Abstract: A receiver in a transmission line communications system, such as a telephone system, receives a carrier signal having upper and lower sidebands. Typically, the carrier signal is modulated with message information signals and the upper sideband is modulated with signaling signals. A sideband cancellation circuit is utilized to cancel the lower sideband which is modulated with the unwanted signaling signals for the upper sideband of the next lower carrier signal in a multi-channel telephone system. The modulated carrier and sideband signals are split into two separate demodulation paths such that the unwanted information signals in each path are one hundred eighty degrees out of phase and the wanted information signals are in phase. The demodulated information signals from the two paths are summed to cancel the unwanted information signals from the lower sideband.

Abstract: A power failure alarm circuit for a transmission line communications system such as a telephone system having means for transmitting and receiving carrier signals connected to a trunk line at an end central office. The alarm circuit monitors the power level of a selected number of the different frequency carriers received on the trunk line and generates an alarm condition signal when the combined power levels fall below a predetermined magnitude. The carrier signal receiving means responds to the alarm condition signal to release the subscribers. After a predetermined delay, the alarm circuit generates an alarm busy signal and the receiving means responds by forcing the trunk lines busy to prevent seizure by new callers. The alarm circuit also monitors the level of power being sent to signal repeaters connected to the trunk line and generates an enable signal when said power is lost.