Difference between ordinary TV and HDTV
The main difference between ordinary TV and HDTV is that HD TV has a wider format picture on the TV screen. Also the picture in an HDTV is very sharp and much denser and clearer than in an ordinary TV picture.
The picture on the right compares an Ordinary TV Vs HDTV picture. HDTV has a width to height ratio of 16:9 compared to a standard definition TV picture width to height ratio of 4:3. The width to height ratio of a TV is known as the aspect ratio. Thus in an HDTV of picture height of say 45 inches the width will be 80 inches. In a standard TV picture of the same height of 45 inches the width will only be 60 inches. So in our example here of a TV picture of same height of 45 inches, since the SDTV and HDTV aspect ratios are different, the SDTV produces a picture of only 60 inches wide compared to 80 inches width of the HDTV. Hence an HDTV picture is one third wider than a standard TV picture of same height.
The main benefit of HDTV over Ordinary TV picture is that the HDTV picture quality is very clear and much better than ordinary TV pictures. When HDTV picture is compared to ordinary TV picture, the HDTV picture quality is crisper and more detailed due to the higher resolution of HDTV. You do not see that grainy sort of picture like on the old ordinary standard definition TVs. HDTV has 1920x1080 pixel resolution compared to old CRT standard television pictures of 640x480 pixels. The wider picture of the HDTV is a pleasure to watch with less eye strain.
HD TV Screen Pixel size
The picture on the screen of all TVs is made of tiny squares known as pixels. These little pixel squares are arranged like a fine net along the width and height of the TV. A Full HD TV has 1920 pixels arranged left to right on the 'X' axis and 1080 pixels arranged from bottom to top on the 'Y' axis. This is usually written as 1920x1080 pixels.
A common question asked is whether a larger bigger HD TV has more pixels than a smaller HD TV. The answer is 'No', all HD TVs have the same number of pixels regardless of their size. In a bigger HD TV each of these pixels will be larger to fit the 1920 pixels along the width of the screen and 1080 pixels along the height of the screen. The reason why you cannot have more pixels in a larger HD TV is because the HD Video picture signals are in 1920x1080 format and each TV pixel must match each of the digital information in the video signals. The Video signals can come from various sources like a DVD player or USB stick or an HD TV broadcast like from Dish TV.
How image formed on TV screen
To explain how an image is formed on the TV screen we use the figure shown here. The image on a TV screen is formed by scanning, by which each TV pixel gets digital information from the video signal. The digital information is fed into the TV pixels starting from the top left corner and going left to right on the first row (1), when the first row scan is over, the scan goes to the left side and starts over from the left side of the second row (2). After the second row, the scanning starts from the third row again from left to right and this row by row scanning continues till the last row of TV pixels at the bottom row are fed with the video digital signal. When the last pixel on the right side of the bottom most row is scanned, one image frame is completed and the scan goes to the top left corner to start scanning the next image (3). TV needs about 60 frames per second to have smooth motion of movements of images on the screen without any flicker. So you can imagine how fast the digital scanning has to be to feed 60 frames of TV image per second.
What are 720p 1080i and 1080p HDTV resolutions
To explain what 720p 1080i and 1080p mean, we have to say that the figures 720 and 1080 refer to the TV screen resolution and the 'i' and 'p' refers to how the video signals are scanned to form the image on the TV screen.
The 720 refers to the 'HD ready' TV resolution of 1280x720 pixels where there are 720 lines of horizontal resolution which requires to be scanned to form each frame of the TV image. 1080 refers to the 'Full HD' resolution of 1920x1080 pixels where there are 1080 lines to be scanned to form each TV image frame. The video below explains what the difference between HD ready and full HD TV.
The 'p' and 'i' in the 1080i and 1080p refers to how the 1080 horizontal lines of the HD image is scanned. The 'p' stands for 'Progressive' and the 'i' stands for 'interlaced'
In progressive scan all the 1080 horizontal lines in a TV image is scanned together in one go and thus there will be 60 full image frames per second on the TV screen.
In interlaced scanning, alternative rows of the 1080 lines of HD TV are scanned in each frame of the image. What this means that in the first scan, rows 1, 3, 5 and all odd rows of the 1080 lines are scanned to form one frame and in the second scan, rows 2, 4, 6 and all the even rows are scanned to form one image frame. Then the two frames are combined to form the complete picture as shown in the image here.
It should be noted that in interlaced scanning, the scanning rate remains at 60 frames per second. Since each frame in interlaced scanning is only half an image frame, there will only be 30 full image frames per second in interlaced scanning compared to 60 full frames in progressive scanning. Thus progressive scanning produces better TV images. So 1080p is better than 1080i