Television – Format conversion – Line doublers type
Reexamination Certificate
1999-10-01
2004-03-02
Kostak, Victor R. (Department: 2611)
Television
Format conversion
Line doublers type
C348S452000
Reexamination Certificate
active
06700622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the processing of video images and, more particularly, to techniques for deinterlacing video images.
2. Description of the Related Art
All major television standards use a raster scanning technique known as “interlacing” or “interlace scanning.” Interlace scanning draws horizontal scan lines from the top of the screen to the bottom of the screen in two passes. Each pass is known as a field. In the National Television System Committee (NTSC) standard used in North America, each field takes approximately {fraction (1/60)}
th
of a second to draw.
Interlace scanning depends on the ability of the cathode ray tube (CRT) phosphors to retain an image for a few milliseconds, in effect acting like a “memory” to retain the previous field while the newer interleaved field is being scanned. Interlace scanning provides a benefit in television systems by doubling the vertical resolution of the system without increasing broadcast bandwidth.
FIG. 1
shows a number of parallel horizontal scan lines
10
on a conventional television display. A first set of horizontal lines
12
is scanned in a first field period and then a second set of horizontal lines
14
is scanned in a second field period. Thus, the first field is temporarily shifted by {fraction (1/60)}
th
of a second from the second field. When rapidly changing images are being displayed, an object in motion may appear to be fuzzy due to the temporal displacement between the two fields.
This temporal displacement typically does not create a problem on conventional television displays, primarily because the image of the “older” field quickly fades in intensity as the light output of the phosphors decays. A secondary reason is that the spatial displacement in the images caused by motion results in a fine detail that television displays do not resolve well. For these reasons, interlace scanning of motion pictures works acceptably well on conventional television displays.
FIG. 2
shows a set of progressively scanned horizontal lines
16
. In progressive scanning, all horizontal lines
16
, are scanned out in one vertical pass
18
, so there is no time displacement of adjacent lines as in interlace scan. Progressive scanning requires a much higher bandwidth signal. Consequently, progressive scanning is typically used for applications where improved image quality and higher resolution are required, relative to conventional television systems. Progressive scanning is widely used in computer CRTs and liquid crystal displays (LCD).
If a motion picture formatted for an interlaced monitor device as in
FIG. 1
is to be displayed on a progressively scanned device as in
FIG. 2
, then it must be converted from the interlaced format to the progressive format. This format conversion is known as deinterlacing.
FIG. 3
is an illustration of a deinterlace process
19
of the prior art. A first series of interlaced video fields
20
is generated by a video source (not illustrated) at {fraction (1/60)}
th
second intervals.
In this example, each of the video fields
20
has a spatial resolution of 720 horizontal by 240 vertical pixels. Each field contains half the vertical resolution of a complete video image. The first series of video fields
20
are input to a deinterlace processor
22
, which converts the 720 by 240 interlaced format to a second series of video fields
24
. In this example, each of the second series of video fields
24
may have 720 by 480 pixels where the fields are displayed at 60 frames per second.
FIG. 4
is an illustration of a prior art method of deinterlace processing, which uses field combination to deinterlace an image using multiple fields. A video field
26
containing scan lines
30
, and a previous video field
28
containing scan lines
32
is fed into a field combination deinterlace processor
34
. The result is a combined frame
36
with scan lines
38
sourced from video field
26
and scan lines
40
sourced from video field
28
. When this simple deinterlacing of the prior art is performed, and a motion picture formatted for an interlace display is converted to a progressive format, a noticeable “artifact” or error arises because the image content of vertically adjacent lines is time shifted by {fraction (1/60)}
th
second as noted previously. The error is most visible around the edges of objects that are in motion.
FIG. 5
is an illustration of a prior art method of deinterlace processing, which uses line interpolation to deinterlace an image using a single reference field. The method
42
interpolates or doubles the number of lines of one field to produce a progressive frame. A video field
44
is scanned from an image to contain a half set of lines
46
. The half set of lines
46
is deinterlaced by line interpolation in a deinterlacing interpolator
48
.
The resulting frame
50
will have all the lines
46
of the original video field
44
. The remaining lines
52
are created by interpolation of lines
46
. The resultant image will not have motion artifacts because all the lines in the image will be created from lines
46
that are time correlated. This alternative method
42
of deinterlacing does not produce motion artifacts, but the vertical resolution of the image is reduced by half.
FIG. 6
shows a deinterlaced image
54
with a stationary object
55
that is rendered without distortion.
FIG. 7
shows an image
56
with the object
55
′ in motion. The edges of object
55
′ create artifacts
57
on the edges of the image
54
because of the aforementioned temporal shift. These artifacts
45
are introduced into the image by the conventional field combination deinterlacing method
25
of FIG.
4
.
Motion artifacts in deinterlaced video images will have characteristics that vary depending on the original source of the motion picture. Video cameras, such as those used for television broadcasts, update motion in each field produced so that fields at 60 Hertz (Hz) will have motion updated at 60 Hz. But motion pictures from other sources are commonly displayed as video, which requires a conversion a different type of motion picture to video. For example, movies originally shot as film must be converted to video for display on a television set. Since film is originally shot at 24 frames per second, maximum update rate for motion for a film source is 24 Hz.
A film source may be viewed as a series of still image frames that are displayed in series at the rate of 24 per second. Film is converted to video in a two step process. First, each of the original film frames must be converted to video fields. Secondly, the video fields must be sequenced in a way that allows them to be displayed at 60 Hz for the NTSC video standard or at 50 Hz for the phase alternation line (PAL) standard. The conversion from film to NTSC video is known as 3:2 pulldown. The conversion of film to PAL video is known as 2:2 pulldown.
FIG. 8
illustrates a diagram of a method
58
for converting film into video of the prior art. A film frame
60
is digitized according to a standard for digital video known at ITR-R BT.601, which specifies the number of horizontal samples per line and the number of lines per field. The film frame
60
is shown with a horizontal by vertical resolution of 720×480, which is the approximate resolution specified by ITU-R BT.601 for NTSC video. (Note: this discussion assumes that film is being converted to a digital format. This may not always be the case but the explanation of 3:2 or 2:2 pulldown applies whether or not the film is digitized.)
Each film frame
60
contains the fall vertical resolution of the image. Since video fields contain half the vertical resolution, each film frame will be converted into two video fields. Assuming the horizontal lines of frame
60
are numbered in sequence from top to bottom, the white bands indicate even numbered lines
62
in the original film frame and the gray bands indicate odd numbered lines
64
. When the frame is converted to fields, the even lines
62
are ass
Adams Dale R.
Thompson Laurence A.
DVDO, Inc.
Kostak Victor R.
Perkins Coie LLP
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