AU640612B2 - Improvements in magnetic recording - Google Patents

Description

OPI DATE 12/01/90 AOJP DATE 15/02/90 APPLN- ID 38366 89 PCT NUMBER PCT/GB89/00668 PCr INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: WO 89/12886 Gl1B 5/02, 20/10, 5/035 Al (43) International Publication Date: 28 December 1989 (28.12.89) (21) International Application Number: PCT/GB89/00668 (81) Designated States: AT (European patent), AU, BE (European patent), BR, CH (European patent), DE (European (22) International Filing Date: 15 June 1989 (15.06.89) patent), DK, Fl, FR (European patent), GB (European patent), IT (European patent), JP, KR, LU (European pat NL Eur n ptent), NO, SE (European pa- Priority data: ten 8814723.6 21 June 1988 (21.06.88) GB 8908975.9 20 April 1989 (20.04.89) GB W Published With international search report.

(71) Applicant (for all designated States except US): THE Before the expiration of the time limit for amending the GOVERNORS OF PAISLEY COLLEGE OF TECH- claims and to be republished in the event of the receipt of NOLOGY [GB/GB]; High Street, Paisley, Renfrewshire amendments.

PA! 2BE (GB), (72) Inventor; and Inventor/Applicant (for US only): PETTIGREW, Archibald.

McGilvray [GB/GB]; "West Glen", West Glen Road, Kilmacolm, Renfrewshire PA13 4DE (GB).

(74) Agent: PACITTI, Pierpaolo; Murgitroyd and Company, Mitchell House, 333 Bath Street, Glasgow G2 4ER (GB).

(54)Title: IMPROVEMENTS IN MAGNETIC RECORDING (57) 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.

1 "ImDrovements in Magnetic Recording" 2 3 This invention re.ates to magnetic recording.

4 Hitherto, magnetic recording involved the use of a.c.

6 bias recording. This a.c. bias is a high amplitude, 7 high frequency signal which is added to the signal to 8 be recorded. The purpose of this bias signal is to .9 reduce or elimina'e the distortion due to hysteresis of the magnetic medium. In audio and video magnetic 11 recording, the desired information signal is registered 12 on the magnetic tape using this additional high 13 frequency, high amplitude bias signal bias) in 14 order to linearise the distortion problems encountered.

16 The disadvantage of a.c. bias is that high frequency 17 signals cannot be effectively recorded.

S..18 S 19 According to'the present invention there is provided a method of magnetic recording comprising: generating an S: 21 analogue signal to be recorded; generating from said 22 analogue signal a recording flux on a recording head of :23 recording apparatus, and transferring said flux to a 24 recording medium to produce a remanant recording flux on said medium, characterised in that said analogue _1: ro 1 signal is modified in inverse proportion to the 2 transfer function of the recording process, said 3 transfer function representing the non-linear 4 relationship between the recording flux from the head and the recorded flux remanent on the recording medium 6 and applying said modified signal to the recording head 7 without using a bias signal.

8 9 The slope of the transfer function of the recording process changes with changes in signal level and the 11 method includes modifying the signal differently for 12 different signal levels.

13 14 As the presence of harmonics in the modified signal is important and as these harmonics undergo phase lag in 16 the recording head, the method provides for phase 17 advance of the harmonics in relation to the frequency 18 of the harmonic'.

19 Further according to the present invention there is 21 provided apparatus for magnetic recording comprising 22 input means for receiving an analogue signal to be 23 recorded, means for generating from said analogue 24 signal a recording flux in a recording head of said recording apparatus, said recording head transferring 26 said flux to a recording medium to produce a remanent 27 recording flux on said medium, characterised in that 28 the apparatus includes means for modifying said signal S29 in inverse proportion to thetransfer function of the 30 recording process, said transfer function representing 31 the non-linear relationship between the recording flux 32 from the head and the recorded flux remanent of the 33 recording medium, and means for applying said modified 34 signal to the recording head without using a bias signal.

S36 2 L P 1 Embodiments of the present invention will now be 2 described, by way of example, with reference to the 3 accompanying drawings, in which:- 4 Fig. 1 illustrates the relationship between 6 recording flux and recorded flux drawn on a 7 linear scale; 8 Fig. 2 illustrates the transfer function of 9 the recording process drawn on a logarithmic scale; 11 Fig. 3 illustrates the inverse transfer 12 function between signal input and recording 13 head flux also drawn on a logarithmic scale; 14 Fig. 4a illustrates a signal to be recorded in the-form of a sine-wave; 16 17 2 2a WO 89/12886 PCT/GB89/00668 1 Fig. 4b illustrates a modified signal; 2 Fig. 4c illustrates the modified signal 3 of Fig. 4b broken down into harmonics; 4 Fig. 5 is a block diagram of circuitry for implementing the method of the 6 present invention; and 7 Fig. 6 is a circuit diagram of circuitry 8 for implementing the method of the 9 present invention.

11 Referring to the drawings, the invention is based on 12 eliminating the distortion characteristic which appears 13 if recording is attempted without a.c. bias. This 14 characteristic is that of a non-linear relationship between the recording flux and the recorded flu:: which 16 approximates to a cubic relationship: This 17 relationship is shown in Fig. 1 which is drawn to a 18 linear scale.

19 Fig. 2 illustrates the same relationship but drawn to a 21 logarithmic scale. The non-linearity of the 22 relationship can be described as follows:- for the 23 first sixteen decibels of input signal, the output 24 rises by a cubic relationship; for the next four decibels of input signal the output rises by the fifth 26 power of the input; for the next twenty decibels of 27 input signal the output increases in direct proportion 28 to the input signal; and thereafter magnetic 29 saturation occurs.

31 Thus, the transfer function of the recording process 32 does not show any hysteresis and is always 33 single-valued. The output will always take the same 34 value for a given input signal strength and is independent of the previous history of the signal, i.e.

WO 89/12886 PCT/GB89/006 68 4- 1 whether the signal is increasing or decreasing. This 2 single-value law results from the virgin remanence 3 magnetisation curve for all ferromagnetic materials.

4 Hitherto, by using a.c. bias techniques for magnetic 6 recording there could be obtained poor high frequency 7 performance with low distortion or there could be 8 obtained good high frequency performance but with high 9 distortion. All audio and video magnetic recording is subject to this compromise. In accordance with the 11 invention, the recording signal is modified in a manner 12 which is inverse to the recording transfer function 13 illustrated in Fig. 2. Thus, by modifying the signal 14 to be recorded and applying this modified signal to the record head, essentially distortion-free recording may 16 be achieved without using any high frequency a.c. bias.

17 18 Referring now to Fig. 3, there is shown a recording 19 signal modifying function which is drawn to a logarithmic scale. This function can be described as 21 follows:- a first stage from 0 to 50 dB of input 22 signal which is cube root function; a second stage 23 from 50 dB to 70dB is a fifth root function, and final 24 stage from 70 dB to 90 dB which is linear. This function is the inverse of the transfer function of the 26 recording process shown in Fig. 2.

27 28 Thus, the signal to be recorded is modified or 29 contoured in a manner inverse to that of the recording transfer function.

31 32 By way of.example, the transfer function of Fig. 3 has 33 the effect of modifying the sine-wave illustrated in 34 Fig. 4a, to the waveform illustrated in Fig. 4b.

By Fourier analysis, the waveform in Fig. 4b can be WO 89/12886 PCT/GB89/00668 1 described by a series of harmonics that is the 2 fundamental frequency, and the 3rd, 5th, 7th, etc.

3 harmonics of different relative amplitudes. This is 4 shown in Fig. 4c.

6 Referring now to Fig. 5, there is shown a block diagram 7 for effecting the modifying process described above.

8 The input signal Vin is applied to a modifying circuit 9 MOD 5 which modifies the signal in a manner inverse to that of the recording transfer function to produce a 11 modified signal Vout.

12 13 This is applied to a recording head drive circuit 14 which first converts Vout to current and then to a flux to be recorded on the moving tape.

16 17 The current through the record head will generate flux 18 in the head gap and recording will result.

19 However, as the frequency increases, phase lag begins 21 to take place in the record head due to the 22 self-capacitance of the windings of the record head.

23 24 At a certain frequency the record head circuit will resonate and as the frequency of the harmonic 26 approaches this resonant frequency the phase of the 27 harmonic at that frequency will be increasingly 28 retarded.

29 This phase lag progressively retards the higher 31 harmonics of the modified signal. However, the 32 harmonics of the signal to be recorded must be 3? precisely in their correct position in time. Thus, in 34 order to produce low distortion recording, the phase of the harmonics of the modified signal is advanced in WO 89/12886 PCT/GB89/00668 1 relati,* to the frequency of the harmonic to compensate 2 for the phase lag caused by the recording head self 3 capacitance.

4 Referring now to Fig. 6, there is illustrated a circuit 6 for carrying out the method of the present invention.

7 8 The circuit comprises a pre-amplifier stage 71 which 9 receives a signal Vin to be recorded. The pre-amplifier stage 71 prepares the input signal for a 11 signal modifying stage 72.

12 13 The signal modifying stage 72 comprises a control 14 amplifier 721 having an input resistor RO which, in combination with the effective resistance of sub-stages 16 723 and 724 control the gain of the amplifier 721. The 17 sub-stage 723 deals with positive signals and the 18 sub-stage 724 deals with negative signals. The 19 sub-stages 723 and 724 form part of negative feedback loops which include the lines 723a, 723b and 724a, 724b 21 respectively.

22 23 Each of the sub-stages comprises sixteen diode stages.

24 In the sub-stage 723 the diode stages are identified by the reference numerals Dl to D16 and in the sub-stage 26 724 the diode stages are identified by the reference 27 numerals D101 to D116.

28 29 Each of the diode stages conducts at a different level which is controlled by the signal level appearing at 31 the output of amplifier 721.

32 33 Each of the diode stages has a preset bias voltage 34 applied which holds the diode non-conducting until the output voltage from amplifier 721 reaches a specific WO 89/12886 PCT/GB89/00668 7 1 valuae. For example, as the output voltage increases 2 from 0 mv to, say, 100 mv the diode Dl in the first 3 stage starts to conduct and the gain of amplifier 721 4 is defined by the ratio of Ri to RO.

6 As the output voltage from amplifier 721 increases 7 further, the diode D2 starts to conduct and the gain is 8 now determined by the ratio to RO of the combined 9 resistance of R1 and R4 in parallel. This is in fact a reduction in the gain and as the signal level increases 11 still further, diodes D3, D4, etc. respectively start 12 to conduct, thus further reducing the gain. The values 13 of the resistors R1, R4, R7, etc. are chosen so that 14 the gain profile of the amplifier 721 is the inverse of the recording process transfer function so as to modify 16 the input signal Vin in the manner described.

17 18 The output V7od from the signal modifying stage 72 is 19 applied to a head drive stage 74 through scaling resistors R39 and VR4. In parallel with the scaling 21 resistors there is connected a phase advance stage 73 22 which comprises a number of capacitor resistor 23 networks.

24 In the phase advance stage 73, the desired frequency 26 range is divided into six stages covering the frequency 27 band of interest. Each of the separate stages in the 28 phase advance stage contributes a small amount of phase 29 advance which is in relation to the frequency of the harmonics making up the signal Vmo d The effect of 31 this is to cancel the phase lag caused by the self 32 capacitance of the recording head inductor 742.

33 34 The net result is that the phase of the harmonics of the signal applied to the recording head is maintained WO 89/12886 PCT/GB89/0066 8 8 1 at 90 degrees over the frequency range of interest.

2 3 Thus, the harmonics of the modified signal are now 4 correctly positioned in time relative to the fundamental.

6 7 Modifications and improvements may be incorporated 8 without departing from the scope of the invention.

9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34

Claims (4)

1. 4. A method as claimed in Claim 1 to 3, wherein the modified signal comprises a number of components 27 including a fundamental and a number of higher frequency harmonics and the phase of each component is 29 advanced in relation to its frequency to compensate for the phase lag to which the component is subjected as a "1 result of the self- capacitance of the inductance of 32 the recording head. 2 5. Apparatus for magnetic recording comprising input means for receiving ai. analogue signal to be 3S recorded, means for generating from said analogue 1 signal a recording flux in a recording head 2 of said recording apparatus, said recording head 3 transferring said flux to a recording medium to produce 4 a remanent recording flux on said medium, characterised in that the apparatus includes means for modifying 6 said signal in inverse proportion to the transfer .7 function of the recording process, said transfer 8 function representing the non-linear relationship 9 between the recording flux from the head and the recorded flux remanent of the recording medium, and 11 means for applying said modified signal to the 12 recording head without using a bias signal. 13 14 6. Apparatus as claimed in Claim 5, wherein said means for modifying said signal includes a plurality of 16 gain modifying stages, each gain modifying 17 stage being responsive to a different signal 18 level. 19
2. 7. Apparatus as claimed in Claim 5 or 6 including means 221 for advancing the phase of the harmonics of the modified signal in relation to the frequency of the 23 hazn.onics.
3. 8. ',pparatus as claimed in Claim 7, wherein said phase advance means comprises a number of phase advance S27 stages each responsive to a different frequency band. 2S.
4. 29. 9. A method of magnetic recording substantially as hereinbefore described with reference to the accompanying drawings. Apparatus for magnetic recording substantially as hereinbefore described with reference to the accompanying drawings. DATED this ELEVENTH day of JUNE 1993 THE GOVERNORS OF PAISLEY COLLEGE OF TECHNOLOGY Applicant. Wray Associates Perth, Western Australia Patent Attorneys for the Applican. .,4