The digital TV system allows a higher resolution image broadcasting, raising the TV image quality a lot. However, to use the new resolutions allowed by digital television you need a high resolution TV set, known as HDTV (High Definition TeleVision).

Older analog TV sets can tune in digital broadcasting using a converter box, but they are limited to shows aired in the standard TV resolution of 480 lines using interlaced scanning (also known as 480i, see table below).

HDTV is a reality in developed countries and there you can watch some shows – even the whole progamming, in some channels – in higher resolution for those who have already HDTV sets. The resolution varies according with the TV station. In the USA, for example, some HBO and CBS shows are aired in 1080i resolution, while the format used by Fox in the shows that are aired in HDTV format is 480p, and on ESPN HD – which is a channel where all their transmitions are in HDTV format – the standard used is 720p.

HDTV sets can run basically in three resolutions: 480, 720 and 1080 lines. The screen scanning technique can be interlaced ("i") or non-interlaced ("p", for progressive). In the table below you can check all posible HDTV resolutions.

Name Resolution Scanning Aspect Ratio
480i 640 x 480  Interlaced  4:3 (1.33)
480p 640 x 480  Progressive  4:3 (1.33)
720i 1280 x 720  Interlaced  16:9 (1.78)
720p 1280 x 720  Progressive  16:9 (1.78)
1080i 1920 x 1080 Interlaced  16:9 (1.78)
1080p  1920 x 1080  Progressive  16:9 (1.78)

The difference in image quality between interlaced scan and progressive scan is huge. The analog TV system uses interlaced scanning. In this method first the odd lines are drawn (line 1, line 3, line 5 and so on) then, after the screen is filled, the even lines are drawn (line 2, line 4, line 6 and so on), and the process starts all over again. In total 60 frames per second are drawn (i.e., the screen is filled 60 times per second), being 30 frames made of odd lines and 30 frames made of even lines, intercalated. So which frame displays only half of the lines the TV set can shown.

In non-interlaced scanning (or progressive scanning, as it is being called now) all lines are drawn. From the 60 frames per second each frame has all lines the TV set is capable of showing.

You have to be very careful when comparing different resolutions. For example, the 1080i resolution is not necessarily better than 720p resolution. Even though 1080i resolution has more pixels on the screen its scanning is interlaced, while 720p uses progressive scanning, making its image quality better (basically better sharpness) than 1080i.

The biggest problem is that HDTV panels, like LCD monitors, have a native resolution (for more information on this issue, please read our tutorial about LCD monitors).

In order to show images from a resolution different from its native resolution, the HDTV set has to convert the image, which can be well done or not, depending on the HDTV model. Let’s pick a random example. 42" plasma TV Panasonic TH42PA20UP has a 848×480 (480 lines) native resolution. To show 720p images it has to convert the image, making its image quality not so good as compared with a HDTV set with a native resolution of 720 lines.

So, if you are an user worried about image quality, when buying a HDTV set pay attention to its native resolution. Preferably it should be equal to the resolution you will watch the most.

It is possible to connect HDTV panels to your PC to use them as video monitors. Let’s see how this can be done.

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.