When talking high definition TV the highest high-definition signal formats is 1080 vertical pixels by 1920 horizontal pixels – that means there are more than two million pixels onscreen (1080 x 1920 = 2,073,600 pixels). However, 1080 pixels comes in two flavours: interlaced (i) and progressive (p) scan.
Interlaced is a hang over from CRT TVs that required the picture to be formed on the screen by scanning in two passes or fields, each lasting 1/50th of a second (in PAL – or 1/60th of a second for NTSC).
The first pass leaves blank spaces between lines, which are filled in by the second pass. Because each field takes 1/50th (1/60th) of a second, and there are two of them, 1080i actually needs 1/25th (1/30th) of a second to convey a full frame – making it a 25-frames-per-second (30-frames-per-second) signal. Because human vision works by retaining images for a brief period of time on the eye’s retina, the interlace scan process makes two separate scans of an image appear as one frame. In the case of 1080i video, this means that there are two fields of 540 lines each that are perceived as a single video image.
The positive aspect of interlaced scanning is that it conserves bandwidth while still providing more than two million pixels on the screen. We adopted interlacing for the 1080-line format in Australia to specifically fit over-the-air HDTV into the same 6MHz bandwidth as an analog TV channel. The downside of interlacing is that it induces motion artifacts and other forms of distortion. The best example would be a horse race. With interlacing, fast action, car wheels, racing horses, footballs or tennis balls become a blur.
Unlike CRTs, LCD or plasma televisions are inherently progressive in nature because they do not project an image onto a screen but rather contain lamps that illuminate to form an image. These television sets have to convert 1080i video to 1080p. Unless they are converting 1080i video, those sets already show images in the sharpest resolution, 1080p.
So, to explain it simply Progressive-scan panels differ by conveying all of the lines of resolution sequentially in a single pass (rather than two scans with interlaced formats), which makes for a smoother, cleaner image, especially with sports and other motion-intensive content.
While the very newest televisions can display 1080p video, their source material has yet to reach that level of resolution. High definition broadcasts are still mostly in the 1080i video format. In addition, these 1080p sets are not equipped to receive an outside 1080p signal, so even though consumers may be paying top dollar for the latest television technology, they will not be able to make full use of it until broadcasters and the new higher definition (Blu-ray and HD DVD) DVD manufacturers begin to release movies in 1080p.