Mis à jour le 23 July 2021.
The format of high definition images is characterized by a resolution 1920 pixels wide and 1080 pixels high. HD resolution can be achieved via two different methods: interlaced (1080i) or progressive (1080p). Although both display methods have the same resolution, they each have advantages and disadvantages, resulting in unequal image quality. Which of these two HD resolutions should you choose? Here is how they work in detail.
Understanding the differences between 1080p and 1080i
1080p and 1080i systems are both HD certified and therefore capable of displaying 1920 x 1080 pixel images. The difference between these two resolutions, however, is in the way the images are displayed. The letters “i” and “p” refer to the display mode used: 1080i refers to “interlaced” and 1080p to “progressive scan”.
1080i uses an interlaced display, whereas 1080p features progressive scan display.
How does a 1080i display work?
The interlaced display of the 1080i format displays each image twice. This is the display mode used by old cathode-ray tube televisions. It was developed to double the number of perceived images per second and improve the contrast of televisions with low scan rates. When the electron beams scanned the screen on old CRTs, there was a significant delay between the first pixel being displayed at the top of the screen and the last pixel at the bottom of the screen. As a result, there was a difference in luminosity between the top and the bottom of the screen and, consequently, a perceptible difference in contrast. Interlaced display was therefore created to display fewer lines simultaneously, thus ensuring better visual comfort. To do this, each image is split into two video fields: the first field contains only the odd lines of the image, and the second contains only the even lines. Each field represents half of the image’s lines. At a scan rate of 50Hz, half an image is obtained per scan, which corresponds to the European standard of 25 frames per second. This system takes advantage of persistence of vision and the brain’s ability to reconstruct the entire image.
As the scan rate of televisions isn’t limited to 50Hz anymore, the interlaced format is no longer relevant. It is no longer used on modern televisions, whether they are LED TVs, UHD 4K TVs, OLED TVs or UHD 8K TVs, for example. However, this format is still used for DTTV. Why? Simply because this technology requires half the bandwidth and, consequently, a less demanding internet connection. Although the full size of the program remains the same, the information transmitted every second is two times smaller, as only half the image is broadcast.
How does a 1080p display work?
Like the 1080i format, 1080p features a resolution 1920 pixels wide and 1080 pixels high. It works differently, however, displaying the whole image at once. Each displayed image field therefore corresponds to an entire image. Progressive display is currently used by all modern computer screens and televisions, whether they are LED TVs, UHD 4K TVs, OLED TVs, UHD 8K TVs, etc.
Choosing between 1080i and 1080p formats
Due to its working principle, 1080p has more advantages than the 1080i format, starting with a superior perceived image quality. Even if an image has the same resolution in both 1080p and 1080i formats, and therefore the same number of pixels vertically and horizontally, the vertical resolution of an interlaced image seems to be almost 60% inferior. This is due to the fact that the even and odd lines of the image aren’t displayed simultaneously. Consequently, a video in a 1080i offers a very similar image quality to that of a file in 720p, meaning you cannot fully enjoy Full HD images.
Note: as mentioned earlier, DTTV programs are broadcast in 1080i in order to optimize bandwidth. A modern television must therefore carry out a deinterlacing process in order to reconstruct the original image from its two image fields. If this deinterlacing process is not performed correctly, it will generate artefacts and an aliasing effect in the image. Therefore, the importance of using a set-top box via internet or satellite and a Blu-ray player or a media player with a progressive scan-enabled video output is evident: the high-definition image produced by the player is ready to be displayed using a progressive scan and doesn’t risk being altered.