Demand for interpersonal video and audio communication at bit rates available on basic and subprimary ISDN accesses is growing and it is expected that service implementation will require large numbers of low-cost high-quality equipment. This paper describes the concept and simulation results of a coding scheme of the hybrid (intraframe transform and temporal DPCM) type whose main feature is the possibility of regaining high resolution for slowly changing pictures. Methods for controlling the spatio/temporal resolutions and simulation results are given. A byproduct of the algorithm allows the reduction of the transmission buffer to negligible values.

Several video codec applications are currently considered and there is a danger that digitisation will lead to more incompatibilities than experienced in the analogue era. A video codec architecture is thus proposed having the property to accomodate a wide range of coding algorithms (hybrid scheme), to operate on a wide bitrate range (by means of an adaptive prefilter) and able to deal with different video standards (by integration of postfilter with standards conversion).

The main problem for vector quantisation is the codebook generation. The LBG method solves it with an iterative approach. This algorithm needs a long training sequence and an initial approximation of the codebook. These problems may be overcome by using space filling curves, also called Peano's curves, which are mappings from the unitary interval (image point) to the unitary hypercube. By using this kind of mapping the generation of the codebook, is reduced to the computation of the one-dimensional (optimum) quantiser associated to the probability density function (pdf) of the image points on the Hilbert curve. The paper describes the way of applying these mappings: a new algorithm is derived, generating codebooks for vector quantisation, and experimental results on image coding are provided.

The paper analyzes the effect of finite-length arithmetic in the calculation of 2-D linear transformations employed in some picture coding algorithms. Since the condition of zero-error in general direct and reverse transformations leads to results of little practical importance, an analysis is carried out on the statistical properties of error in 2-D linear transformation with given length of arithmetics. Then the important case of discrete cosine transform (DCT) applied to real images is considered in detail. The results of the paper allow a circuit designer to determine the representation accuracy of the one- and two-dimensional coefficients required to satisfy a preassigned reconstruction error on the image.

This paper describes a subjective test for comparing different video intraframe coding methods based on adaptive orthogonal transformations. A set of images were objectively measured, in terms of mean square error, by means of computer simulation using a 16×16 Discrete Cosine Transform (DCT) in the following situations: - non adaptive transform coding - adaptive coding obtained through multiplicative coefficient for the quantiser level, according to the activity classes - adaptive coding with a constant mean square error per sub-block. Subjective and objective measures show a low correlation if a non adaptive coding scheme is used, while the correlation is higher if adaptive coding schemes are used. In particular, the uniform impairment distribution (the same quantisation error for each block) supplies a very high correlation between the two measures. Subjective evaluations were carried out using a SSR (Single Stimulus Rating) method; results are presented in terms of mean and standard deviation. Analysis of variance (ANOVA) was also performed to evaluate factor and interaction contribution to variability.