Video Processors - film mode deinterlacing

Film mode deinterlacing is where the material was originally recorded progressively on film cameras. The important process here is lining up the correct interlaced fields for progressive display. Since the material started progressively and then underwent a process of interlacing, the deinterlacing process should be able to recreate the original progressive frames exactly. The diagram below shows the process for a 25fps signal (what we use in PAL land!), ultimately being displayed as 50Hz progressive:

- the original 25 progressive frames are shown twice each in the cinema to avoid flicker
- to convert this to broadcast standard the double frames are still used, but the odd-numbered frames exist only of odd-numbered lines of information, and vice versa
- this is the exact format a traditional TV would use too as our brains will reassemble the image for us
-but for a progressive display it must merge the two lots of information back together again (creating just 25 individual frames), and as with the cinema display each frame twice

However, for NTSC the process is a little more complicated. The film material is recorded as 24 fps rather than 25fps, which is displayed as 48 fps in the cinema i.e. each frame shown twice as with PAL sources above. But in America the television system is broadcast at 60Hz refresh rate, which doesn't match the 48 fps the material is now in. This means for broadcast to 60Hz displays a 3:2 display sequence is employed which repeats every other frame to create a total of 60 frames in each second in order to fill the gaps. There is some image judder because of this process which is evident on all TVs, although most of you will have never noticed this:

As above shows, the process between 24fps and display in the cinema at double the rate is the same, but each frame is only taking up 1/48th of a second when it should be 1/60th of a second (60Hz means 60 times per second). So this 3:2 sequence is employed to fill the time line with enough frames to make a 60Hz signal:

- the top line of this diagram shows the interlaced 48Hz signals from earlier now converted to an interlaced 60Hz one. Notice that field 2a and 4a appear twice...
- the processor must correctly identify these double frames in order to be able to match each half back together (matching 2a to 3a for instance would create a mess!)
- finally the processor must still display with a 3:2 sequence in order to correctly fill the 60Hz timeline. So every other group of frames is shown thrice rather than twice, hence 3:2

Cadence detection is this process of correctly identifying the frame sequence and applying the appropriate pulldown method to ensure the original frames are analysed in the correct order, and that repeated frames are ignored. The processor will have “film mode” or “video mode” detection to detect whether the material might be film mode and hence is able to be deinterlaced perfectly, and then cadence detection analyses and interprets the frame sequence of the material to ensure correct pulldown is applied as above.

Better processors will include the ability to detect not just 2:2 and 3:2 cadences, but also the more weird and wonderful ones found in broadcast. In some animations the frame sequence might be 5:5, with some broadcasts in an attempt to speed up movies (and maximise advertising breaks!) the broadcaster might remove every 12th frame creating a 3:2:3:2:2 sequence!! This was something originally progressive, and so should be able to be recreated perfectly using film mode deinterlacing - however because the broadcaster altered the cadence of the material only better processors will recognize this new cadence and correctly reproduce the original frame. Without proper detection the processor might misalign the wrong fileds, or the processor reverts to video mode deinterlacing which doesn't take into account the full resolution of the image….