IMPROVEMENTS IN MAGNETIC RECORDING

Abstract: There is described a method and apparatus for magnetic recording wherein the signal to be recorded is modified in inverse proportion to the transfer function of the recording process. In addition, the harmonics of the modified signal are subjected to phase advance in relation to their frequency.

Description: "Improvements in Magnetic Recording" This invention relates to magnetic recording. Hitherto, magnetic recording involved the use of a.c. bias recording. This 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. In audio and video magnetic recording, 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. The disadvantage of a.c. bias is that high frequency signals cannot be effectively recorded. According to the present invention there is provided 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. As the presence of harmonics in the modified signal is important and as these harmonics undergo phase lag in the recording head, the method provides for phase advance of the harmonics in relation to the frequency of the harmonic. Further according to the present invention there is provided 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. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- 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; and Fig. 6 is a circuit diagram of circuitry for implementing the method of the present invention. Referring to the drawings, 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. Thus, 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. Hitherto , by using a.c. bias techniques for magnetic recording there could be obtained poor high frequency performance with low distortion or there could be obtained good high frequency performance but with high distortion. All audio and video magnetic recording is subject to this compromise. In accordance with the invention, the recording signal is modified in a manner which is inverse to the recording transfer function illustrated in Fig. 2. Thus, by modifying the signal to be recorded and applying this modified signal to the record head, essentially distortion-free recording may be achieved without using any high frequency a.c. bias. Referring now to Fig. 3, there is shown 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. Thus, the signal to be recorded is modified or contoured, in a manner inverse to that of the recording transfer function. By way of example, 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. By Fourier analysis, 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. Referring now to 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 . This is applied to a recording head drive circuit 50 which first converts Qut to current and then to a flux to be recorded on the moving tape. The current through the record head will generate flux in the head gap and recording will result. However, as the frequency increases, phase lag begins to take place in the record head due to the self-capacitance of the windings of the record head. At a certain frequency 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. This phase lag progressively retards the higher harmonics of the modified signal. However, the harmonics of the signal to be recorded must be precisely in their correct position in time. Thus, in order to produce low distortion recording, 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. Referring now to 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. In the sub-stage 723 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. As the output voltage from amplifier 721 increases further, 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. The output ^ fro: the signal modifying stage 72 is applied to a head drive stage 74 through scaling resistors R39 and VR4. In parallel with the scaling resistors there is connected a phase advance stage 73 which comprises a number of capacitor resistor networks. In the 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. The net result is that the phase of the harmonics of the signal applied to the recording head is maintained at 90 degrees over the frequency range of interest. Thus, the harmonics of the modified signal are now correctly positioned in time relative to the fundamental. Modifications and improvements may be incorporated without departing from the scope of the invention.

Claims: CLAIMS 1. 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. 2. A method as claimed in Claim 1, wherein said signal modification includes generating the cube root of the signal to be recorded. 3. A method as claimed in Claim 1 or 2, wherein modifying signal includes generating the 5th root of the signal to be recorded. 4. A method as claimed in any preceding claim, wherein the signal is modified by means of a transfer function the slope of which differs for different signal levels. 5. A method as claimed in any preceding claim and including the step of first determining the transfer function of the recording process and modifying said signal in inverse proportion to the determined transfer function. 6. A method as claimed in any preceding claim and including the step of altering the phase of the modified signal in relation to the frequency of the harmonics of the signal. 7. A method as claimed in Claim 6, wherein the signal is phase advanced to compensate for the phase lag caused by the self-capacitance of the inductance of the recording head. 8. A method as claimed in any preceding claim, wherein the modified signal comprises a fundamental and a number of higher frequency harmonics and the phase of each harmonic is advanced in relation to its frequency. 9. A method as claimed in Claim 1, wherein the modified signal comprises a number of components including a fundamental and a number of higher frequency harmonics and the phase of each component is advanced to compensate for the phase lag to which the component is subjected as a result of the self- capacitance of the inductance of the recording head. 10. 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. 11. Apparatus as claimed in Claim 10, wherein said means for modifying said signal includes a plurality of gain modifying stages, each gain modifying stage being responsive to a different signal level. 12. Apparatus as claimed in Claim 10 or 11 and further including means for advancing the phase of the harmonics of the modified signal in relation to the frequency of the harmonics. 13. Apparatus as claimed in Claim 12, wherein said phase advance means comprises a number of phase advance stages each responsive to a different frequency band. 14. A method of magnetic recording substantially as H hereinbefore described with reference to the accompanying drawings. 15. Apparatus for magnetic recording substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: PETTIGREW ARCHIBALD MCGILVRAY, PETTIGREW, ARCHIBALD, MCGILVRAY
Applicants: PAISLEY COLLEGE TECHNOLOGY
Application year: 1989
Main classification: G11B5
IPC codes: G11B5