Analog signals are uniquely described by their frequency, amplitude and phase. The combination of phase and amplitude represents a vector whose magnitude is changed by changing the phase angle and/or amplitude.
In vector modulation, the vector is modulated by means of amplitude or frequency modulation and reconstructed in phase angle and magnitude at the receiving end after transmission at the transmitting end. For this purpose, however, a reference phase angle must also be transmitted, which forms the phase reference for the vector angle at the receiving end.
Such a method is used for the transmission of color difference signals in analog television. The color subcarrier forms the reference. The phase angles of all demodulated signals refer exclusively to the phase reference of the color subcarrier.