Object based bitstream transcoder

Details Object based bitstream transcoder Object based bitstream transcoder

2000-02-14

2002-12-10

Britton, Howard (Department: 2613)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C382S243000, C375S213000

Reexamination Certificate

active

06493386

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to information delivery systems, and more particularly to delivery systems that adapt information encoded as object-based compressed bitstreams to available bit rates of a network.
BACKGROUND OF THE INVENTION
Recently, a number of standards have been developed for communicating encoded information. For video sequences, the most widely used standards include MPEG-1 (for storage and retrieval of moving pictures), MPEG-2 (for digital television) and H.263, see ISO/IEC JTC1 CD 11172, MPEG, “Information Technology—Coding of Moving Pictures and Associated Audio for Digital Storage Media up to about 1.5 Mbit/s—Part 2: Coding of Moving Pictures Information,” 1991, LeGall, “MPEG: A Video Compression Standard for Multimedia Applications,” Communications of the ACM, Vol. 34, No. 4, pp. 46-58, 1991, ISO/IEC DIS 13818-2, MPEG-2, “Information Technology—Generic Coding of Moving Pictures and Associated Audio Information—Part 2: Video,” 1994, ITU-T SG XV, DRAFT H.263, “Video Coding for Low Bitrate Communication,” 1996, ITU-T SG XVI, DRAFT13 H.263+Q15-A-60 rev.0, “Video Coding for Low Bitrate Communication,” 1997.
These standards are relatively low-level specifications that primarily deal with the spatial and temporal compression of video sequences. As a common feature, these standards perform compression on a per frame basis. With these standards, one can achieve high compression ratios for a wide range of applications.
Newer video coding standards, such as MPEG-4 (for multimedia applications), see “Information Technology—Generic coding of audio/visual objects,” ISO/IEC FDIS 14496-2 (MPEG4 Visual), Nov. 1998, allow arbitrary-shaped objects to be encoded and decoded as separate video object planes (VOP). The objects can be visual, audio, natural, synthetic, primitive, compound, or combinations thereof. Video objects are composed to form compound objects or “scenes.”
The emerging MPEG-4 standard is intended to enable multimedia applications, such as interactive video, where natural and synthetic materials are integrated, and where access is universal. MPEG-4 allows for content based interactivity. For example, one might want to “cut-and-paste” a moving figure or object from one video to another. In this type of application, it is assumed that the objects in the multimedia content have been identified through some type of segmentation process, see for example, U.S. Pat. No. 6,400,846“Method for Ordering Image Spaces to Search for Object Surfaces” filed on Jun. 4, 1999 by Lin et al.
In the context of video transmission, these compression standards are needed to reduce the amount of bandwidth (available bit rate) that is required by the network. The network may represent a wireless channel or the Internet. In any case, the network has limited capacity and a contention for its resources must be resolved when the content needs to be transmitted.
Over the years, a great deal of effort has been placed on architectures and processes that enable devices to transmit the content robustly and to adapt the quality of the content to the available network resources. When the content has already been encoded, it is sometimes necessary to further convert the already compressed bitstream before the stream is transmitted through the network to accommodate, for example, a reduction in the available bit rate.
Bit stream conversion or “transcoding” can be classified as bit rate conversion, resolution conversion, and syntax conversion. Bit rate conversion includes bit rate scaling and conversion between a constant bit rate (CBR) and a variable bit rate (VBR). The basic function of bit rate scaling is to accept an input bitstream and produce a scaled output bitstream which meets new load constraints of a receiver. A bit stream scaler is a transcoder, or filter, that provides a match between a source bitstream and the receiving load.
As shown in
FIG. 1
, typically, scaling can be accomplished by a transcoder
100
. In a brute force case, the transcoder includes a decoder
110
and encoder
120
. A compressed input bitstream
101
is fully decoded at an input rate Rin, then encoded at a new output rate Rout
102
to produce the output bitstream
103
. Usually, the output rate is lower than the input rate. However, in practice, full decoding and full encoding in a transcoder is not done due to the high complexity of encoding the decoded bitstream.
Earlier work on MPEG-2 transcoding has been published by Sun et al., in “Architectures for MPEG compressed bitstream scaling,” IEEE Transactions on Circuits and Systems for Video Technology, Apr. 1996. There, four methods of rate reduction, with varying complexity and architecture, were presented.
FIG. 2
shows an example method. In this architecture, the video bitstream is only partially decoded. More specifically, macroblocks of the input bitstream
201
are variable-length decoded (VLD)
210
. The input bitstream is also delayed
220
and inverse quantized (IQ)
230
to yield discrete cosine transform (DCT) coefficients. Given the desired output bit rate, the partially decoded data are analyzed
240
and a new set of quantizers is applied at
250
to the DCT blocks. These re-quantized blocks are then variable-length coded (VLC)
260
and a new output bitstream
203
at a lower rate can be formed. This scheme is much simpler than the scheme shown in
FIG. 1
because the motion vectors are re-used and an inverse DCT operation is not needed.
More recent work by Assuncao et al., in “A frequency domain video transcoder for dynamic bit-rate reduction of MPEG-2 bitstreams,” IEEE Transactions on Circuits and Systems for Video Technology, pp. 953-957, December 1998, describe a simplified architecture for the same task. They use a motion compensation (MC) loop, operating in the frequency domain for drift compensation. Approximate matrices are derived for fast computation of the MC blocks in the frequency domain. A Lagrangian optimization is used to calculate the best quantizer scales for transcoding.
Other work by Sorial et al, “Joint transcoding of multiple MPEG video bitstreams,” Proceedings of the International Symposium on Circuits and Systems, May 1999, presents a method of jointly transcoding multiple MPEG-2 bitstreams, see also U.S. patent application Ser. No. 09/410,552“Estimating Rate-Distortion Characteristics of Binary Shape Data,” filed Oct. 1, 1999 by Vetro et al.
According to prior art compression standards, the number of bits allocated for encoding texture information is controlled by a quantization parameter (QP). The above papers are similar in that changing the QP based on information that is contained in the original bitstream reduces the rate of texture bits. For an efficient implementation, the information is usually extracted directly in the compressed domain and may include measures that relate to the motion of macroblocks or residual energy of DCT blocks. This type of analysis can be found in the bit allocation analyzer.
Although in some cases, the bitstream can be preprocessed, it is still important that the transcoder operates in real-time. Therefore, significant processing delays on the bitstream cannot be tolerated. For example, it would not be feasible for the transcoder to extract information from a group of frames, then transcode the content based on this look-ahead information. This would not work for live broadcasts, or video conferencing. Although it is possible to achieve better transcoding results in terms of quality due to better bit allocation, such an implementation for real-time applications is impractical.
It is also important to note that classical methods of transcoding are limited in their ability to reduce the bit rate. In other words, if only the QP of the outgoing video is changed, then there is a limit to how much one may reduce the rate. The limitation in reduction is dependent on the bitstream under consideration. Changing the QP to a maximum value will usually degrade the content of the bitstream significantly. Another alternative to reducing the spatial quality is to redu

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