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An externally modulated transmitter was linearized for AM subcarrier multiplexing (AM-SCM) video transmission. Linearization was accomplished by combining the outputs of two lithium niobate Mach-Zehnder interferometers which were simultaneously modulated. By controlling the optical and RF power at each modulator, more than 20 dB of third-order distortion cancellation was demonstrated for a multitone video spectrum. Results are presented for both NTSC and PAL B/G frequency plans. The effects of modulator nonideality on system performance are analyzed and confirmed experimentally. In particular, overall system performance is shown to be very sensitive to acoustic effects and to optical and electrical multipath interference. System modeling is used to provide tolerances on various critical parameters.