Digital magnetic recording

In a system for retrieving data from a recording medium employing MFM or FM encoding forms, a decoding method and circuit employs the technique of delaying the third harmonic of the data signal relative to the fundamental of the data signal by 180 degrees. This delaying is effected by the use of two resonant networks each delaying the third harmonic relative to the fundamental by 90 degrees. The delaying technique enables a differentation stage to be eliminated.

This invention relates to a method of and to a circuit for improving 
digital magnetic recording. 
In the recovery of digital data in magnetic recording, in which the most 
common encoding forms used are MFM or FM, the following stages are 
present: 
(1) Obtain a voltage proportional to the rate of change of flux. 
(2) Amplify and filter with a low pass Bessel type filter. 
(3) Differentiate to change peaks to zero crossings. 
(4) Obtain a digital pulse from every zero crossing. 
As the gain in the differentiator stage is proportional to frequency, all 
white noise is increased resulting in a net loss of signal to noise ratio 
of approximately eight decibels. 
It is an object of the present invention to obviate or mitigate this 
disadvantage. 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 above-mentioned differentiation stage may be 
eliminated. 
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 recording medium, delaying the 
phase of the third harmonic of said data signal relative to the phase of 
the fundamental of said data signal by 180 degrees with reference to the 
frequency of said harmonic to produce a modified signal, and deriving a 
digital information-carrying pulse train from said modified signal wherein 
the delaying of said third harmonic is effected by passing the data signal 
through two resonant networks whose resonant frequency is the frequency of 
said third harmonic. 
Preferably also, said resonant networks comprise resonant low pass filters. 
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, means for 
delaying the phase of the third harmonic of said data signal relative to 
the phase of the fundamental of said data signal by 180 degrees with 
reference to the frequency of said third harmonic to produce a modified 
signal, and means for deriving an information-carrying pulse train from 
said modified signal, wherein said phase delaying means comprises two 
resonant networks whose resonant frequency is the frequency of said third 
harmonic.

Referring to the drawings, and in particular FIGS. 5 and 6, a circuit for 
recovering data in digital magnetic recording includes a magnetic reading 
head H which reads information from a magnetic recording medium such as a 
disc or tape. 
The signal derived from the head H is amplified in amplifer A and passed to 
a low pass filter F to give the waveform a shown in FIG. 6. The output of 
the filter F is passed to a zero crossing detector ZCD to give the pulse 
train shown by waveform b of FIG. 6, these pulses being passed to a first 
monostable M1 having a period of defined length to give waveform c. 
The signal thereafter branches into two paths, the first of these paths 
being to recover a coherent clock signal. To this end the waveform c is 
applied to a phase lock PLL, the output signal d of the VCO of the PLL 
being passed to a square rooter R1 to generate a clock signal e at the 
correct frequency for application to a data window comparator D. 
In its second path, the rising edge of the waveform c sets a variable 
period monostable M2 which in effect delays the signal before presentation 
to the data window comparator D for multiplication with the clock signal 
e. 
The delayed data pulse from the monostable M2 (waveform g) is passed to a 
first inverted D-latch circuit L1 where it is multiplied with the clock 
signal e. As L1 is an inverted D-latch circuit the data pulse g is fed to 
the "clock" input of L1 whereas the clock signal e is fed to the "data" 
input, the output of L1 being a latched data signal j, thus on every 
rising edge of waveform g the output of L1 (j) is set to the value 
appearing at that instant on the data input i.e. if the value of clock 
waveform e is high at that instant, then the output j is set high and if 
the value of clock waveform e is low then the output j is set low. In the 
cases where j is set high it remains so till a reset pulse is fed to the 
"reset" input. The reset input can conveniently be derived from the system 
clock d, each rising edge of the waveform d resetting the output of L1 to 
zero. 
The output of the inverted D-latch circuit L1 is synchronised in a second 
D-latch circuit L2 using the opposite phase of the system clock, that is 
waveform e' to give fully synchronised NRZ system output k. L2 is a normal 
D-latch circuit and thus the inverted clock signal e' is fed to the 
"clock" input of L2 and the latched data signal j is fed to the "data" 
input. 
It will have been noted that the decoding technique just described does not 
include the differentiation stage prior to zero crossing detection, thus 
eliminating the loss of signal to noise ratio which occurs when such a 
differentiation stage is included. However, as the differentiation in 
effect alters the relative phase of the third harmonic of the data signal 
by 180 degrees relative to the phase of the fundamental, this phase 
difference has to be restored if the differentiation stage is to be 
eliminated. 
This restoration of phase difference between the third harmonic and the 
fundamental is achieved by passing the head signal through two resonant 
networks whose resonant frequency is the frequency of the third harmonic. 
The resonant networks may take the form of resonant filters or any other 
circuit which delays the third harmonic by 90 degrees relative to the 
fundamental. 
In a preferred form one of the resonant networks is constituted by the 
reading head itself and the second network is constituted by a resonant 
filter. 
Referring to FIG. 3 there is shown in greater detail the electrical 
components of the head H of FIGS. 1 and 5. The inductance L.sub.h 
represents the given inductance of the turns of wire wound on the core and 
the head coil, by virtue of interaction of adjacent turns has a natural 
capacitance C.sub.h. To these two components there is added an additional 
capacitance C.sub.a and a damping resistor R.sub.d such that the resonant 
frequency of the head coincides with the frequency of the third harmonic 
of the data signal derived from the magnetic medium. Thus the reading head 
constitutes a second order low pass filter having a resonance at the 
frequency of the third harmonic. At the frequency of the third harmonic, 
the effect of the resonance is to delay the phase of the third harmonic by 
90 degrees relative to the phase of the fundamental. 
In FIG. 4, there is shown one design for the filter F of FIGS. 1 and 5. The 
filter F is a second order low pass filter having an inductance L.sub.f 
and capacitance C.sub.f and resistance R.sub.f, the values of the 
components being chosen such that the filter F has a resonant frequency 
coinciding with the frequency of the third harmonic of the data signal 
derived from the magnetic medium. The filter F again delays the phase of 
the third harmonic by a further 90 degrees relative to the fundamental. 
Thus at the output of the filter F there is obtained a signal having a 
bandwidth extending from the fundamental frequency to the third harmonic. 
Also, in this signal the phase of the third harmonic has been delayed by 
180 degrees relative to the phase of the fundamental. Further, the energy 
of the third harmonic has been increased, and by suitably adjusting the 
gain of the head H and filter F the relative amplitudes of the fundamental 
and the third harmonic can be optimised to reduce peak shift. It has been 
found that a third harmonic amplitude which is one third of the 
fundamental amplitude gives optimum results. 
There has been provided an improved data recovery technique in which the 
differentiation stage has been eliminated thus reducing the white noise 
normally generated in this stage. 
Modifications and improvements may be incorporated without departing from 
the scope of the invention.