This paper presents a review of the advances in digital coding of video signals during the last four years. Displacement estimation algorithms for coding applications are compared first and the relationship between the algorithms is pointed out. The developments in predictive and transform coding are described and discussed with view to broadcast television and video-conferencing applications. One chapter summarizes the first promising results of motion adaptive frame interpolation. Some problems to be solved in the future are pointed out in the conclusions.

Advances in picture coding

Hybrid predictive/transform coding is studied. The usual formulation is to first apply a unitary transform and then code the transform coefficients with independent DPCM coders, i.e., the prediction is performed in the transform domain. This structure is compared to spatial domain prediction, where a difference signal is formed in the spatial domain and then coded by a transform coder. A linear spatial domain predictor which minimizes the mean square prediction error also minimizes the mean square of each transform coefficient. The two structures are equivalent if the transform domain prediction scheme is extended to a more general predictor. Hence, the structure that gives the easiest implementation can be chosen. The spatial domain structure is preferred for motion compensation and for line interlaced video signals. Interframe hybrid coding experiments are performed on interlaced videophone scenes using an adaptive transform coder. Motion compensation gives a rate reduction of 25-35 percent compared to frame difference prediction with the same mean square error. The subjective advantage is even greater, since the "dirty window" effect is not present with motion compensation. It is important to perform the motion estimation with fractional pel accuracy. Field coding with a switched predictor using previous field in moving areas is an interesting alternative to frame coding with frame difference prediction.

Fixed and Adaptive Predictors for Hybrid Predictive/Transform Coding

Hybrid coding of color television images at broadcast standards is investigated. This involves two-dimensional discrete cosine transform (2-D DCT) of each field, followed by the differential pulse code modulation (DPCM) between successive fields. Two 22frame sequences of "Water Skier" and "Wheel of Fortune" in component form, i.e., luminance Y and chrominance I and Q , were utilized as the database. A statistical study of the prediction error in the 2-D DCT domain is carried out. Three different algorithms for the 2-D DCT system are simulated and analyzed. In the first algorithm, optimum (nonuniform) quantizers are used in DPCM loops followed by fixed-wordlength coders. Uniform quantizers along with variablewordlength coders are implemented in the second system. The performances of different deterministic coders are investigated and compared with Huffman coders. The third scheme is an adaptive system. In this system each 8 × 8 block is divided into four subblocks. The activity of each subblock is monitored and when it exceeds some threshold, the subblock is considered to be spatially active. More bits are assigned to active subblocks and the ranges of corresponding quantizers are expanded. The prediction error of the dc coefficient is monitored to determine the temporal activity of a block. For a temporally active block, the number of bits assigned to the two lower frequency subblocks is increased. Performance of these three systems for unmatched statistics and in the case of a scene change are studied. Mean square error (MSE) between the original and reconstructed images, bit rate, entropy, essential maximum, and other parameters are utilized as the performance criteria. Also, a subjective evaluation of the processed images is carried out. It is shown that the interfield adaptive hybrid coding of color TV signals in component form results in significant savings in bit rates for transmission over a digital link.

Interfield Hybrid Coding of Component Color Television Signals

A non-adaptive scheme for encoding an image sequence is described that yields better results than any previous comparable interframe image coding method. The scheme uses a hybrid of two-dimensional transform coding for the spatial dimension and DPCM for the time dimension similar to some previous works [e.g., 1]. Unlike the previous efforts, which use discrete cosine transforms (DCT's) of image subblocks and suboptimal quantization, we utilize a full image DFT and an optimal quantization algorithm. The results with the standard sixteen frame sequence of Walter Cronkite are visually superior to previous transform coding efforts, especially at rates lower than 1 bit/pixel/ frame. The signal-to-noise ratios are higher than those of nonadaptive hybrid DCT/DPCM in [1] by 2.6 to 10.5 dB depending on rate.

P. Jakatdar

Papers

Hybrid DFT/DPCM interframe image quantization