Detection of recorded control signals upon reproduction from recording medium

Claim 1

In a reproducing system, apparatus for detecting a control signal of a given frequency which is recorded on and reproduced from a recording medium together with an informational signal, which control signal is within the frequency band of the informational signal, wherein the recorded amplitude of the informational signal is lower than the recorded amplitude of the control signal at and below the frequency of the control signal, comprising detecting means for detecting when a signal reproduced from a said recording medium includes a component of at least a minimum amplitude correlated to the recorded amplitude of the control signal, wherein said detecting means comprises a bistable circuit which resides in a quiescent state to produce a first amplitude output signal when not detecting a reproduced signal component having an amplitude greater than said minimum amplitude, and which resides in a non-quiescent state to produce a second amplitude output signal when detecting a signal reproduced component greater than said minimum amplitude; and determining means coupled to the detecting means to be enabled when the detecting means detect that said reproduced signal includes a component of at least a minimum amplitude, to determine whether said component of at least a minimum amplitude also has a cyclic period of at least a minimum duration correlated to the frequency of the control signal, wherein said determining means includes both an integrating circuit which in response to said second amplitude output signal produces an analog signal, the amplitude of which is directly proportional to the duration of said second amplitude output signal, and a trigger circuit for producing a response signal such as an operation-advance signal when the amplitude of said analog signal reaches a specified level.