Abstract: An incoming signal (tone) and the feedback signal of a biquadratic filter are converted into square waves by zero crossing comparators. The phase state of the square waves is decoded and the decoded result is phase compared to determine if a 90.degree. phase shift exists between the incoming and feedback signals. If a 90.degree. phase shift does not exist, a counter is incremented or decremented, depending upon whether the phase comparison indicates that frequency of the incoming signal is greater, or less than the feedback signal. The output of the counter is decoded and utilized through a switching circuit to control the center frequency of the biquadratic filter (determined by the impedance of its feedback path). The switching circuit changes the impedance of the feedback path so that the center frequency is moved toward the frequency of the incoming signal.

Abstract: A link is connected between each linefinder/first selector combination of a telephone system. The links, in turn, are selectively connected to a group of ports, several links to one port, and the ports are connected to one or more telephone signal processors. When a telephone goes OFF HOOK, the sleeve or C lead of the linefinder at which that telephone's scanning switch is then pointing (determined by the associated link being connected to an available port) is grounded. When the address of this link next occurs in a time multiplex cycle, the link is latched, and the telephone's tip and ring leads are connected to the link's port. After signal processing has ended, or some other release action occurs, the port generates a release signal that disconnects the link from the port. While any link is connected to its associated port, a make-busy ground circuit forming part of eack link applies a false busy signal to the linefinders associated with the other links common to the same port.