Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-10-11
2003-11-25
Le, Vu (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240020
Reexamination Certificate
active
06654417
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for encoding moving pictures. In particular, the present invention relates to a method and apparatus for performing variable bit rate control for a digital video encoder.
BACKGROUND OF THE INVENTION
Digital compression has been applied to moving pictures for the purposes of transmissions bandwidth or storage size reduction. One current art of such compression techniques can be derived from the ISO/IEC MPEG Standards, the ISO/IEC 11172-3 (MPEG-1), the ISO/IEC 13818-2 (MPEG-2) and the MPEG-2 TM5 (test model 5), developed by the Moving Picture Experts Group of the International Organisation for Standardization. The disclosures of those standards documents are hereby expressly incorporated into this specification by reference.
In a standard MPEG compliant video encoder, a sequence of moving pictures (e.g. video) is input to the encoder where it is compressed with a user defined target bitrate. The target bitrate is set according to the communication channel bandwidth in which the compressed video is to be transmitted, or the storage media capacity in which the compressed video sequence is to be stored.
Several different forms of coding can be employed depending upon the character of the input pictures, referred to as I-pictures, P-pictures, or B-pictures. The I-pictures are intra-coded pictures used mainly for random access or scene update. The P-pictures use forward motion predictive coding with reference to previously coded I- or P-pictures (anchor pictures), and the B-pictures use both forward and backward motion predictive/interpolative coding with reference to previously coded I- or P-pictures. Furthermore, a group of picture (GOP) is formed in encoded order starting with an I-picture and ending with the picture before the next I-picture in the sequence.
The pictures are partitioned into smaller and non-overlapping blocks of pixel data called Macroblocks (MBs) before encoding. Each MB from a P- or B-picture is subjected to a motion estimation process in which forward motion vectors, and backward motion vectors in the case of a B-picture MB, are determined using reference pictures from a frame buffer. With the determined motion vectors, motion compensation is performed where the intra-or inter-picture prediction mode of the MB is first determined according to the accuracy of the motion vectors found, followed by generating the necessary predicted MB.
The predicted MB is then subjected to discrete cosine transform (DCT) and DCT coefficients quantization based on quantization matrices (QM) and quantization stepsize (QS). The quantized DCT coefficients of the MB is then run-length encoded with variable length codes (VLC) and multiplexed with additional information such as selected motion vectors, MB coding modes, quantization stepsize, and/or picture and sequence information, to form the output bitstream.
Local decoding is performed by inverse quantizing the quantized DCT coefficients, followed by inverse DCT, and motion compensation. Local decoding is performed such that the reference pictures used in the motion compensation are identical to those to be by an external decoder.
The quantization stepsize (QS) used for quantizing the DCT coefficients of each MB has direct impact on the number of bits produced at the output of the VLC encoding process, and therefore the average output bit rate. It has also a direct impact on the encoding quality, which is output picture quality at the corresponding decoder. In general, larger QS generates lower output bit rate and lower encoding quality. In order to control output bit rate and picture quality so that the resulting bitstream can satisfy channel bandwidth or storage limitations as well as quality requirements, rate control and quantization control algorithms are used.
Some methods for rate control and quantization control can be found in the abovementioned MPEG-2 TM5 (Test Model 5). These methods comprise generally a bit allocation process, a rate control process, and an adaptive quantization process. In the bit allocation process, a target number of bits is assigned for a new picture to be coded according to a number of previously determined and present parameters. The rate control step then calculates a reference quantization stepsize QS
ref
for each MB based on the target bits for the picture, the number of bits already used from the target bits in encoding MBs from that picture, and a virtual buffer model as given in MPEG-2 TM5. In the adaptive quantization process, the calculated QS
ref
is then scaled according to local activities of the MB, and an average MB activity determined from the previously coded picture. This scaling is done according to a level of masking effects of coding noise by human perception for MB with high or low activities within a picture. An example of an adaptive quantization technique is disclosed in U.S. Pat. No. 5,650,860, entitled “Adaptive Quantization”. A video buffer verifier (VBV) may also be employed in such a way that underflow and overflow are prevented as required by the MPEG standard to ensure the target bit rate is maintained. Techniques for underflow detection and protection are also disclosed in U.S. Pat. No. 5,650,860.
It is apparent that the fixed target bitrate in the process outlined above has little or no relationship to the actual or varying complexity of the video scenes contained in the input picture sequence. The target bitrate is actually defined by the communication channel bandwidth, or by the target storage capacity for the picture sequence, but the perceptual quality of the resulting pictures when decoded may vary from good to annoying from scene to scene according to scene complexity.
For applications where picture sequences are compressed for storage and retrieval, for example DVD (Digital Video Disc or Digital Versatile Disc), variable bit rate (VBR) may be applied on individual segments of the picture sequences depending on its scene complexity to maximize bit rate allocation and encoded picture quality. Data bits may be reduced for less complex scene to save storage space and increase potential recording duration of the medium, or the resulting storage saving can be used for coding of more complex scenes.
Similarly, VBR can also be applied to other applications such as a multi channel video broadcasting network. Such channel bandwidth may be dynamically allocated to individual video sequences to be multiplexed together so that higher percentage of the bandwidth is used adaptively by sequences with complex scenes.
Existing VBR control algorithms such as that disclosed in U.S. Pat. No. 5,650,860 require multiple encoding passes to properly distribute data bits. In the first coding pass, the bit utilization information is determined for each scene or each picture in the input picture sequence. This may be done by fixing the reference quantization stepsize and disabling the VBV control.
The determined bit utilization information is then be used to generate a bit budget for each scene or picture such that an overall target number of bits to code the sequence is fixed, and so that a maximum bit rate is not violated. To accomplish this, the bit budget for each picture is modified so that the VBV buffer does not underflow. In cases that initial bit utilization information obtained is unrealistic for generating the bit budget, steps from the first coding pass must be repeated with an adjusted reference quantization stepsize. The input sequence is coded in a final pass using the generated bit budget information to achieve the target bits or bit rate. This form of multiple-pass VBR encoder requires very large storage memory for storing the intermediate bit utilization information, and large computational capacity for the additional passes and the bit budget generation. Furthermore, such VBR technique such as this cannot process the input sequence in real-time.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a one-pass variable bit rate control technique for coding of moving pic
Carlson David V.
Jorgenson Lisa K.
Le Vu
SEED IP Law Group PLLC
STMicroelectronics Asia Pacific Pte. Ltd.
LandOfFree
One-pass variable bit rate moving pictures encoding does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with One-pass variable bit rate moving pictures encoding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and One-pass variable bit rate moving pictures encoding will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3181422