A method of transmitting on a transmission line a set of carrier signals of which at least one component represents a respective data value and the carrier signals are of different frequencies ranging from a lowest frequency to a highest frequency, in which the respective component of each carrier above the lowest frequency is adjusted on transmission in accordance with a selected relationship between that respective component and the corresponding component of the next lower frequency so as to compensate for the frequency-dependent effect of the transmission line on those components. The components may be amplitudes. The relationship may be a ratio of the respective and corresponding components. Alternatively each respective component may be adjusted in accordance with a scaling factor dependent on the previous data value presented by the corresponding component of next lower carrier frequency, the scaling factor being a ratio between a predetermined value and the said corresponding component, the predetermined value depending on the said previous data value. The components may be phases and the phase of each carrier after the first may be set from the phase of the previous lower frequency carrier augmented by the ratio of the data value in bits and the number of bits that can be carried by the respective component.