Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-11-24
2003-05-27
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240130
Reexamination Certificate
active
06570922
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a digital video transcoder and method for allocating bits for encoding successive pictures in a group of pictures (GOP) without a priori knowledge of the picture types in the GOP.
With digital video coding standards such as MPEG, input pictures can be coded in three different picture types, namely I, P and B. The three pictures require quite different numbers of bits for encoding because of the different nature of their temporal processing. Hence, an intelligent bit is allocation strategy should assign a number of bits for encoding according to the picture's type. This implies a requirement of a priori knowledge of the picture types for a given bit budget. This requirement is not a problem for a standalone encoder as the encoder can determine the picture type for each input picture.
In fact, the encoder can plan ahead for the types of the input pictures. In contrast, a transcoder has no such a priori knowledge regarding a picture's type before actually processing the picture. This creates difficulty in allocating an appropriate number of bits for encoding pictures in a transcoder.
Accordingly, it would be desirable to have a method and apparatus for allocating bits for encoding pictures in a transcoder without a priori knowledge of the picture type. The system should avoid a processing delay of about one GOP which would otherwise be incurred to extract the complete GOP structure information from a pre-compressed bit stream. In addition, the system should avoid the need to store the data corresponding to the GOP, thereby reducing the memory required for transcoding.
The system should be compatible with both variable bit rate and constant bit rate bitstreams.
The system should be compatible with statistical multiplexing and remultiplexing systems.
The present invention provides an apparatus and method having the above and other advantages.
SUMMARY OF THE INVENTION
The present invention relates to a digital video transcoder and method for allocating bits for encoding successive pictures in a group of pictures (GOP) without a priori knowledge of the arrangement of picture types in the GOP.
1. Progressive Refresh Sequence
A method for allocating bits for coding a progressive refresh sequence of pictures in a bitstream received at a digital video transcoder includes the steps of: (a) providing an assumed distance M′ between a first picture of the bitstream and the next closest subsequent P-picture of the bitstream (in display order); (b) providing an assumed bit budget as a function of the assumed distance M′; (c) coding the first picture in accordance with the assumed bit budget; (d) determining a picture type of a second picture that immediately follows the first picture in the bitstream; (e) adjusting the assumed bit budget according to the picture type of the second picture; and (f) allocating bits for coding the second picture in accordance with the adjusted bit budget.
Thus, a bit budget for coding the pictures is initially assumed for the first picture, and the bit budget is updated as the second picture type is known. A target number of bits is allocated for coding each picture according to the bit budget. The first and second picture types generally indicate the picture distribution in the sequence.
The assumed bit budget is proportional to the assumed distance and a bit rate of the bitstream, and inversely proportional to a frame rate of the bitstream.
The pictures in the bitstream may form a progressive refresh sequence, where there is no GOP structure.
1.1 P-picture Followed by B-picture
When the first picture is a P-picture and the second picture is a B-picture, the method includes the further step of: determining an actual distance M″ between the first picture and the next closest subsequent P-picture (in display order) according to the picture type of the first picture and the picture type of the second picture.
The actual distance M″ is determined according to a difference between a temporal reference of the first picture, in a display order of the bitstream, and a temporal reference of the second picture, in the display order, plus one picture.
The method may include the further step of allocating bits for coding the remaining M″−1 pictures following the first picture in accordance with the adjusted bit budget.
The assumed bit budget is adjusted in the adjusting step (f) by +(M″−M′)*“bit rate”/“frame rate”, where “bit rate” is a bit rate of the bitstream, and “frame rate” is a frame rate of the bitstream.
1.2 P-picture Followed by P-picture
When the first and second pictures are P-pictures, the method includes the further step of: determining an actual distance M″ between the first picture and the next closest subsequent P-picture (in display order) according to the picture type of the first picture and the picture type of the second picture. The actual distance M″ is determined according to a difference between a temporal reference of the second picture and a temporal reference of the first picture. The temporal references are determined in relation to a display order of the bitstream.
The assumed bit budget is adjusted in the adjusting step (f) by −(M′−1)*“bit rate”/“frame rate”, where “bit rate” is a bit rate of the bitstream, and “frame rate” is a frame rate of the bitstream.
The method includes the further step of allocating bits for coding a series of M″ pictures following an initial M″ pictures that includes the first and second pictures in the bitstream in accordance with the adjusted bit budget. Thus, the adjustments to the bit budget in one series of pictures is carried over to the next series to allow correct coding of the next series.
2. Non-progressive Refresh Sequence
Generally, the invention enables transcoding to begin by assuming a reasonable set of GOP parameters, including M, the distance between each I-picture and the next P-picture, and N, the GOP length. M and N are adjusted during transcoding as additional information becomes available regarding the GOP structure, e.g., the distribution of picture types in the GOP and the length of the GOP.
The assumed M can be corrected to the actual value within two frames, and the assumed N can be corrected to the actual value within one GOP. M should be verified, and adjusted if necessary, after each I- or P-picture, and N should be verified, and adjusted if necessary, after each I-picture. When M and/or N are adjusted, a bit rate R for coding the pictures, and n
I
, n
P
, and n
B
, the remaining numbers of I, P and B-pictures in the current GOP, respectively, are also adjusted.
A method for allocating bits for coding a non-progressive refresh sequence of pictures in a bitstream received at a digital video transcoder, includes the steps of: (a) providing an assumed length N′ of a particular group of pictures (GOP) of the bitstream; (b) providing an assumed bit budget as a function of the assumed length N′; providing an assumed distance M′ between the first picture and the next closest subsequent P-picture of the bitstream (in display order); (c) allocating bits for coding a first picture of the particular GOP in accordance with the assumed bit budget, and N′ and M′; (d) determining a picture type of a second picture that immediately follows the first picture; (e) adjusting the assumed bit budget according to the picture type of the second picture; and (f) allocating bits for coding the second picture in accordance with the adjusted bit budget.
The assumed bit budget is proportional to the assumed length N′ and a bit rate of the bitstream, and inversely proportional to a frame rate of the bitstream.
M″ may change within a GOP. Thus, the method may include the step of periodically verifying M″ throughout the GOP. For example, M″ may be calculated at every I-picture and/or P-picture that follows the first I-picture. The assumed bit budget is adjusted each time in accordance with M&Prime
Eifrig Robert O.
Luthra Ajay
Wang Limin
General Instrument Corporation
Kelley Chris
Lipsitz Barry R.
McAllister Douglas M.
Parsons Charles
LandOfFree
Rate control for an MPEG transcoder without a priori... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rate control for an MPEG transcoder without a priori..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rate control for an MPEG transcoder without a priori... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3068459