Sharad Singhal

Windowed motion compensation

A new motion compensation technique using a window which satisfies the perfect reconstruction condition is proposed. THe conventional motion compensation using rectangular blocks often gives discontinuities between neighboring motion compensation blocks in the predicted image. The proposed method is based on a window operation to the data which overlaps an area of the conventional motion compensation block. Computer simulation is carried out using MPEG video coding algorithm to evaluate the proposed method. The performance of the proposed method is better than the conventional method in terms of mean square error, and large improvement can be obtained at the block boundaries. This gives a smooth predicted image for a typical MC + DCT coding scheme.

High quality audio coding using multipulse LPC

Experiments are described in coding broadband audio using multipulse linear predictive coding (LPC). It is possible to obtain stable LPC filters that model sinusoids closely and to include perceptual masking in these coders. The quantization of both the LPC and multipulse parameters is also examined, and it is found that multipulse can compensate for quantization error in LPC filters. With appropriate perceptual masking, these coders can provide high quality and audio output. At 128 kb/s, the coders achieved typical SNR values of 35-40 dB in simulations.<<ETX>>

Adaptation of the MPEG video-coding algorithm to network applications

The Motion Picture Experts Group (MPEG) video-coding algorithm is regarded as a promising coding algorithm for coding full-motion video. However, since MPEG was originally designed for storage applications, some problems must be solved before the algorithm can be applied to interactive services. Due to the use of periodic intraframe coding and bidirectional interframe prediction, the end-to end delay of the MPEG algorithm is much larger than that of the H.261 algorithm. In packet video transmission, the large peak in bit rate caused by periodic intraframe coding may lower performance of statistical multiplexing. In this paper, real-time video transmission using the Hybrid Extended MPEG (Bellcores proposal to ISO/MPEG) is considered. First, the end-to-end delay of the Hybrid Extended MPEG algorithm is analyzed. Then several schemes to reduce the delay are considered and compared with regular coding schemes in terms of image quality, end-to-end delay and performance of statistical multiplexing. Error resilience of the presented schemes is also tested by simulations assuming cell loss. It is shown that the presented schemes improved the end-to-end delay and performance of statistical multiplexing significantly. >

Hybrid extended MPEG video coding algorithm for general video applications

Abstract This paper is an analysis of a video coding algorithm designed and optimized for video compression at bit-rates from 3 up to 10 Mbit/s. This algorithm is suitable for different applications ranging from communication services to video broadcasting. This is a hybrid DCT/DPCM coding scheme originally based on an MPEG1 (Moving Picture Experts Group phase 1) algorithm modified for interlaced CCIR601 resolution pictures coded at higher rates. Some important features of the algorithm include field-based motion compensated prediction and interpolation, frame-based DCT coding and quantization, optimized frame bit allocation and quantizer assignment, and adaptive Huffman code tables for transform coefficients. This coding scheme allows easy implementation of common VCR functions, and can also operate in a low-delay mode as required for interactive video. This paper presents a brief analysis of the coding/decoding delay. The result of a subjective test conducted at Bellcore concludes that this algorithm achieves a quality better than NTSC at 4 Mbit/s and close to component quality at 9 Mbit/s. This algorithm was submitted as an MPEG2 (MPEG phase 2) proposal, and showed high performance among 21 525-format proposals.

A New Rate Control Strategy for the MPEG Video Coding Algorithm

Abstract In this paper, we describe a new rate control strategy for the MPEG video coding algorithm. The presented strategy assigns quantizer step sizes considering the spatial activity of the coded area, and changes the bit allocation to each coding mode dynamically so that the ratio of quantizer step sizes among different coding modes becomes stable at the given target ratio. The strategy was implemented on the Hybrid Extended MPEG (Bellcores proposal to ISO/MPEG) in simulations. In the simulations, it was shown that the resulting ratio of quantizer step sizes among different coding modes is always close to the given target ratio while the coder also satisfies the constraint of the given bit rate.

Interframe video coding using overlapped motion compensation and perfect reconstruction filter banks

Interframe video coders using the discrete cosine transform (DCT) and motion compensation (MC) produce block artifacts in the decoded video at low bit-rates. Results on lapped orthogonal transforms (LOTs) suggest that they can reduce these artifacts. However, LOTs are difficult to use efficiently with motion compensation because of block overlap. The authors propose a new video coding algorithm that forms a natural connection between LOTs and motion compensation using two novel concepts: overlapped motion compensation (OMC) and overlapped macroblocks using frequency domain coefficients.<<ETX>>

Reducing computation in optimal amplitude multipulse coders

Multipulse excitation provides excellent quality speech at medium bit rates. However, the analysis-by-synthesis method used to obtain the excitation is computationally expensive. Although the computational complexity of multipulse coders can be reduced by simplifying the model, the simplification usually reduces the degree of optimization in the excitation and the resulting coders do not achieve the performance promised by the multipulse method. In this paper we describe an algorithm that keeps the amplitudes of all pulses optimum while searching for the pulse locations. The algorithm requires O(Nm3/6 + Nm) multiply-adds and O(2Nm) divisions, where m pulses are placed in a frame of length N samples. The algorithm requires O(Nm+3N) words of storage. In addition, we show that it is possible to obtain the best pulse location at a given stage without an exhaustive search. A simple sampling technique enables us to drop a substantial fraction of possible pulse locations from the search at each stage without reducing the optimization.

On encoding filter parameters for stochastic coders

Stochastic coders provide a way of encoding the excitation to the synthesis filter at bit rates of about 2 kbit/s, thus leading to fhe possibility of high quality speech coding at 4.8 kbit/s. In these coders, the excitation is encoded as an index into a codebook of random excitation waveforms and the coder transmits the parameters of a short-term filter (LPC all-pole predictor), the parameters of a long-term filter (pitch predictor) and the excitation gain to the receiver. Although the coders give excellent speech quality with unquantized parameters, the output degrades significantly when the filter parameters are coarsely quantized. For a 4.8 kbit/s coder, the short-term filter parameters have to be quantized at 1 kbit/s or less and conventional scalar quantizers at this bit rate result in severe degradation of output speech. In this paper we describe the performance of stochastic coders when the short-term filter parameters are quantized using direct vector quantization and vector quantization with predictive coding and eigenvector rotation. Our results indicate that good performance can be achieved with relatively small codebooks for the quantizers and that predictive coding with eigenvector rotation gives a small but consistent improvement over direct vector quantization.

Design of a multifunction video decoder based on a motion-compensated predictive-interpolative coder

ABSTRACTIt is now possible to encode VCR quality video and stereo audio at only 1 .5 Mbit/s. In addition, standards have been defined for compressing full color still images and teleconferencing video at bit ratesof 64 to 1920 kbit/s. Finally there is great interest in the next generation of PCs which will incorporate multimedia displays and have capabilities to edit, store and transmit video and images over communication networks. Although the standards defined for video teleconferencing and those beingdefined for storage of video and images are different, they still have substantial parts that are common. Inthis paper, we describe the design of a multi-function decoder that is capable of decoding bit streams fromthe different encoders. By sharing functional modules that are common to the different algorithms, thedecoder can cope with the different standards with only a minimal increase in complexity required overthat needed for any one standard. In addition, it allows transparent display of video information coded atdifferent frame rates and using different aspect ratios, thus facilitating exchange of information betweenNTSC and PAL-based systems as well as film material.