Patent ID: 8213437

Claim:
A method of transmitting a plurality of modulated signals, the method comprising: modulating a plurality of carrier waves having frequencies, each frequency being set at ½ n (n is a variable positive integer such as 0, 1 or 2) of a reference frequency, with respective transmission signals different from one another to produce a plurality of modulated signals having respective modulated carrier waves; and sending the modulated signals to a transmission line common to the modulated signals to transmit the modulated signals, multiplexed by frequency division multiplexing, to a synchronous detector in which synchronous detection is performed for the carrier waves by receiving the modulated signals of the transmission line as an input signal, calculating a moving average of the input signal every sampling period of time equal to one-half or one-fourth of a reference cycle corresponding to the reference frequency, and performing an addition and subtraction calculation, corresponding to a cycle of one carrier wave, for the moving averages for each of the carrier waves, wherein each of the transmission signals modulates the corresponding carrier wave such that, as the signal level of the transmission signal is lowered, the frequency of the carrier wave modulated with the transmission signal is lowered.