This paper describes fixed-point design methodologies and several resulting implementations of the Inverse
Discrete Cosine Transform (IDCT) contributed by the authors to MPEG's work on defining the new 8x8 fixed
point IDCT standard - ISO/IEC 23002-2. The algorithm currently specified in the Final Committee Draft (FCD)
of this standard is also described herein.
This paper analyzes the drift phenomenon that occurs between video encoders and decoders that employ different
implementations of the Inverse Discrete Cosine Transform (IDCT). Our methodology utilizes MPEG-2, MPEG-4
Part 2, and H.263 encoders and decoders to measure drift occurring at low QP values for CIF resolution video
sequences. Our analysis is conducted as part of the effort to define specific implementations for the emerging ISO/IEC
23002-2 Fixed-Point 8x8 IDCT and DCT standard. Various IDCT implementations submitted as proposals for the new
standard are used to analyze drift. Each of these implementations complies with both the IEEE Standard 1180 and the
new MPEG IDCT precision specification ISO/IEC 23002-1. Reference implementations of the IDCT/DCT, and
implementations from well-known video encoders/decoders are also employed. Our results indicate that drift is
eliminated entirely only when the implementations of the IDCT in both the encoder and decoder match exactly. In this
case, the precision of the IDCT has no influence on drift. In cases where the implementations are not identical, then the
use of a highly precise IDCT in the decoder will reduce drift in the reconstructed video sequence only to the extent that
the IDCT used in the encoder is also precise.
This paper presents a straightforward multiplier-less approximation of the forward and inverse Discrete Cosine Transform
(DCT) with low complexity and high accuracy. The implementation, design methodology, complexity and performance
tradeoffs are discussed. Particular, the proposed IDCT implementations, in spite of simplicity, comply with and
can reach far beyond the MPEG IDCT accuracy specification ISO/IEC 23002-1, and also reduce drift favorably compared
to other existing IDCT implementations.
Audio Video coding Standard (AVS) is established by the Working Group of China in the same name. AVS-video is an application driven coding standard. AVS Part 2 targets to high-definition digital video broadcasting and high-density storage media and AVS Part 7 targets to low complexity, low picture resolution mobility applications. Integer transform, intra and inter-picture prediction, in-loop deblocking filter and context-based two dimensional variable length coding are the major compression tools in AVS-video, which are well-tuned for target applications. It achieves similar performance to H.264/AVC with lower cost.
Adaptive Block-size Transforms (ABT) has been widely used in image/video coding, since it exploits the maximum feasible signal length for transform coding. However, if the transforms in an ABT coding system are Integer Cosine Transforms (ICT), not only separate transform units but also different scaling matrices are required, which consume a vast amount of resources in practical implementations. In this paper, a new approach to compatible ABT is presented, by which 8x8, 8x4, 4x8 and 4x4 transforms can be processed in one transform unit. Furthermore, with Pre-scaled Integer
Transform (PIT), the compatibility of scaling matrices especially for 8x4 and 4x8 ICT can be achieved and a single scaling matrix is required. Simulation results and analysis reveal that this approach greatly saves hardware resources and makes the implementation of ABT much easier without loss of performance.