1. A text video-transmission system comprising an information emitting center and receiving stations, the emitting center comprising: means for composing a magazine consisting of pages organized into lines of characters, if such means are not already contained in the information source, a circuit for memorizing the magazine in the form of numerical signals grouped in octets (8 binary elements), these octets comprising control octets and data octets, the control octets indicating in particular the beginnings and ends of pages and the beginnings and ends of lines, the data octets corresponding to characters contained in each line, all these control and data octets comprising a binary element of high weight which is an odd-numbered element, the octets wherein the 6th and 7th binary elements are zero being the control octets, a junction connected to the circuit, and a distribution multiplexer serving to insert the information in the lines of a television signal, this emitting center further comprising encryption means for encoding the information using an operating key, each receiving station comprising: a circuit for receiving and demodulating said television signal, a line for processing the video picture signals, a line for processing the numerical signals, containing in particular a numerical signal decoder, and display means, each receiving station further comprising decryption means for decoding the encrypted information and using said operating key, wherein, in this system: (A) the encryption means of the emitting center comprise: (a) a generator 22 delivering a numerical signal representing an operating key K, this key changing randomly at predetermined intervals, (b) automatic encryption means 24 comprising: (i) a discriminator with an input connected to the magazine memorizing circuit from which it receives clear octets, this discriminator being capable of distinguishing, among these clear octets, those wherein the 7th and 6th binary elements are zero, this discriminator having two outputs, the first carrying these octets wherein the 7th and 6th binary elements are zero and connected to the distribution multiplexer via the junction, and the second carrying the clear octets d.sub.j wherein the 7th and/or 6th binary elements are not zero, (ii) an OR-exclusive logic circuit with two inputs, one of which is connected to the second output of the discriminator from which it receives the clear octets d.sub.j wherein the 6th and/or 7th binary elements are not zero, this logic circuit having an output which carries odd-numbered encrypted octets D.sub.j, the encrypted octets then being directed toward the distribution multiplexer via the junction; (c) a generator of encoding octets controlled by the automatic encryption means from which it receives octets indicating the page and line numbers of the data to be transmitted and the signal corresponding to the operating key, this octet generator delivering, for each data octet d.sub.j of a displayable line, an encoding octet C.sub.j, this octet having an even-numbered 8th binary element and having 7th and 6th binary elements of zero, this octet being applied to the other input of the logic circuit, (B) the decryption means of each receiving station comprise: (d) a generator of a numerical signal representing the operating key K in use in the emitting center, (e) an automatic decryption means comprising: (i) a discriminator with an input connected to the numerical signal decoder of the receiving station from which it receives the coded octets, this discriminator being adapted to distinguish, among these coded octets, those wherein the 7th and 6th binary elements are zero, this discriminator having two outputs, the first carrying these octets wherein the 7th and 6th binary elements are zero, this first output being directly connected to the display means, the second carrying the coded octets D.sub.j wherein the 7th and/or 6th binary elements are not zero, (ii) an OR-exclusive logic circuit with two inputs, one connected to the second output of the discriminator from which it receives the encrypted octets D.sub.j, this logic circuit having an output which carries decrypted octets d.sub.j which are then directed towards the display means; (f) a generator of decoding octets controlled by the automatic decryption means which transmits to it the octets indicating the page and line numbers of the data transmitted and the signal corresponding to the operating key K in use, this generator of decoding octets having an output which delivers, for each coded octet received, a decoding octet C.sub.j having a heavy weight binary element which is forced to zero and 7th and 6th binary elements of zero.