Since its creation, France 24 has developed a 100% digital video processing system. But this bet faced several obstacles. The transition from analog video to digital video is not achieved without problems. In fact, interlaced video is a common source of confusion in the world of digital video.

A video is made up of numerous snapshots, called frames. The frame rate is the number of frames displayed each second (25 fps in PAL based system). A television, however, deals with video in terms of fields which are half of frames. Each frame consists of two fields. The “Odd field” contains the odd lines of the original frame. The second field contains the even lines.

A television displays the video field by field. The “Odd field” is displayed first then the “Even field”, and so on. Each field is displayed 1/50^th of a second after the preceding field.  This scan is called “Interlaced scan”.

A computer monitor performs “Progressive Scan “which means that the monitor updates the image only once for each frame of video. The two fields are displayed simultaneously.

In the other hand, there are two ways to record videos: “Progressive video” and “Interlaced video”.

In the “Progressive video”, both fields are taken in the same time and they represent the same point of time. However, in the “Interlaced video”, each field represents a different instance of time. So, in this case, each video frame is composed, as a result, of two fields taken in two different times, and does not correspond to a single instant in time.

In the case of playing “Interlaced video” on an “Interlaced screen”, it is very important to know the field order. Because the fields represent series of progressing moments in time, there can only be one correct order in which to display them.  If the order is reversed, the video will quickly alternate between going forwards and backwards in time, leading to jerky motion.  Because of the fact that there is not consistent field dominance (each capture card and each video source can lead to different field dominance*), the only way to know the field dominance was, formerly, to play the video on TV monitor assuming a field dominance. If the motion is jerky, then the right field order is the opposite of the assumed order. If not, the assumed order is the right one.

But, from now on, France 24 has taken the decision to establish its proper application to detect and correct the field order, without human interaction. So, we have developed a specific application of video analysis. It detects the right order of fields.

For every frame, we separate the two fields. Then we create two sets of successive fields:

-          O1-E1-O2-E2-O3-E3… : “Odd field”

-          E1-O1-E2-O2-E3-O3… : “Even field”

Only one of these sets represents the right chronological order. The other represents the bad order. To guess what is good and what is bad, we compare the difference of luminance between successive fields.

We notice that for one of sets, the curve of the difference of luminance by time is smooth, and for the other, this curve is rough. Also, the smooth one is situated under the rough one because the pixels are closer so the difference is smaller.

We can conclude that the green curve represents the right order, and the blue curve represents the bad one.

Once we know the field dominance, we can encode the video with the detected parameters.