1. A process for realizing a stereophonic coder, characterized in that it consists in employing signals S.sub.1 and S.sub.2, already pre-emphasized, relative to the left and respectively the right channel of a stereophonic system, by modifying and combining their levels in successive time intervals so as to obtain a waveform without the harmonics up to the 6th one included; this result being obtained by switching actions occurring at time intervals equal to 1/8f, f being the frequency of the subcarrier, in such a way that in the successive intervals a signal S.sub.u is generated proportional to the following values: ##EQU4## which are then repeated in reverse order, then again in the same order and so on.
2. A process according to claim 1, characterized in that said frequency f is 38 kHz.
3. A process according to claims 1 or 2, characterized in that the pilot signal of the stereophonic coder is obtained by employing switching actions which follow one another at time intervals of 1/152000 sec so as to give sequential levels proportional to the following values: ##EQU5## these values being then repeated in reverse order, in direct order, in reverse order and so on.
4. A stereophonic coder characterized in that it comprises more than two switches synchronously driven and associated to voltage or current dividers so as to obtain the suppression of the harmonics up to the 6th one by applying the process according to claim 3.
5. A stereophonic coder according to claim 4, characterized in that it comprises switching and dividing means arranged to generate the pilot signal through a multilevel switching, eliminating the harmonics of said signal up to the 6th one included.
6. A stereophonic coder according to claim 5, characterized in that the switching actions which serve to generate the pilot signal follow one another at a rate of 1/152000 sec.