In our beloved cameras, there are strange data, we do not always understand. Why do we find data like "1080p" or "1080i" or "50p" or "50i". What does this little "i" or a small "p"? This is what we will see together.
The INTERLACED Mode - "I"
The principle of the interlaced mode is simple. Is displayed during a first frame, all pairs of image lines; and secondly, it shows all the odd lines of a second image.
This method has emerged with CRTs, and to avoid the flicker effect. Indeed, at the time, there was a high amount of time between the first pixel display, top left, and the last pixel (bottom right) of the same image. Result, the beginning of the picture had already lost intensity when viewing the end of the image. Interlacing was therefore a solution such as for displaying fewer lines. Indeed, it shows only one line at 2. The lines are very small and tightened, it can not be seen, and it has a better visual comfort that if we showed all the lines.
Interlacing therefore be a way to reduce the data transmission rate. This is not entirely true. The transmission rate depends on the number of frames per second multiplied by the definition. Interlacing allows a buffer twice smaller than in progressive mode. This memory, every moment contains a half-image, not the full picture.
However, this interlaced mode has several flaws. During rapid sequence, bands (or comb) appear. This phenomenon is reduced during slow sequences, because the even frames are very close to the odd frames.
The PROGRESSIVE MODE - "P"
Progressive scanning is a method used by electronic displays such as computer monitors and HDTVs. The principle is simple: The progressive scan displays the entire image at one time. Each frame corresponds here to an entire image.
The progressive display has some advantages over the interlaced:
although a progressive or interlaced image have the same number of vertical and horizontal lines, one has the feeling that the vertical resolution is lower in interlaced images. Indeed, the fact that the odd and even lines are not displayed at the same time, there is a perception of poorer details. The resolution of an interlaced image is estimated at 60% of that of a progressive image. So the perception of an image in 1080i correspond to that of an image in 720p.
interlaced display can lead to frequent blinking horizontal patterns, suddenly, progressive scan offers greater image stability.
Moreover, it would be easier to process and compress progressive images in the world of digital everything.
Despite all these advantages for progressive mode, why there is always interlaced mode proposed in our expensive cameras? As we have seen above, the interlaced mode reduces bandwidth. This is particularly useful when the latter is limited. It is for this reason that we continue to see this proposed mode in our expensive cameras.
For example, some reflex are not able to offer the 50 FPS in progressive mode. However, they can offer 50 FPS interlaced (50i), thus offering the videographer, the ability to shoot with a number of high SPF and still smooth slow.
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