Time base control system for a spindle servo

Claim 1

A time base control system for a rotating recording medium, comprising: means for rotating a recording medium; accessing means for accessing recording tracks on said rotating recording medium and having a detection point on said recording medium, said accessing means providing a detection signal corresponding to a relative speed between said detection point and a circumferential position of said rotating recording medium; a source of a reference signal; means for generating an error signal in accordance with a comparison between said reference signal and said detection signal, said error signal having first components with a first characteristic and second components with a second characteristic, different from said first characteristic; means for jumping said detection point radially across tracks of said rotating recording medium; tracking means for causing said accessing means to follow a selected one of said tracks of said rotating recording medium; time base correction means for correcting, in accordance with said error signal and said reference signal, a time base according to which said rotating means rotates said recording medium; coarse tuning means responsive to said error signal for correcting said time base, said coarse tuning means including a first servo loop passing through said rotating means; fine tuning means responsive to said error signal for correcting said time base, said fine tuning means including a second servo loop not passing through said rotating means, said fine tuning means being capable of controlling said time base over a variable range thereof; and control means operative during said jumping mode for alternately operating said tracking means and said jumping means, said control means widening said variable range of said fine tuning means and gradually increasing a supply of said second components of said error signal under a predetermined level to said coarse tuning means, whereby rough tuning of said time base is performed according to both said first and said second components of said error signal.