Video Processors - interlaced and progressive

We've talked about a signal being interlaced, or being deinterlaced to become a progressive signal. But what does that mean??? Your traditional CRT television (the ugly box in the corner of your lounge you used to have before you got that plasma screen!) displays its image in an interlaced fashion. If you imagine a PAL derived picture as a series of 576 lines, the CRT gun in the TV will quickly draw all the odd numbered ones, and then a split second later all the even numbered ones. This is done so quickly that the brain interprets the two fields of information together to assemble the full image. These two “fields” of odd-numbered and even-numbered lines, when combined, are known as a single video “frame”. A 50Hz signal, as we have in the UK, will show 50 frames in a single second, these will be 25 odd-numbered line fields and 25 even-numbered line fields and will be displayed as such on an interlaced display.

With today's digital displays, such as a plasma screen for example, there isn't the ability for the screen to show interlaced fields in the same interlaced fashion as a CRT TV does. With a plasma screen, the display is made up of a fixed number of pixels, all of which must be lit at once to show one whole video frame at a time. So, the same 50Hz signal must be displayed as 50 “progressive” frames i.e. the odd-numbered and even-numbered lines are combined, or deinterlaced, to produce complete frames for progressive display. These progressive frames are then showed twice each to achieve the 50 frames per second or 50Hz frequency.

But this joining process isn't always so easy. When the source is a film it is easier, the material has usually been recorded on a movie camera at 25 frames per second (24 for US), and each frame is progressive already (in the cinema you see each full frame twice as 25 frames in one second flickers quite noticeably). So the 25 frames are split into 50 odd or even lined fields i.e interlaced. Your average CRT TV having no problem displaying this in interlaced fashion. The plasma screen however has to merge the two sets of fields together to recreate the original 25 frames that it was original recorded in. Then to avoid showing a juddery image, each frame is shown twice to reach the 50 frame refresh rate.