Adaptive end of tape detection apparatus and method

The changing diameters of the respective tape packs wound on the supply and take-up reel are continually measured. The measured diameters of each reel are compared to a stored minimum diameter value corresponding to an end of tape on that particular reel. When the apparatus is in a learning mode, the stored minimum diameter values for each reel are updated every time when a new measured value is smaller than the previously stored value. The last stored value is utilized for comparison in an end of tape detection mode, to allow stopping the tape transport mechanism before the end of tape would run off that reel to prevent damaging the transducers or the tape.

The invention relates to end of tape detection and, more particularly, to 
an apparatus and method for detecting end of tape on reels utilized in 
tape recording or reproducing devices. 
In tape recording or reproducing devices it is generally known to transport 
an information signal carrying tape between two reels on which the two 
opposite ends of the tape are secured. The tape is transferred by 
transport means from one of the reels, generally referred to as a supply 
reel, to the other reel, generally referred to as a takeup reel. When the 
tape moves longitudinally in a forward direction, the effective diameter 
of the tape pack wound on the supply reel is decreasing while the 
effective diameter of the tape pack on the takeup reel is increasing. 
When the tape is transported as indicated above in a magnetic recording or 
reproducing device, it passes in close proximity of one or more 
electromagnetic transducers which may record an information signal thereon 
or reproduce a previously recorded information signal therefrom, 
respectively. During the recording or reproducing operation the 
longitudinal tape speed is accurately controlled, usually by a capstan 
drive which drives the tape at a selected speed past the transducers. The 
rotation of the capstan is accurately controlled by a capstan drive servo 
circuit as it is well known in the art of magnetic tape recording and 
reproduction. 
During the above-mentioned transfer of tape between the two reels it is 
important to detect continuously the amount of tape on each reel so that 
it may be determined, for example, how much tape is remaining for 
recording or reproduction on a reel or in which direction should the tape 
move in a search mode to locate a certain portion of the recorded 
information. Therefore, it is well known to measure the instantaneous 
diameter of a tape pack on each reel, for example by counting the number 
of linear units of tape for a given number of whole or fractional number 
of reel revolutions. 
As an example, it has been known to measure the instantaneous tape pack 
diameter as a ratio of capstan or idler tach pulses, shortly tachs, and 
the supply or takeup reel tachs, respectively. As well known, the smaller 
the diameter of the tape pack wound on the reel, the faster are the reel 
revolutions. However, the capstan revolutions remain substantially the 
same for a selected tape speed, regardless of the changing speed of either 
the supply or the takeup reel. 
It is also important to measure the amount of tape remaining on the reels 
to permit advanced determination of the end of tape being wound off a 
reel. If the end of tape is permitted to be wound uncontrollably off a 
reel, it often leads to damage of the tape or delicate parts of the tape 
recording/reproducing device located in close proximity to the tape 
transport path. In some devices, the unwound end of the tape has been 
known to strike and damage the rotating transducers. Moreover, the 
uncontrolled end of the tape can lead to the tape impacting and being 
scratched or otherwise damaged by the tape guide elements along the path 
of transport. 
For example, helical scan recording or reproducing devices employing a 
single capstan/pinch roller mechanism for controlling the transport of the 
tape are particularly susceptible to these harmful effects. In such 
devices, the capstan/pinch roller mechanism is located between one of the 
tape reels (often the takeup reel) and the rotating head/tape guide drum 
assembly, commonly, referred to as a scanner. When transporting the tape 
in the forward direction from the supply reel to the takeup reel under 
control of the engaged capstan and pinch roller, the tape can be brought 
to a stop rapidly when its end leaves the supply reel without endangering 
the tape or delicate elements of the recording/reproducing device. When 
transporting the tape in the reverse direction from the takeup reel to the 
supply reel, the transported tape can be brought to a stop without 
endangering the tape or delicate elements of the transport mechanism, 
because the engaged capstan and pinch roller holds the loose end of the 
tape when wound off the takeup reel. However, in other operating modes of 
the recording/reproducing device when the capstan and pinch roller are out 
of engagement, such as tape shuttle when tape is rapidly transferred from 
one reel to the other, there is danger of harm to the delicate elements of 
the transport mechanism and/or the tape. 
In one type of such devices, the scanner rotates in a direction opposite to 
the direction of travel of the tape around the rotating scanner when the 
tape is transported in the forward direction from the supply reel to the 
takeup reel. This establishes a pressure gradient at the location where 
the tape leaves the scanner in its travel towards the capstan and 
following takeup reel that creates a force on the tape tending to draw the 
tape towards the rotating scanner. As long as the tape is wound on both 
reels, this force is overcome. However, in the reverse tape shuttle mode 
of operation (capstan and pinch roller disengaged and the tape transported 
from the takeup reel to the supply reel), this force can draw the end of 
tape wound off the takeup reel into the rotating scanner. When this 
occurs, the transducers carried by the rotating scanner and the tape 
itself are often damaged. 
To prevent such damages, it has been known to compare the measured diameter 
of the decreasing tape pack on the supply reel to a safe minimum value. 
When the measured diameter reached that minimum diameter value the tape 
transport has been stopped. As the minimum diameter value, a constant 
value has been utilized which has been determined from a largest possible 
reel hub diameter of commercially available reels and minimum amount of 
tape which has to be wound on the reel to secure the end of tape thereon. 
Because the reel hub diameters of commercially available reels for a given 
reel size vary significantly, generally by as much as forty percent, that 
safe minimum diameter value did not allow stopping the tape transport at a 
location corresponding to a desired minimum amount of tape wound on the 
reel hub. Therefore, in instances where the hub diameter was much smaller 
than presumed by the safe value, a considerable amount of tape remained on 
the supply reel when the end of tape has been detected by the prior art 
circuits. Consequently, the tape could not be shuttled or cued to a 
material previously recorded on that portion of the tape. 
SUMMARY OF THE INVENTION 
The present invention overcomes the shortcomings and disadvantages of the 
prior art by providing an adaptive end of tape detection method and 
apparatus. In accordance with the invention the respective tape pack 
diameters on the supply and takeup reel are continually measured and 
compared to a stored minimum diameter value corresponding to an end of 
tape on that particular reel. When, for example, a reel of tape is 
inserted into the device, the end of tape detection circuit compares the 
measured diameter of the tape pack wound on that reel to a stored safe 
minimum diameter value. That stored value corresponds to a largest 
possible minimum diameter value which is necessary to assure that the end 
of tape will remain on the reel. When the measured diameter value is 
smaller than the stored value, the stored value is updated by that 
measured value. It will be further referred to the above-described 
operation as "learning mode". In accordance with the present invention, 
the learning mode is selectively enabled when the tape is transported 
under such conditions that no harm can be done thereto or to any portion 
of the recording/reproducing device, if the tape is unwound from a reel. 
For example, in a recording or reproducing device having a single 
capstan/pinch roller mechanism briefly described hereinbefore, a learning 
mode can be enabled during any capstan controlled tape transport operating 
mode or a forward tape shuttle operating mode. The learning mode would be 
disabled in the reverse tape shuttle mode. It should be appreciated, 
however, the particular operating mode of a recording or reproducing 
device that would endanger the tape or elements of the device as a result 
of being wound off a reel depends upon the construction of the transport 
mechanism and operating modes of the device. For example, helical scan 
recording and reproducing devices can have various relative tape transport 
and transducer rotation arrangements. In some of these devices, the 
transducer is rotated in a direction opposite to the forward direction of 
travel of the tape about the rotating scanner, as previously described. In 
others, the transducer is rotated in the same direction as the travel of 
the tape about the rotating scanner. Helical scan recording and 
reproducing devices also can have tape transport mechanisms utilizing 
other than capstan/pinch roller mechanisms for controlling the transport 
of the tape. Vacuum capstans and vacuum columns are examples of such other 
mechanisms. Moreover, the capstan/pinch roller mechanism can be located at 
either side of the rotating scanner. Such variations in the transport 
mechanism can alter the operating mode that endangers the tape and 
delicate elements of the recording or reproducing device when the end of 
the tape is wound off a reel. In the preferred embodiment of the 
invention, the end of tape detection apparatus operates in the learning 
mode when the tape is transported under capstan control such as during 
recording or playback. Although the learning mode can be executed in some 
other operating modes as well, such as, the forward tape shuttle mode when 
the capstan and pinch roller are disengaged, it is preferred to execute 
the learning mode only when the tape transport is under capstan control. 
Thus in the learning mode the stored diameter value for each reel is 
updated every time when the measured diameter value is smaller than the 
stored value. When the tape is transported during shuttle or cue 
operation, when the capstan control is not utilized, the learning mode is 
disabled. When the measured diameter of a decreasing tape pack reaches the 
last stored minimum value, during a shuttle or cue operation, the end of 
tape detection circuit applies a control signal to the tape transport 
means which in turn causes the transport means to stop. Consequently, the 
tape transport can be stopped precisely at a desired location for each 
individual reel while only a minimum amount of tape is left on the supply 
reel which is necessary to prevent tape run-off. It is a significant 
advantage of the invention that the foregoing operation is applicable to 
individual reels which may have significantly different hub diameter 
sizes. 
Additional features and advantages of the invention include resetting the 
apparatus and restarting the learning operation every time a particular 
reel is removed from the recorder or the tape is outside the prescribed 
tape path in the recorder. This assures that the minimum diameter value 
which has been obtained for a particular reel will not be utilized for a 
different reel, which may have a different hub diameter. A new minimum 
value for that new reel will then be obtained through a new learning 
process. 
A further feature which prevents erroneous diameter values from being 
stored includes comparing two subsequently measured diameter values. The 
stored value is allowed to be updated only if there are no significant 
differences between these two values, since abrupt changes between these 
values would indicate measuring errors.

DETAILED DESCRIPTION 
Referring now to FIG. 1, a magnetic tape 20 is shown as being transported 
between a supply reel 22 and a takeup reel 24 in a forward direction shown 
by arrow 26. The tape 20 passes around a rotating scanner drum 28 as it is 
well known from rotary scan recorders, for example helical scan recorders. 
One or more transducers (not shown) are arranged around the periphery of 
the scanner drum 28, and engage the tape 20 for recording thereon or 
reproducing therefrom an information signal, as it is well known in the 
art. 
A tape timer idler 30 is arranged along the tape path in contact with the 
tape 20. Respective tach pulse detection circuits 32, 34 and 36 for the 
supply reel 22, the takeup reel 24 and the tape timer idler 30 are 
utilized to sense the rotation of these respective elements in a manner 
well known in the art. A capstan 31 and pinch roller 33 are shown as being 
located along the tape path as well known in the art. 
The continually changing diameters of the tape packs on each reel 22, 24 
are continuously measured by respective diameter measuring circuits 38, 40 
which circuits will be described later in more detail. The circuit 38 
receives the supply reel tach pulses via line 42 and the idler tach pulses 
via line 44 and it provides an output signal A on a data line 41, 
representing the tape pack diameter on the supply reel 22. Similarly, the 
takeup reel tape pack diameter measuring circuit 40 receives the tach 
pulses via line 48 and the idler tach pulses via line 44 and it provides 
an output signal A' on a data line 45, representing the current tape pack 
diameter on the takeup reel 24. To simplify the description, it will be 
further referred to these respective diameters as supply and takeup reel 
diameter. 
A tape detector 25 is utilized to detect absence of tape from the 
prescribed tape path within the recorder. As the detector 25 for example a 
well known photo emitter and photo detector can be utilized, preferably 
type GE 23B1 manufactured by General Electric Corporation. The photo 
emitter and photo detector 25 are each located on an opposite side of tape 
20 as shown in FIG. 1. 
It is noted that lead lines conveying control signals are designated by a 
single line while lines conveying data are designated by double lines 
throughout the drawing FIGURES. 
During the tape movement the currently measured diameter values A, A' are 
compared in comparators 50, 52 with minimum diameter values B, B' 
presently stored in memories 54, 56, respectively. 
Prior to each end of tape detection operation, for example when a tape reel 
has been removed from the tape recorder and thereafter it is reinserted 
therein, when a new tape reel is inserted instead of the removed reel or 
when the tape has been detected as being outside its prescribed path, a 
predetermined minimum diameter value for both the supply and takeup reel 
is stored in the memories 54, 56, respectively. This initially stored 
value may, for example, correspond to the previously described safe 
minimum tape pack diameter value utilized by the prior art circuits. This 
value, further being referred to as "default value", represents the worst 
case of a minimum tape pack diameter obtained by considering the largest 
hub diameter size of commercially available reels. In FIG. 1 the default 
value is stored on lines 71, 72, for each reel 22, 24 for example as a 
hard-wired binary value, in a well known manner. 
For example, prior to a recording or playback operation, when a full reel 
of tape such as a supply reel 22 is inserted in the recorder, it is 
threaded through a prescribed tape path, including various tape guides 
(not shown in FIG. 1), the rotating scanner 28, the capstan 31, tape 
detector 25, and the idler 30, as well known. The end of tape is attached 
to a hub of an empty takeup reel 24 by winding a few turns of tape on the 
hub for example three or more turns, as needed to secure the tape end. 
Thereafter, a learning mode is initiated automatically by the device by 
applying for example a high control signal on line 61. For example, the 
signal on line 61 may be obtained from a well known capstan drive servo 
circuit, such as shown at 116 in FIG. 4, as it will be described later. 
In accordance with an important feature of the invention in the learning 
mode the end of tape detection circuit is allowed to "learn" at least once 
whenever a currently measured reel diameter A or A' is smaller than the 
previously stored minimum diameter B or B'. If A is smaller than B, or A' 
is smaller than B', a respective comparator 50 or 52 applies a control 
signal on line 57 or 59 to a control logic and memory circuit 58 or 60, 
respectively. When the control signal on line 61 indicates that continuous 
learning is allowed and a newly measured value A or A' is smaller than the 
previously stored value B or B', then B or B' will be replaced by A or A', 
in the memory 54 or 56, respectively. 
It follows from the foregoing description that when the apparatus of the 
present invention is in learning mode, the memories 54, 56 are continually 
updated whenever a smaller than the stored diameter is measured. It is 
then desired to stop the tape transport when the reel diameter becomes 
smaller than the last stored diameter to prevent tape running off the reel 
when the apparatus operates in other than the learning mode. 
It will be appreciated that regardless of the direction of tape movement, 
each time when a smaller than the presently stored diameter value for that 
particular reel is measured, both memories 54, 56 are continually updated 
by that smaller value. 
It will be understood with respect to the foregoing description that in 
case of the takeup reel 24 whose tape pack diameter is continuously 
increasing when the tape 20 is transported in the forward direction 26, 
preferably only the first valid measurement is considered by the circuit. 
Analogously, only the first valid measurement of the supply reel tape pack 
diameter is considered when the tape is moving in reverse direction. The 
foregoing features allow updating the default diameter value once after 
the tape has been reinserted into the device while eliminating unnecessary 
and repetitious diameter comparison operations for increasing diameter 
values, as it will be described later in more detail. 
In the presently described preferred embodiment, utilized in a helical scan 
tape recorder, the learning mode preferably takes place during recording, 
playback and variable speed playback. During these operation modes the 
longitudinal tape movement is controlled by the capstan 31, as well known. 
Consequently when the end of tape becomes detached from the reel, the tape 
is being held in position between the capstan 31 and the pinch roller 33. 
Therefore there is no significant risk involved in damaging the 
transducers or the tape, as it would occur without the capstan control. 
When the tape 20 is running in shuttle or cue operation and the capstan 
control is disengaged, the relative transducer-to-tape speed is generally 
in the order of 1500 inches per second in forward and 500 inches per 
second in reverse direction. During the above indicated operation 
continuous learning is disabled by applying for example a low signal on 
line 61, indicating that the capstan drive servo circuit is disabled. When 
now value A is smaller than the updated value B stored in memory 54, the 
comparator 50 applies a control signal on line 57 to the control logic and 
memory circuit 58. In response thereto the circuit 58, applies a stop 
signal via line 62 to stop the tape transport. 
It will be understood with respect to the similarity between the circuit 
portions of FIG. 1 related to the supply reel and takeup reel, 
respectively, that the foregoing description is also applicable to the 
takeup reel circuit portion and therefore will not be repeated herein. 
It is seen from the foregoing description that the present invention has 
the advantage of stopping the tape transport for every individual reel at 
an optimum position while leaving just enough tape wound on the reel to 
secure its end on the hub. 
It is noted that while in the present description it is generally referred 
to reel 22 as supply reel and to reel 24 as takeup reel, these denotations 
do not limit the tape movement to the forward direction shown by arrow 26. 
Thus when the tape is transported in a reverse direction, that is opposite 
to arrow 26, learning is also allowed in the preferred embodiment, 
provided the tape is under capstan control. 
With further reference to FIG. 1, when the tape 20 becomes slackened or is 
removed from the prescribed tape path, for example when changing reels or 
reinserting a reel, a control signal from tape detector 25 is applied via 
line 69 to the logic and memory circuits 58, 60. These circuits 58, 60 in 
turn reset via lines 66, 68 the respective contents of both memories 54, 
56 to the previously mentioned default diameter value on lines 71 and 72, 
respectively. Memory input enable control lines 73, 74 are activated by 
the respective logic and memory circuits 58, 60 whenever the contents of 
memories 54, 56 are being updated or reset. The control signal on line 69 
also disables the diameter measuring circuits 38, 40, respectively, to 
prevent obtaining erroneous values therefrom. 
The present invention will be further described with reference to a more 
detailed functional block diagram of the end of tape detection circuit 
shown in FIG. 2A, and corresponding to FIG. 1. 
It is noted that like circuit elements are designated by like reference 
numerals in all the drawing FIGURES. 
It follows from the above description of FIG. 1 that in the preferred 
embodiment the circuit portions utilized for each reel 22, 24 are 
substantially identical. Therefore, the diagrams of FIGS. 2A and 3 depict 
only circuit portions pertaining to one reel, for example the supply reel 
22, to simplify the circuit representation as well as the description. 
However, the diagram of FIG. 2B pertains to both the supply and takeup 
reel circuit portions, respectively. 
In FIG. 2A a portion of the control logic and memory circuit 58 of FIG. 1, 
indicated 58b, is shown in more detail and it will be described below. 
When the end of tape detection circuit operates in a mode where learning is 
not allowed, for example in a shuttle mode, a control signal received on 
line 75 is low. That signal is provided by a common portion 58b, 60b of 
the control logic and memory circuits 58, 60, which portion is shown in 
FIG. 2B and will be described later. When at the same time the measured 
current diameter value A on line 41 is smaller than the minimum diameter 
value B currently stored in memory 54 and applied via line 45 to 
comparator 50, a high control signal on line 57 is applied to connected 
first inputs of AND gates 81, 82. The signal on line 75 is applied to 
connected second inputs of AND gates 81, 82, of which the second input of 
gate 81 is inverted. The respective input signals on lines 57, 75 effect a 
high output signal on line 62 from gate 81 which in turn is applied to 
stop the tape transport mechanism (not shown), thereby preventing tape 
run-off from the supply reel 22, as previously described. At the same time 
the output signal from gate 82 on line 89 is low. Thus in absence of a 
reset signal on line 69 the output signal on line 73 from OR gate 83 is 
low and the memory 54 is prevented from loading a new value. 
In other operating modes, for example during record, playback or variable 
speed playback, learning is allowed and the control signal on line 75 is 
high. If a high control signal on line 57 is obtained indicating that the 
currently measured diameter A on reel 22 is smaller than the value B 
previously stored in memory 54, the output signal from AND gate 81 on line 
62 is low and the tape transport will not be stopped. The respective high 
signals on lines 57, 75 effect application of a high output signal on line 
89 from AND gate 82 to one input of OR gate 83. This is turn effects a 
load signal on line 73 to go high which signal enables updating the memory 
54 with a new value. 
The memory 54 will be now updated by replacing the previously stored value 
B by the currently measured value A as follows. An adder 84 is preferably 
utilized to add a predetermined constant value C obtained on line 85 to 
the currently measured value A on line 65. The value C is selected to 
compensate for an inertia of the tape transport mechanism which causes the 
tape movement to continue after the stop signal on line 62 has been 
applied thereto. Thus the value C is determined considering a particular 
inertia provided by a particular tape transport, a particular tape speed 
and reel size utilized. The thusly compensating tape diameter value A+C is 
applied via line 86 to one input of a switch 87. The other input of switch 
87 is coupled to receive the previously mentioned default value on line 
71. The output of switch 87 is coupled via a data line 66 to a data input 
of memory 54. Because the respective signals on lines 71 and 85 represent 
constant values they may be for example preset or hard wired binary values 
as it is well known in the art. 
The switch 87 is controlled by the previously described reset line 69 to 
apply one of its inputs to its output 66. When the control signal on the 
reset line 69 is low, the switch is controlled to apply the value A+C via 
line 66 to memory 54 where it replaces the previously stored value B. At a 
high reset signal on line 69 indicating, for example, that the tape is 
outside its predetermined path, the switch 87 is controlled to apply the 
default value on line 71 via line 66 to the memory 54 where it replaces 
the previously stored minimum tape pack diameter value B. The signal on 
line 69 is obtained for example from the previously described tape 
detector 25 shown in FIG. 1. The latter signal serves to prevent a stored 
minimum diameter value which has been detected for a particular reel from 
being utilized for a different reel which may have a significantly 
different reel hub diameter, as previously described. It also prevents the 
use of invalid diameter numbers from entering the memory. 
It will be understood from the foregoing description that in applications 
where it is not necessary to compensate for the inertia, the line 85 and 
adder 84 in FIG. 2 will be deleted from the circuit diagram. It is noted 
that when the adder 84 is utilized, memory 54 in FIG. 2A should only be 
allowed to "learn" when the diameter of reel 22 is decreasing to avoid 
continuously increasing the stored diameter number. 
FIG. 3 shows an example of a detailed functional block diagram of the 
diameter measuring circuit 38 of FIGS. 1 and 2A and will be described now. 
The previously described elements 31, 33 and 25 shown in FIG. 1 as being 
located along the tape path are deleted from FIG. 3 for simplicity. 
As it has been previously described with reference to FIG. 1, the supply 
reel 22 tach pulses from detector 32 are applied via line 42 to one input 
of the diameter measuring circuit 38. The tape timer idler 30 tach pulses 
detected by detector 36 are applied via line 44 to a second input of the 
circuit 38. 
In the preferred embodiment during one revolution of the supply reel 22 
sixteen tach pulses are generated by the tach detector circuit 32 and 
transmitted on line 42 to a clock input of a first counter 91 of FIG. 3. 
The counter 91 is utilized as a divide by 16, that is, it provides a "once 
around" low output pulse on line 93 for each 16 clock pulses received on 
line 42. Thus counter 91 divides by a number n corresponding to the number 
of supply reel tach pulses per revolution. The output pulses on line 93 
are applied to an input of a second counter 95 whose clock pulses are 
provided by the tape timer tach pulses on line 44. Thus the second counter 
95 counts the number of tape timer tach pulses on line 44 occurring 
between two consecutive once around output pulses on line 93. The 
resulting count A on line 97 in the form of a binary number is applied 
during each once around pulse on line 93 to a current diameter memory 98 
and is stored therein as it will be described below. 
To facilitate the description, the circuit of FIG. 3 will be first 
described without considering the encircled circuit portion 101. For the 
simplified description a control line 102 will be considered, shown by an 
interrupted line. 
In accordance with that simplified description, a reel diameter value A 
which has been previously stored in memory 98 is updated by a currently 
measured value A as follows. 
During each once around low pulse on line 93 the counter 95 stops counting. 
The duration of that pulse is preferably one sixteenth of a supply reel 
revolution. The pulse on line 93 is applied via line 94 to an inverting 
input of an AND-gate 100. The other input of AND-gate 100 is kept high 
unless there is a change in the tape direction as indicated by the control 
signal on line 111a, as it will be described later. A resulting high 
output signal on line 102 from AND-gate 100 is applied as a load control 
signal to the current diameter memory 98. In response to the signal on 
line 102 the memory 98 updates its contents with the currently measured 
diameter value A applied thereto on line 96. 
It follows from the foregoing description that the memory 98 is updated 
once per revolution of the supply reel 22, unless the control signal on 
line 111a indicates change of direction. The control signal 111a is 
obtained from a tape direction output signal applied by a tach processor 
88 connected to the tape timer idler tach circuit 36 shown in FIG. 1. 
When a change in the direction of longitudinal tape movement occurs, that 
is from forward to reverse movement or vice versa, both the supply and 
takeup reel 22, 24 will first come to a stop and thereafter they will 
start to rotate in opposite direction. During that transition period the 
tape idler 30 shown in FIG. 1 may continue to rotate even when the reels 
22, 24 are at a temporary standstill. Consequently the counter 95 would 
provide erroneous tape diameter measurements. To prevent the foregoing 
errors, the previously mentioned tape direction change control signal on 
line 111a resets both a D-flip-flop 106 and the first counter 91 via an 
OR-gate 104. The flip-flop 106 has its D-input permanently connected to a 
logic high signal and its clock input connected via line 92 to the output 
of the first counter 91. Thus the Q-output of flip-flop 106 is high 
whenever a rising edge of a previously described low pulse from counter 91 
occurs on line 93. However, when a reset pulse on line 111a is received, 
the flip-flop 106 provides a low going pulse at its Q-output and the load 
signal on line 102 is disabled, thereby preventing a currently measured 
diameter value A from updating the memory 98. 
The circuit portion 101 serves to further improve the operation of the 
circuit of FIG. 3. Prior to updating a previously stored value in the 
memory 98 with a new value A, the circuit portion 101 compares the last 
obtained measured value A1 with the new value A. Only when it is 
determined that the new value A does not significantly differ from the 
last measured value A1, then it is loaded in memory 98. Because these 
values A, A1 are being compared after each revolution of the reel, any 
significant change in these subsequent diameter values from one revolution 
to the next would indicate an error in the current diameter measurement 
and therefore the memory 98 should not be updated in that case. 
In the embodiment of FIG. 3 the circuit portion 101 utilizes a last 
measured value memory 110 and a comparator 113. It is noted that when the 
circuit portion 101 is utilized, the interrupted line 102 is deleted from 
the circuit diagram. Thus the current measured diameter value A is applied 
from the second counter 95 via line 97 to the last measured value memory 
110. The previously described memory load control line 102 is now 
connected to memory 110 instead to the memory 98. In operation, at every 
once around output pulse on line 93 from counter 91 and when no change in 
the direction signal is received on line 111a, the value A from counter 95 
is loaded into the memory 110 and held until the arrival of the next pulse 
on line 93. The comparator 113 receives the currently measured value A via 
line 96 and the stored value A1 from the last value memory 110 via line 
103 and compares these values. When the two values are only slightly 
different while the difference basically corresponds to one revolution of 
the reel, then the comparator 113 provides a load signal on line 108. That 
load signal in turn enables loading the output signal A on line 96 into 
memory 98, thereby updating the current diameter measurement therein. The 
content of the current diameter memory 98 is applied via line 41 to a 
comparator 50 as it has been previously described with respect to FIG. 2A. 
To assure proper operation of the circuit of FIG. 2A, the previously 
described transfer of value A on line 41 from circuit 38 to the comparator 
50 is also controlled by the load control signal on line 102 or 108, 
respectively. 
FIG. 2B shows a further control circuit portion 58a, 60a which is common to 
both control and memory circuits 58, 60 of FIG. 1. The circuit of FIG. 2B 
controls the previously described learning operation modes of each 
respective reel 22, 24 and it will be described below. 
The circuit of FIG. 2B receives the previously mentioned direction control 
signal on line 111 from the tach processor 88 shown in FIG. 1, indicating 
whether the tape 20 moves in forward or reverse direction. The signal on 
line 111 is applied to one input of a first NAND gate 216 and to an 
inverting input of a second NAND gate 217. The other inputs of NAND gates 
216, 217 receive the previously described control signal on line 61 shown 
in FIG. 1, indicating that continuous learning is allowed. In the 
preferred embodiment the signal on line 61 is applied from a capstan servo 
circuit, which circuit is well known in the art and it is shown for 
example at 116 in FIG. 4. 
The respective output signals from each NAND gate 216, 217 are applied on 
lines 218, 219 each to a respective input of a D-flip-flop 220, 221. The 
D-flip-flops receive the previously described control signal on line 102 
or 108 of FIG. 3 as a clock signal and they also receive a reset signal on 
line 69, corresponding to the previously described control signal from the 
tape detector 25 of FIG. 1. 
The output signal on line 75 from D-flip-flop 220 corresponds to the 
control signal received on line 75 of FIG. 2A indicating to the supply 
reel 22 circuit portion that learning is allowed. Analogously, the output 
signal on line 76 from D-flip-flop 221 indicates that learning is allowed 
and that control signal is applied to the takeup reel 24 circuit portion, 
which is analogous to the circuit of FIG. 2A. 
In operation, a reset signal on line 69 resets both D-flip-flops 220, 221, 
causing the output signals on lines 75, 76 to go high and thereby allowing 
learning on both reels 22, 24. If thereafter continuous learning is not 
allowed, the signal on line 61 is low and both outputs on lines 218, 219 
from NAND gates 216, 217, and thus both D-inputs of the flip-flops will be 
high. However, in case continuous learning is allowed, the signal on line 
61 is high and one D-input of the flip-flops 220, 221 will be high while 
the other one is low, depending on the high or low status of the tape 
direction line 111. The foregoing prevents continuous learning on both 
reels simultaneously. This additional feature eliminates unnecessary 
continued comparison between a stored diameter value and continuously 
increasing measured diameter values, for example, of the supply reel when 
the tape is moving in reverse direction or of the takeup reel when the 
tape is moving in forward direction. 
The above-described control signal on line 218 which is present on the 
D-input of flip-flop 220 is transferred to the inverting Q-output thereof 
at the occurrence of the previously described control signal on line 102 
or 108 of FIG. 3. Analogously, the signal on line 219 is transferred to 
the inverting Q-output of flip-flop 221 at a clock signal received on line 
102a or 108a, shown in FIG. 1, from the takeup reel diameter measuring 
circuit 40. Consequently, every time a valid measured diameter value A is 
stored in the current diameter memory 98 of FIG. 3, the respective control 
signals on lines 75, 76 will indicate whether learning for each particular 
reel is allowed. In the preferred embodiment, the flip-flops 220, 221 
operate at the negative edge of the clock signal to allow loading the 
value A on line 96 to memory 98 of FIG. 3 prior to applying the control 
signals on lines 75, 76, respectively. The above-indicated provision also 
assures proper loading of value A from the diameter measuring circuit 38 
to the comparator 50 simultaneously with loading the memory 98. 
While the foregoing description of the adaptive end of tape detection 
circuit of the invention has been made with reference to the functional 
block diagrams of FIGS. 1 to 3, the circuit is preferably implemented as 
shown in FIGS. 4 to 5D and as it is described below. With respect to the 
similarity between the circuits of FIGS. 1 and 4 only the differences will 
be described to avoid repetition. 
In FIGS. 4 to 5D the previously described memories 54, 56, control logic 
and memory circuits 58, 60 and the comparators 50, 52 of FIG. 1 are 
implemented by a microprocessor and memory circuit 115. The circuit 115 
comprises a microprocessor, a random access memory (RAM) and a read only 
memory (ROM). These and other circuit elements of FIG. 4 are shown in 
detail in the schematic electrical circuit diagram of FIGS. 5A to 5D. A 
servo data and control bus 117 connects the circuit 115 with the 
respective counters 91, 95, 91a, 95a and with the rest of the device. As 
it is well known from digitally controlled magnetic recording/reproducing 
devices, the bus 117 also connects the respective reel servo, capstan 
servo and other servo circuits (not shown) of the device with the 
microprocessor, RAM and ROM of circuit 115, respectively. The capstan 31 
is shown as being connected via line 114 to a well known capstan servo 
circuit 116 which circuit in turn is connected also to the bus 117. When 
the capstan servo 116 is disabled, it applies a control signal to the 
microprocessor of circuit 115 via bus 117. The microprocessor in turn 
provides a previously described low control signal on line 61 indicating 
that learning is not allowed. Analogously, it applies a high signal on 
line 61 when the servo 116 is operating. 
Well known digital-to-analog converters 119, 121 are utilized, one for each 
reel 22, 24 to convert various digital control signals received on the bus 
117 into analog signals on line 124, 125 suitable for controlling analog 
motor drive amplifiers (MDA) 80, 81. The MDA's 80, 81 in turn control the 
respective supply and take-up reel motors 82, 83 in a well known manner. 
The previously described output signal on line 44 from the tape timer idler 
tachometer circuit 36 is applied to a tach processor 88 which has an 
output connected to the bus 117 via a control line 111. The tach processor 
circuit receives from the tach detector 36 via line 44 two tach signals 
which are 90 degrees phase shifted with respect to each-other, in either 
direction, depending on the direction of tape 20 movement and thus on the 
direction of rotation of the idler 30. In response thereto the tach 
processor 88 generates a direction signal on line 111 corresponding to the 
actual direction of tape movement. Thus when a change of tape direction is 
indicated by the signal on line 111, the microprocessor in circuit 115 
will reset the contents of the counters 91, 95, 91a and 95a as previously 
described with respect to FIG. 3, to avoid measuring errors. 
In addition to the above-described servo data and control bus 117 shown in 
FIG. 4 a machine controlled data bus 186 is utilized in the device. The 
bus 186 is primarily utilized to convey various data and control signals 
between the machine control (not shown), servos, microprocessor and memory 
circuit 115 and other portions of the device. To enable transfer of data 
between the respective data busses 117, 186 a communication interface 187 
is utilized in a well known manner, as it is also shown in the detailed 
circuit diagram in FIGS. 5B and 5C. 
For example, the machine controlled data bus 186 conveys the previously 
described control signal on line 69 obtained from sensor 25, informing the 
machine control that the tape is out of path. The latter control signal is 
also transmitted via the communication interface 187 and bus 117 to the 
microprocessor and memory circuit 115. Other control signals which are 
transferred via the machine controlled data bus 186 include mode command 
signals indicating whether the device is in record, playback, shuttle or 
cue operation, and signals applied from the microprocessor circuit 115, 
such as the previously described stop signals on lines 62, 64 of FIG. 1. 
FIGS. 5A to 5D are consecutive portions of a detailed schematic circuit 
diagram corresponding to a portion 185 of FIG. 4 as it is indicated by an 
interrupted line to facilitate comparison and which detailed diagram is 
described below. 
To avoid repetition only those portions of the detailed diagram of FIGS. 5A 
to 5D will be described which are different from FIG. 4 or are not shown 
therein. 
To provide a more complete disclosure, as an example, in FIGS. 5A to 5D the 
respective integrated circuit elements are designated by type numbers 
under which these elements are commercially available. 
Thus in FIG. 5A the respective counters 91, 95 and 91a, 95a are implemented 
by two integrated counter circuits, preferably type 8253-5. Each 
integrated circuit comprises three counters and an interface 191, 191a 
connected to the servo data and control bus 117. 
With further reference to FIG. 5A the respective tach signals on lines 42, 
48 from the supply and take-up tach detection circuits 32, 34 shown in 
FIG. 4 are each applied to one first counter 91, 91a via a squaring 
amplifier 171, 172, respectively. These amplifiers receive the analog tach 
signal on lines 42, 48 and provide a square wave output signal suitable 
for applying to the counters 91, 95, 91a, 95a as a clock signal. 
FIG. 5A also shows the previously described digital-to-analog converters 
119, 121, preferably implemented as type 6080. An output port 194, 
preferably type 74LS273, is utilized for connecting respective control 
lines 195 of the counters 91, 95, 91a and 95a to the bus 117. 
In FIGS. 5B to 5D the microprocessor and memory circuit 115 of FIG. 4 is 
shown as utilizing a microprocessor circuit 196, a RAM circuit 197 and two 
ROM circuits 198a and 198b, respectively. The microprocessor is preferably 
type MC6802, the RAM type TC5517APL and the ROM's type 27128, 
respectively. The RAM 197 is utilized to store, among other values, the 
continually changing respective reel diameter values, as it has been 
described previously with respect to the memories 54, 56 of FIG. 1, and 
memories 98 and 110 of FIG. 3, respectively. 
The ROM's 198a and 198b are utilized to store, besides other values, 
programs controlling the operation of the microprocessor 196 in accordance 
with the flow charts of FIGS. 6 to 8 which will be described later. These 
ROM's also store the previously mentioned default diameter value and the 
constant value C utilized to compensate for the inertia of the tape 
transport mechanism. 
A system reference clock, which in the preferred embodiment has a frequency 
of 4 MHz is applied on line 215 to the microprocessor 196 from a well 
known reference clock generator (not shown). 
An interface device 188, preferably type LS245 is utilized to connect the 
respective circuits 196, 197, 198a and 198b to the bus 117 as well known 
in the art. 
The communication interface 187 of FIG. 4 is implemented in FIGS. 5B and 5C 
by four input/output ports, preferably type LS374, connected between the 
servo data and control bus 117 and the machine controlled data bus 186, as 
previously described. 
Address decoders 190, shown in FIGS. 5B to 5D, are preferably implemented 
by four integrated circuits type LS138 and are utilized to address various 
input/output ports 187 by the microprocessor 196. An address buffer 199, 
shown in FIG. 5D, preferably type LS541, is connected between the circuits 
115, and 190 to buffer the address lines going from the circuit 115 
addressing the decoders 190. 
Control lines 201, 202 and 203 in FIG. 5B are read/write lines connected to 
the microprocessor 196. On lines 204, 205 of FIG. 5B an internal clock 
signal of 1 MHz is provided by the microprocessor 196, obtained by 
dividing the previously mentioned 4 MHz system clock by four. Lines 206, 
207 represent buffered address lines which are utilized together with 
device select lines 210, 211 shown in FIG. 5C from address decoders 190 to 
address the individual counters 91, 95, and 91a, 95a, respectively. 
The operation of the circuit of FIGS. 4 to 5D will now be described with 
reference to the various flow chart diagrams of FIGS. 6 to 8. These 
respective flow charts show various operation steps performed by the 
circuit of FIGS. 4 to 5D. These operations are controlled by the 
microprocessor 196 and they are performed in accordance with programs 
stored in ROM's 198a and 198b. The operation described and shown in these 
flow charts is analogous with the operation previously described with 
reference to the functional block diagrams of FIGS. 1 to 3. 
In the description below the following flags will be utilized: 
Flag 1 indicates a valid diameter measurement. It is set by block 132 of 
FIG. 6. 
Flag 2 indicates that a diameter measurement has been seen before. It is 
set by block 134 of FIG. 6. 
Flag 3 indicates that the next diameter measurement has to be skipped. It 
is set by block 150 of FIG. 7. 
Flag 4 indicates that learning is allowed once and it is set by block 147 
of FIG. 7. Flag 4 is then updated in block 170 of FIG. 8 depending on the 
status of an external signal indicating whether continuous learning is 
allowed. 
Now with reference to the flow chart of FIG. 6, it shows steps 
corresponding to an operation which is analogous with the method of 
continuously measuring the tape pack diameter on one reel, as it has been 
described with reference to FIG. 3. Two identical subroutines are 
utilized, one for each reel 22, 24, and each corresponding to the flow 
chart of FIG. 6. Therefore, to avoid repetition, the following operation 
will be described with respect to the supply reel 22 only. In the 
preferred embodiment this operation is performed approximately 900 times 
per second to insure that every measurement taken is considered by the 
microprocessor 196. The timing of this operation is derived from the 
system reference clock and it is entered by an interrupt signal. 
As indicated by block 124 of FIG. 6, the microprocessor 196 determines 
continuously whether a once around reel tach output pulse on line 93 from 
counter 91 has been obtained. If not obtained, the circuit repeatedly 
checks until the presence of that pulse is determined. Then block 126 
determines whether the same pulse has been already detected before by 
testing whether a flag 2 has been previously set. The foregoing step 
assures that at slow rotational speed of the reel the same pulse on line 
93 will not be repeatedly considered by the circuit. 
If the pulse on line 93 has not been detected before, a validity check in 
box 128 is made by testing whether flag 3 has been set previously. If flag 
3 is present, that measurement will be skipped. As indicated by block 136, 
flag 3 is then cleared to allow to consider the next measurement. Block 
134 then sets the previously mentioned flag 2 to indicate that the 
presently considered measurement has been seen. 
Returning now to block 128 of FIG. 6, in case flag 3 is not present, then 
the obtained diameter measurement will not be skipped, and it will be read 
as it is indicated by block 130. 
To simplify the following description, the steps indicated by blocks 127, 
131 enclosed by an interrupted line will be considered later. These steps 
correspond to the previously described operation of the circuit portion 
101 of FIG. 3 and represent an additional feature of the circuit operation 
in accordance with the invention, as it has been described before. 
As shown by block 129, the current diameter value is saved by updating a 
previous diameter value in RAM 197 and thereafter a flag 1 is set 
indicating that the measured diameter is valid, as shown of block 132. 
Thereafter flag 2 is set by block 134 to indicate that the currently 
considered measurement has been seen as previously mentioned. The 
foregoing operation corresponds to updating the memory 98 of FIG. 3 
without considering the circuit portion 101. 
Returning now to the previously skipped block 127, prior to saving the 
current diameter value in RAM 197, the newly measured value is compared to 
the last measured value. The latter step corresponds to the operation of 
the circuit 101 of FIG. 3. If the compared values are sufficiently close, 
the previously described step 129 takes place. If the compared values are 
not close, the operation continues as indicated by block 131, which step 
is analogous with replacing a last measured value with a current value in 
memory 110 of FIG. 3, every time a new measurement has been taken by 
counter 95. 
Block 125 of FIG. 6 clears flag 2 if the once around reel tach pulse is not 
detected. 
FIG. 7 is a flow chart related to a control operation of the previously 
described diameter measuring process shown in FIG. 6, and it corresponds 
in part to the operation of the circuit portion of FIG. 2B. Block 142 
determines whether the tape is out of path. If yes, block 144 disables the 
diameter measuring hardware, that is, the counters 91, 95 and 91a, 95a, 
and clears flag 1. An analogous operation has been described before with 
reference to FIG. 3 when a reset signal on line 69 has been received. 
Because of the reel servos being turned off when the tape is out of path, 
as it is well known for example in the art of helical video tape 
recorders, block 146 continuously checks whether the reel servos have been 
reactivated. When they are reactivated after the tape has been reinserted 
in the prescribed tape path, block 147 sets a flag 4 indicating that one 
learning cycle is allowed to take place and the diameter measuring 
hardware is enabled by block 148. This corresponds to the operation of the 
circuit of FIG. 2B after a reset signal on line 69 is received. 
The previously mentioned flag 3 is set by block 150 to skip the first 
measured value from being considered, for example after the tape has been 
reinserted into its path. Flag 3 assures that the first measurement after 
the tape being out of path will be discarded because of the high 
probability of error while the tape may not yet be properly tensioned, as 
it has been previously described with respect to the operation of 
D-flip-flop 106 of FIG. 3. 
When it is determined by block 142 that the tape is not out of path, and by 
block 152 that no change in tape direction has occurred, no further action 
is necessary. 
However, when block 152 detects a change in the tape direction the diameter 
measuring hardware is being reset by block 154. The latter operation is 
analogous to that of FIG. 3 upon receiving a control signal on line 111 
which causes resetting the counter 91 and D-flip-flop 106. 
FIG. 8 shows operation related to updating a currently stored diameter 
value in the memory, RAM 197, which operation is analogous with updating 
the previously described memory 54 of FIGS. 1 and 3 or memory 56 of FIG. 
1, respectively. 
Block 162 first determines whether learning is allowed by examining whether 
the previously described flag 4 has been posted by block 147 of FIG. 7, 
indicating that learning is allowed. If learning is allowed, block 164 
further examines whether the current diameter measurement provided by the 
counters 91, 95 is valid by examining the previously described flag 1 
provided by block 132 of FIG. 6. If flag 4 or 1 is absent the program 
waits for the next valid measurement, that is until both flags are set. 
For example, the first measurement after the tape has been out of path is 
always skipped as it has been described before. When a valid diameter is 
indicated, its value is compared in block 166 with the presently stored 
minimum diameter value in the RAM 197. If the new value is smaller than 
the stored value, a constant value C is added to the new diameter value in 
block 168, to compensate for the tape transport inertia as it has been 
previously described with reference to FIG. 2. The memory 197 is then 
updated with the thusly obtained value as it is indicated by block 169, 
that is, the previously stored minimum diameter value is replaced by the 
value obtained by block 168. 
Thereafter as indicated by block 170, flag 4 which has been previously set 
by block 147 of FIG. 7 is updated, depending on an external control signal 
value indicating whether continuous learning is allowed. The latter 
operation is analogous with the previously described operation of the 
circuit of FIG. 2B. Consequently, in block 170 flag 4 remains set if 
continuous learning is allowed and is cleared if continuous learning is 
not allowed, respectively. Various modifications of the disclosed 
embodiments, as well as alternative embodiments may become apparent to 
those skilled in the art without departing from the spirit and scope of 
the invention defined by the appended claims.