Reel servo for tape transport

A supply reel servo for a tape transport in which a motor-driven capstan of low inertia draws tape past a cylindrical scanning drum includes a tension servo arm which provides a signal denoting tape tension in a loop adjacent the supply reel. This signal is compared with a reference to develop an error signal to control the supply reel motor. The reference is modified in response to a signal which represents the energization of the capstan motor and accordingly the torque output thereof so as to provide automatic compensation for variation in tape tension at the scanning drum.

BACKGROUND TO THE INVENTION 
This invention relates to tape transports and in particular to the control 
of a supply reel in a tape transport for regulating the tension in a 
magnetic tape for which the transport provides movement in a tape path 
extending around a cylindrical guide drum by means of which tape is guided 
in a helical path for scanning by a recording or playback head. 
The need to regulate the tension in a magnetic tape in a tape transport is 
well-known, particularly when the transport forms part of a machine for 
recording signals on magnetic tape or playing back signals from such a 
tape. The stretchability of magnetic tape is notorious and is particularly 
susceptible to changes in temperature and humidity. The control of the 
tension in the tape is particularly important, because its variation 
affects the physical wavelengths of signals recorded on the tape and 
thereby causes undesirable variation in the frequencies played back from 
the tape and tends to cause deterioration in the quality of a played back 
signal. 
It is well-known to provide means for sensing the tape tension in a tape 
path extending between a guide drum and, for example, a supply reel. 
Typically the means for sensing the tape tension comprises means for 
forming a loop of tape and for measuring the tape tension in that loop. A 
signal representing the tape tension may be used to control, in known 
manner, a motor driving the supply reel so as to tend to maintain the tape 
tension substantially constant. For this purpose it is known to provide a 
pivoted arm which carries a guide roller around which the tape path 
extends to form a loop so that variation in the tape tension causes 
pivoting of the arm. The pivoting can be sensed to provide a signal 
representing the tape tension and constituting, after comparison with a 
reference signal, an error signal for driving a servo amplifier for the 
motor which drives the reel. It is also known to correct for variations in 
tape tension by servomechanically controlling a take-up reel in accordance 
with detected tension errors and variations in the load experienced by a 
capstan disposed in the tape path between the take-up reel and the 
scanner. Although control of the loop tension in a buffer loop is an 
important feature of most tape transports, in order to ensure that a 
capstan driving the tape longitudinally is not loaded by the inertia of 
the tape reel and to ensure that the tape reel does not outrun the 
capstan, ordinary arrangements do not provide compensation for variation 
in tape tension at the scanner drum itself. One of the difficulties in 
doing so is the impracticability of monitoring the tape tension at or 
across the guide drum directly. 
It is one object of the present invention to provide an improved tape 
transport. 
It is another object of the present invention to provide compensation for 
variation in tension of a tape extending around a cylindrical guide drum 
in a tape transport for a video tape recorder. 
SUMMARY OF THE INVENTION 
The basis of the present invention resides in a tape transport in which a 
capstan drives tape around a cylindrical guide drum, the tape having a 
substantial angle of wrap around the guide drum and usually in a helical 
path thereabout, and servomechanical control of the tape tension in the 
tape at the side of the drum opposite to that on which the capstan is 
located is governed by the load on the capstan. In normal practice, the 
capstan is located in the tape path between the scanner drum and a take-up 
reel and accordingly a supply reel is preferably, according to the present 
invention, governed by feedback from the capstan. Thus in preferred 
embodiments of the invention a supply reel is driven by a servo to 
maintain the tension in a loop of tape between the guide drum and the 
supply reel substantially constant, and the servo receives feedback 
representing the torque required of a capstan downstream from the drum. 
This torque varies according to the friction at the scanner drum and 
thereby varies according to the tension of the tape around the drum, 
provided that the capstan is maintained at substantially constant speed, 
the capstan and its motor being preferably of low inertia. The present 
invention provides, in a preferred form, a control system in which a 
supply reel is controlled servomechanically, in accordance with the 
tension of a loop of tape formed between the drum and the reel, a capstan 
disposed downstream of a scanner drum is maintained at a constant speed 
and a reference for the servo control of the supply reel is varied in 
accordance with the capstan torque. Thus, for example, as the tape tension 
around the guide drum increases, and the consequent increase of friction 
at the drum requires, for the same speed of the capstan, an increase in 
the torque produced by the capstan, the energisation of the reel motor may 
be increased in accordance with the change in capstan torque so as to 
relieve the increase in tension. 
It is usual to provide servomechanical control of the speed of the capstan 
and for this purpose the capstan is usually provided with a tachometer 
which is used to provide a signal representing the speed of the capstan. 
That signal may be compared (at least during recording) with a reference 
to provide an error signal for energising a motor for the capstan. The 
error signal is a measure of the torque required of the capstan and may be 
used to modify the reference employed for controlling the speed of the 
motor driving the supply reel. Control of the capstan during playback may 
be effected from the control track of the tape but the error signal which 
is formed by the capstan servo will be available in analogous manner as an 
indication of the capstan torque. 
Other objects and advantages of the invention will be apparent from a 
consideration of the following description of a preferred embodiment of 
the invention with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates in simplified form the principal parts of a video tape 
recorder. As is shown in FIG. 1, the recorder 1 is intended for recording 
signals on or playing back signals from a magnetic tape 2 which is 
supplied by a supply reel 3 driven by its own motor. A preferred drive 
circuit for the motor forms part of the circuit shown in FIG. 4. From the 
supply reel the path of the tape 2 extends around a fixed guide 4 and 
thence to a guide 5 which is carried at one end of an arm 6 pivoted 
coaxially with the guide 4. The purpose of the pivoted arm is to provide 
sensing of the tension of tape in the loop extending around the guide 4. 
If the tension in the tape in this loop increases, the arm 6 will rotate. 
The rotation of the arm can be sensed in any convenient manner to provide 
a signal representing the tape tension in the loop. However, a specific 
preferred form of circuit is shown in FIG. 3 and is intended for use with 
a sensing arm 6 constructed and arranged as described in our copending 
International Application of even date entitled "Tape Loop Sensing Arm and 
Tape Guide for Magnetic Tape Recording and Playback Machines". 
From the guide 5 the path of the tape extends past a guide 7 and a video 
erase head 8 to a pair of guides 9 controlling the entrance of the tape to 
a helical path extending around a scanner drum 10 of which the axis 11 is 
slightly tilted relative to the general plane of the path of the tape. 
Within the drum is a motor driving around the periphery of the drum a 
scanning head for the scanning of the tape in oblique tracks. Although the 
proper control of the entrance and exit of the tape to its helical path 
and its following of the correct helical path are necessary for achieving 
a constant tape tension at the scanner the particular construction of the 
scanner and its associated guides is not part of the present invention. 
One suitable construction is disclosed in our International Application of 
even date entitled "Video Tape Recorders and Guide Assemblies Therefor". 
At the end of the helical path of the tape around the drum 10 the tape 
passes around a pair of guides 12 and extends past a guide 13 to a capstan 
14 which is provided with a pinch roller 15 for the maintenance of the 
tape in close proximity to the capstan. The tape path extends from the 
capstan past erase, audio and control track heads 16 to 18 and then around 
a further guide 19, a guide 20 mounted at one end of the pivoted arm 21 
rotatable about a pivot, a further guide 22 which is coaxial with the 
pivot for the arm 21 and finally to a take-up reel 23. The guide 20 acts 
in a manner similar to the guide 5, pivoting movement of the arm 21 
providing a measure of the tension in the loop of tape around the guide 20 
and providing a tension-representative signal for controlling a motor (not 
shown) which drives the take-up reel 23. 
FIG. 2 illustrates in schematic form a control system which is intended for 
use with the machine briefly illustrated in FIG. 1 and similar machines in 
which a capstan draws tape in a helical path around a cylindrical guide 
drum to which tape is supplied from a supply reel, there being a loop 
formed in the tape path between the cylindrical guide drum and the supply 
reel. In such machines, the capstan has, as previously mentioned, a 
tachometer which provides a signal of which the frequency represents the 
speed of the capstan. One suitable tachometer is described and illustrated 
in our International Application of even date entitled "Magnetic 
Tachometer Assembly." The particular construction of the tachometer is not 
relevant to the present invention. 
In the system shown in FIG. 2, a capstan tachometer 24 provides a signal of 
which the frequency varies in accordance with the speed of rotation of the 
capstan. This signal is fed to a servo circuit 25 of which the principal 
parts are shown in FIG. 2 but which will be described in more detail with 
reference to FIG. 3. 
In the servo circuit 25, the signal from the tachometer 24 is amplified by 
a preamplifier 26 so as to produce an output in the form of spike pulses 
at a frequency representative of the speed of the capstan. These pulses 
are fed to a frequency meter 27 providing an output in the form of a 
voltage likewise representing the speed of the capstan. In the specific 
embodiment to be described later, the frequency meter comprises a 
generator of a ramp signal which is sampled and restarted on the 
occurrence of each spike pulse derived from the tachometer output. The 
sampled output of the frequency meter is retained in a temporary store 28 
(which may be constituted by a capacitor) and used as the input for an 
amplifier 29 which drives a servo-amplifier 30 coupled to provide 
energisation for a motor 31 driving the capstan 14. In a preferred 
embodiment the capstan motor 31 may be a brushless DC motor incorporated 
within the capstan, as set forth in our International Application of even 
date entitled "Tape-Driving Capstan". Although the particular construction 
of the capstan motor is not crucial, it is desirable that the capstan and 
its motor have low inertia. 
The servo circuit 25 includes an integrator 32 coupled to augment the speed 
signal stored in capacitor 28 by the integral of the output of the 
amplifier 29. As is explained in our International Application of even 
date entitled "Control System for Electric Motor", this provides 
interpolation for the speed signal and enables the capstan servo to have a 
high open loop gain without instability; however, this feature is not 
crucial to the present invention. 
The output of the amplifier 29 represents, before amplification, the signal 
which actually drives the motor 31 and is accordingly representative of 
the energisation of the motor 31 and the torque of the capstan. The signal 
at the output of the amplifier 29 is used, after it is passed through a 
low-pass filter 33, as an input to a reel servo circuit 34 driving a motor 
35 coupled to drive the supply reel 3 of the tape transport. The manner in 
which the output from the amplifier 29 is used will be described in more 
detail later. 
The construction and manner of operation of the motor 35 is not of 
consequence to the present invention, although the motor is preferably of 
the kind described in our International Application of even date, entitled 
"Reel Hub Assembly for Tape Transport". 
The main control for the motor 35 is derived from a loop sensor 36. The 
construction and manner of operation of the loop sensor is not important 
to the present invention. However, it is preferred to employ a sensor of 
the kind described in our International Application of even date entitled 
"Tape Tension Sensor and Servo embodying same". In any event, it is 
sufficient for the sensor, in conjunction with a circuit illustrated 
generally within the dashed lines 37, to produce a signal which varies 
between appropriately chosen datums in accordance with the tension in the 
tape path formed adjacent the supply reel. 
As is set forth in the aforementioned International Application of even 
date entitled "Tape Tension Sensor and Servo embodying same", the loop 
sensor includes a member which is mounted to rotate with the pivoted 
sensing arm (6) previously mentioned. The sensor includes a plate carrying 
lossy strips and the plate moves in a gap of a core carrying a coil which 
forms part of an oscillator 38 forming part of the circuit 37. The 
movement of the pivoted arm serves to alter the quality factor of the 
aforementioned coil and thereby to alter the amplitude of an oscillation 
produced by the oscillator 38. This amplitude is sensed by a detector 39 
which thereby provides a direct voltage varying between appropriately 
chosen limits in accordance with the tension in the loop which is engaged 
by the loop sensor. A specific circuit arrangement for the circuit 37 is 
set forth in FIG. 4. 
The output of the circuit 37 is fed to one input of an amplifier 40 forming 
part of the circuit 34. The amplifier 40 compares the "loop tension 
signal" with a reference shown as being supplied from a reference circuit 
41 so as to provide at the output of the amplifier 40 a signal which is an 
amplified form of the difference between the signal denoting loop tension 
and a reference value. The output of the amplifier 40 is used to drive a 
servo-amplifier 42 coupled to drive the motor 35 associated with the 
supply reel 3. 
If the effect on the circuit 34 of the output of the amplifier 29 is 
ignored, then, as will be apparent, the motor 35 drives the supply reel 3 
so as to maintain a substantially constant loop tension. This is generally 
in accordance with known practice. For the sake of simplicity, various 
switching networks which may be provided at the input of the amplifier 40 
have been omitted. These switching networks are provided because the 
control of the reel motor 35 in the manner described is not normally 
required for all the possible modes of operation of the transport, 
particularly for fast rewind, and accordingly the switching network is 
arranged to provide coupling of the various signals aforementioned to the 
amplifier only for those modes of operation for which control in the 
manner set forth herein is required. However, such switching networks are 
a commonplace feature of tape transports and will not be described. 
It is found in practice that the output of the amplifier 29 fluctuates 
considerably, normally under-going. a step change in consequence of each 
new tachometer pulse and in order that the output of the amplifier 29 
should not cause unwanted transient disturbance of the driving of the 
motor 35, the low-pass filter 33 is preferably disposed between the 
amplifier 29 and the circuit 34. 
As will be apparent in more detail from a consideration of FIG. 5, the 
coupling of the output of the amplifier 29, representing the torque 
required of the capstan, to the reel servo circuit is to vary, at least 
for a range of variation of the capstan torque, for modification of the 
reference provided by the circuit 41 to the amplifier 40 which governs the 
energisation of the motor 35. 
As is mentioned above, although it is preferable to provide the low-pass 
filter 33, if the energisation of the capstan were sensed at some other 
point, for example by the use of a sensing resistor in the final drive 
circuit for the motor 31, such a low-pass filter 33 would not be 
necessary. The arrangement described is convenient because the low-pass 
filter can readily be arranged to produce a signal varying between 
convenient datums, as described hereinafter. 
It is also found in practice that the signal representing the tape tension 
fluctuates considerably and in order to avoid excessive transient 
disturbance of the motor 35 it is convenient to provide the amplifier 40 
with a transfer characteristic which is partly proportional and partially 
integrating. This is conveniently achieved by the provision of a feedback 
circuit 43 including a series resistor and series capacitor, as 
specifically illustrated in FIG. 5. 
FIG. 3 illustrates in greater detail the capstan servo circuit denoted 25 
in FIG. 2. 
In practice the capstan servo is controlled during recording by comparing a 
speed signal from a tachometer directly driven by the capstan with a 
predetermined reference and during playback the capstan servo is 
controlled by signals taken from the control track on the tape. The 
particular source of the control of the capstan is not relevant to the 
present invention and accordingly only the capstan control during 
recording will be described in detail. 
One input to the capstan servo circuit comprises a tachometer coil 74 which 
feeds the input of stages 75 which serve to shape and amplify the signal 
from the tachometer. In the present embodiment of the invention it is 
presumed that the tachometer signal is sinusoidal, as it would be provided 
by the tachometer described in the aforementioned International 
Application entitled "Magnetic Tachometer Assembly". 
The stages 75 comprise a low-pass filter 76 which serves to cut off 
frequencies substantially above the ordinary range of variation of the 
fundamental frequency produced by the tachometer and 
differentiating/limiting stages 77 which serve to convert the sinusoidal 
signal into spike pulses which are amplified in an amplifier stage 78. 
This stage feeds the input of one stage 80 of a dual monostable circuit of 
which the Q output triggers the second stage 81 of the dual monostable. 
The monostables control two switches 82 and 83 which would normally be 
constituted by transistor switches, shown as mechanical switches for the 
sake of simplicity. These two switches are connected to one plate of a 
capacitor 84 of which the other plate is grounded. The switch 82 normally 
connects the capacitor 84 through a variable resistor 85 to a positive 
supply rail 86. The lower end of the resistor 85 is connected by way of a 
diode 87 and a resistor 88 of very high resistance to a junction point 89 
connected to the non-inverting input of an FET amplifier 90. The junction 
point 89 is connected to one plate of a capacitor 91 of which the other 
plate is connected to one plate of a capacitor 92 of which the other plate 
is grounded. The capacitor 92 is of substantially greater capacity than 
the capacitor 91, for example 47 microfarads as compared with 3.3 
microfarads so that charge can be rapidly transferred from capacitor 84 to 
capacitor 91 without substantial effect on the charge held by capacitor 
92. 
The junction point 89 is also connected to the second output of the switch 
82. The switch 83 normally connects the upper plate of the capacitor 84 to 
a first output which is isolated. The second output of the switch 83 is 
connected by way of a low value resistor 93 to ground. 
Before the remainder of the circuit is described the operation of the 
switches 82 and 83 and the components associated therewith will be 
described. 
The capacitor 84 constitutes, in conjunction with the resistor 85, a ramp 
generator which is rendered operative when the switch 82 connects the 
capacitor 84 to the first output terminal of the switch 82. When the first 
monostable 80 is triggered by derived tachometer pulse from the output of 
the amplifier 78, the Q output of the monostable 80 momentarily operates 
switch 82 so as to connect the capacitor 84 to the second output terminal 
of the switch 82. Immediately thereafter the Q output of monostable 80 
triggers the monostable 81, of which the Q output operates switch 83 to 
connect the capacitor 84 to resistor 93, the switch 82 having meanwhile 
reverted to its former state in which capacitor 84 is connected to 
resistor 85. This switching action serves firstly to transfer charge from 
the capacitor 84 to the capacitor 91, the capacitor 91 thereby receiving a 
voltage representative of the ramp voltage which appeared across the 
capacitor 84, then to effect complete discharge of the capacitor 84 by way 
of the resistor 93, and finally to restart the generation of a ramp signal 
by means of the charging of capacitor 84 from the positive supply rail 86 
through the resistor 85. 
A voltage divider constituted by resistors 94 and 95 provides a reference 
voltage at the inverting input of the comparator amplifier 90 by way of a 
comparatively large value resistor 96. The amplifier 90 has a mainly 
low-pass feedback circuit 97 which provides some derivative control of 
stability in the forward direction of the amplifying circuit. Thus the 
sampled signal representing the speed of the capstan is compared with a 
reference and the difference is amplified to provide drive for the capstan 
motor by way of the further stages of amplification to be described 
hereinafter. 
The comparator amplifier 90 feeds by way of a resistor 98 and a resistor 99 
the inverting input of a further FET amplifier 100 of which the 
non-inverting input is referenced by a voltage divider comprising 
resistors 101 and 102 connected between the positive rail 86 and ground. 
Some phase adjustment, of no particular consequence to the present 
invention, is provided by a resistor-capacitor branch 104 shunting the 
resistor 99 and a shunt capacitor 103 between ground and the junction 
between resistors 98 and 99. 
Feedback is taken from the output 105 of the amplifier 100 back to the 
store constituted by the capacitor 91 by way of a loop constituted by a 
passive integrator comprising a resistor 136 and the capacitor 92. The 
purpose of this integrator is explained in the aforementioned 
International Application entitled "Contrgl System for Electric Motor" and 
is, briefly, to provide anticipatory interpolation of the speed signal in 
the intervals between the sampling times at which it is measured. This is 
advantageous when the capstan and its motor are of low inertia and are 
driven at a low speed. 
In order to monitor the energisation of the capstan so as to obtain an 
indication of the torque applied by the capstan and an indication of 
tension in the tape extending around the scanning head, the amplifier 
stage 100, which forms part of the amplifier 29 indicated in FIG. 2, is 
monitored by a circuit 106 coupled to the output line 105 of the amplifier 
100. This circuit 106 comprises a comparator amplifier 107 of which the 
non-inverting input is referenced by a voltage divider comprising 
resistors 108 and 109. The inverting input of amplifier 107 is connected 
to the line 105 by way of a resistor 110 and the amplifier 107 has a 
parallel resistive/capacitative feedback circuit 113 and feeds by way of a 
resistor 111 an output terminal 112. The circuit 106 constitutes the 
low-pass filter 33 of previous mention and in practice is arranged to 
provide, at the output terminal 112, a voltage which varies between 8 
volts and 2 volts as the torque required of the capstan varies from a 
maximum to a minimum. The output terminal 112 is connected to provide one 
of the inputs to the amplifier 40 of the reel servo circuit 34, which is 
shown in detail in FIG. 5. 
The line 105 is connected by way of a resistor 114 to the non-inverting 
input of an amplifier 115 which has a capacitative feedback impedance 116 
and of which the inverting input is connected by way of a resistor 117 to 
the positive rail 86. This amplifier 115 is an input stage for a power 
amplifier 119 of which a first stage is constituted by two complementary 
transistors 120 and 121 of which the emitters are connected in common to 
the output of amplifier 115 by way of the resistor 118. The bases of the 
transistors 120 and 121 are connected together to the junction of a 
voltage divider constituted by resistors 122 and 123 connected between the 
positive rail and ground. The collector of the transistor 120 is connected 
to the base of a power transistor 124 of which the emitter is connected to 
the base by way of a resistor 126 and is also connected to a positive rail 
127. The collector of the transistor 121 is connected to the base of a 
power transistor 125 of which the emitter is connected to the base of the 
same transistor by way of a resistor 128 and is also connected to a rail 
129 at zero volts. Between a junction 130, connected to the collectors of 
transistors 124 and 125, and the zero volts rail 129 is connected a 
capstan motor commutator circuit 131. This circuit is of no consequence to 
the present invention but is preferably an electronically switched circuit 
for energising the stator windings of the motor 31, which is preferably a 
brushless commutator motor. However, other forms of motor can readily be 
energised by the power amplifier in known manner. 
Inserted between the commutator circuit 131 and the zero volts rail 129 is 
a current sensing resistor 132 the ends of which are connected 
respectively through resistors 133 and 134 to the non-inverting and 
inverting inputs respectively of the amplifier 115. This arrangement 
provides current feedback for the motor and provides some reduction of 
switching transients produced by the commutator circuit 131. 
FIG. 4 illustrates the circuit associated with the tape loop sensor. The 
circuit shown in FIG. 4 receives an electrical supply from a positive rail 
173 and a zero volts rail 174. The voltage between the rails is stabilised 
by a capacitor 175. A resistor 176 connects the positive rail to the 
emitter of a transistor 177 of which the collector is connected to the 
negative rail by way of two capacitors 178 and 179 in series. The output 
of an amplifier 180 is connected by way of a resistor 181 to the base of 
the transistor 177. The base of this transistor is connected to the zero 
volts rail 174 by way of a capacitor 182 and the emitter of the transistor 
177 is connected to the inverting input of amplifier 180 by way of a 
resistor 183. The emitter of transistor 177 is connected to the junction 
184 of the capacitors 178 and 179. The non-inverting input of the 
amplifier 180 is connected to the tap of a variable resistor 185 which is 
connected to the positive rail 173 by way of resistor 186 and to the 
negative rail 174 by way of resistor 187. 
The amplifier 180 and the transistor 177 constitute the active components 
of an oscillator of which the operating frequency is determined by the 
capacitors 178 and 179 together with the coil 138. The amplitude of the 
oscillation varies according to the quality factor of the coil 138 which, 
as previously mentioned, is varied according to the position of the lossy 
strips 136 which are carried on a plate mounted for movement with the 
sensing arm 6 shown in FIG. 1. 
In order to monitor the amplitude of the output of the oscillator, the 
detector 39 is connected between the rail 174 and the point 188 between 
the capacitor 178 and the collector of transistor 177. The detector 
comprises a diode 189 which couples the point 188 to one plate of a 
capacitor 190 of which the other plate is connected to the rail 174. The 
upper plate of the capacitor 190 is connected by way of a resistor 191 to 
the non-inverting input of an amplifier 192. This amplifier is, as is the 
amplifier 180, preferably of type 358. The non-inverting input of the 
amplifier 192 is connected to the positive rail 173 by way of a diode 193 
and a resistor 194. The inverting input of the amplifier 192 is connected 
to the zero volts rail 174 by way of a resistor 195 and to an output line 
of the amplifier 192 by way of a variable resistor 196. The output line of 
the amplifier 192 extends to an output terminal 197, nominally at five 
volts. 
The purpose of the detector 39 constituted by the amplifier 192 and the 
associated components is to provide an output varying from 2 volts (when 
the tape tension is at a maximum) to 8 volts (when the tape tension is at 
a minimum). 
FIG. 5 illustrates in more detail the amplifier 40 of the reel servo 
circuit 34 and the components associated therewith. 
One input to the amplifier circuit 40 is the terminal 197 already mentioned 
in reference to FIG. 4; at this input terminal appears a voltage which 
varies in accordance with the tension of the tape in the loop sensed by 
the sensing arm 6. The voltage varies between 2 volts for maximum tape 
tension and 8 volts for minimum tape tension. The terminal 197 is coupled 
to the non-inverting input of an amplifier 198 by way of an 82 kilohm 
resistor 199. The output of the amplifier 198 is connected by way of a 330 
kilohm resistor 200 to an output line, which may feed the input of a power 
amplifier 42 (FIG. 2) of which the construction is not important for the 
present invention. 
The circuit 43 in FIG. 2 is in FIG. 5 represented by the series RC circuit, 
constituted by the resistor 201 and the capacitor 202 connected between 
the resistor 200 and the input terminal 197. The resistor 201 may be a 33 
kilohm resistor and the capacitor 202 a 0.33 microfarad capacitor. 
The input to the non-inverting terminal of the amplifier 198 will be at 
approximately five volts when the sensing arm 6 is in its mean position 
but, as previously mentioned can vary in a range extending from a minimum, 
selected to be two volts when the tension in the tape is a maximum, to a 
maximum, selected to be eight volts, when the tension in the tape sensed 
by the arm 6 is a minimum. This input, representing the tape tension, is 
compared with a reference signal provided by the reference circuit 41 
previously discussed with reference to FIG. 2. For simplicity the 
aforementioned switching networks which are disposed between the reference 
circuit 41 and the amplifier 198 have been omitted. 
The reference circuit 41 receives at the terminal 112, corresponding to the 
terminal 112 in FIG. 3, the signal which represents capstan torque. This 
signal, which varies between two volts and eight volts is coupled by way 
of a resistor 203 to the junction 204 between two resistors 205 and 206 
which are connected between ground and a positive rail 207 at ten volts. 
The junction point 204 is coupled to the inverting input of the amplifier 
198 by way of an input resistor 208. 
Between the output terminal of the amplifier 198 and the inverting input 
terminal is a feedback circuit 211 comprising a resistor 210 shunted by a 
series combination of a resistor 209 and a capacitor 212, so that the 
amplifying characteristic of the amplifier 198 is partly proportional and 
partly integrating. 
The purpose of the reference circuit 41 is to provide for comparison with 
the signal representing tension in the tape engaged by the guide 5, a 
reference signal which varies in a range in accordance with the signal 
which represents the capstan torque, so that when the capstan torque 
indicates that the tape-to-scanner friction is low, the reference is such 
as to prescribe a relatively high tape tension on the supply side, and 
when the capstan torque indicates that the tape-to-scanner friction is 
high, the reference is such as to prescribe a relatively low tape tension 
on the supply side. The tape tension is, of course, regulated to the 
reference by the control of the speed of the supply reel which is driven 
in accordance with the error signal developed by the amplifier 198. 
The range of variation of the reference should preferably be considerably 
less than the total possible range of tension which can be accommodated by 
the sensing arm. In practice, it is convenient to arrange the system so 
that during normal tape transport the supply tape tension should vary in 
an upper half of a range and so that during, for example, fast wind the 
supply tension varies in a lower half of the range. Since the signal which 
represents tape tension is at two volts when the tape tension is a 
maximum, the resistive values should be chosen for the reference circuit 
and the amplifier so that the reference effectively varies within the 
range from five volts to two volts, and preferably within a slightly 
narrower range than this. Suitable values for the components are indicated 
in FIG. 5.