Abstract: A speaker telephone system having hands-free telephone sets with microphones and loud-speakers and including circuitry for preventing feedback between the loud-speakers and the microphones in both the transmitting and receiving modes by automatically controlling the amplification of the signals coming from the microphones and going to the loud-speakers, as a function of the signal direction, while still maintaining the microphones and loud-speakers electrically coupled to the telephone lines.

Abstract: A solid state subscriber station network which provides transmitting, receiving and regulating functions using a single active element. The transmitted signals, the received signals and the regulatory functions can be separately varied within proscribed limits without adversely affecting the other functions.

Abstract: An emergency traffic control system is disclosed for use with emergency vehicles to control traffic signal lights. A coded transmitter employing frequency shift keying is provided in the emergency vehicle and upon actuation, transmits a coded signal to a traffic signal light which is provided with a receiver. Upon reception of a verified signal, the receiver causes the signal light to indicate a steady red in all directions. The emergency system further includes a test circuit for positively determining whether the traffic signal is under control of the emergency vehicle to prevent an accident if two emergency vehicles are simultaneously trying to control the same traffic signal light.