Patent ID: 6990155

Claim:
A transmitting circuit apparatus comprising: a first digital modulator and a second digital modulator for performing sigma-delta modulation on an I signal and a Q signal which are multi-valued digital baseband modulation signals, into a digital I signal and a digital Q signal, respectively, having the number of bits smaller than that of said baseband modulation signals, a first E/O converter and a second E/O converter each for converting the output signal of each of said first and second digital modulators into an optical signal having a wavelength different from each other; a first O/E converter and a second O/E converter each for converting the optical signal transferred from each of said first and second E/O converters into an electric signal; and a quadrature modulator for performing quadrature modulation on a carrier wave by the output signal of each of said first and second O/E converters, as an I signal and a Q signal, respectively.