Motion video signal processing for recording or reproducing – Local trick play processing – With randomly accessible medium
Reexamination Certificate
1999-01-26
2002-06-04
Boccio, Vincent (Department: 2615)
Motion video signal processing for recording or reproducing
Local trick play processing
With randomly accessible medium
C386S349000, C348S459000
Reexamination Certificate
active
06400895
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to multimedia processors and particularly to MPEG decoders.
2. Related Art
Encoded digital video streams are used in a variety of applications that allow videos to be distributed on a variety of media. For example, movies can be stored on Digital Video Disc (DVD) for distribution or rental purposes. Similarly, digital video streams can be broadcast (DVB) to multiple subscribers on pay-per-view systems. Alternatively, digital video streams can be transmitted over a computer network, such as the Internet, and viewed on remote computers without the need for a dedicated DVD player or a DVB receiver.
Several encoding standards, such as MPEG, MPEG-2, MPEG-4, AVI, QuickTime, etc. have been developed for encoding and distribution of digital video streams. Different standards allow for varying degrees of functionality versus storage requirements. For example, MPEG-2 is primarily designed for encoding movies and other audio-visual works. Similarly, MPEG-4 is designed to handle video streams transmitted over low bandwidth communication channels.
Movies, on the other hand, are typically shot on optical film at rate of 24 frames/second by means of a photographic process, while audio is captured on a separate medium, with audio-video synchronization added to either recording medium. To store movies as digital video streams, therefore, each frame must be first captured in electronic format and then encoded according to a pre-selected encoding standard.
A system
100
for capturing and displaying an encoded digital video stream is illustrated in FIG.
1
. First, a video camera
110
captures a scene
120
on film
130
.
An encoder
140
then transfers the images stored on film
130
onto an encoded video stream
150
stored on an electronic storage medium. A decoder
160
, in turn, decodes video stream
150
that is thus displayed on a screen of a display device
170
. Decoder
160
can be either a dedicated device such as a DVD player or a general purpose device such as a computer. The display device
170
can be either a television set or a computer monitor.
Video streams encoded according to the MPEG-2 standard, for example, are often encoded for playback at a rate of 60 fields/second. A field
220
(
FIG. 2
) consists of only half the pixels of an image
210
. By displaying successive fields
220
in rapid succession, the human eye perceives the entire image
210
as being displayed at once, while in reality only half image
210
is refreshed at any one time. Since only 24 frames where shot in the original movie for all 60 fields to be displayed during playback, each movie frame must be displayed an average of 2.5 times (60/24). This is typically, accomplished by displaying images
210
of successive frames
200
for a duration of 3 and 2 fields
220
,in an alternating pattern, as shown in FIG.
2
. This technique is typically referred to as “3-2 pulldown.” The MPEG-2 standard provides a repeat_first_field flag
310
on each frame
200
to indicate whether image
210
is to be displayed for
2
or 3 fields
220
,as shown in FIG.
3
. For example, in
FIG. 3
, when repeat_first_field flag
310
has a value “0” image
210
is displayed for two fields
220
, while when repeat_first_field flag
310
has a value “1” image
210
is displayed for three fields
220
.
In addition, a time stamp (not shown) is included in each frame
200
of the encoded video stream representing a desired playback time for each frame
200
. The decoder, in turn, examines the time stamps of each frame
200
to determine whether the timing relationship among the frames in the encoded video stream is preserved during playback or whether playback timing must be adjusted to compensate for variations in the decoding/display process.
The MPEG-2 standard also provides a progressive_frame flag
410
(
FIG. 4
) that indicates whether each encoded image
210
represents an interlaced or a non-interlaced image. An interlaced image is one in which consecutive fields
220
are captured at successive points in time. Thus, to preserve correct timing, each field
220
in an interlaced image must be displayed at a different point in time. Interlaced images are typically displayed correctly on interlaced display devices (e.g., televisions sets, interlaced computer monitors, etc.). However, displaying interlaced images on non-interlaced (progressively-scanned) monitors is problematic. Progressive displays, in fact, display complete images
210
at each refresh interval. Therefore, each field
220
in an interlaced image must be “filled” by interpolating values for the missing lines in each field
220
. The interpolation process, however, results in a loss of image resolution and the introduction of artifacts which, in turn, degrade the overall image quality.
A non-interlaced image, by contrast, is an image
210
in which consecutive fields
220
are captured at the same time. Thus, playback of non-interlaced images on progressive monitors results in higher quality images because there are no missing lines.
Since the two fields in each movie frame are always shot at the same time, progressive_frame flag
410
should always be set to
1
,as shown in FIG.
4
. However, some encoders only set the progressive_frame flag to
1
when repeat_first_field flag
310
is also set to
1
, as shown in FIG.
5
. Movies encoded in such a fashion, therefore, are played back inconsistently on non-interlaced monitors, due to the alternation of display modes.
SUMMARY OF THE INVENTION
The present invention provides a method for improving playback consistency of an encoded video stream. The method of the present invention first determines whether a series of values of the progressive_frame flag of consecutive images of the encoded video stream are arranged in a predetermined pattern. If the values are arranged in the predetermined pattern, the image of at least one of the frames is displayed progressively regardless of the value of the progressive_frame flag for that frame. As a result, the video stream is played back as if the movie had been consistently encoded in the first place.
REFERENCES:
patent: 5594552 (1997-01-01), Fujinami et al.
patent: 5835672 (1998-11-01), Yagasaki et al.
patent: 6148035 (2000-11-01), Oishi et al.
patent: 6236806 (2001-05-01), Kojima et al.
Lightstone Michael L.
Webb Richard Whitby
ATI International SRL
Boccio Vincent
Vedder Price Kaufman & Kammholz
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