Abstract: 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.

Abstract: A method and apparatus for recording data on a rotating magnetic disk at plural density rates on different areas of the disk is disclosed. The method is particularly useful in magnetic disk recording systems in which the header or control fields of the recorded information are recorded in "single" density FM recording and the data fields of the recorded information are in "double" density modified frequency modulation (MFM) code. The technique involves changing the conventional MFM coding rules to prevent certain data patterns from appearing to the data recording system read circuitry as part of the control fields. In particular, the data pattern 011110 is encoded according to the invention as a data pattern which consists of 000000 with synchronization bits between the first and second, third and fourth, and fifth and sixth data bits. This encoding modification prevents any misinterpretation of the data field as part of a control field.