMI inputs for 1080P signals at either 24 or 50 frames per second.

If a 720p signal is received by a 1080p tv it is oversampled (or upscaled) to fill the resoltion of the 1080p High definition television by means of sophisticated software.

Full HD

Older full HD flat panel tv’s may not fulfill all ‘HD Ready 1080P’ requirements.

What is an Interlaced or Progressive picture ?

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 two times the rate of the actual frame and at half the resolution.

Interlaced pictures on video have more fluid movement 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.

The UK broadcast standard is known as PAL which operates with 25 full frames per second or 50 fields per second. Interlacing the signal means that half the picture information is in each field so half the bandwidth is required for broadcasting in comparison to a Progressive signal at 50 full frames per second.

Interlaced pictures on recordings made for television or with a video camera aren’t able to be displayed on normal definition LCD tvs and Plasma tvs. This is because the picture isn’t produced with an electron scan like tube tv’s so LCD televisions and Plama TVs don’t benefit from the interlaced picture signal. Flat screen widescreen tv’s have internal processing to create a progressive scanned image from a interlaced picture – i.e. Deinterlacing.

Progressive scan delivers benefits.

This is also known as non-interlaced scanning. It is a technique of storing, displaying or transmitting a moving image where all of the lines of each frame are shown sequentially instead of odd lines in one field and then even lines in the subsequent field as per Interlaced signals.

Progressive scan has the benefit of superior vertical resolution than interlaced images with the same frame rate and no interlace artifacts or blurring, and therefore less eye strain. Also better results are possible for scaling to higher resolutions than the comparable interlaced sources. For the best scaling results full frames work the best but interlaced video sources have to be deinterlaced prior to being scaled and this can produce very noticeable combing artifacts.

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

On static images a 1080i/25 (1920 x pixels 1080 pixels) produces a a little higher horizontal picture resolution than a 720p/50 (1280 x pixels 720 pixels) picture. Moving pictures on an interlaced picture have a lower subjective line resolution because of line twitter or artifacts caused by the fields being somewhat dissimilar that create a frame. Broadcasters tend to opt for 1080i/25 over 720p/50 because it requires less bandwidth, but both systems are used for HD tv broadcasts.

Progressive scanning 720p gives more fluid movement, particularly on slow-motion, than an interlaced 1080i signal. But interlaced 1080i signals give better static resolution. If the internal processing is competent enough a 1080i signal on a 1080 display will still appear 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.

When a HD Ready TV receives a 1080p/50 signal it can change the picture into a 1080i/25 image much easier than a full HDTV can alter a 1080i signal into 1080p.

What are the advantages of 1080p/24?

This is 1080p at 24 frames per second. This gives the definitive picture quality with the 1080p picture decoded straight from the BlueRay disc at 24 frames per second, and then sent to the flat screen HD TV. The 24 frames per second is the same rate as the original cinema film. The TV will then create additional frames to increase to either 48 or 72 frames per second creating middle frames that make the image more fluid.

Sources of HDTV

A HD Ready TV can accept all current broadcast formats of 720p/50 or 1080i/25. HD is obtainable on Freesat, Sky Digital HD, BT Vision, and Virgin media cable. The only sources of Full HD 1080P signals are Blueray players, Playstation 3 and by download on the internet. The games on an Xbox 360 are at 720P.

Conclusion

High definition enhances the viewing experience and makes TV more engaging. If you don’t intend to use a Blueray player, Playstation 3, or to download films from the webthen a HD ready television will probably suffice. However if you need to future proof yourself against the chance of future full HD 1080p broadcasts then the full HD 1080p models are the ones to buy. Obviously, if you already possess, or are going to buy a Blueray player, Playstation 3 or to you are going to down load full HD films then the full HD 1080p tv is the logical choice.

Like this post? Subscribe to my RSS feed and get loads more!