| 摘要 |
<p>A decoder is provided for four channel FM stereophonic composite signals including a first pilot signal, a main channel signal, a first sub-channel signal suppressed carrier amplitude modulated on a first subcarrier having a frequency twice that of the first pilot signal, a second sub-channel signal suppressed carrier amplitude modulated on a second subcarrier having the same frequency as the first subcarrier and having 90.degree. phase difference with respect to the first subcarrier, a third subchannel signal suppressed carrier amplitude modulated on a third subcarrier having a frequency twice that of the first subcarrier, and a second pilot signal indicative of the four channel stereophonic signal broadcasting mode and with a frequency interposed between the frequency bands of the first and third subcarriers; which decoder comprises a control signal generator controlled by the first pilot signal and producing first and second control signals in predetermined phase relation to the first pilot signal and with frequencies that are respectively equal to, and twice the frequency of the first pilot signal, a signal combining circuit supplied with the first and second control signals for producing a switching signal, and a switching circuit controlled by the switching signal for detecting the second pilot signal in the four channel FM stereophonic composite signals. When the second pilot signal is thus detected, signals from the control signal generator are employed for demodulating the four channel FM stereophonic composite signals into the corresponding four discrete audio signals; whereas, when the second pilot signal is not detected, for example, in the case of the reception of two channel stereophonic broadcast transmission, the aforesaid second control signal is employed for demodulating the two channel FM stereophonic composite signals into the two corresponding discrete audio signals.</p> |
