a u03c0 occurs and since the circuitry is polarity sensitive as will be described hereinafter in further detail the receiver r a u03c0 will not respond and a false call on will be prevented it can be further appreciated that in the situation of interference from the receiver transmitter pair at one location with the receiver transmitter pair at another location the frequency and phase designations shown in fig1 provide for an accurate solution as can best be explained by way of an example it should first be recognized that due to the low number of different carrier frequencies being used a legitimate safety concern regards the possibility of a transmitter falsely energizing a receiver at another block location a receiver for example r b u03c0 2 shown at point 2 is synchronized with its complementary transmitter t b u03c0 2 shown at point 3 however since transmitter t bo at point 5 is of the same carrier frequency under certain conditions receiver r b u03c0 2 at point 2 could see the signal from transmitter t bo as will be described hereinafter in further detail and with the aid of the signal comparisons of fig5 b the signal from transmitter t bo will be 90 u00b0 out.