Magnetic disk device

A magnetic disk reading apparatus utilizing a modulation/demodulation system for recording and reproducing data wherein a data field of a recorded sector includes a demodulation start synchronization mark, indicative of a demodulation start position of said data field, having a predetermined pulse train pattern including (i) a first plurality of pulse trains having a frequency equal to one half of frequency of a clock synchronization PLOSYNC signal, and (ii) a second plurality of pulse trains, interspersed with said first plurality of pulse trains, having a frequency equal to said frequency of said clock synchronization PLOSYNC signal, said apparatus comprising:

• (a) means for demodulating data recorded in said data field of said recorded sector in accordance with a demodulation start signal;
• (b) means, for generating said demodulation start signal, comprising means for (i) detecting said demodulation start synchronization mark, (ii) dividing said demodulation start synchronization mark into N blocks during one period of said PLOSYNC signal, (iii) determining whether said block-divided demodulation start synchronization mark completely coincides with said predetermined pulse train pattern, (iv) determining whether a predetermined number M (M<N) of said blocks of said block-divided demodulation start synchronization mark coincide with said predetermined pulse train, and (v) generating said demodulation start signal when either said block-divided demodulation start synchronization mark completely coincides with said predetermined pulse train pattern or said predetermined number M of said blocks of said block-divided demodulation start synchronization mark coincides with said predetermined pulse train pattern.