An interlaced video is a succession of approximately 50 or 60 fields per second with each of the fields carries only the alternating rows displayed in each frame of the video. FIG. 1 shows an interlaced video clip with panning. Interlaced video is often derived from motion picture film materials photographed in a 24 or 30 frames-per-second progressive manner for display on cathode ray tube (CRT) type of displays. Interlacing is generally performed to achieve bandwidth reduction, but results in a reduced picture display quality. However, since the reduction in picture display quality from interlacing is not very noticeable on CRT displays, interlacing has been used as an elementary compression technique for CRT displays.
As most of today's digital television displays require progressive video inputs video materials in the interlaced format need to be converted to a progressive video format in order to be properly displayed on the digital TV displays. This process is referred to as deinterlacing. One typical deinterlacing technique is known as field combination (or weaving) which simply combines the appropriate fields together to restore the original images in the case where one original frame is used to generate two fields.
To weave the correct fields together, the deinterlacer detects cadence of the interlaced video. Cadence generally refers to a pattern of successive fields that correspond to the frames of the original video (i.e., conversion pattern). If the cadence cannot be properly detected, the deinterlacer will not be able to properly weave the correct fields together and may discard video data erroneously.
The detection of cadence may be complex and difficult. For example, motion picture film is typically progressive and is based on 24 frame-per-second sequence while the NTSC format for TV broadcast is 60 fields per second. To convert a motion picture film into an interlaced video in NTSC format, a 3:2 pull-down repeating cadence is used to generate three fields from one film frame and two fields from the next film frame. In addition, sometimes every twelfth field is dropped to accelerate the film and fit the film within a given time slot. This loss results in a 3:2:3:2:2 repeating cadence.
Moreover, although the 3:2 repeating cadence is the most common format, other repeating cadences (e.g., 2:2, 2:3:3:2, and 3:3) may also be used to interlace the progressive original film. Furthermore, it is possible that an interlaced video may have one portion of the video interlaced with a 3:2 cadence while having another portion of the interlaced video converted with a different cadence (e.g., 2:2 or 3:3). It is also possible that an interlaced video does not have any cadence. False detection of cadence may occur when two fields that were not generated consecutively are weaved together. 2:2 cadence is another common format. This often originates where film destined for television broadcast in phase alternating line (PAL) or sequentiel couleur a memoire (SECAM) formats is photographed at 25 frames per second and is broadcast at 50 fields per second, where each frame is the progenitor of an odd and even field. 2:2 cadence is also common for television broadcast in National Television System Committee (NTSC) format where a show has been photographed at 30 frames per second and then broadcast at 60 fields per second.