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giải mã được phát triển. Đạt được hiệu suất tốt trong thực hiện thực tế đòi hỏi thiết kế cẩn thận và lựa chọn cẩn thận các thông số mã hóa. Trong chương này, chúng tôi cung cấp cho một cái nhìn tổng quan về các vấn đề thực tế liên quan đến việc thiết kế các phần mềm hoặc phần cứng triển khai thực hiện các tiêu chuẩn mã hóa. | 7 Design and Performance 7.1 INTRODUCTION The MPEG-4 Visual and H.264 standards include a range of coding tools and processes and there is significant scope for differences in the way standards-compliant encoders and decoders are developed. Achieving good performance in a practical implementation requires careful design and careful choice of coding parameters. In this chapter we give an overview of practical issues related to the design of software or hardware implementations of the coding standards. The design of each of the main functional blocks of a CODEC such as motion estimation transform and entropy coding can have a significant impact on computational efficiency and compression performance. We discuss the interfaces to a video encoder and decoder and the value of video pre-processing to reduce input noise and post-processing to minimise coding artefacts. Comparing the performance of video coding algorithms is a difficult task not least because decoded video quality is dependent on the input video material and is inherently subjective. We compare the subjective and objective PSNR coding performance of MPEG-4 Visual and H.264 reference model encoders using selected test video sequences. Compression performance often comes at a computational cost and we discuss the computational performance requirements of the two standards. The compressed video data produced by an encoder is typically stored or transmitted across a network. In many practical applications it is necessary to control the bitrate of the encoded data stream in order to match the available bitrate of a delivery mechanism. We discuss practical bitrate control and network transport issues. 7.2 FUNCTIONAL DESIGN Figures 3.51 and 3.52 show typical structures for a motion-compensated transform based video encoder and decoder. A practical MPEG-4 Visual or H.264 CODEC is required to implement some or all of the functions shown in these figures even if the CODEC structure is different H.264 andMPEG-4 Video
