Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-01-20
2001-10-23
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240120, C375S240140, C382S236000
Reexamination Certificate
active
06307886
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to compression of digital visual images, and more particularly, to a technique for using image statistics during encoding of a video sequence to dynamically determine group of picture (GOP) size as a function of picture change activity within the sequence of video frames.
BACKGROUND OF THE INVENTION
Within the past decade, the advent of world-wide electronic communications systems has enhanced the way in which people can send and receive information. In particular, the capabilities of real-time video and audio systems have greatly improved in recent years. In order to provide services such as video-on-demand and video conferencing to subscribers, an enormous amount of network bandwidth is required. In fact, network bandwidth is often the main inhibitor in the effectiveness of such systems.
In order to overcome the constraints imposed by networks, compression systems have emerged. These systems reduce the amount of video and audio data which must be transmitted by removing redundancy in the picture sequence. At the receiving end, the picture sequence is uncompressed and may be displayed in real-time.
One example of an emerging video compression standard is the Moving Picture Experts Group (“MPEG”) standard. Within the MPEG standard, video compression is defined both within a given picture and between pictures. Video compression within a picture is accomplished by conversion of the digital image from the time domain to the frequency domain by a discrete cosine transform, quantization, and variable length coding. Video compression between pictures is accomplished via a process referred to as motion estimation and compensation, in which a motion vector plus difference data is used to describe the translation of a set of picture elements (pels) from one picture to another.
The ISO MPEG-2 standard specifies only the syntax of bitstream and semantics of the decoding process. The choice of coding parameters and tradeoffs in performance versus complexity are left to the encoder developers.
In video applications, it is advantageous to optimize encoding of digital signals in order to obtain the best density or compression of data. There are a number of known techniques to accomplish encoding, however there is an increasing need for techniques to compress video data at low bit rates to overcome bandwidth constraints on networks, while still maintaining picture quality. This invention seeks to meet this need by providing an encode technique which optimizes variable bit rate bandwidth while maintaining picture quality, especially at low bit rates and low motion video applications such as surveillance, distance learning, and video conferencing.
DISCLOSURE OF THE INVENTION
Briefly summarized, this invention comprises in one aspect a method for encoding a sequence of video frames which includes: encoding the sequence of video frames to produce an encoded bitstream; and simultaneous with the encoding, dynamically determining as a function of activity within the sequence of video frames a group of picture (GOP) length for at least one GOP of the encoded bitstream. More particularly, the invention may include (in one embodiment) encoding a frame of the sequence of video frames as an intra-coded (I) frame and employing forward prediction motion estimation based on the encoded I frame for each frame of a variable number of frames temporally subsequent to the encoded I frame. Wherein, the I frame and the variable number of frames temporally subsequent thereto comprise the GOP length of the at least one GOP. Preferably, each frame of the variable number of frames is encoded as a bi-directionally predictive-coded (B) frame.
In another aspect, the invention comprises a method for encoding a sequence of video frames. This method includes: encoding a frame of the sequence of video frames for use as a reference frame; encoding subsequent frames of the sequence of video frames using motion estimation based on the reference frame; and during the encoding of subsequent frames, for each frame thereof comparing at least one frame characteristic of that frame to a preset threshold and dynamically determining therefrom an encode picture type for a next frame of the subsequent frames. Again, the reference frame preferably comprises an intra-coded (I) frame and each frame of the subsequent frames is a bi-directionally predictive-coded (B) frame.
Systems and articles of manufacture corresponding to the above-outlined methods are also described and claimed herein in accordance with the principles of this invention.
To restate, presented is a technique for dynamically determining a group of picture size during encoding of a sequence of video frames. By dynamically varying the group of picture size, bit rate bandwidth can be optimized and picture quality maintained, especially at low bit rates and low motion within the video sequence. Thus, this invention comprises a technique to maximize temporal redundancy in low bit rate, low motion video applications such as surveillance, distance learning, and video conferencing. The technique is described below with reference to the MPEG video compression standard, but is equally applicable to other standards involving motion estimation and motion compensation. Dynamic generation of GOP size within an MPEG video stream is obtained by stringing forward predicted only “B” pictures together until a threshold number of intra-macroblocks are coded, triggering a new “I” frame encode, and initiating a new GOP. The number of intra-macroblocks within a frame comprises one example of an intraframe characteristic which can be monitored as representative of picture change activity, and used to detect picture degradation.
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Heslin & Rothenberg, P.C.
International Business Machines Corp.
Kelley Chris
Radigan, Esq. Kevin P.
Steinberg, Esq. William H.
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