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Video Codec Design for Real Time Applications

Abstract:

Multimedia industries are witnessing a rapid evolution toward integrating

complete systems on a single chip. The most important characteristic of multimedia

applications is the rapid growth of the complexity of the design. As an example, the new

videophone and teleconferencing standard (H.263+) is much more complex than the

previous H.263 and H.261 standards, and the continued evolution of video coding

standards from MPEG-1 and MPEG-2 to MPEG-4 [3] is accompanied by a significant

complexity increase with user flexibility. The conventional approach for system-level

design and integration of video processing technology, combined with the level

complexity and extremely short product development cycles, cannot found robust, cost-

effective solutions simply by implementing large parts of the system functionality in

software running on application-specific instruction set processor (ASIP) cores.

The project focuses on the system-level modeling of image and video processing

systems. Based on the models given, image and video compression and decompression

standards (e.g. H.263 and MPEG-4) will be implemented in a formal, consistent and

extensible framework with well-defined and optimized processing primitives. The models

of computation and an implementation of image and video processing algorithm will also

help automate partitioning of the design into hardware and software.

Video Codec Structure

The Video Codec is split into two programs Encoder and Decoder. Both of them

are organized into three layers according to the functionality, interfaces, and the coding

strategy followed.

Block diagram of Encoder:

This diagram explains the flow of data and control in the Encoder. The decoder has a

similar structure with the data flowing the reverse direction.

Top Layer

Intermediate Layer

Input

Raw frame Recon. frame

Bottom Layer

Bit stream

Residue frame Recon. Residue frame

EncoderBlocks

ME, MCBlocks

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Top Layer: This layer serves as interface between user and the actual encoder. It

loads the input into frames and feeds them to the lower layer sequentially. This layer also

performs the necessary initializations (such as Loading Huffman tables). The input

sequence is read and passed as frames to the lower layer. The very first frame is forced to

be INTRA. The subsequent frames are by default directed to be coded as P frames. The

lower layer may dynamically decide to code such a frame as an I-frame according to

various parameters (scene change / INTRA refresh).

Intermediate Layer: This Layer performs the frame level manipulations and also

takes care of the frame-level and macro block-level decision making in the encoder. This

layer performs the Motion Estimation and Compensation or removes 128 from the frame

according to whether the frame is to be coded as an I-frame or a P-frame. The residue

frame is input to the frame-encoder layer. The reconstructed residue frame received from

the encoder layer is used to reconstruct the current frame for future prediction.

Bottom Layer: This layer performs the actual coding. The frame is coded

according to the set of parameters which are passed by the higher layer. These parameters

are set by the higher layer according to the coding strategy formed. The bits generated are

put in the bit stream. One way to modify the encoder to perform better is to keep track of

the bits generated per macro block and adaptively change the quantization step size in

order to keep the bit rate to a desired value. In this case it is necessary to include the

Quantizer information in the macro block header. This is a possible extension to the

existing implementation.