| HD stands for High Definition (HD) and is essentially a | | | | video lag. |
| video format which is digital in nature and offers the | | | | In the PAL system images are shown using a |
| promise of sharper, clearer pictures and sound than | | | | method known as interlace (i). |
| currently available using analogue video and television | | | | Interlace shows the video images as two fields which |
| formats. | | | | we call odd and even. The odd fields is lines |
| There are two standards (commercially current) of | | | | 1,3,5,7...575 and even fields are 2,4,6,8,...576. |
| HD which are 720 and 1080. | | | | The full image is made up of both fields but to show |
| Each can be shown and recorded in two different | | | | both "halves of the image" at the same time. As the |
| ways, Interlaced and Progressive. | | | | display of moving video is actually a sequence of stills |
| This gives rise to the four commonly stated | | | | shown in sequence (25 per second) the engineers |
| standards which your display device is capable of | | | | devised a method which showed for the 1 of 25 stills |
| showing namely:- | | | | just the odd lines and for the 2 of 25 stills just the |
| 720i, 720p, 1080i, 1080p where (i) stands for | | | | evens, whilst leaving the odd fields up from the first |
| interlaced and (p) stands for progressive. | | | | image. The next still shows the odds of the 3rd |
| To understand which is better and which standard | | | | image and the evens from the previous still. This iss |
| you should seek out it is worth first looking at the | | | | repeated and gives the appearance of motion, it is |
| common PAL system which is used currently in the | | | | also why when you freeze frame the image is never |
| majority of VHS, DVD and television broadcasts. | | | | as clear as you'd expect. |
| The amount of information contained in a picture is | | | | This works because the human eye has a thing called |
| directly related to the quality of the image able to be | | | | latency, in which the image it sees remains in the |
| displayed. For PAL we have a picture which consists | | | | retina and fades as a new image is seen. An extreme |
| of 576 lines and 720 columns per line. This in | | | | example of latency is to look at a bright light and |
| computer "speak" gives us a number of possible | | | | then look away - you still see the bright light although |
| pixels per image of 414,720. Pixel shape is rectangular. | | | | it is not present. |
| If this raw information is displayed on a screen the | | | | The use of interlaced transmission and processing |
| clarity will decrease as the screen size increases - will | | | | tricks the brain into thinking it is seeing a full motion |
| look fuzzier. This is because the same number of | | | | image when it is not. |
| pixels will still be present but each will be larger for a | | | | The alternative method is to show the whole of |
| larger screen size. | | | | each image completely, however this requires a lot of |
| There are clever algorithms which will divide these | | | | processing power and is now possible due to the |
| pixels into smaller units so that the image on a large | | | | technical advances made in electronics. This is called |
| plasma screen will look better, however the raw | | | | Progressive display. |
| information remains the same. | | | | So the difference between PAL, 720i and 1080i is the |
| For 720 HD the resolution (number of pixels) | | | | amount of data with which the eye is presented. |
| increases to 720 lines by 1280 columns giving us a | | | | 720i and 1080i are a vast improvement on PAL and |
| total of 921,600 using square pixels | | | | you will notice a significant improvement in the clarity |
| For 820 HD the resolution (number of pixels) | | | | of the image and the amount of detail able to be |
| increases to 1080 lines by 1920 columns giving us a | | | | seen. |
| total of 2,073,600 using square pixels | | | | The same level of improvement again would be |
| Thus PAL has half the resolution of 720 HD and a | | | | displayed with 720p and 1080p which do not use |
| fifth of 1080 HD. | | | | interlaced video display methodology. |
| The next problem is how to get all of this information | | | | DO NOT however think that the selection of 1080p |
| to the screen to show a smooth motion which looks | | | | is automatically a superior image to that of 720i. The |
| natural and has no jerky movements and in sport for | | | | size of the screen, how far way the viewer is and |
| instance the football moves across the screen | | | | how the original footage was obtained is very |
| without leaving a ghostly trail or shadow, known as | | | | relevant. |