Abstract: In order to obtain a stereophonic coder, the levels of the signals S.sub.1, S.sub.2 relative to the two channels of a stereophonic system are modified and combined so as to generate a waveform in which the harmonics up to the 6th included have been suppressed; this is achieved by switching actions which occur at intervals of 1/8 f (f being the frequency of the subcarrier) and are such that a signal is obtained which in the successive intervals is proportional to: ##EQU1## then to the above values repeated in reverse order, then again to the values repeated in the direct order, and so on. The invention concerns also the coder performing such a process.

Abstract: For measuring the group delay of amplitude modulated transmitters a variable measurement frequency is modulated by a fixed frequency so as to obtain the sum and difference frequencies which are sent to the input of the transmitter under test; the two waves corresponding to the same side-band are obtained at the transmitter output, are frequency shifted, filtered and detected to give their beat, filtered again to obtain only the beat at a frequency twice the modulation frequency; and said beat is compared with a signal of the frequency but coherent with the modulating signal, the comparison providing a direct voltage component which is a linear function of the group delay of the side-band corresponding to the measurement frequency. The invention also concerns the apparatus for realizing the method, comprising also the known units allowing to measure the amplitude of each side band.