| 摘要 |
Apparatus for reading data from a rotating magnetic disk whereon data has been recorded at either "single" or "double" density rate, or at both rates on different sectors of the same disk. Density rate information is used to control the frequency of a timing signal generated by a phase locked loop (PLL). The PLL times the reading of data according to a synchronizing signal recorded on the disk. To accommodate the bit shift associated with such plural density data recording, the phase locked loop is provided with a dual gain low pass filter. A high gain is used in the phase locked loop filter until lock-in is achieved, to increase acquisition range and reduce acquisition time; the gain of the loop filter is reduced upon acquisition, to provide a greater lock-in range and to reduce system sensitivity to bit shift of the data, thereby obviating the need for pre-compensation of the bit recording positions.
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