Abstract: Coding and decoding of digital images by transformation of pixel blocks reduce the data bit rate. Transformation of blocks leads to discontinuities in brightness level for pixels bordering on block boundaries. These discontinuities are detected for each given bordering pixel by a change of sign of two level gradients respectively computed with the levels of two pixels adjacent to the bordering pixel. A brightness level of the bordering pixel is then corrected by a discontinuity correction factor evaluated from a discontinuity divergence equal to the absolute value of the difference between the brightness level of the bordering pixel and an arithmetical average of brightness levels of the two adjacent pixels, as a function of a quantization pitch associated with a block to which belongs the bordering pixel. The block is then corrected in its entirety to attenuate the differences of average brightness levels between two adjacent blocks generated by the coding and decoding.

Abstract: A method for the digital transmission of coded video signals with a fixed number of binary digits per frame dot, wherein each line of the video signal comprises a line synchronization word multiplexed during the line fly-back period, comprising using as line synchronization-word a pseudorandom sequence the self-correlation function of which is such that the synchronization word can be detected at the receiving equipment within a window of at least 3 binary digits, and stuffing each line of the video signal by eventually adding or subtracting one binary digit during the line flyback period at a fixed location in relation to the line synchronizing word.

Abstract: A matrix transform system for coding and decoding images. The system comprises means for sampling the lines and the columns of an image and forming with these samples a sample square matrix and storing the same. This sample square matrix is split into component square matrices of order 3N .times. 3N having a central part of order N .times. N. Each of these component matrices is multiplied by a first rectangular coefficient matrix of order N .times. 3N having a square central part in which the coefficients are equal to unity and two square lateral parts in which the coefficients are selectively equal to zero and a predetermined factor smaller than unity, which gives an intermediate matrix of order N .times. 3N. This intermediate matrix is multiplied by a second rectangular coefficient matrix of order 3N .times. N which is the transpose of the first rectangular coefficient matrix, which gives an output matrix of order N .times. N which is the transform of the central part of the component matrix.

Abstract: A transmission system of the kind comprising, on emission, a code converter for coding said signals into variable length words according to their occurrence probabilities, the word rate being determined by an emission clock, a synchronizing word generator operating at a frequency bound to said emission frequency, means for multiplexing words from the code converter and synchronizing-words, and an elastic memory connected to said multiplexing means, and, on reception, an elastic memory connected to said emission elastic memory, reading on emission and writing on reception being carried out at a rate determined by a transmission clock, means for detecting synchronizing-words at the output of said reception-memory and a decoder for deriving said signals from said variable length words, said system further comprising, on emission, means for supplying to said multiplexing means the drift of said emission frequency with respect to a frequency bound to said transmission frequency, and, on reception, a controlled osc