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CODEST (coding for definition of significant states and transitions) is a system for the transmission of telegraph signals and digital data in coded form. CODEST is a completely "transparent" system; i.e., it restitutes the position of significant instants of a modulation whatever the speed and whatever the operating mode, whether synchronous or asynchronous. Furthermore, the telegraph distortion of restituted signals is independent of the transmission channel, and distortion is maintained below a given rate which can be as small as required. The CODEST system is the equivalent for telegraphy of the pulse-code telephone systems (pulse-code modulation) (PCM). The required bandwidth for CODEST may extend, according to the case, from 4 to 10 times the bandwidth required for frequency division transmission, but CODEST is particularly adapted to PCM links. It is well known that PCM systems compensate bandwidth expansion by such great advantages as immunity to noise, simplicity of signal regeneration, and low cost of equipment. Another essential advantage of CODEST is that it replaces data or telegraph signals by a synchronous binary sequence of fixed frequency without any strict relationship to the time base or speed of the original telegraph or data sequences.