摘要 |
PURPOSE:To remove the influence slight displacement in the tuning frequency of the filter by using a filter generating change in the transmission phase characteristics at substantially intermediate frequency of both the modulated frequencies in order to detect the modulated frequency of the control signal. CONSTITUTION:The rectifying output voltage level of the control signal (b) is discriminated by the comparison switching circuit 21 to obtain the change-over signal (m). The amplitude detector 4 of output (c) is fed to the filter circuit 22 generating the change in transmission phase charactristics between 922 5Hz and 982, 5Hz, thereby the change over signal (n) for discriminating whether the modulated frequency is 922.5Hz or 982.5Hz. These two change-over signals (m), (n) are fed to the change-over switch circuit 13 to operate the desired control signal discrimination and when the level of the control signal does not reach the predetermined level, it is enforcibly monoral operated. Accordingly, if there is change slightly in the filter characteristics, both the modulated frequencies can be effectively discriminated. |