US 4,231,071 A · Reader for data recorded on magnetic disks at plural densities

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.

Claims19

Dependent claims

In apparatus for reading data from a rotating magnetic disk whereon binary data has been recorded on a plurality of data fields, the data being recorded on each data field at a known density rate selected from among a set of substantially differing density rates, and wherein a control signal is provided to select the density rate for reading data from each field, the improvement comprising:

said control signal comprising, for each data field, a density-selection signal selected from among a plurality of available signals, each of which represents one of the available density rates of the set;
and
means responsive to the control signal for reading each data field at the density rate represented by the density-selection signal for that field, the density rate for reading each data field thereby being independent of the density rate for reading each other data field,
whereby any combination of data fields on a disk record may be read in any arbitrary, desired order, in a single pass over the disk, regardless of the density rate variations from one data field to another.

1 more element

In apparatus for reading data from a rotating magnetic disk whereon binary data has been recorded on a plurality of data fields, each of said data fields having data thereon recorded at a known density rate selected from among a set of available density rates, the improvement comprising:

means responsive to a control signal for reading each of said data fields independently of the other data fields, at one of said available rates;
said control signal comprising one signal selected from among a plurality of available signals, each of which is associated with one of said available density rates;
the means for reading including a phase locked loop for synchronously detecting the recorded data;
the phase locked loop including a variable frequency oscillator which operates at a nominal frequency or bit rate which is at least as high as the highest density rate of the set of density rates, and wherein the other available density rates in said set of density rates are integral submultiples of the highest rate;
the phase locked loop providing a synchronization signal in phase with the recorded data and having a frequency corresponding to the density rate associated with the control signal, the synchronization signal being provided by the output of a frequency divider which is driven by the output of the variable frequency oscillator, the frequency divider comprising a divide-by-N counter including means for selecting the division factor N responsive to said control signal.

2 more elements

A phase error detector for use in a phase locked loop for comparing the phase difference between a pair of signals comprising digital pulse trains having nominally the same frequency but varying in relative phase, the phase error detector comprising

(a) means for comparing the phase of a pulse in the first pulse train with a corresponding pulse in the second pulse train;
(b) means for providing an output signal of a first type responsive to the detection of said pulse in the first pulse train prior to the detection of the corresponding pulse in the second pulse train;
(c) means for providing an output signal of a second type responsive to the detection of said pulse in the second pulse train prior to the corresponding pulse in the first pulse train;
and
(d) said means for providing the output signal of the second type including means for suppressing the output signal of the second type until the detection of the corresponding pulse in the first pulse train, whereby the output signal of the second type is not provided until and unless the corresponding pulse in the first pulse train is detected.

2 more elements

An apparatus for reading data from a magnetic disk whereon binary data has been recorded on a plurality of data fields, each of said data fields having data thereon recorded at a known density rate selected from among a set of available substantially differing density rates, and including means for reading the data recorded in each data field at said known density rate, the means for reading including a phase locked loop for synchronously detecting the recorded data, the phase locked loop providing a synchronization signal locked in phase with the recorded data and, for each data field, having a frequency corresponding to the density rate of the data recorded therein, the improvement comprising:

the phase locked loop including a phase error detector for comparing the phase difference between a pair of signals comprising digital pulse trains having nominally the same frequency but varying in relative phase, such phase error detector comprising:
A. means for comparing the phase of a pluse in the first pulse in the first pulse train with a corresponding pulse in the second pulse train;
B. means for providing an output signal of a first type responsive to the detection of said pulse in the first pulse train prior to the detection of corresponding pulse in the second pulse train;
C. means for providing an output signal of a second type responsive to the detection of said pulse in the second pulse train prior to the corresponding pulse in the first pulse train;
and
D. said means for providing the output signal of the second type including means for suppressing the output signal of the second type until the detection of the corresponding pulse in the first pulse train, whereby the output signal of the second type is not provided until and unless the corresponding pulse in the first pulse train is detected.

3 more elements

Dependencies are inferred from the claim text.

Reader for data recorded on magnetic disks at plural densities

Life of this patent

Reader for data recorded on magnetic disks at plural densities

Life of this patent

Abstract

Claims19