An arrangement for digital encoding of a colour television video signal for transmission by delta-modulation, in which the choice of the sampling for quantising and coding an transmission of information depends upon the magnitude of the variations of the moment of the brightness signal and of the colour signal and upon the variations appearing during the next samplings. The method is used for retransmission of the signals E''Y, D''B and D''R of the SECAM system from a communication satellite. This invention relates to an arrangement for digital encoding particularly but not exclusively the brightness signals and colour signals of colour television pictures for transmission in a frequency band of moderate width in the form of a rapid succession of short signals, each having a given, elementary significance said succession being grouped in ''''words'''' of variable length by the application of pulse delta-modulation. It is known that the basic principle of delta-modulation signal transmission consists in transmitting solely the intensity variations of the signal instead of transmitting at any instant an electric magnitude representing in analogue form the required information. The variations to be transmitted result from the comparison of signal magnitudes at two different instants lying very near each other and corresponding to two consecutive signal samples. In order to avoid the cumulative errors which might result from a transmission disturbed by background noise of the successive variations of the signals to be transmitted, the corresponding information is usually transmitted in coded pulse form, which transmission mode provides the best immunity to disturbances likely to be introduced by a given level of background noise. This immunity to background noise is, however, opposed by the fact that delta pulse-code-modulation transmission in order to maintain a satisfactory picture definition requires a much wider frequency band, than the frequency band required by the direct transmission of the signals in analogue form; this is a disadvantage especially so if the signal to be transmitted is a wide band signal, such as a television video signal. Among the variants various arrangements for the transmission of information in the form of a difference signal there is disclosed in French Pat. specification No. 1,041,766 an arrangement in which the amplitude variations of a signal are transmitted in the form of successive information signals, each of which corresponds to a variation quantum selected among a given number of predetermined variation levels, each one of these levels being represented for information transmission by a suitable group of coded pulses. The quality and the fidelity of a signal transmitted with the aid of different signals and of pulse code transmission depend upon the number of quantised levels such that the use of a greater number of levels representative of the variations to be transmitted provides an improved transmission but leads to a code of longer words requiring a higher total number of pulses and a larger bandwidth for the transmission. A method providing an appreciable improvement in the quality of the pictures obtained, while a comparatively small number of representative levels is used, is disclosed in applicant''s French Pat. application filed the 31st of Dec., 1969 registered under No. 6945677. Said method allows an appreciable reduction of the frequency band required for the transmission, but the use of this method alone does not permit reducing the bandwidth required for the transmission of brightness and colour information contained in a colour television video signal to an extent as required for said transmission to be possible in a standard 8 MHz channel. The invention has for its object to reduce the frequency band occupied by the brightness and colour information of a colour television video signal with a basic interval between samplings of 100 nsec and coded transmission of information in accordance with the ''''quadriphase'''' transmission method, in a sequential system such as the SECAM system, to some 10 MHz with a picture of 625 lines and 50 frame periods a second, while an accurate time transmission of the most important transitions of brightness and chrominance is maintained. The invention is based on the recognition of the fact that the correct reproduction of a picture depends largely upon the transmission of the important variations of the brightness signal at instants as accurate as possible and that the regular sampling being random with respect to the signal it is very likely to detract from the correct reproduction of the important transitions at the receiver end. According to the invention the method of digital encoding of a video signal, particularly but not exclusively of colour television pictures, is mainly characterized in that with simultaneous sampling for information and quantisation of the instantaneous value of the brightness signal to be transmitted and the values it will have in 1 to n time steps or sampling, the choice of sampling in favour of quantisation and coding is made in accordance with the values of the variations determined and with the times required for the transmission of corresponding code words and in that information transmission priority is given to either that brightness signal or that colour signal whose variation is proportionally the most important at the instant of carrying out a given set of simultaneous samplings. In order to ensure that the transmission of code signals corresponding to the most important variations of the image contents are carried out at the optimum instants, the transmission of a small or mean variation (corresponding, for example, to a code word of two digits) preceding a transition of high amplitude is replaced by the transmission of a very small variation (code word on one digit) so that at the most suitable instant a word of three digits can be transmitted and in this way an accurate reproduction of the contours in brightness and in position is ensured. The instants of information sampling remain evenly distributed in time and provide to the circuitry the necessary indications for the quantisation, coding and transmission samplings to be carried out at the most appropriate instants, while the transmission of a very small variation (code word of one digit) may be considered as that of a waiting signal preceding the coding and the transmission of a much more important variation. In order to ensure optimum coincidence between the end of the transmission of the corresponding code word and the scanning of the place of the transition in line scan, the sampling of a great transition (code word of three digits) for quantisation and coding is advanced by a time unit relatively to the sampling for coding a mean or small transition (code word of two digits). The transmission of a very small variation by a code word of one digit is finally advanced by a time unit relatively to the transmission of a small, mean or large variation without the structure of the image at the receiver end being perceptibly affected thereby. The arrangement according to the invention permits utilising to the optimum the frequency band occupied by high-frequency signals to be transmitteD , the phase of said signals indicating the elementary information of each of them, which information may be represented in a conventional code by an arabic digit (intrinsic meaning of the term digit in data processing) or by a code letter (extended meaning of the term digit). It is known that in a normal image the information of brightness variations to be transmitted, for example, is not likely to have the same probability factor and that with a number of samples of about 500 on one line roughly 50 percent of transitions between samples represents less than 3 percent of the total scale from black to white and that less than 1
percent of the transitions corresponds to a variation of more than 30 percent. While it leads to the use of a frequency band of moderate width, the method according to the invention permits, for example, the transmission of numerous small variations expressed by a digit, which indicates at the same time that the ''''word'''' has only one digit, the transmission of mean variations of two different representative levels, which are considerably less numerous and expressed by words of two digits and the transmission of a few variations of great amplitude expressed by words of three digits. The simultaneous samplings of the actual value and the successive values of the sampled signal permit knowing in advance the great variations to be transmitted and, while inhibiting the transmission of preceding small variations, permit beginning at the desired time the transmission of words of three digits representing the same in a logical transmission code. The colour information, which provides the colouring of the image in compatible television systems and which does not directly affect the fineness of the definition, lies in a comparatively narrow pass band as compared with the brightness signal and does not require a very high transmission rate: when this transmission is performed at the instants when the colour signal variations are larger than the brightness signal variations at all instants when the transmission of the brightness signal has a comparatively doubtful efficiency the very principle of delta-modulation is utilised as best as possible so that what is not transmitted subsequent to a sampling leads to an ''''overtake'''' during the next sampling. The lower transmission rate of colour information permits using code words of three digits which is otherwise required for distinguishing them from brightness information.