Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2001-08-15
2004-10-12
Philippe, Gims (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C348S700000
Reexamination Certificate
active
06804301
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved system and method for encoding digital images within frames for digital television transmission.
Digital television offers viewers high quality video entertainment with features such as pay-per-view, electronic program guides, video-on-demand, weather and stock information, as well as Internet access. The video images, packaged in an information stream are transmitted to the user via a broadband communication network over a satellite, cable, or terrestrial transmission medium. Due to bandwidth and power limitations, efficient transmission of film and video demands that compression and formatting techniques be extensively used. Protocols such as MPEG1 and MPEG2 maximize bandwidth utilization for film and video information transmission by adding a temporal component to a spatial compression algorithm.
Each individual image in a sequence of images on film or video is referred to as a frame. Each frame is made up of a large number of picture elements (pixels) that define the image. Within each frame, redundant pixels describe like parts of a scene, e.g. a blue sky. Various types of compression algorithms have been used to remove redundant spatial elements thereby decreasing the bandwidth requirements for image transmission. Sequences of frames on film or video often contain pixels that are very similar or identical. In order to maximize bandwidth utilization, compression and motion compensation protocols, such as MPEG, are typically used to minimize these redundant pixels between adjacent frames. Frames referenced by an encoder for the purpose of predicting motion of images within adjacent frames are called anchor frames. These anchor frames can be of type Intra-frame (I-frame) or Predicted-frame (P-frame). Groups of pixels (macroblocks) that are mapped without reference to other frames make up I-frames, while P-frames contain references to previously encoded frames within a sequence of frames. A third type of frame referred to as a Bi-directional (B-frame) contains macroblocks referred from previously encountered frames and macroblocks from frames that follow the frame being currently analyzed. This entails a type of look-ahead scheme to describe the currently analyzed image in terms of an upcoming image. Both B-frame and P-frame encoding reduce duplication of pixels by calculating motion vectors associated with macroblocks in a reference frame, resulting in reduced bandwidth requirements. MPEG-2 encoding and MPEG-1 encoding differ in their support of frame slices. Slices are consecutive groups of macroblocks within a single row defined for a frame that can be individually referenced. Typically slices are of the same type, i.e. all P-frame encoded or all I-frame encoded. The choice of encoding type for a particular frame is dependent upon the complexity of that image.
In MPEG-2 digital video systems, the complexity of a video frame is measured by the product of the quantization level used to encode that frame and the number of bits used for coding the frame. This means the complexity of a frame is not known until it has been encoded. As a result, the complexity information always lags behind the actual encoding process, which requires the buffering of a number of frames prior to encoding, thereby adding expense and complexity.
Furthermore, selection of I-frame versus P-frame encoding protocol typically requires multiple encoding passes on a single frame to determine the complexity of the encoding. If a P-frame encoding results in a greater complexity than would be realized using I-frame encoding, then I-frame encoding would be selected. Ideally, an anchor frame should be coded twice in the first pass encoder to generate the complexity measure for both I and P cases, but computational overhead typically limits such an approach. From a bandwidth utilization viewpoint, it would be most effective to code for P-frames except where the image complexity would call for I-frame encoding, e.g. at scene changes. One problem with requiring multiple encoding passes on a single frame is the increased computational complexity introduced, thereby reducing the throughput of the encoder. Another problem with this approach is the inherent inefficiency of having to encode a frame twice.
Accordingly, there is a need for an improved complexity encoding system. The system should enable effective scene change detection to be performed. Furthermore, the system should be usable with essentially any type of video data, including high-definition (HD) and standard-definition (SD) television (TV). The present invention provides a solution for solving these problems while providing enhanced throughput of film or video frame encoding.
SUMMARY OF THE INVENTION
A method and accompanying apparatus for specifying the digital video frame type and Group-Of-Pictures (GOP) length for a succession of video signals is presented.
The present invention alternately encodes both I-frame and P-frame macroblocks within a single frame. By doing so, both I and P encoding complexity can be computed without encoding the same frame twice. This arrangement allows the I-frame decision to be made at the second pass encoder instead of at the first pass encoder, thus taking advantage of a look-ahead pipeline to more effectively align the I-frames with scene changes. This method also reduces the computational encoding complexity.
The invention comprises a two-pass video encoding system whereby the first pass encoding entails assigning to each successive anchor frame a Predicted frame (P-frame) encoding type alternating, e.g., with two successive Bi-directional encoded frames (B-frames). Generally, frame encoding type assignments can either be Intraframe (I-frame), Predicted frame (P-frame) or Bi-directional frame (B-frame) encoding.
For the purpose of computing the complexity of each video frame in a single pass, each P-frame is partitioned into interleaving Intra-frame encoded macroblocks; e.g. I-slices and Predicted-frame encoded macroblocks, e.g. P-slices. Between two adjacent P-frames, these slices are encoded in alternating positions. For each of the encoded frames, a complexity measure is calculated and sent to a second-pass encoder for further processing. The complexity measure for each frame type is equal to a product of the total number of bits generated by the slices within a frame and a value associated with a nonlinear mapping of the relationship between a quantizer level and the generated bits. The step of calculating the complexity measure for both P-frame encoding and I-frame encoding for a single anchor frame is performed in one pass, allowing an I-frame to be specified at the second pass encoding instead of the first-pass encoding. This is advantageous because of possible scene change frames that might be introduced in later frames that would require I-frame type assignment. It is more efficient to extend the Group-Of-Pictures (GOP) from it's default length and to include a scene change frame with an assigned I-frame type. Scene change frames are detected by the first pass encoder using a scene change detection algorithm, however, frame type assignment is performed during a second pass encoding. A scene change frame is identified by calculating the relative difference between a P-frame complexity measure and an I-frame complexity measure for a frame, and evaluating the calculation with respect to a threshold value. A scene change notification associated with each of the scene change frames is sent to the second-pass encoder for processing.
The pipeline architecture of the second pass encoder provides a look-ahead buffer capability for efficient encoding of successive video frames. It is used both for P-frame and B-frame encoding algorithms and for identifying a scene change frame in a forthcoming frame and thereby inhibiting the assignment of an I-frame until the forthcoming frame is processed. A counter is incremented for each frame processed. In the second-pass encoder, the scene change notifications associated with each of the scene change
Liu Vincent
Wu Siu-Wai
Lipsitz Barry R.
McAllister Douglas M.
Philippe Gims
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