The process of magnetic recording depends fundamentally on two properties of steel, namely its magnetic remanence and the fact that neighbouring elements in a length of the metal can be to a large extent independently magnetised.
A detailed theoretical examination of the operation of the system is too complex to be attempted here. The following elementary analysis should therefore be read with some reserve since it involves certain assumptions that do not apply rigidly in the actual working of the process.
In this machine the recording is carried out by passing a steel tape at constant velocity through a device known as a 'recording head' which induces into it an alternating flux corresponding to the programme A.C. The alternations with respect to time occurring in the recording head result in spatial variations in the distribution of the remanent longitudinal flux in the tape after it has passed through the head. Reproduction is carried out by passing the magnetised tape at the same velocity through a device known as a 'reproducing head' which is sensitive to the changes of the flux in the tape. The spatial alternations are thereby transformed back into alternations with respect to time corresponding with the original alternations applied to the recording head. Before recording it is necessary to ensure that any previous flux variations distributed in the tape have been removed and this is effected by a 'wiping head.'
The cycle of operations can best be followed by reference to the hysteresis loop of the steel tape, which is shown right.
The tape enters the wiping head in an unknown state of magnetisation and is there saturated by the D.C. field to a state of magnetisation represented by point 1. The tape leaves the head with the maximum residual magnetisation corresponding to point 2.
As the tape passes through the recording head it is subjected to the influence of the field produced by a D.C. modulated in accordance with the programme to be recorded. In order to secure the maximum effect on the residual magnetisation of the tape by this current, the field produced by it must always be in the opposite direction to that produced by the wiping head. For this reason the D.C. component of the current in the recording head is usually referred to as the 'demagnetising' current. With zero modulation this current will bring the magnetisation of the tape to point 3 and its residual magnetisation on leaving the head will correspond to point 3'. When modulation occurs, the magnetisation of the tape will be brought to points indicated, say by 4 and 5 respectively, and the tape will leave the head with residual magnetisation corresponding to points 4' and 5'.
It should be realised that the tape is not subjected to an alternating cycle, but that each element is demagnetised separately to a degree dependent on the instantaneous value of the demagnetising current. The tape thus reaches the reproducing head in a state of varying magnetisation and the lines of force from the tape induce e.m.f.'s in the coils of the head, which are applied to a valve amplifier.
In all systems of recording there are limiting factors inherent in the recording medium and in the system itself which tend to produce various forms of distortion. In this case the total volume range which may be used is bounded on the one hand by noise, since it is essential to keep the ratio of programme/noise above a certain value, and on the other hand by amplitude non-linearity, which results in the production both of harmonics and also of inter-modulation tones.
If consideration is given to the matter it will be seen that it is desirable to adjust the recording frequency characteristic in such a way that the maximum input level likely to be found in any programme at any frequency will bear a definite relationship to the maximum level that may be applied to the system at that frequency.
To do this it is necessary to determine not only the maximum permissible amplitude-frequency characteristic of the system but also how the maximum levels found in any programme which may be applied to the system will vary with frequency. As regards the latter point, a considerable amount of work involving the lengthy analysis of a very large number of programmes has been done in America, and curves have been published showing the relation between frequency and the levels occurring in various types of programme. These curves are all fairly similar in character, and it is generally accepted that the highest levels are liable to occur in the region of 250 c/s. By combining a frequency characteristic of this kind with the maximum permissible amplitude-frequency characteristic, an input characteristic can be derived which will give the highest possible programme/noise ratio.
The curve actually used includes a slight bass tip-up, the object of which is to ensure that overloading will occur first in the bass and thus provide the engineer on control with a warning and a margin of time in which to make suitable adjustment before 'blasting' occurs over the whole band of frequencies.
The frequency characteristic of the reproducing amplifier is adjusted to give a sensibly flat overall frequency characteristic to the system within the required range. In order to maintain a satisfactory programme/noise ratio, a low pass filter is provided, cutting off between 5,000 and 6,000 c/s.
A tungsten steel tape, 3mm. in width and 0.08 mm. thick, is used and runs at a speed of 90 m. a minute. The tape is wound on spools and is of sufficient length to provide a continuous record lasting about half-an-hour.
Five heads are provided, one wiping head and duplicate recording and reproducing heads. They are all similar in mechanical design. The recording and reproducing heads are also identical electrically but the wiping head has a larger number of turns. Their construction can be seen by reference to the picture, right. (See previous page for a larger version of this picture, with a key to the various parts.) They consist of upper and lower portions which are hinged so that they can be closed over the tape. When the two halves are closed in this manner and clamped together, a slot is formed between them for the tape to pass through. In each half a strip of magnetic material known as a pole-piece may be inserted. When the head is closed the pole-piece is held at right angles to the plane of the tape and its end is held in mechanical contact with the tape by the action of a spring-loaded plunger.
In the case of the wiping and recording heads the pole-pieces are made of stalloy and one is inserted in each half. In the case of the reproducing heads the pole-pieces are made of permalloy and only the lower half of the head is used.
The pole-pieces (right) are 3 mm. in width and 0.4 mm. in thickness. The end in contact with the tape is made flat for wiping, but is bevelled and tapered for both recording and reproducing. The taper reduces the width of the pole-piece from 3 mm. to 1.8 mm. and the bevel, which is at an angle of 60°, reduces the thickness at the end to only 1/6th of its original value. The pole-piece is inserted with the bevel on the leading side, that is to say, as the tape travels forward it first encounters this side. Provision is made for moving one half of the head relative to the other so as to form a longitudinal gap between the two pole-pieces, and in the cases of the wiping and recording heads the upper pole-piece is given a lead, of 1.5 mm. and 3 mm., respectively.
The most important mechanical requirement is that the velocity at which the tape moves through the heads should be kept constant. If this is not so, variations in pitch, known as 'wow,' will be heard in the reproduction. It is also important that the tape should not be subjected to undue strain when being wound from one spool on to the other.
Experience has shown that the foregoing requirements are best covered by three separate mechanical drives, one for unwinding the tape from the trailing reel, another for maintaining the velocity of the tape constant through the heads, and the third for winding the tape on to the leading reel. The constant-speed drive is virtually isolated from the unwinding and winding systems by the provision of reservoirs, one preceding it and one following it, and the speed of the winding and unwinding motors is automatically regulated so that both reservoirs always contain a loop of tape.
These two motors are of the induction type, but their rotor windings instead of being closed are brought out to three slip-rings with their brushes connected to external resistances arranged as a three-phase star-connected system (above). With the full resistance in circuit the speed of the motor is such as to make the speed of the tape less than it is in the constant-speed drive system, but provision is made for short-circuiting part of the resistance. The winding and unwinding systems thus each have two forward speeds, one greater and one less than that of the constant-speed drive. By causing the motors to alternate between their two speeds a sufficient loop of tape is always maintained in both reservoirs. The switching in each case is carried out by a relay, the operation of which is controlled by a gas-filled thermionic valve of the thyratron type. This consists essentially of a half-wave rectifier, but by applying a sufficient negative bias to its control grid the anode current can be completely suppressed. Such a bias is normally applied and, the relays being unoperated, the unwinding motor runs normally at its higher speed, and the winding motor at its lower speed. Therefore loops build up in both reservoirs. In either case when, in due course, the loop of steel tape touches a contact at the bottom of the reservoir, which is connected to the grids of the associated thyratrons, it short-circuits the grids to the cathodes and removes the bias. Anode current forthwith strikes, causing the relay to operate and change the speed of the motor. The size of the loop is now reduced but the process is checked when the tape is pulled clear of the contact, for the bias is thus reapplied to the thyratrons causing the relay to release and restoring the original condition.
When recording or reproducing the tape travels from left to right, being pulled off a reel placed on the left-hand hub and wound on to an empty reel placed on the right-hand hub. Since the reproducing heads follow the recording heads an immediate playback can be obtained at the time of the recording. However, before the record can be re-played it is necessary first to wind the tape back on to the left-hand reel, which operation, to save time, is carried out at approximately double the forward speed. It may then be run forward again through a reproducing head. When recording, the wiping head one recording head and one reproducing head will be in use, but for the re-play the wiping and recording heads must be open and only the reproducing head closed. During the rewind none of the heads should be closed.
The constant-speed drive system is not required during the re-wind and an automatic release clutch is provided for disconnecting it. The rewinding motor, running in the reverse direction and coupled to the tape drive through its double-speed pulley, pulls the tape off the right-hand reel and feeds it into the large tape reservoir. The winding motor, with its direction reversed, now drives the left-hand reel on to which the tape is being rewound, and its speed is regulated as before by the thyratron circuit. The speed of the unwinding motor is not varied during the rewinding operation.
The trailing reel in both directions of working is restrained by a simple brake-band, and provision is made for increasing the brake tension in order to overcome the momentum of the reel when stopping the machine.
For the purpose of description the three driving systems are distinguished as follows:
No.1 System (Winding). This includes the large tape reservoir with the associated thyratron circuit, and No.1 motor together with the system of belting and pulleys on the lower tier for driving the hub carrying the leading reel, that is to say, for driving the right-hand hub when the machine is running forward and the left-hand hub when it is in reverse.
No.2 System. (Unwinding). This includes the left-hand tape drive system and the small tape reservoir on the front of the machine, No.2 motor and the system of pulleys and belts associated with it on the top tier, and the thyratron circuit associated with No.2 reservoir.
No.3 System (Main Tape Drive). This includes the constant speed motor with its flexible coupling and oil-damped flywheel for providing mechanical smoothing, and the right-hand tape drive system which includes the automatic clutch for cutting out this system for reverse running.
In addition to the heads and the winding systems, the machine also includes the Starter System which consists of a central starting handle, with an Off, two Forward and two Reverse positions, and mechanical devices for operating the starting switches, for applying braking to the trailing reel, and for operating the clutch of the main tape drive system.
In order to set up the machine for running in the forward direction a full tape reel is put on the left-hand hub and an empty reel on the right-hand hub. The hub must first be slackened by turning its handle anti-clockwise about one turn, and the reel then placed on the hub with its back face making full contact with the hub flange. The hub must then be expanded and lightly tightened by clockwise rotation of the handle.
The heads and No.1 reservoir entry doors are then opened, the glass of No.2 reservoir lifted, and the tape drive belts released, by operating the quick-release toggles.
Tape from the left-hand reel is threaded over the left-hand spring jockey, under the left-hand tape tension brake, and vertically upwards on to the No.2 tape driving system. The tape is then passed straight through No.2 reservoir, the small friction pads, and the tape guides, to No.3 drive system. It is then led vertically downwards, under the lower guide wheel and tape tension brake, over the right-hand spring jockey, and on to the right-hand tape reel. No special device is provided for attaching the tape which is simply lapped round the centre of the reel.
The belts of the tape drive systems are then tightened up by the toggles, the glass front of No.2 reservoir is lowered and the doors of No.1 reservoir are closed. The friction pads are closed on to the tape.
The main switch (on the wall), which is provided to enable the machine to be isolated for maintenance, must be closed before the machine can be started. Before this is done, however, it must be checked that the control handle is in the Off position. When the main switch is made a small white lamp on the left of the machine will come up and will remain alight during the whole time that the power is switched on. At least one minute must be allowed to elapse after switching on before the control handle is moved from the Off position, in order to let the thyratron cathodes warm up. Failure to observe this delay may result in the thyratrons being ruined.
Before starting up in either direction any tape loops that exist, particularly upon the trailing side, must be taken up by rotating the tape reels by hand.
To start the machine running forwards the control handle should be turned clockwise into the No.1 Wind position, pushed forward through the gate and further turned in the same direction into the No.2 Wind position. To stop the machine the process is reversed, but before the control handle is moved into the Off position it must be retained in the No.1 position until the machine has come to rest.
To start the machine in the reverse direction the control handle is turned anti-clockwise into the No.1 Rewind position, then pushed through the gate and turned to the No.2 Rewind position. To stop the machine after rewinding the reverse process is followed, but the control handle should be moved from the No.2 to the No.1 position only when the tape is clear of the thyratron contact in No.1 reservoir, and from the No.1 to the Off position only when both tape loops have disappeared and the machine is at rest. If it should be required to stop the machine within one minute of commencing the rewind, extra braking should be applied by hand to the left-hand reel in order to prevent the tape loop from being pulled out from No.1 reservoir too quickly.
The behaviour of the machine for the various positions of the control handle is given in the following schedule.
CONTROL HANDLE OFF. Power is applied to the heaters of the thyratron cathodes, and to the metal oxide rectifier circuit providing the grid bias.
H.T. is switched on to the thyratrons and the L.T. and grid bias supplies continued.
No.1 WIND. No.1 motor is switched on for forward running at its slower speed, engages with the belt drive of the R.H. reel by means of No.1 automatic dog clutch, and tends to drive this reel in an anticlockwise direction.
Maximum braking is applied to the trailing (L.H.) reel.
The conditions of the No.1 Wind position are maintained except that the breaking on the L.H. reel is reduced to the normal running value.
No.2 motor runs forward at its higher speed, engages with the lower geared belt system through No.2 automatic dog clutch, and thus causes No.2 tape drive to pull the tape off the L.H. reel and feed it forward at a rate slightly faster than the feed of the main tape drive system.
NO.2 WIND. No.3 motor is switched on and runs forward at the constant speed of 250 r.p.m. pulling the tape through the heads at a steady rate of 90 metres per minute and feeding it into No.1 reservoir from the R.H. side.
The higher speed of No.2 motor is maintained until the tape loop in No.2 reservoir touches the thyratron contact thereby short-circuiting the grid bias and causing the operation of No.2 relay. The speed of No.2 motor is thus changed to the lower value which allows the tape loop in reservoir No.2 to be reduced. The lower speed is maintained until the loop ceases to make contact with the thyratron control contact when the motor reverts to its higher speed and the process is repeated. The R.H. reel, now having no tape tension to restrain it, is driven by No.1 motor and begins to wind up the tape, but at a slower rate than it is being fed into No.1 reservoir by No.3 system. However, when the loop in No.1 reservoir touches the thyratron contact, No.1 motor runs at its higher speed and continues so until the tape loop in No.1 reservoir is pulled off the contact. The speed then returns to the lower value and the process is repeated.
NO.1 REWIND. H.T. is switched on to the thyratrons and the L.T. and grid bias supplies are continued.
No.3 Drive system clutch is disengaged.
No.1 motor is switched on for reverse running at its slower speed, engages with the higher geared belt drive of the L.H. reel by means of No.1 automatic dog clutch, and tends to drive this reel in an anti-clockwise direction.
Maximum braking is applied to the trailing (R.H.) reel.
NO.2 REWIND. The conditions of the No.1 Rewind position are maintained, except that the braking on the R.H. reel is reduced to the normal running value.
No.2 motor runs backwards at its higher speed, engages the higher-geared belt system through No.2 automatic dog clutch and thus causes No.2 tape drive to pull the tape off the R.H. reel and feed it into No.1 reservoir from the left-hand side at approximately twice the normal forward speed. No tape loops occur in No.2 reservoir hence the speed of No.2 motor is not controlled.
The L.H. reel, driven by No.1 motor and relieved of restraint by the existence of the loop in No.1 reservoir, then begins to wind up the tape at a slow rate until the loop in No.1 reservoir touches the thyratron contact. When this contact is made No.1 motor runs at its higher speed and continues so until the loop in No.1 reservoir is pulled off the contact. The speed then returns to the lower value and the process is repeated.
No.3 motor does not run at all during the rewinding.
MAINTENANCE AND ADJUSTMENT.
The following parts should be oiled once a week with Mobiloil BB: Tape jockey arm bearings (Two). Tape-tensioning brake arm bearings (Two). Quick release mechanisms for tape-drive system (Two). No.3 clutch-operating arm pivots (one on front of machine and the other inside). Spring-loaded belt jockey arm bearings (Two). Brake operating mechanisms (Two). Pedestal bearings for control handle shaft and switch operating mechanism (Three).
The following should be oiled or greased every three months:- Bearings of drum in switch box with Mobiloil BE. Operating chain for switch box with Mobiloil BB. Threads of automatic dog clutches (Two) with Mobiloil BB (Sparingly). Cams on control handle shaft (Three) with Tecalemit. Threads of expanding hubs (Two) with Tecalemit. The tape should be kept lightly oiled by occasional application of oil to the friction pads between No.2 reservoir and the heads.
Care should be taken to avoid getting oil on the linings of the brakes on the reel hub shafts.
All rotating shafts are supported on ball bearings and only need to be packed with grease during the general overhaul of the machine.
The following require daily attention: Relay contacts to be burnished. The surfaces in each head that come into contact with the tape to be cleaned with benzine. The peripheries of the tape drive wheels to be cleaned with benzine, care being taken not to exert very great pressure. The following require attention once a week: Tape guides between the heads to be cleaned with benzine. The slip rings of Nos.1 and 2 motors to be cleaned with benzine and left lightly smeared with vaseline. The contact surfaces of the expanding hubs to be lightly cleaned with paraffin. The thyratron contact in No.2 reservoir should be cleaned with benzine two or three times a week or as often as is found necessary, but that in No.1 reservoir need only be cleaned about once a month. For this purpose the glass case of the reservoir must be taken out, which can be done by removing the side and top fixing angles.
The surface of the belt pulleys inside the machine should not be cleaned, except for the removal of any surplus dope exuded at the edges or collected at spots on the surface to form lumps.
The thyratrons should be checked once a month. One of each pair of paralleled thyratrons should be removed and the machine should still function satisfactorily. The first two thyratrons should then be replaced and the other two removed and the operation of the machine again checked.
Care must be taken when making these tests to allow the thyratrons at least one minute for their cathodes to heat up before the control handle of the machine is moved from the Off position.
Tape Tensioning Brake Blocks
These should be inspected every week and the front and back edges bevelled off whenever they show signs of developing a sharp edge, since this may catch in joints in the tape and cause them to open.
The speed settings of Nos.1 and 2 motors will not normally require to be altered. In the event, however, of the machine losing its adjustment the braking should first be checked and readjusted if necessary to the values specified (see Spring Tensions). The motor speed control should then be checked and the resistances adjusted as necessary in order to satisfy the conditions given below. The amounts of resistance in circuit for the two positions of the thyratron-controlled relay are adjustable by means of movable contact clips. Since a three-phase star-connection is used it is necessary for the resistance in each phase to be the same and the tappings must therefore be made at similar positions on each resistance.
The two speeds of No.2 motor must be such that, when the machine is running in its No.2 Wind position, a loop of tape forms in No.2 reservoir. This loop must increase, touch the thyratron contact, decrease to about half its maximum, and then increase again to repeat the cycle, the average period of which should be about 5 seconds. With either a full or empty reel on the L.H. hub or one in any intermediate condition, there must always be a loop in No.2 reservoir when the machine is running forward.
The higher speed of No.1 motor should be such that, with a full tape reel on the L.H. side, the initial loop formed in No.1 reservoir when the machine is started up for forward running is drawn off the thyratron contact within 8 seconds. The lower speed must be such that, when the machine is running forward with a nearly full reel on the R.H. side, the tape loop is never quite pulled out. But the torque of the motor when running at its lower speed must be sufficient to overcome the total braking applied to the reel that it is driving when the control handle is on either of its No.1 positions, so as to ensure that the tape loop in No.1 reservoir will disappear before the machine comes to rest.
After the adjustment has been made for the forward direction, further slight adjustment may be found necessary in order to prevent looseness of the tape at the spring jockeys and also to enable similar conditions to be satisfied by No.1 motor for rewinding.
A spring balance, calibrated in kilogrammes, is available for measuring the various spring tensions and braking loads on the machine. The methods of measuring these forces are specified below, together with approximate working values for the various tensions.
The readings should always be made with the pull of the spring balance along the line or motion of the part to which it is attached.
Tape Tension Brake. The spring balance is hooked into the hole on the tape tension brake arm and a force is applied to the balance, at right angles to the brake arm and in the plane of its motion. The force is increased gradually until the brake arm just commences to move. The reading of the spring balance then gives the value of the tape tensioning force.
Tape Jockey Arm. A small loop of string is placed over the tape jockey wheel and the spring balance is hooked into this loop. A pull is then exerted on the balance, at right angles to the jockey arm and in the plane of its motion. This pull is increased gradually until the jockey arm has been moved exactly 1" from its back-stop. The reading of the spring balance then gives the value of the restoring force acting on the tape jockey arm.
Reel Braking. The spring balance is hooked over one of the spokes of the reel as near the outer rim as possible. When the normal braking is to be measured the spring balance should be on the left of the reel and force applied to produce an anti-clockwise motion. For the additional braking the balance should be attached at the right-hand side and force applied to impart a clockwise movement to the reel. A measurement of the braking is given by the indication on the spring balance when this is pulled down just sufficiently to produce motion of the reel. Normal braking is measured on both reels with the starter handle of the machine in the Off position. The additional braking is measured on the L.H. reel with the starter handle in the No.1 Wind position, and on the R.H. reel with the starter handle in the No.1 Rewind position. It must be ascertained in each case that No.1 automatic dog clutch is in the position in which the reel under consideration is disengaged from No.1 motor.
Approximate Working Values. The values given below for the various spring tensions are to be taken as typical values only and indicate roughly the order of the values which will be used in practice. These will, of course, be adjusted from time to time as found necessary in order to secure the most satisfactory working conditions for the particular machine.
|Tape Tension Brake Arms||0.5 kg.|
|Tape Jockey Arms||2.0 kg.|
|Braking Extra||2.0 kg.|