The CCITTconsideringthat on FDM systems non-linear crosstalk may cause mutual interference between the several types oftransmission channels; that the interference depends on the total loading of the FDM system; that the interference in a channel can be measured as a noticeable deterioration of the signal-to-noise ratio; that for setting realistic performance limits of interference, a conventional test signal imitating the soundprogramme channel loading is desirable.