(presented by Philips)
A simple explanation for how TV works is that a transmission signal sends information to your TV.
The information tells the TV what the picture is supposed to look like. Once received, the TV paints that picture onto the screen pixel by pixel, forming horizontal rows, called ‘scan lines,’ across the screen. The TV continues to paint and re-paint these scan lines multiple times every second. This refreshing of the scan lines happens so fast that the human eye perceives the changing picture as fluid movement.
Interlaced
Devices that utilize interlaced scanning paint half of the scan lines (referred to as a field) and then paint the second half. Each field is updated 60 times per second. And a frame, made up of the two fields, is completely refreshed 30 times per second. While interlacing works well on smaller sets, on the bigger TV screens that are out today, the refreshing process may be visible to the eye, so that some people notice a perceptible flickering.
Progressive
Progressive scanning paints all of the scan lines onto the screen at the exact same time, 60 times per second. Progressive scanning was developed for computer monitors in order to decrease flickering. Progressive scan has fewer jagged edges and can show movement in better detail with fewer trails (called artifacts).
The way that these scanning methods are expressed is: a number that represents the number of scan lines, and a letter that indicates interlaced or progressive. For example, 480p means 480 scan lines updated using the progressive scanning method.
Along came DTV
Interlaced and progressive probably wouldn’t be a topic of discussion if digital television (DTV) hadn’t come along. As you may or may not know, many TV broadcasters have become fully digital, and there are channels broadcasting with digital signals in all major markets.
DTV standards allow broadcasters to select from several different transmission formats. Knowing about the format differences is also critical to choosing the right TV. The specific formats are:
| 480i | Picture transmits at 60 interlaced fields per second (30 frames per second). Same number of scan lines and frames per second (fps) as analog, but provides a slightly improved image merely because of digital transmission method. | Provides surround sound-capable digital audio that matches CD-quality | Known as Standard Definition Television (SDTV) |
| 480p | Same number of scan lines as analog, but sent progressive-style (60 complete fps) resulting in an even more improved picture. | Capable of Dolby Digital Audio. | Known as Enhanced Definition Television (EDTV) |
| 720p | Higher resolution picture with 720 scan lines, sent at 60 complete fps. | Provides Dolby Digital 5.1 Surround | This is HDTV |
| 1080i | Even higher resolution (1080 scan lines), but fewer frames per second. (60 fields per second, equal to only 30 complete fps). | Provides Dolby Digital 5.1 Surround | This is HDTV |
| 1080p | This is the ultimate format goal for the future: the best of both worlds with maximum scan lines and maximum fps. No broadcasters have adopted this format, and not many TVs can display it properly. |
Because broadcaster can have either scan method, many HDTVs have features that allow them to scale to the non-native scanning method. For instance, an incoming 720p signal is translated to the 1080i format. The best HDTVs will show 720p as 720p, rather than scaling it to 1080i and vice versa. Neither interlaced or progressive will disappear, or fail to be supported. And you will still be able to watch an interlaced broadcast if you purchase a progressive scan TV. The better the TV, the better the image, even if the scan types are not the same.
i and p in DVD
DVD players and recorders come in both progressive and interlace scan modes, meaning that they use one or the other to send their signals to your TV. Some let you select the output depending on the scan method you want (to match to your TV). And many have features like 3-2 pulldown processing, which improves the look of film-based sources when being converted to progressive-scan.
Conclusion
So now you know a little about the i’s and p’s of TV. Devices are not better or worse than each other simply because they are progressive or interlaced. Neither will be going away and in the end, you will definitely be bringing home a better TV if you take advantage of DTV features.
