08_principles of magnetic tape

8/12/2019 08_Principles of Magnetic Tape

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Principles of Magnetic Tape Recording

8PRINCIPLES OFMAGNETIC TAPE RECORDING

Introduction

Magnetic tape recording system has got many special features, which makes it unique inSound Broadcasting, Television and Computer field. These are :

i! "nstant and simultaneous replay during recording.

ii! The recording medium i.e., the magnetic tape can #e used again and again aftererasing the previous recordings, which generally takes place along with therecording of the new programme.

iii! The editing is simple and accurate. This can also #e done electronically, withoutphysically cutting the tape.

These facilities com#ined with e$cellent quality and relia#ility has made magneticrecording system very popular in the field of entertainment and all direct recordings arefirst done on magnetic tape.

The System

The magnetic tape recording system may #e studied under three su#systems :

a! The magnetic system comprising the magnetic tape plus record, replay anderase heads.

#! Tape transport system comprising the two spooling motors, the capstan motor,the #rake mechanism and the control cum interlock system.

c! The electronic system comprising the amplifiers equalisers and power suppliesetc.

a) The Magnetic System

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Induction Course (General)

or #etter understanding of the magnetic system, a review of the magnetic principles, asapplica#le in the magnetic system is necessary and is attempted in the following su#sections.

Magnetic Permeability

-hen magnetic materials like iron, co#alt, nickel, manganese and some of theircompounds are #rought in a magnetic field, the num#er of flu$ lines that pass through across section of such materials will #e much greater than those passing through vacuumor air. Such materials are said to have high permea#ility as compared to air or vacuumwhich is taken as unity. The permea#ility is also defined as the ratio of magnetic

induction to the magnetic field producing the magnetic induction. "t is denoted #y .

)(

)(

oerstedHFieldMagnetic

gaussBInductionMagnetico =

erro magnetic materials are those whose permea#ility is very much greater than that ofair or vacuum and their permea#ility values may differ to a very great e$tent. Thus thepermea#ility of transformer limitations may #e from '' to ',''' and that for metal/','''. &ermea#ility for special magnetic materials called &01M23345 can #e high as6,'','''. errites, 2lperms, 2lnicos etc. used in the manufacture of magnetic headshave very high permea#ility. Strictly speaking all materials affect magnetic field.Materials having a permea#ility only slightly higher than vacuum, are called paramagnetic, the others less than that of vacuum are diamagnetic.

Hysteresis

7ormally, the permea#ility of a magnetic material is the ratio B)8, when the material hasno previous magnetic history. 8owever, if we plot a B)8 curve for various values of 8, itwill not #e a straight line, i.e. the permea#ility is not constant. "f 8, the magnetising field,is increased from 9ero in one direction and magnetic induction measured, it will #efound to rise slowly and in proportion to the magnetic field, then it will rise rather quickly,then it will slow down until a point is reached at which any further increase will produceno further increase in the magnetic induction. This is called saturation induction, if thematerial has never #een su#ected to magnetisation earlier, i.e. has no magnetic history.This curve will #e called normal or initial magnetisation curve %"MC!. "f the magnetisingfield is now reduced from this saturation value to 9ero, the induction will not #e reducedto 9ero #ut takes a value B as in fig.6. This is called, 1emanance or magnetic inductionremaining #ehind after the magnetising field is removed. To reduce the a#ove

remanance to 9ero, a magnetising field in the reverse direction is required to #e applied,as denoted #y point C. this reverse magnetic field required to remove the remanantmagnetism of the matrial is called coercivity. "f the magnetic field applied in the reversedirection is further increased, the magnetic induction will reach saturation in reversedirection, denoted #y point ;. this reverse field, if further reduced to 9ero, a remanantmagnetic induction will occur in the reverse direction denoted #y point 0. if the field isagain increased in the forward direction, it reached the saturation point 2 again, thoughtracing a different path. "f the magnetising cycle is repeated in forward and reversedirection continuously, the magnetic induction will follow the closed path 21C;2. This

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is called the 8ysteresis cycle and the curve is called the 8ysteresis loop. The8ysteresis curve %commonly called B8 curve! is a measure of the magnetic nature ofthe material. The intercepts B or 0 on the 5a$is represents 10M2727C0 and theintercept C or on the


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Fig. 2

Fig. 3

Fig. 4

The difference can #e well appreciated and understood from the 8ysteresis curves ofthe two types of magnetic materials. "t should #e noted that energy is consumed in themagnetising cycle and this appears in the form of heat. "t can #e shown that the areaenclosed #y the 8ysteresis loop represents the energy lost per magnetic cycle.

Magnetic esistances !eluctance)

This term is in some ways similar to resistance in electric circuits. "t is directlyproportional to the magnetic path length and inversely proportional to area of cross

section. Thus .a

l1m

= "t may #e remem#ered that even a very small non

magnetic shim or air gap in a high permea#ility magnetic path may increase itsreluctance manifold and all the magnetic flu$ will appear across the air gap and spreadoutward from it. This is used with advantage in recording head manufacture, where asmall #ack gap is used to sta#ilise the field)flu$ characteristic and a still smaller gap in

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the front with sharp edges, of reduced cross section, to o#tain high flu$ concentration,where the recording magnetic tape comes in contact with the head as shown in thefigure /.The head core is made from high permea#ility magnetic materials to reduce hysteresis

losses. The laminations %a#out .1000

1

! reduce eddy current losses, as the internally

generated short circuit currents do not flow through the #ody of the core. "t may #erecalled that these short circuit currents are produced due to e.m.f. generated #y fastchanging magnetic fields in the #ody of the core and unless eliminated or greatlyreduced, will cause core heating and loss of high frequency power, causing poorefficiency.

ecording Princi"les

The magnetic material used in recording is magnetic o$ide of iron e(4/and e(4>or asuita#le mi$ture of the two with small quantities of the o$ides of 7ickel and Co#alt. Thisis mi$ed with suita#le adhesives, plasticisers, fillers etc. and applied in the form of an

e$tremely smooth, even and thin coating %'.> to '.+ mils! on to a &=C #acking %6.' to6. mils thick!. This magnetic coated tape has a remanance of a#out '' to 6'''gausses, coercivity of a#out /'' to '' oersteds. The permea#ility is rather low % to6'!. This tape gets magnetised when it comes in contact with a recording head withaudio frequency signal currents flowing through the head windings, and as it passes onforward, retains the magnetism induced, due to the magnetic properties of remananceand coercivity. Thus if the tape is moved across the head at a constant speed of =cm)sec. and the signal current is of frequency ?f? 89, the signal current variations in time,will #e recorded as magnetic intensity variations along the tape length. Thus a singlecycle will #e recorded on a tape length =)f cm. This is called recorded wavelength andwill #e given #y

.

f

v=

So in tape recording, we really record wavelengths and from a recorded wavelength anyfrequency signal can #e o#tained #y running the tape at play #ack speeds different fromthe one at the time of recording. 7ormally, the record and replay speeds are e$actly thesame for a faithful reproduction of the recorded signal. or a fi$ed frequency audiosignal pure tone, the magnetic conditions on the so magnetised tape can #eappro$imately depicted in the form of recorded wave lengths condition of an array of halfwave #ar magnets placed end to end along with the tape length as shown in figure >.

The magnetised tape when passes over the play #ack head, which is almost similar tothe record head, will induce tiny voltages in the coil depending upon the intensity of

magnetisation of the tape, which itself is representative of the signal current fed to therecording head at the time of recording. Thus, the signals recorded magnetically arereproduced from the tape. The a#ove are the elementary principles of the magnetic taperecording. The set up of the system in a tape recorder is shown in ig. .

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Fig. #

Fig. $

The %rase Process

0rasing the previously recorded signal is essential for using the tape repeatedly. 2

satisfactory method for this is to feed the erase head with a high amplitude signal ofa#out 6'' k89 and the tape passes over this erase head #efore it passes on to therecord head %see fig. +!. "n this arrangement every part of the tape passes the erasehead gap %a#out 6 mil! and is su#ected to a#out ('' cycles of alternating magneticfield, starting from low value at the start of the gap, increasing to saturation value in themiddle of the gap and again steadily dropping to low value of the field, as the tapeleaves the gap. These repeated magnetising cum demagnetising cycles erase thesignal completely and leave the tape in completely unmagnetised form similar to a virgintape without a magnetic history.

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-e note that for effective erasure, the magnetic material should #e su#ected to a#out6'' to ('' alternating magnetic cycles. 4n erase head this is achieved #y a current ofa#out 6'' k89 frequency as the tape stays in the erase head gap for a very shortduration a#out one mili second. "n contrast, we use the line current of ' 89 in #ulkerasure and the whole of the magnetic tape remains in the magnetic field for a#out ( to /seconds to complete the erasure with the same result.

The ecording Process

Magnetic recording is made possi#le due to magnetism remaining #ehind after themagnetising force is removed. "n ig. @,

Fig. &

Fig. '

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Fig. (

Fig. 1

Fig. 11

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Fig. 12

the curve showing the relation #etween remanant magnetism %Br! and magnetising field%8!, indicates that the relation is nonlinear in the #eginning. Therefore a signal recordedas such will have a high component of harmonic distortion of the wave form as shown inthe ig.A and is not useful. This can #e partially improved #y using ;.C. #ias to avoidthe instep of the initial Magnetisation curve %".M.C! and shifting the operating point to a

more linear middle region as shown in the fig. . This method of ;.C. #ias does give afairly satisfactory quality of recording #ut the noise and distortion in the reproducedsignal leaves much to #e desired. 2lso it may #e seen that only one half of the Br8used suggesting scope of further improvement. This a#ove method of ;.C. #iasrecording is still used in cheaper version or cassette recorders for domestic use. 2greater improvement in recording quality was achieved during forties when the highfrequency #ias, a tap off from the erase head was also applied to the record head alongwith the audio signal to #e recorded. This 8.. #ias current agitates the magneticparticles on the tape, sufficiently that they settle down to the flu$ value, corresponding tothe superimposed signal current, overriding the in step nonlinear portion of the ".M.C.This is diagrammatically depicted in ig. 6'. Both halves of the Brh curves are utili9ed,flu$ recorded is high, signal to noise ratio is good and distortion is low as a result of 8..

#ias.

"t may #e noted that the #ias signal is not recorded or reproduced. "t only acts as avehicle to carry the audio signal #eyond the nonlinear portion. "ts frequency is normally to 6' times higher than the highest signal frequency and amplitude also a#out five timehigher than the audio signal amplitude. 2 normal audio record head has a signal currentof a#out 6 m2 and #ias current a#out to + m2, where as the erase current is of theorder of 6 to (' m2.

*ias +"timi,ation

4ptimum #ias is determined #y high signal level, low distortion and low noiseconsideration. *enerally tapes of higher coercivity require higher #ias current and canrecord hither signal levels. The graph of ig. 66 shows the effect of changing the #iasamplitude on output signal distortion and noise level. "t is normal to set the #ias not at 2,#ut slightly #eyond at point B %over #ias! where although the output is down #y 6 dB, thenoise has improved from ( dB to + dB and the harmonic distortion is still at a#out(. This #ias optimi9ation is normally done #y recording 6 k89 tone.

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ecording -osses

"f we feed a constant current to the recording head at various frequency, say from ' 89to 6' k89, the flu$ recorded on the tape will #e less for higher frequencies as shown inthe curve %refer ig. 6(! due to various losses in the record process. The most importantones are :

- 8ead losses, due to hysteresis and eddy current #eing more at higher frequency.

- Self demagneti9ation losses also are higher at higher frequencies as therecorded wave length and the associated half wave #ar magnets #ecome smallerand smaller in length and are closely packed and try to get demagneti9ed #ymutually canceling effect of the #ar magnets.

Fig. 13

Fig. 14

Fig. 1#

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Fig. 1$- The high frequency #ias current also contri#utes towards loss of recorded flu$ in

the gap. This process is similar to the erase process in the erase head and

#ecomes more prominent as the frequency of recorded signal increases.

- 8igh frequency flu$ is not a#le to penetrate the full depth of the magnetic coatingon the tape. 4nly the layer near the surface effectively contri#utes towardsrecorded flu$. This also causes loss of recorded flu$ as the signal frequencyincreases.

- &oor head to tape contact also causes losses towards 8 end. That is why,head cleaning is given so much importance in tape recording. The loss in dB isgiven #y the relation :

3oss D d)

-here is recorded wavelength and d is head to tape separation.

- "f the record head gap is not truly %normal! to the tape travel direction, the flu$recorded and reproduced is less towards high frequency end. Therefore, headalignment is also of great importance.

ecording %uali,ation

;ue to a#ove reasons the flu$ recorded towards high frequency end #ecomes less andless. This is partially compensated #y providing #oost at high frequency end. The

equali9ed response curve of the flu$ recorded at various frequencies resem#les theimpedance curve of a ;C, 1C network in parallel at various frequencies. The time

constant of the 1C network in CC"1 system at @. inches)sec. speed is @' sec. This

is shown in ig. 6/.

The high frequency #oost in the recording chain can #e provided at the output of therecord amplifier and #efore the record head input, as shown in ig. 6>.

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The e"lay Process

-hen the recorded magnetic tape passes over the low reluctance %almost a magneticshort circuit! replay head, almost whole of the surface flu$ passes through the head coreand links with the coil wound over it. This induces a small voltage across the coil endswhich is proportional to the rate of change of flu$ passing through the core and linkingthrough the coil. There is no #ack gap in the replay head and num#ers of coil turns arelarger as compared to the record head.

"t may #e remem#ered that even though the recorded flu$ along the tape surface isconstant for various frequencies %taking an ideal case!, the flu$ density normal to thesurface per unit recorded track length increases with frequencies. "t is this surface flu$density which passes through the head core, links with the head coil and produces the&B head output. Therefore, the &B head output rises with frequency, when the flu$recorded at various frequencies is constant. This is the + dB per octave characteristicwhich simply means that under ideal record replay conditions %no losses! for theconstant current input of the record head at various frequencies, the flu$ recorded alongthe tape length is constant and the &B head output is proportional to the frequency i.e.when frequency #ecomes dou#le, the output also #ecomes dou#le.

3et the recording current #e i D " sin ( ft

Then recorded flu$ r D E" sin ( ft

Surface flu$ density Ddt

dr

D ftCosfK 221

=oltage induced in &B 8ead Ddt

drK

2 D E(E6(f cos (ft

D E/( f Cos (ft.... %6!

Thus the &B head output is proportional to frequency %E, E 6, E(, E/#eing constants ofproportionality!.

Play *ac/ -osses

Theoretically + dB)octave play #ack characteristic is actually not possi#le due to thevarious losses in the process, the main ones are :

- The gap is of finite si9e and at a frequency where recorded wave length #ecomesequal to the head gap, there is no output from the &B head, even when themagnetic flu$ is there on the tape i.e. the frequency at that wave length #ecomes

e$tinct. This is called e$tinction frequency. This happens due to the reason thatthe two half wave #ar magnets in the gap at or near the e$tinction frequencycause the flu$ to flow in the core in opposite directions, there#y cancelling thetotal effect as shown in ig. 6@.

"t can #e proved mathematically that the e$pression %6! giving the &B headoutput gets modified in the case of a finite gap as given #elow :

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( )

l

lSinftCosfKe 22=

where is gap width and is recorded wavelength. 2t longer wave lengths orlower frequencies, the gap effect is negligi#le, the head output is ma$imumwhere the head gap equals the half wave #ar magnet and #ecomes 9ero%0$tinction frequency! when the gap equals two half wave #ar magnets i.e. therecorded wave length.

Therefore, the &.B. head gap and tape speed should #e such that the e$tinctionfrequency should fall well outside the useful high frequency range. or use inaudio system 6' k89 may #e considered as the high frequency edge. Therefore,the e$tinction frequency is a#out 6 k89, giving half e$tinction frequency of @.k89 and response is e$tended upto 6' k89 #y providing a selective #oost from @to 6' k89 in equali9ation.

Fig. 1&

Fig.1'

or #est reproduction the tape should #e in firm contact with &B head, as otherwise the

separation loss will amount to d)as already discussed during recording. 2lso &B

head a9imuth alignment should #e truly perpendicular to the direction of the tap motion,

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as was during recording process. Misalignment of the 8ead a9imuth causes severe 8losses. Therefore, head cleaning and a9imuth alignment form important maintenanceaspects.

Playbac/ %uali,ation

or constant input to the recording amplifier, the output from the play#ack head is risingwith frequency. This has to #e equali9ed for an over all record.)replay flat frequencyresponse. The equali9ing characteristics is to #e falling with frequency with some 8#oost near the 8 end to compensate the various losses as enumerated a#ove. Therecord characteristic and the play #ack characteristic visavis frequency is given alongwith the play #ack equali9ation required. %ig. 6A!.

b) The Ta"e Trans"ort System

;uring record or replay, the tape has to pass across the heads. 2lso there should #e

provision for spooling fast rewind. 2 standard tape transport format used in allprofessional machines is shown #elow.

The rewind and forward motors are ordinary induction motors. Their torque depends onthe voltage applied. The rewind and ) motors freely rotate in opposite direction asindicated. "n fast forward mode, the ) mode is supplied full voltage and the 1)-motor, a fraction of it, so the tape moves forward under a small reverse drag to keep thetape taut. "n rewind mode, the 1)- motor is given full voltage and the ) motor a smallvoltage. The tape rewinds fast under a small reverse drag. "n play #ack mode, themotors are given equal voltage and a#out half the full voltage. The tape as suchremains taut and should not move in any direction, if freed from the capstan motor pinchroller. ;uring record)replay mode the tape must move with constant speed, as otherwise

the frequency reproduced will #e different from the one recorded. 2lso the speed shouldremain constant and not vary around a mean average speed. This simply means that ifthe nominal tape speed is @. inches)sec. The tape should not only move forward @.inches)sec #ut also @. mili inches every milisecond to say. This is achieved #y thecapstan motor and pinch roller com#ination. The capstan motor is heavy duty, underloaded synchronous motor. "ts speed of rotation i.e. no. of revolutions per second, aresynchronous to the power supply frequency. =oltage variations have no effect on su#multiple of it, depending upon the pair pole windings. The tape is pinched #etween thecapstan motor shaft and pinch roller and can move forward only when the capstan shaftrotates at the precise rate of FdF inches per rotation where FdF is the capstan shaftcircumference. Thus the tape speed is maintained constant as short duration frequencyvariations mains power supply frequency %frequency fluctuations! are rare.

0a"stan Sero 0ontrol

or most purposes, the synchronous capstan motor synchronised, with mains frequencygives a good speed accuracy. 8owever, these days the servo control of capstan motoris #ecoming increasingly common. The #asic method consists in attaching to thecapstan motor shaft a toothed wheel which is passing over a magnetic coil or an opticalpath generates a pulse each time the tooth passes over the magnetic or optical sensor.

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The pulse rate is an accurate measure of the instantaneous speed of rotation of themotor. This pulse rate is processed and compared with the accurate pulse rate o#tainedfrom a reference source. "f there is a difference #etween the two, an error signal isgenerated which is fed #ack to the motor control circuit, till correct nominal speed iso#tained. The servo controlled capstan motors are light weight and offer #etter speedaccuracy.

Ta"e Trans"ort Interloc/

The tape transport mechanism can have one of the three modes, namely, rewind, fastforward and record)replay in addition to the stop functions. Supplies to the motors isapplied in such a way, that no two transport modes are energised simultaneously. Thisis generally achieved through using main and au$iliary contacts of multi contact relayinterlocks. These days in modern machines, digital gates are used frequently forinterlock and control functions, as these are more compact and relia#le. 0ach machinehas its own interlock and control electronic circuitry which should #e studied from thesuppliers manual. 2nother important interlock is the tape lift and shift off arrangement,when the tape shuttles at high speed in rewind and fast forward mode. The fast movingtape is shifted a little away from the heads #y a shift lever actuated automatically, toavoid wear and tear to the heads. 8owever, for quick location of the end of a programpiece or in #etween pauses, the shift away can #e disa#led and a cue output o#tainedfrom the &B head.

*ra/e Mechanism

or stopping the tape from any transport mode, the supply is switched off to the motors#y pressing the stop #utton and simultaneously applying the #rakes to kill the movementof the shutting motors due to inertia. "t may #e noted that the capstan motor is notswitched off. The pinch roller engages with the capstan shaft in record)replay mode only

and in shuttling mode, this remains disengaged and as such capstan motor thoughrunning continuously does not play any part in tape shuttling. The #rake system used isinvaria#ly of differential #raking type where the #raking torque depends upon thedirection of rotation of the shuttling motors. The idea is that the tape supply spool motorshould have higher #raking torque %almost dou#le! as compared to the take up spoolmotor. This ensures that the tape supply spool stops a #it second earlier than the takeup spool and the tape always remains tight over the threading path and no tape spillover takes place.

Measurements and dustments

"n the tape transport system following quantities are specified #y the manufacturers.

These should #e checked at least once a week and adustments carried out if themeasured values are not within the tolerance limits specified. -orn out #rake #andsand pinch rollers need replacement as and when necessary. The measurements andadustments are :

- 1ewind reel #rake torqueclock and anti clockwise.- )orward reel #rake torqueclock and anti clockwise.- Capstan &inch roller pressure.

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- 1ewind and )orward motor torques in all the three modes separately in pullingand release form.

o5 6 Flutter

-ow G flutter is caused #y speed variation of tape transport across the record)replayhead during record or replay. -ow is the term given to speed variation occurring at therate of '.6 89 to (' 89. lutter is the term given to speed variations occurring at the rate#etween (' 89 to ('' 89. These shortterm speed variations cause a peculiarroughening of the sound and change in its pitch. 2ctually, these speed variations causefrequency modulation of the signal #eing recorded)reproduced. These M componentsgenerate multiple order side#ands which are responsi#le for the peculiar roughening ofthe reproduced signal. The wow and flutter is measured #y recording / k89 tone andplaying it #ack. The output is passed through a limiter to remove amplitude variations.The amplitudelimited output is then passed to M discriminator which transformfrequency variations into amplitude variations. This 1MS value is read as wow andflutter on a cali#rated meter.

The wow and flutter figure for good quality machines these days is #etter than '.6.0ar can detect a wow and flutter of './ easily. "f wow and flutter #ecomes 6, thereproduced quality is positively annoying and o#ectiona#le.

The tape speed may fluctuate for several reasons, a few important ones are given #elow

- "nsufficient pressure on pinch roller.- lats on capstan shaft and pinch roller due to normal wear and tear.- ;irty capstan or pinch roller or tape guides.- -o##ling of pinch roller or capstan due to improper mounting due to eccentricity.- riction #etween tape and guides.- Tape sticking- 8unting of the capstan motor. This is minimised #y using fle$i#le coupling and

heavy flywheel #etween the motor shaft and the capstan.

c) TH% %-%0T+7I0 S8ST%M

The electronic system in tape recorders comprises of power supplies, recording amplifierchain including equalising circuits, play#ack amplifier chain with play #ack equaliser, 8#ias oscillator, metering and monitoring amplifier etc.

The individual circuit details vary from model to model and detailed study should #edone from the particular equipment manual. 8owever, the circuits are quiteconventional. 2 typical #lock schematic of recording and play#ack chains is given infigure 6.

The ecording 0hain

2s is evident from the a#ove #lock diagram, the input may #e from a microphone or froma high level controlled, amplified, correct amount of 8 #oost is provided to precompensate for the 8 record process losses as descri#ed earlier. 2lso, in addition to

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amplification and preequalisation, it converts constant voltage input into constantcurrent output. This is required #ecause the record head is a current operated deviceand the magnetic flu$ is proportional to the current flowing in the record head coil.

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through the au$iliary contact of the record interlock system. This ensures that therecorded program does not get erased #y mistake during replay and tape shuttle mode.

2 typical circuit arrangement is shown in the diagram separately.

Metering 6 Monitoring

"t may #e seen from the #lock diagram that the metering and 3)S monitoring point istaken #efore equalisation in the case of recording chain and after the equalisation in play#ack chain, so that a natural quality program is availa#le for su#ective aural monitoring.The same reasoning is valid for input to the =H meter.

%lectronic System lignment

"t is o#viously necessary that for every machine the overall record replay chain responseversus frequency should #e flat. This can #e achieved in various ways #ycomplementary equalisation in the recording and replay chains. 8owever, suchmachines will not reproduce satisfactorily the recording made on other machines i.e. the

equipment will not #e compati#le for programme e$change. or this reason, a standardrecorded flu$ characteristic is generally specified keeping in view the #est signal to noiseratio and minimum distortion. The CC"1 characteristics is followed #y large num#er ofcountries all over the world including "ndia and the recording machines are standardisedusing the standard tape and then the recording chain is standardised using the standardplay #ack chain as reference.

The Standard Ta"e: The CC"1 standard tape for @. inches)sec. %6' cm)sec! speedhas three sections as discussed #elow :

-eel Section

This section has 6 k89 signal recorded on it at a recorded flu$ level of /( miliMa$well)mm recorded track length.

This is the ma$imum permissi#le level recorded on present day commercially availa#letapes and such tapes should give a total harmonic distortion of / or less whenreproduced from a standard play#ack chain. The average or normal recorded level is 6'dB #elow this ma$imum level. or chain alignment the play#ack output attenuator is setat (' dB mark and play #ack amplifier gain control adusted to give ' dB reading on the=H meter. The attenuator is advanced #y 6' dB i.e. to a reading of 6' dB and leftundistur#ed. 7ow when the recording input gain control is so adusted that the play #ackchain output reads 9ero ensures that the recorded level on the tape is 6' dB #elowreference level or /( m Ma$well.

Head ,imuth Section

This section has 6' k89 signal recorded on the standard tape with a record head havingtrue a9imuth. The tape is played and the play head a9imuth adusted for ma$imum quiteconstant i.e. output variations not more than '. dB. The &B head is left undistur#ed.7ow a 6' k89 signal is given to the record chain. The record head a9imuth adusted forma$imum and sta#le output from the play #ack chain, which was earlier a9imuth,

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standardised with standard tape. This completes a9imuth alignment of the completerecord replay chain.

Freuency es"onse Section

This section of the standard tape has recordings on it, various frequencies from +' 89 to6'' k89 corresponding to the specified recorded flu$ versus frequency characteristic, asper the CC"1 specified impedance =s. frequency characteristic of an 1C parallelconnected network of @' micro second time constant. The reference flu$ at 6 k8 #eing6' dB #elow the ma$imum specified flu$ of /( mili. Ma$well)mm. This standard tapesection is played and play #ack equalisation adusted to o#tain flat frequency responsewithin permissi#le limits as given #elow in ig. (.

+erall 0hain es"onse

By the a#ove procedure using standard tape, the play #ack chain is standardised forlevel, a9imuth and equalisation. 7ow for over all chain response, a set of specific

frequencies from ' 89 to 6' k89 are fed to the recording chain at constant level andrecord equalisation adusted till the output monitored at the play #ack chain output is flatwithin specified limits as a#ove. -e should #e careful that during this measurement, theplay #ack chain is not to #e distur#ed at all as it has already #een standardised withreference to the standard tape.

*ias +"timisations

-e have already discussed the importance of 8 #ias in improving S)7 ratio andreducing distortion. The #ias is optimised as follows. 7ormal level of 6 k89 is fed to therecording chain and output monitored at the &B chain output point. The #ias control isturned down to the minimum. This results in reducing the output to a very low value.

7ow #ias is slowly increased. This increases the &B output level. Bias increase iscontinued till the play #ack output #ecomes down #y 6 dB. Thus we over #ias slightly, soto say. Some manufacturers recommended #ias optimisation at 6' k89 rather than at 6k89, speciallty with the low noise, high coercivity tapes as they claim that it gives #etterresults. So when the instructions so provided for a special grade of tape, it should #efollowed. 4rdinarily for medium coercivity tapes, the optimisation at 6k89 is preferred,as at 6' k89, the other factors like noise, a9imuth equalisation etc. make accurateadustments difficult.

M>7%TI0 TP%

Before we conclude the discussions on magnetic recording system, some knowledge

a#out the magnetic tape qualities and defects are considered essential for professionalgrade sound recording and reproduction. Some important mechanical and magneticproperties are given #elow for guidance.

Tensile Strength The tape should stand a steady pull of /. Eg. -t. and impulseload test of 6'' gm. falling from a height of (' mm.

0lastic 0longation 1esidual elongation should not #e more than './ for a steady

ST"%T! &u#lication 117 ''()"C%*!)(''+


20/20


load of 6 Eg. -t. for (> hours.

4verall Thickness '.'' mm I '.'' mm.

Coating Thickness Ma$. '.'6 mm min. '.'6 mm.

Smoothness of Coating Better than '.''6 mm.

Cup and Curl Tape should #e free from coupling and curling defects on visualinspection.

3ayer to layer 2dhesion Tape shall show no sticking

Magnetic 2nchorage The magnetic layer should show no evidence of anchoragefailure, when an adhesive tape of 6? length is stuck and pulledaway.

&lasticity The tape should fall against a sharp edge under its own weight of

cm length.

0rasa#ility -hen a recording at ma$imum record level is passed through anerase field of 6''' oersteds, the residual signal level should #e+' dB #elow the recorded level.

;istortion 2t the ma$imum recorded level of /( mili Ma$well)mm at 6 k89,the total harmonic distortion should not #e more than / and at6' dB #elow the a#ove, the total distortion should #e less than6.

;ynamic 1ange Should #e #etter than +' dB i.e. from ' dB to I 6' dB %Ma$.

level!. This means that the inherent noise level should #e #etterthan +' dB.

&rint through &rint through resulting from inter layer signal transfer shall #e#etter than dB.

requency 1esponse -ith reference to a standard record)replay chain, the responsevariation due to tape should #e within 6 dB from ' 89 to 6' k89.

Temperature and8umidity

-hen the tape is su#ected to a cold test at A oC and dry heattest at oC for short duration, it should not show any noticea#ledeterioration.

Care and Storage The tapes should normally #e stored at an am#ient temperatureof ('oC and 'oC humidity in a dust free atmosphere, away frommagnetic field and kept in their cartons, when not in use.

ST"%T! &u#lication 118 ''()"C%*!)(''+

08_principles of magnetic tape

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