GB2149190A - Reading digital data from a magnetic medium - Google Patents

Abstract

A method and circuit for data recovery in digital magnetic recording ensures that the relative phase difference between the fundamental frequency signal and each of the harmonic frequency signals follows a desired profile by feeding the signal from the head via a first resonant circuit R<1>, a first phase shift circuit P1, a second resonant circuit R2 and optionally a second phase shift circuit P2. <IMAGE>

Description

SPECIFICATION Improvements in digital magnetic recording This invention relates to a method of and to a circuit for improving digital magnetic recording.

More particularly, the invention relates to an analogue resonant filter technique for MFM signals in magnetic recording.

In the recovery of digital data in magnetic recording, in which the most common encoding forms used are MFM or FM, a read-back pulse signal is amplified, the signal filtered with a low pass linear phase filter and then the waveform is differentiated to change the peaks of the signals to the zero crossings.

Digital processing then takes place to recover the data. A standard read-back data channel is shown in Fig. 1. A number of disadvantages exist in this approach. Firstly, the "peaks" do not become accurate zero crossing points simply by passing them through a differentiator since any loss in harmonic energy results in peak shift or phase distortion. Secondly, it is not possible to build a Bessel linear phase filter with a sharp roll-off characteristic. Thus, system noise which is well beyond the required signal passband is permitted through the filter. This effect is exaggerated by the differentiator stage, in which all higher frequency noise components are increased in proportion to the frequency.

Thirdly, both the head inductance and the differentiator have finite turn-over frequencies, the first due to the natural head resonance and the second caused by design. Although these bandwidth limiting effects marginally improve filtering action, they introduce extra phase lags to the higher harmonics, which reduce the rate of change of amplitude at the ZCD. This results in higher peak shift levels.

Care must be taken in the design of the recording head in order to maximise the natural resonant frequency by minimising the self capacitance. The value of the inductance is a compromise between the natural frequency of resonance and self capacitance. This compromise limits the maximum size of the read head inductance and hence the signal size.

Further, in combination with the differentiator stage, the self capacitance of the head causes additional non-linear phase distortion of the MFM signal which exacerbates white noise and peak shift.

It is an object of the present invention to obviate or mitigate these disadvantages. More particularly, it is an object of the present invention to provide a method of and a circuit for data recovery in digital magnetic recording wherein the differentiation stage may be eliminated.

According to the present invention there is provided a method of data recovery in digital magnetic recording, wherein the relative phase difference between the fundamental frequency signal and each of the harmonic frequency signals follows a desired profile.

Further according to the present invention there is provided a method of data recovery in digital magnetic recording, comprising the steps of deriving an information-carrying data signal from a recordng medium, passing the data signal through a first resonant circuit, a phase shift circuit, and a second resonant circuit to produce a modified signal, and deriving a digital information-carrying pulse train from said modified signal.

Still further according to the present invention there is provided a circuit for data recovery in digital magnetic recording comprising means for deriving an information carrying data signal from a recording medium, a first resonant circuit, a phase shift circuit and a second resonant circuit, for producing a modified signal form said data signal, and means for deriving an information-carrying pulse train from said modified signal.

Preferably, said first and second resonant circuits have a resonant frequency in the range of three times to four times the fundamental frequency of the system.

Preferably also, said first resonant circuit formes part of the read head of the system.

Preferably also, the phase shift circuit is in the form of a high pass filter.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 illustrates a known read-back channel employing a differentiation stage; Figure 2 illustrates a read-back data channel made in accordance with the present invention; Figure 3 illustrates a preferred embodiment of a read-back data channel made in accordance with the present invention; Figure 4 shows a series of phase delay/frequency curves illustrating desired phase difference profiles for a number of frequencies; and Figure 5 shows the amplitude response of the circuit of Fig. 2.

Referring to Fig. 2 of the drawings, a data signal from the recording medium is derived from a read head H which forms part of a first resonant circuit R1 which has a resonant frequency within the range of 3F to 4F to the system fundamental frequency.

This signal is passed to an amplifier stage A which is part of a phase shift circuit P in the form of a high pass filter and thence to a second resonant circuit R2 which again has a resonant frequency in the range 3F to 4F. The output of the resonant circuit R2 is passed to a zero crossing detector ZCD to give the desired pulse train.

Referring to Fig. 3 of the drawings the data signal from recorded medium and derived from the read-back H passes through the first resonant circuit R 1 of which the read-back forms part. In an MFM recording system in which the fundamental frequency is 62.5 KHz it has been found that a suitable resonant frequency for the circuit R1 is 220 KHz. Also, the resonant circuit should have a damping coefficient of 0.6. In this embodiment the phase shift circuit is in the form of two circuits P1 and P2 each having its own amplifier Al and A2 and both being in the form of a high pass filter having a cut-off frequency of 31 KHz. Between the two phase shift circuits P1 and P2 there is connected the second resonant circuit R2 which preferably has a resonant frequency of 220 KHz and a damping coefficient of 0.3.The output of the phase shift circuit P2 is passed to the zero crossing detector ZCD to give the desired pulse train.

The arrangement of providing a two pole filter design allows a more acclerated tailoring of the phase delay/ frequency characteristics of the curves of Fig.

4.

The two resonant circuits R1, R2 serve to amplify the higher harmonics, principally 3F, thus maximising the harmonic energy and minimising peak shift. However, each of the resonant circuits results in phase delays which differ for each harmonic and the desired profile as illustrated in Fig. 4 is restored by the relative phase delay or advance which occurs in the phase shift circuits P, P1 and P2.

The read-back channel of the invention operates by defining a relative time advance or delay between the five harmonics in MFM (1, 1 1/3, 2, 3, 4F) and the two resonant circuits together with a phase shift network are designed to give a best fit to the desired phase difference profile whilst maximising the harmonic energy.

Fig. 5 illustrates the limited bandwidth response of the circuit of Figs. 2 and 3 whereby white noise is greatly supporessed thus increasing the signal-to-noise ratio of the system whilst minimising peak shift by amplifying the higher harmonic portion of the signal.

Modifications and improvements may be incorporated without departing from the scope of the invention. For example, provided the delay/frequency profile remains unaltered, as shown in the different curves of Fig. 4, no phase distortion in introduced and a number of circuit designs can be employed to effect the solution of the present invention.

Claims (12)

1. A method of data recovery in digital magnetic recording, wherein the relative phase difference between the fundamental frequency signal and each of the harmonic frequency signals follows a desired profile.

2. A method of data recovery in digital magnetic recording, comprising the steps of deriving an information-carrying data signal from a recording medium, passing the data signal through a first resonant circuit, a phase shift circuit, and a second resonant circuit to produce a modified signal, and deriving a digital information-carrying pulse train from said modified signal.

3. A method as claimed in claim 2, wherein the phase shift circuit is in the form of a high pass filter.

4. A method as claimed in either claims 2 or 3, wherein the said phase shift circuit is made up of two a high pass filters.

5. A circuit for data recovery in digital magnetic recording comprising means for deriving an information-carrying data signal from a recording medium, a first resonant circuit, a phase shift circuit, and a second resonant circuit, for producing a modified signal from said data signal, and means for deriving an information-carrying pulse train from said modified signal.

6. A circuit as claimed in claim 5, wherein said first and second resonant circuits have a resonant frequency in the range of three times to four times the fundamental frequency of the system.

7. A circuit as claimed in either claims 5 or 6, wherein said first resonant circuit forms part of the read head of the system.

8. A circuit as claimed in any one of claims 5 to 7, wherein the phase shift circuit is in the form of a high pass filter.

9. A circuit for data recovery in digital magnetic recording comprising means for deriving an information carrying data signal from a recording medium, a first resonant circuit, a first phase shift circuit, a secon resonant circuit and a second phase shift circuit for producing a modified signal from said data signal, and means for deriving an informationcarrying pulse train from said modified signal.

10. A circuit as claimed in claim 9, wherein said phase shift circuits are in the form of high pass filters.

11. A method of data recovery in digital magnetic recording substantially as herein be fore described with reference to the accompanying drawings.

12. A circuit for data recovery in digital magnetic recording substantially as hereinbefore described.