Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2001-03-26
2004-09-28
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240020, C375S240150, C375S240260
Reexamination Certificate
active
06798835
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method of switching moving-picture bitstreams that have been coded to a small amount of codes by highly efficient coding, such as, MPEG-coding with inter-picture predictive coding for efficient video data transfer, storage and displaying.
Moving-picture bitstreams are switched in digital TV broadcasting or video data transfer, according to change in TV programs or content of data to be transferred. These systems require multiplexing (switching) of moving-picture bitstreams into one bitstream in edition. The edited bitstream requires high-fidelity decoding.
Pictures under MPEG-coding are divided into three different types I-, P- and B-pictures. I-pictures (intra-coded pictures) are coded independently, entirely without reference to other pictures. P-pictures (unidirectionally predictive-coded pictures) are compressed by coding the differences the pictures and reference preceding I- or P-pictures. B-pictures (bidirectionally predictive-coded pictures) are also compressed by coding the differences the pictures and reference preceding or upcoming I- or P-pictures. B-pictures are not used as reference pictures.
Inter-picture predictive coding requires an output buffer to follow a big difference in amount of codes generated for each picture. Coding is controlled so that a generated code amount becomes adequate for the buffer storage capacity, thus buffer occupancy being changed for each group of pictures.
Bitstream switching is performed so that the bitstream just after switching starts with an I-picture.
Switching of sequential two bitstreams generated by inter-picture predictive coding results in that the code amount overflowing or underflowing the buffer because the last buffer occupancy for the anterior bitstream and the initial buffer occupancy for the posterior bitstream at the switching timing are different from each other.
The difference in buffer occupancy is compensated for by inputting stuff data to increase an effective code amount or by inserting empty frames to decrease an effective code amount. The former technique does not affect reproduced pictures whereas the latter causes reproduction of no pictures for a period corresponding to empty frames.
Empty frames are mostly inserted at a bitstream-switching timing because the posterior bitstream starts with an I-picture of large amount of codes.
A well-known bitstream-switching apparatus switches MPEG-2 bitstreams “A” and “B”, using a blank inserter and a blank controller.
Suppose that each of the input bitstreams “A” and “B” carries 480 p(progressive)-pictures (720×480 pixels, about 60 frames per second) coded for every sixth-frame period of P(I)-pictures by MPEG-2 inter-picture predictive coding.
The input bitstreams “A” and “B” are supplied to the bitstream-switching apparatus and switched from the bitstream “A” to “B” at a predetermined timing. The switching timing is decided so that the anterior bitstream “A” is switched at the end of a P-picture to the posterior bitstream “B” starting with an I-picture, otherwise causing problems in inter-picture prediction for reproduction.
The switching is thus performed with detection of frame period and picture type to meet such requirement for switching timing, as illustrated in FIG.
1
.
The output bitstream after switching has displacement of several frames at the switching timing, thus resulting in difference in buffer occupancy at the switching timing to cause overflowing or underflowing decoder buffer.
To avoid such difference in buffer occupancy, the output bitstream after switching is supplied to a blank inserter for inserting stuff bits or empty frames to the output bitstream in accordance with control data sent by a blank controller.
Stuff bits increase the number of bits for each predetermined reproduction period but will not be used in decoding. On the contrary, empty frames carrying no video data decrease the number of bits for each predetermined reproduction period. Insertion of empty frames causes delay in bitstream reproduction timing.
The amount of codes controlled by insertion of stuff bits or empty frames corresponds to the storage capacity of an output buffer. The storage capacity usually corresponds to the amount of bits to be transferred for about 0.2 to 0.5 seconds.
Therefore, insertion of stuff bits or empty frames sometimes causes no reproduction of pictures for about 0.2 to 0.5 seconds at maximum.
The bitstream processed by the blank inserter is supplied to an output buffer. The buffer stores the intermittently input bitstream and outputs it at a predetermined transfer bit rate. The buffer further detects change in amount of codes of the bitstream to send code amount data to the blank controller.
The bitstream after switching as described above is further supplied to a multiplexed-bitstream separator. The separator obtains buffer occupancy BO that is set as video buffering verifier VBV in MPEG, for each frame of the bitstream.
The BO data is supplied to the blank controller. Based on the BO data, the controller detects the displacement of frames at the switching timing. The controller sends control data in accordance with the displacement of frames to the blank inserter.
The inserter inserts stuff bits or empty frames to the bitstream after switching in accordance with the control data until the displacement of frames is cancelled, as illustrated in FIG.
2
.
In
FIG. 2
, buffer occupancy BO is illustrated only at frame change. Buffer occupancy BO indicates decoder-buffer occupancy BO according to the MPEG standard. Thus, the larger the amount of codes for each frame, the lower the BO whereas the smaller the amount of codes for each frame, the higher the BO. The change in BO when raised becomes maximum with respect to transfer bit rate.
The bitstreams “A” and “B” have their own BO values which causes difference in buffer occupancy when switched according to the requirement for switching timing which is discussed above.
Insertion of empty frames to the bitstream at the switching timing promotes increase in buffer occupancy with respect to transfer bit rate due to no code amount for the empty frames.
Empty frames are inserted in unit of frame while stuff bits are inserted in unit of picture portion smaller than a frame so that there is a small difference in buffer occupancy as small as possible for the successive bitstreams “A” and “B”.
In
FIG. 2
, at the switching timing, the last buffer occupancy BO for the anterior bitstream “A” is indicated by the sign “a” and the initial buffer occupancy BO for the posterior bitstream “B” is indicated by the sign “b”. The difference in buffer occupancy for the successive bitstreams “A” and “B” at the switching timing is expressed as “b-a”.
Buffer occupancy for the bitstream “B” after switching becomes lower than “b” for the input bitstream “B” before switching by (b-a). In
FIG. 2
, buffer occupancy at a portion “c” of the input bitstream “B” is lowered to that at a portion “d” of the bitstream “B” after switching, thus underflowing the decoder buffer.
Several empty frames that correspond to the BO difference (b-a) are inserted into the output bitstream after switching to make less the BO difference.
Buffer occupancy at the portion “d” of the bitstream “B” before empty frame insertion is then raised to that at a portion “e” of the bitstream “B” after empty frame insertion, or returned to that at the portion “c”, thus change in code amount before and after switching becoming small for the storage capacity of the decoder buffer.
Insertion of empty frames for making small the difference in buffer occupancy at the switching timing, however, causes delay in reproduction start timing for the posterior bitstream “B”.
Such delay results in no pictures to be decoded after the switching timing for a period that corresponds to the inserted empty frames, thus causing discontinuity in moving picture when reproduced.
SUMMARY OF THE INVENTION
A purpose of the present invention is to provide an apparatus and a method of switching moving-picture b
Kelley Chris
Rekstad Erick
Victor Company of Japan Ltd.
LandOfFree
Apparatus and method of switching moving-picture bitstream does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method of switching moving-picture bitstream, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method of switching moving-picture bitstream will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3268141