Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
  • G11B5/02—Recording, reproducing, or erasing methods; Read, write or erase circuits therefor

Definitions

• This invention relates to magnetic recording.
• a.c. bias is a " high amplitude, high frequency signal which is added to the signal to be recorded.
• the purpose of this bias signal is to reduce or eliminate the distortion due to hysteresis of the magnetic medium.
• the desired information signal is registered on the magnetic tape using this additional high frequency, high amplitude bias signal (a.c. bias) in order to linearise the distortion problems encountered.
• a method of magnetic recording comprising: generating a signal to be recorded; modifying said signal in inverse proportion to the transfer function of the recording process; and applying said modified signal to a recording head of recording apparatus.
• the slope of the transfer function of the recording process changes with changes in signal level and the method includes modifying the signal differently for different signal levels.
• the method provides for phase advance of the harmonics in relation to the frequency of the harmonic.
• apparatus for magnetic recording comprising input means for receiving a signal to be recorded; means for modifying said signal in inverse proportion to the transfer function of the recording process; and means for applying said modified signal to a recording head of recording apparatus.
• Fig. 1 illustrates the relationship between recording flux and recorded flux drawn on a linear scale
• Fig. 2 illustrates the transfer function of the recording process drawn on a logarithmic scale
• Fig. 3 illustrates the inverse transfer function between signal input and recording head flux also drawn on a logarithmic scale
• Fig. 4a illustrates a signal to be recorded in the form of a sine-wave
• Fig. 4b illustrates a modified signal
• Fig. 4c illustrates the modified signal of Fig. 4b broken down into harmonics
• Fig. 5 is a block diagram of circuitry for implementing the method of the present invention
• Fig. 6 is a circuit diagram of circuitry for implementing the method of the present invention.
• the invention is based on eliminating the distortion characteristic which appears if recording is attempted without a.c. bias.
• This characteristic is that of a non-linear relationship between the recording flux and the recorded flux which approximates to a cubic relationship. This relationship is shown in Fig. 1 which is drawn to a linear scale.
• Fig. 2 illustrates the same relationship but drawn to a logarithmic scale.
• the non-linearity of the relationship can be described as follows:- for the first sixteen decibels of input signal, the output rises by a cubic relationship; for the next four decibels of input signal the output rises by the fifth power of the input; for the next twenty decibels of input signal the output increases in direct proportion to the input signal; and thereafter magnetic saturation occurs.
• the transfer function of the recording process does not show any hysteresis and is always single-valued.
• the output will always take the same value for a given input signal strength and is independent of the previous history of the signal, i.e. whether the signal is increasing or decreasing.
• This single-value law results from the virgin remanence magnetisation curve for all ferromagnetic materials.
• the recording signal is modified in a manner which is inverse to the recording transfer function illustrated in Fig. 2.
• essentially distortion-free recording may be achieved without using any high frequency a.c. bias.
• a recording signal modifying function which is drawn to a logarithmic scale.
• This function can be described as follows:- a first stage from 0 to 50 dB of input signal which is cube root function; a second stage from 50 dB to 70dB is a fifth root function, and final stage from 70 dB to 90 dB which is linear.
• This function is the inverse of the transfer function of the recording process shown in Fig. 2.
• the signal to be recorded is modified or contoured, in a manner inverse to that of the recording transfer function.
• the transfer function of Fig. 3 has the effect of modifying the sine-wave illustrated in Fig. 4a, to the waveform illustrated in Fig. 4b.
• the waveform in Fig. 4b can be described by a series of harmonics - that is the fundamental frequency, and the 3rd, 5th, 7th, etc. harmonics of different relative amplitudes. This is shown in Fig. 4c.
• Fig. 5 there is shown a block diagram for effecting the modifying process described above.
• the input signal V ⁇ n is applied to a modifying circuit MOD 5 which modifies the signal in a manner inverse to that of the recording transfer function to produce a modified signal V out .
• the current through the record head will generate flux in the head gap and recording will result.
• phase lag begins to take place in the record head due to the self-capacitance of the windings of the record head.
• the record head circuit will resonate and as the frequency of the harmonic approaches this resonant frequency the phase of the harmonic at that frequency will be increasingly retarded.
• phase lag progressively retards the higher harmonics of the modified signal.
• the harmonics of the signal to be recorded must be precisely in their correct position in time.
• the phase of the harmonics of the modified signal is advanced in relation to the frequency of the harmonic to compensate for the phase lag caused by the recording head self capacitance.
• FIG. 6 there is illustrated a circuit for carrying out the method of the present invention.
• the circuit comprises a pre-amplifier stage 71 which receives a signal V ⁇ n to be recorded.
• the pre-amplifier stage 71 prepares the input signal for a signal modifying stage 72.
• the signal modifying stage 72 comprises a control amplifier 721 having an input resistor RO which, in combination with the effective resistance of sub-stages 723 and 724 control the gain of the amplifier 721.
• the sub-stage 723 deals with positive signals and the sub-stage 724 deals with negative signals.
• the sub-stages 723 and 724 form part of negative feedback loops which include the lines 723a, 723b and 724a, 724b respectively.
• Each of the sub-stages comprises sixteen diode stages.
• the diode stages are identified by the reference numerals Dl to D16 and in the sub-stage 724 the diode stages are identified by the reference numerals D101 to D116.
• Each of the diode stages conducts at a different level which is controlled by the signal level appearing at the output of amplifier 721.
• Each of the diode stages has a preset bias voltage applied which holds the diode non-conducting until the output voltage from amplifier 721 reaches a specific value. For example, as the output voltage increases from 0 v to, say, 100 mv the diode Dl in the first stage starts to conduct and the gain of amplifier 721 is defined by the ratio of Rl to R0.
• the diode D2 starts to conduct and the gain is now determined by the ratio to R0 of the combined resistance of Rl and R4 in parallel. This is in fact a reduction in the gain and as the signal level increases still further, diodes D3, D4, etc. respectively start to conduct, thus further reducing the gain.
• the values of the resistors Rl, R4, R7, etc. are chosen so that the gain profile of the amplifier 721 is the inverse of the recording process transfer function so as to modify the input signal V ⁇ n in the manner described.
• phase advance stage 73 the desired frequency range is divided into six stages covering the frequency band of interest. Each of the separate stages in the phase advance stage contributes a small amount of phase advance which is in relation to the frequency of the harmonics making up the signal mod . The effect of this is to cancel the phase lag caused by the self capacitance of the recording head inductor 742.
• phase of the harmonics of the signal applied to the recording head is maintained at 90 degrees over the frequency range of interest.

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• Recording Or Reproducing By Magnetic Means (AREA)
• Digital Magnetic Recording (AREA)

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• 1989
  • 1989-06-15 EP EP19890907224 patent/EP0423166A1/de not_active Withdrawn
  • 1989-06-15 AU AU38366/89A patent/AU640612B2/en not_active Ceased
  • 1989-06-15 WO PCT/GB1989/000668 patent/WO1989012886A1/en not_active Application Discontinuation
  • 1989-06-15 JP JP50702689A patent/JPH04502976A/ja active Pending

Non-Patent Citations (1)

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