An Adaptive Control for the Propagation Errors Incurred by DCT Coefficient-Dropping Transcoder

Abstract

This paper presents a new distortion control scheme with a simple estimation model for the propagation errors incurred by dropping some parts of the bitstream in a frame dropping-coefficient dropping (FD-CD) transcoder. The primary goal of this paper is to facilitate bit-rate conversions and rate-distortion controls in the compressed domain without introducing a full decoding and reencoding system in the pixel domain. First, the error propagation behavior over several frame sequences due to coefficient dropping is investigated on the basis of statistical and empirical properties. Then, such properties are used to develop a simple estimation model for the CD distortion accounting for the characteristics of the underlying coded-frame. Finally, the proposed estimation model allows us to determine the amount of coefficient dropping and to effectively allocate rate-distortions into coded-frames. Experimental results show that the proposed estimation model accurately describes the characteristics of propagation errors adaptively in the compressed domain and can be easily applied to distortion control over different kinds of video sequences.Keywords: Coefficient dropping, propagation error, distortion control. Manuscript received Jan. 10, 2007; revised Aug. 14, 2007. Jin-soo Kim (phone: + 82 42 821 1147 and Mong-han Yun (email: monghan@pinetelecom.com I. IntroductionIn the framework of universal multimedia access (UMA), one challenge for video transmission is to deliver video content through heterogeneous network channels matching the diversity of client devices [1]. As one approach, video transcoding schemes are commonly used [2]. In particular, transcoding based on dropping some parts of the bitstream in the compressed domain is used mainly due to its low computational complexity and simple implementation [3], [4]. Since a frame memory is not required, and there is no need for an inverse discrete cosine transform (IDCT), regardless of the techniques used to achieve the reduced bit-rate, these schemes are relatively simple. This paper focuses on the frame dropping-coefficient dropping (FD-CD) scheme, which provides trade-offs between spatial and temporal qualities and extends the range of rate reduction. By combining framedropping and DCT CD, it is possible to simply adapt the bit rate of a pre-coded video to the available dynamic bandwidth, especially in streaming applications. Furthermore, the FD-CD scheme can achieve rate shaping across a wide range of rate reduction while maintaining spatial-temporal quality as much as possible. For a particular FD-CD operation, in which nonreference frames (namely, B frames) are dropped to avoid the decoding dependency, CD is applied to meet the available bandwidth quite accurately by adjusting the number of dropped coefficients.Some previous studies on FD-CD transcoders have focused on the optimal selection of the number of dropped coefficients within a frame [1], [5]- [9]. The dynamic rate shaping (DRS) approach was introduced as a technique to adapt the rate of compressed v...