Digital Signal Processing Reference
In-Depth Information
6.4 Cadence Detection
6.4.1 Another feature that is becoming standard
in advanced deinterlacers
Interlaced video can be even more complex than the trans-
mission of odd and even fields. Motion picture photography is
progressive and is based on 24 fps, whereas the NTSC format is
60 fps. The conversion of motion picture photography into
interlaced video requires us to convert 24 frames (per second)
into 60 fields (per second). Since there is no direct factor, first
let's consider what happens if each frame is converted into two
fields.
24 frames would convert into 49 fields
not the 60 fields that
e
we are looking for.
One method takes the first frame and converts it to three
fields (repeat one field); takes the next frame and converts that
to two fields. This is called a 3:2 pull-down technique
or
e
“cadence”.
Frame 1
3 Fields
/
Frame 2
2 Fields
/
Frame 3
3 Fields
/
Frame 4
2 Fields
/
Frame 5
/
2 Fields
Repeat
since 12 frames
contribute 24 fields and the other 12 frames contribute 36
fields.
Although 24 fps film, and its associated 3:2 video cadence, is
the most common format, professional camcorders and various
types of video processing use different types of cadences.
For example, Panasonic produced a different cadence for their
camcorders. Instead of converting the frames into fields using
a repeating 3:2 pattern, the frames are converted into a 2:3:3:2
pattern. The first frame is converted into two fields, the second
into three fields, the third into three fields, and the fourth into two
fields. It then repeats this pattern for every group of four frames
that follows.
Cadence detection is important for deinterlacers because if
the correct cadence is not detected video data may be thrown
away by the deinterlacer, or processing is done on the wrong field.
This will give you 60 fields per second
e
