Compressed video editor with transition buffer matcher

Details Compressed video editor with transition buffer matcher Compressed video editor with transition buffer matcher

2001-08-07

2004-04-20

Tran, Thai (Department: 2615)

Motion video signal processing for recording or reproducing

Local trick play processing

With randomly accessible medium

C386S349000, C375S240040, C375S240050

Reexamination Certificate

active

06724977

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and system for editing video bit streams. In particular, the present invention relates to a method and system for encoding transitions between encoded video segments so that the ending buffer fullness of one segment is sufficiently large so that there will not be a buffer underflow while the following segment is decoded.
BACKGROUND OF THE INVENTION
Encoder
A video encoder system
10
is illustrated in FIG.
1
. The system
10
includes a source of video
12
, a preprocessor
14
, a video encoder
16
, a rate buffer
18
and a controller
20
. The source
12
of video is, for example, a video camera, or a telecine machine which converts a sequence of film images into a sequence of video frames, or other device which outputs a sequence of video frames. The frames may be interlaced or progressive. An interlaced frame picture comprises top and bottom field pictures. The preprocessor
14
performs a variety of functions to place the sequence of video frames into a format in which the frames can be compressed by the encoder. For example, in the case where the video source is a telecine machine which outputs 30 frames per second, the preprocessor converts the video signal into 24 frames per second for compression in the encoder
16
by detecting and eliminating duplicate fields produced by the telecine machine. In addition, the preprocessor may spatially scale each picture of the source video so that is has a format which meets the parameter ranges specified by the encoder
16
.
The video encoder
16
is preferably an encoder which utilizes a video compression algorithm to provide an MPEG-2 compatible bit stream. The MPEG-2 bit stream has six layers of syntax. There are a sequence layer (random access unit, context), Group of Pictures layer (random access unit, video coding), picture layer (primary coding layer), slice layer (resychronization unit), macroblock (motion compensation unit), and block layer (DCT unit). A group of pictures (GOP) is a set of frames which starts with an I-picture and includes a certain number of P and B pictures. The number of pictures in a GOP may be fixed.
The encoder distinguishes between three kinds of pictures, I, P, and B. The coding of I pictures results in the most bits. In an I-picture, each macroblock is coded as follows. Each 8×8 block of pixels in a macroblock undergoes a DCT transform to form an 8×8 array of transform coefficients. The transform coefficients are then quantized with a variable quantizer matrix. The resulting quantized DCT coefficients are scanned using, e.g., zig-zag scanning, to form a sequence of DCT coefficients. The DCT coefficients are then organized into run, level pairs. The run, level pairs are then entropy encoded. In an I-picture, each macroblock is encoded according to this technique.
In a P-picture, a decision is made to code the macroblock as an I macroblock, which is then encoded according to the technique described above, or to code the macroblock as a P macroblock. For each P macroblock, a prediction of the macroblock in a previous video picture is obtained. The prediction is identified by a motion vector which indicates the translation between the macroblock to be coded in the current picture and its prediction in a previous picture. (A variety of block matching algorithms can be used to find the particular macroblock in the previous picture which is the best match with the macroblock to be coded in the current picture. This “best match” macroblock becomes the prediction for the current macroblock.) The predictive error between the predictive macroblock and the current macroblock is then coded using the DCT, quantization, scanning, run, level pair encoding, and entropy encoding.
In the coding of a B-picture, a decision has to be made as to the coding of each macroblock. The choices are (a) intracoding (as in an I macroblock), (b) unidirectional backward predictive coding using a subsequent picture to obtain a motion compensated prediction, (c) unidirectional forward predictive coding using a previous picture to obtain a motion compensated prediction, and (d) bidirectional predictive coding wherein a motion compensated prediction is obtained by interpolating a backward motion compensated prediction and a forward motion compensated prediction. In the cases of forward, backward, and bidirectional motion compensated prediction, the predictive error is encoded using DCT, quantization, zig-zag scanning, run, level pair encoding, and entropy encoding.
B pictures have the smallest number of bits when encoded, then P pictures, with I pictures having the most bits when encoded. Thus, the greatest degree of compression is achieved for B pictures. For each of the I, B, and P pictures, the number of bits resulting from the encoding process can be controlled by controlling the quantizer step size. A macroblock of pixels or pixel errors which is coded using a large quantizer step size results in fewer bits than if a smaller quantizer step size is used. Other techniques may also be used to control the number of encoded bits.
After encoding by the video encoder, the bit stream is stored in the encoder buffer
18
. Then, the encoded bits are transmitted via a channel
21
to a decoder, where the encoded bits are received in a buffer of the decoder or stored in a recording medium for later transmission to a decoder.
A decoder system
30
is shown in FIG.
2
. An encoded video bit stream arrives via the transmission channel
21
and is stored in the decoder buffer
32
. The size of the decoder buffer
32
is specified in the MPEG-2 specification. The encoded video is decoded by the video decoder
34
which is preferably an MPEG-2 compliant decoder. The decoded video sequence is then displayed using the display
36
.
The purpose of rate control is to maximize the perceived quality of the encoded video sequence when it is decoded at a decoder by intelligently allocating the number of bits used to encode each picture. The sequence of bit allocations to successive pictures preferably ensures that an assigned channel bit rate is maintained and that decoder buffer exceptions (overflow or underflow) are avoided. The allocation process takes into account the picture type (I, P or B) and scene dependent coding complexity. To accomplish rate control at the encoder, the controller
20
maintains a model of the decoder buffer. This model is known as the video buffer verifier (vbv). The vbv is described in detail in Annex C of the MPEG-2 video standard the contents of which are incorporated herein by reference. (MPEG-2 Video Specification, Annex C). Bits are entered into the vbv in a manner which models the transmission channel. Bits are removed from the vbv in a manner which models removal of the bits by the decoder. That is, all the bits belonging to a picture are removed instantaneously. Based on the occupancy or fullness of the vbv, the controller
20
executes a rate control algorithm and feeds back control signals to the encoder
16
(and possibly to the preprocessor
14
, as well) to control the number of bits generated by the encoder for succeeding pictures, and for succeeding macroblocks within each picture.
The rate control algorithm executed by the controller
20
controls the encoder
16
by controlling the overall number of bits allocated to each picture. The controller allocates bits to successive pictures to be encoded in the future so that the occupancy of the vbv is controlled thereby preventing exceptions at the decoder buffer
32
. The predicted occupancy of the vbv buffer at any time depends on the number of bits which enter the vbv based on a model of the transmission channel and the number of bits removed from the vbv based on the predicted number of bits used to encode each picture.
One conventional rate control algorithm is the MPEG-2 Test Model (TM). The TM rate control is designed to expend a fixed average number of bits per group of pictures (GOP). If too many bits are spent on one GOP, then the excess will be remedied by allocating fewer bi

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