Chi-Min Liu

Objective image quality measure for block-based DCT coding

Block-based discrete cosine transform (DCT) coding has been widely adopted in image/video compression standards, such as JPEG and MPEG. Such coding will produce a noticeable artifact known as the blocking effect at very low bit rate applications. In the literature, there have been various postprocessing methods proposed to reduce the blocking effect. However, there is no suitable objective criterion evaluating the effectiveness of these various methods. This paper presents a new objective measure evaluating the postprocessing methods. The proposed measure also provides an analytical result on the underlying sources of artifacts.

A New Postprocessing Method For The Block-based DCT Coding Based On The Convex-projection Theory

Abssfracf- The block-based DCT coding has been adopted in image compression standards such as JPEG and MPEGs. This type of coding causes some noticeable artifacts known as the blocking effect. Various post processing methods have been devoted to removing the blocking effect; however, these methods also unavoidably blur the decoded image. Removing the blocking effect while avoiding blurring is the main issue of the postprocessing. This paper presents a new postprocessing method based on convex-projection theory. This method is shown to have keep good tradeoff for the blocking effects and the blurring effects.

A new intensity stereo coding scheme for MPEG1 audio encoder-layers I and II

In the MPEG 1 audio compression standard, the intensity stereo coding scheme is a key technology to achieve a bit rate lower than 2/spl times/128 kbit/s for stereophonic signals while preserving the high audio quality. However, the intensity coding scheme has difficulties in the signal cancellation and scale-factor consistency. This paper shows that the Karhunen-Loeve (KL) transform can be applied to the intensity coding for solving the two problems. We present two methods used to implement the Karhunen-Loeve transform in the MPEG 1 layers I and II. Among the two methods, one is fully compatible with the MPEG 1 while the other need a slight modification on the MPEG coding process. The two methods are tested and shown to have a better audio quality than the technique indicated by the MPEG 1 layers I and II.

A fast bit allocation method for MPEG layer III

The MPEG/audio layer III has been widely used to store the CD-quality music sequences. The layer III is an asymmetric codec in the sense that the complexity of the audio encoder is much higher than that of the decoder. A key task leading to the higher complexity is the bit allocation. The MPEG/audio layer III encoder has adopted schemes such as nonuniform quantization and Huffman coding to achieve high coding efficiency. However, all these schemes do not have the problem in quality and bit-rate controls. The usual method follows an iterative approach to perform bit allocation to achieve both quality and bit rate tuning. This method has immense complexity and will sacrifice the quality to fit bit rate. This paper presents an efficient allocation method, which has merits in both complexity and quality control.

Design Of The Coupling Schemes For The Dolby Ac-3 Coder In Stereo Coding

When applying the Dolby AC-3 coder for stereo music compression, the coupling scheme that combines the two channels of stereo audio signals at high frequency into one channel is the key technology for Dolby AC-3 to achieve the bit rates lower than 96/spl times/2 kbits/sec while preserving high stereo audio quality. This paper proposes four coupling methods for the AC-3 encoder. These four methods vary with the complexity and performance. These four methods are compared through both subjective and objective tests. These four coupling methods are also combined with the dithering scheme and examined through subjective and objective tests. The result shows that the dithering scheme can effectively ease the coupling artifacts and enhance the audio quality.

Bit allocation method for AC-3 encoder

Dolby AC-3 is an audio coder selected in the United States high definition television (HDTV) standards and widely adopted for the audio codec in DVD films. For audio codecs, the complexity of the audio encoders is always higher than that of the decoders. The main module leading to the higher complexity is the bit allocation process. The objective of the bit allocation is to allocate restricted bits to encoded information. Since the AC-3 coder has adopted a floating point representation which the magnitude of each spectral line is represented with an exponent and a mantissa, the bit allocation process not only has to decide the suitable quantization method for the mantissa similar to the process in other coding standards such as MPEG 1, but also the exponent strategies and hence the parameters in psychoacoustic models. This coding process has been referred to as a hybrid coding (Todd et al. 1994) and has led to immense complexity for an encoder. This paper proposes an efficient method for the bit allocation process.

The design of a hybrid filter bank for the psychoacoustic model in ISO/MPEG phases 1, 2 audio encoder

The ISO/MPEG phases 1 and 2 audio compression are receiving a wide range of applications. In the MPEG encoding process, the psychoacoustic model exploits the audio irrelevancy which has the key role in achieving a high compression ratio without losing the audio quality. However, the Fourier transform (FT) which has been used by the two psychoacoustic models suggested in the draft standard requires a high computational complexity, and hence leads to high hardware and software cost for real-time applications. This paper presents a new design named the hybrid filter bank to replace the FT. The hybrid filter bank can be integrated with the psychoacoustic models and provides a much lower complexity than the FT. Also, this paper shows that the hybrid filter is more suitable for stereo coding and hence can provide a better quality for the intensity stereo coding, which is the key technology for the MPEG 1 to achieve a near transparent quality lower than 96/spl times/2 kbits for two stereo channels.

Bit Allocation Method For Dolby Ac-3 Encoder

The complexity of the audio encoders is always higher than that of the decoders. A main process leading to the higher complexity is the bit allocation. Especially, AC-3 has adopted a hybrid coding strategy (4) which leads to higher allocation difficulty. This paper proposes an efficient method for the bit allocation process.