Read circuit providing two different reference levels for reading the

Abstract

A hard disk drive is disclosed in which the disk has tracks each divided into alternating data sectors and servo sectors, with the control data such as track addresses on the servo sectors being normally recorded more stably than the main data on the data sectors. In order to obtain read pulses representative of the peaks of the output waveform of a transducer, this waveform is directed on one hand into a differentiator circuit and thence into a wave shaping circuit for the provision of data pulses which rise and decay in synchronism with the peaks of the transducer output waveform. On the other hand, the transducer output waveform is sent into a gate pulse circuit, in which the waveform is compared with a reference signal in order to provide gate or window pulses. A logic circuit is provided for ascertaining whether the data pulses rise and decay within the durations of the gate pulses and, if they do, for producing read pulses indicative of the rises and decays of the data pulses. The duration of the gate pulses is made shorter for recovery of the control data than for main data recovery, with a view to more precise detection of the peaks of the transducer output waveform representative of the control data.