Television – Basic receiver with additional function – Multimode
Reexamination Certificate
1999-07-06
2002-10-22
Kostak, Victor R. (Department: 2611)
Television
Basic receiver with additional function
Multimode
C348S497000
Reexamination Certificate
active
06469744
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for encoding, decoding and displaying images and, more particularly, to methods and apparatus for encoding, decoding and displaying images in a manner that provides relatively smooth motion.
BACKGROUND OF THE INVENTION
Video sequences comprise a series of images, e.g., frames. In the case of motion pictures, where film is used, each frame corresponds to a frame of film.
During filming, frames which are also sometimes called pictures, are captured at a pre-selected rate, e.g., 24 frames a second. The rate at which images are converted to frames, e.g., by the taking of a sequence of pictures, is refereed to as the image capture rate. When using rolls of film to show an image sequence, e.g., in a movie theater, pictures are normally displayed at the same rate at which they were initially captured. However, the shutter of the projector may be shuttered multiple times per picture display time to provide a flicker rate which is higher than the video capture rate. This is because higher flicker rates tend to be less annoying than lower flicker rates.
Significantly, in the case of film, pictures are normally displayed at a uniform rate which is usually a function of the picture capture rate. Thus, motion in film tends to be relatively smooth since each new picture corresponds to the same amount of time as the preceding picture.
The advent of television and, more recently, computers, has greatly expanded the number of devices which use media other than film to store, transmit and display video images. Most television sets are capable of displaying images at a single rate determined by the television set's horizontal and vertical refresh rates. Computer display devices, e.g., multi-sync monitors, are frequently capable of responding to synchronization signals which may fall within a range of refresh rates supported by the monitor. As a result, multi-sync monitors are capable of supporting one or more display refresh rates while television sets usually support a single display refresh rate.
While video images are now commonly transmitted as analog signals, e.g., in the case of NTSC television, the use of digital data to represent video images is growing in popularity. For example, digital video disk players, digital satellite broadcasts to the home, and digital high definition television, currently rely on the transmission of video images as digital data.
A frame is generally used to describe a complete image, or an image composed of two interleaved fields, which are to be displayed on a display device. Frames may be coded as either progressive or interlaced images. In the case of progressive images, all of the lines of a frame are coded to be displayed in sequence. Thus, progressive frames may be coded so that the lines of the frame will be displayed in sequence starting at the top of a display screen and ending at the bottom of the display screen each time the display is refreshed. Interlaced frames normally comprise two fields, a first field corresponding to the even lines of a frame and a second field corresponding to the odd lines of a frame. During display, the lines of the first field of an interlaced display are normally refreshed and then the lines of the second field are refreshed. In this manner, with each updating of an interlaced display, every other line of a frame is updated. In the case of non-film video sequences, the term picture is often used to refer to either a progressive frame or a field of an interlaced frame.
Various standards relating to the encoding and transmission of digital video signals now exist. One such standard is MPEG-2 which is described in detail in “INFORMATION TECHNOLOGY—GENERIC CODING OF MOVING PICTURES AND ASSOCIATED AUDIO”, Recommendation H.262 ISO/IEC 13818-2, published by INTERNATIONAL ORGANISATION FOR STANDARDIZATION ISO/IEC JTC1/SC29 (Nov. 1993).
Frames of a video sequence are normally encoded and then transmitted to a destination device once. While encoded frames are usually transmitted only once, frames or fields of a frame may be displayed repeatedly. Since frames tend to be encoded and decoded on average at a rate corresponding to the rate at which the original images were collected, the rate at which digitally represented video frames are encoded or decoded over a period of time normally corresponds to, or matches, the image capture rate. The rate at which frames are coded is sometimes referred to as the coded frame rate.
The MPEG-2 standard provides for a frame rate indicator in the encoded bitstream. The frame rate indicator provides the “indicated frame rate”, i.e., the rate at which frames are to be displayed. Significantly, MPEG-2, as well as other digital video standards, allows for the actual number of coded frames per second, which is coded and transmitted as part of a digital bitstream, to differ from the indicated frame rate specified by the frame rate indicator which is included in the bitstream. Normally, discrepancies between the coded frame rate and the indicated frame rate are resolved through the use of a field or frame repeat mechanism, e.g., one or more field or frame repeat commands included in a transmitted bitstream. MPEG-2 supports a repeat_first_field decoder instruction.
Frequently, an encoded bitstream's indicated frame display rate differs from the actual coded frame rate. This is because video images which are captured using a first media, e.g., film, are often subsequently coded for display using a different media, e.g., analog or digital television. The frequent discrepancy between indicated and coded frame rates is not surprising given the large number of possible frame rates. Consider for example, the ATSC Digital Television Standard, described in “ATSC DIGITAL TELEVISION STANDARD”, Doc. A/53, published by the Advanced Television Systems Committee (Sep. 16, 1995) which is based in part on the MPEG-2 standard. The ATSC standard permits a plurality of indicated frame rates to be supported including 23.976 Hz, 24.000 Hz, 29.970 Hz, 30,000 Hz, 59.940 Hz, and 60.000 Hz. The actual coded frame rates may differ from any one of these indicated frame rates requiring the use of a repeat field or repeat frame operation to achieve the indicated frame rate at the time the video images are output for display.
One technique, often called the 3:2 pull down technique, for adapting film images recorded at 24 HZ to be displayed at 30 HZ is illustrated in FIG.
2
. In the first row identified by reference numeral
20
, film pictures
1
-
12
(P
0
-P
11
) corresponding to a one half second period of time, are shown. In the second row identified by reference numeral
22
, numbers
0
through
29
are used to indicate the 30 fields which will be displayed in a half second period of time. A box is used in row
22
to group together fields which correspond to the same film picture. Note how, in row
22
, three fields are displayed for every odd numbered film picture while two fields are displayed for every even numbered film picture. In row
4
of
FIG. 2
, indicated by reference numeral
24
, a capital F is used to indicate a frame and the sub-script is used to indicate the number of the frame. Note how a total of 12 frames, F
0
-F
11
, are used to represent the
12
film pictures P
0
-P
11
. The fourth row of
FIG. 2
, indicated by reference numeral
26
shows the sequence of displayed fields. A review of row
26
shows how the third field displayed for each even numbered film picture, and thus even numbered frame, is a repeat of the first field while the first and second fields of even numbered film pictures are displayed only once. The field repetition rate illustrated in
FIG. 2
may be achieved, e.g., by the inclusion of repeat_first_field commands in an encoded MPEG-2 bitstream.
In the
FIG. 2
example, the coded frame rate is 24 frames per second while the indicated frame rated included in an encoded bitstream would be 30 frames/sec since 60 fields/sec are displayed in one time period as the result of the illustrated field repetition.
Hitachi America Ltd.
Kostak Victor R.
Straub Michael P.
Straub & Pokotylo
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