A method and apparatus for modulating both quadrature components of each of a plurality of RF signals comprising a multichannel signal is disclosed. By employing certain digital signal processing techniques, a pair of composite baseband signals is produced in manner that is uniquely associated with the modulation content of the RF signals. According to one aspect of the invention, by using the composite baseband signals to modulate in quadrature a single quadrature modulator, the composite baseband signals are shown to have a baseband frequency span that is substantially half of the frequency range spanned by the multichannel signal. An application for the CATV field in which a 6 MHz channel spacing between the RE signals is also disclosed.