Cassette tape player having circuit for detecting reverse rotation of take-up reel

A system for preventing improper tape feed in a cassette tape player uses pulse width measurements from the take-up reel output signal to sense normal or aberrant tape feeding conditions. The pulse width of the output signal is compared with a predetermined range of pulse widths associated with normal tape operation. The range is variable as a function of the rotational frequency of the take-up reel and is updated periodically to reflect changes in the diameter of the take-up reel as the tape plays. If the pulse width of the output signal falls outside of the predetermined range, it denotes an aberrant tape feeding condition.

TECHNICAL FIELD 
The present invention relates to cassette tape players with a system for 
preventing improper tape feed and more particularly to a circuit and 
algorithm having a variable detection means for detecting reverse or 
erratic rotation of the take-up reel of a cassette tape. 
BACKGROUND OF INVENTION 
Vehicles and home entertainment systems commonly use audio cassette tapes 
for recording and playback of music and other audio recordings. Cassette 
tape cartridges vary widely in quality of material and design. The best 
cassette tape cartridges are made from impact and heat resistant materials 
which are durable and provide thousands of hours of quality playback. 
However, many manufacturers use inferior cassette tape cartridges due to 
their low cost. "Bargain" cassettes may not comply with minimum standards 
for cassette design, testing and manufacturing. If a cassette is not 
manufactured to precise tolerances, it will not maintain uniform rolling 
friction of internal parts and may result in improper tape guidance. In 
severe cases, this may result in tape spilling into the transport 
mechanism and wrapping around the capstan. A condition commonly referred 
to as "tape eating" occurs when tape spills from the cassette and wraps 
onto the capstan, pinch roller or other parts of the cassette player. 
"Tape eating" occurs relatively frequently in home and vehicle cassette 
tape players. 
Three principal factors are associated with "tape eating": (i) the cassette 
take-up hub may not rotate freely; (ii) the tape may be loosely wound on 
the tape reels or separated from the tape pack; (iii) the cassette may 
have thin tape commonly used in long-playing C-120 cassettes. 
When take-up hub rotation is impaired, tape pulled by the capstan from the 
supply reel does not wind onto the take-up hub. Tape then spills into the 
tape transport mechanism and wraps onto the capstan or pinch roller. The 
cause of impaired take-up reel rotation is generally related to one of the 
following conditions and cassette cartridge defects: 
the hub may be dislodged out of position and jammed as a result of the 
cassette being dropped. 
the cassette shell may be made from low-grade plastic which warps upon 
exposure to high temperatures. 
improper sonic welding of a cassette may cause misalignment between the two 
shell halves which reduces internal dimensional tolerances. 
inadequate internal support between the two half shells of the cassette may 
result in shell deformation when placed in a tape player. 
Step formations in the tape pack may cause excessive friction, especially 
when repeated changes in play and fast forward or rewind modes occur in 
cassettes having poor internal tape guidance. 
"Tape eating" caused by loose tape is more frequently encountered with 
vehicle cassette tape players than in home tape players due to the extreme 
conditions in which the tapes are used and stored. When the cassette tapes 
are stored in the vehicle outside their protective case, vehicle 
vibrations can be transferred to the cassette and loosen the tape. Upon 
insertion, the tape may spill into the mechanism, at which time the tape 
can wrap onto the capstan. 
"Tape eating" caused by the use of thin tape, like C-120 and C-90 tapes, 
can result from an initial formation of a small loop in the tape caused by 
the tape following the curvature of the capstan upon initial insertion. If 
a cassette tape cartridge does not have ramps or tape strippers required 
by the industry standard, a portion of the tape will be able to follow the 
capstan rotation, catch under the pinch roller, and subsequently wrap 
around the capstan. 
It is estimated that a significant percentage of all cassette system 
failures in vehicle warranty claims are in some way related to defective 
or improperly stored cassette cartridges. Cassette system failures are 
costly to equipment manufacturers, particularly during the warranty 
period. When a consumer returns a vehicle to the dealer for repair, it is 
frequently necessary to entirely remove and disassemble the tape player 
from the vehicle for service leading to consumer dissatisfaction. 
Several attempts have been made to prevent "tape eating" in the prior art. 
One example is disclosed in Taraborrelli U.S. Pat. No. 4,348,702 which 
describes a device for preventing tape windup on the capstan of a tape 
deck. The Taraborrelli device incorporates a rotation-sensing switch and 
switch wiper mounted on the bottom of a take-up spindle. The rotation 
sensing switch only allows rotation when the take-up spindle rotates in a 
proper direction. If the take-up spindle changes direction due to tape 
windup on the capstan, a tab on a rotation-sensing switch engages a 
vertical edge at the bottom of the take-up spindle and stops the sensor 
switch from rotating. When the rotation-sensing switch stops, the 
rotation-sensing circuit disables the drive mechanism to prevent 
additional tape windup on the capstan. However, the rotation sensing 
switch is not sensitive enough to detect the tape windup as soon as it 
happens and additional tape may wind up during the time the switch moves 
to engage the vertical edge. 
Another approach is disclosed Tarpley, Jr. et al U.S. Pat. No. 4,597,547 
which describes a logic circuit for detecting reverse rotation of a 
take-up reel in a tape transport mechanism wherein three motion sensing 
switches are added to a tape player adjacent the take-up reel to detect a 
sequence of switch actuation according to the sequence of A-B-C. 
Subsequent switch activation sequences are then monitored by a logic 
circuit which can sense and react to an improper switching sequence. The 
switches and logic circuit are additional elements which must be added to 
a cassette tape player, thereby increasing cost. Additionally, there is no 
way to adjust the sensitivity of the motion sensing switches to compensate 
for changes in the rotational velocity of the take-up reel, making early 
detection difficult. 
In Tarpley, Jr. U.S. Pat. No. 4,632,333 another circuit sensing improper 
rotation of a take-up reel is disclosed wherein three switches are 
provided on the cassette player adjacent to the take-up reel. The sensing 
circuit outputs a pulse each time the sensor switches are actuated. 
Monostable multivibrators receive the pulses and provide output signals 
into a gate which responds by providing a control signal to a sensor 
switch. Improper rotation of the take-up reel interrupts this control 
signal, causing the sensor switch to respond by stopping or reversing the 
tape deck mechanism. The need for switches and monostable multivibrators 
in a special sensing circuit again increase the cost of the cassette tape 
deck equipped with such a system. 
The present invention is directed to overcome the above disadvantages noted 
in conjunction with prior art systems and to provide a new system which 
surpasses the prior art in efficiency and simplicity. 
SUMMARY OF INVENTION 
The present invention incorporates in a tape player a circuit for detecting 
aberrant operational conditions soon after they occur which can be 
implemented without incurring significant additional cost for additional 
hardware or circuitry. It is an object of the invention to provide such a 
system wherein an algorithm can be programmed into any 
microprocessor-controlled tape deck or microprocessor-based AM/FM radio 
utilizing inputs from already existing spindle rotation sensor inputs to 
detect proper operation and the aberrant operational conditions. 
It is also an object of the invention to use already existing hardware and 
microprocessor capacity to reduce the cost of implementing a "tape eating" 
or aberrant operational condition system with little or no additional cost 
except for the cost of programming existing microprocessor capacity with 
the algorithm disclosed. 
Another object is to provide a detection system which can adjust its 
defined range of normal operation to account for varying operating 
conditions in the tape player and to enable early detection of aberrant 
tape conditions regardless of where on the tape the aberration occurs. 
According to the present invention, a cassette tape player having a 
capstan, a take-up reel spindle and a supply reel spindle is described. An 
actuator attached to the bottom of the take-up reel spindle in the 
cassette tape player rotates as the spindle rotates, and the rotating 
actuator actuates a sensor mounted on the cassette tape player as the 
take-up reel spindle rotates to produce a periodic output signal. As the 
diameter of the take-up reel increases with tape accumulation thereon, the 
period and therefore the pulse width of the signal also increases. A 
microprocessor control receives the output signal and compares the pulse 
width of the output signal to a pulse width range derived from a prior 
signal. Time constants are both added to and subtracted from one or more 
selected pulse widths of the prior signal to determine a range in which 
the pulse width of the output signal should fall during normal operation. 
This range is compared to the pulse width of the output signal. A control 
signal in response to an aberrant tape condition is generated when the 
pulse width of the output signal is outside the normal operating range, 
thereby indicating reverse or erratic rotation of the take-up reel caused 
by the tape beginning to wind around the capstan of the cassette tape 
player. If the pulse width of the output signal falls within the normal 
operating range, the range is updated before measuring the next pulse 
width in order to accommodate the subsequent change in pulse width of the 
output signal and maintain a narrow normal range of operation. 
The invention will become apparent upon review of the attached drawings in 
conjunction with the following detailed description of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
A tape deck generally indicated by reference numeral 20 is shown. The tape 
deck 20 includes a supply reel 22 secured to a supply reel spindle 24. A 
magnetic tape 26 is transferred between supply reel 22 and a take-up reel 
28 which is secured to a take-up reel spindle 30. The tape 26 is guided by 
feed alignment roller 32 past the playback head 34. A capstan 36 and pinch 
roller 38 move the tape at a constant speed past the playback head 34. A 
take-up alignment roller 40 guides the tape 26 as it is fed from between 
the capstan 36 and pinch roller 38 to the take-up reel 28. 
FIG. 2 illustrates a common condition wherein a loop 42 forms between the 
capstan roller 36 and the take-up alignment roller 40. This occurs upon 
initial start-up of the tape deck 20 or if the take-up reel 28 stops 
rotating. The loop 42 forms as tape follows the curvature of the capstan 
roller 36. Normally the loop 42 is automatically eliminated as the take-up 
reel 28 takes up the excess tape 26. 
Referring now to FIG. 3, an aberrant tape feed condition is illustrated 
wherein the loop 42 becomes caught around the capstan roller 36 and forms 
a roll 44 of tape which rapidly accumulates on the capstan roller 36. If 
the loop becomes caught between the capstan roller 36 and the pinch roller 
38, the rotation direction of the take-up reel 28 is reversed and tape 
feeds in the reverse direction from the take-up reel 28 as it winds about 
the capstan roller 36. Any delays in detecting the reverse rotation of the 
take-up reel 28 enables additional tape to wrap around the capstan roller 
36, making the roll 44 larger and more likely to damage the tape deck. 
Ideally, the aberrant tape condition is detected as soon as the loop 42 
catches between the capstan roller 36 and the pinch roller 38. 
FIG. 4 shows the structure of a take-up reel spindle 30 in cross-section. A 
spindle drive 50 frictionally engages a rubber ring 52 which is secured to 
the take-up reel spindle 30 to drive the spindle. A drive pulley 54 forms 
part of the spindle drive 50 and is rotated by means of a drive belt 56. A 
multi-pole magnetized disc 46 is secured to the take-up reel spindle 30 as 
shown in FIG. 4. A slip clutch 58 prevents excess tension from occurring 
on the tape. A sensor such as a reed switch 48 responds to changes in the 
magnetic field caused by the rotation of the multi-pole magnetized disc 
46. The multi-pole magnetized disc 46 and reed switch 48 currently exist 
in conventional tape decks for sensing end-of-tape and other spindle 
rotation conditions. 
Referring to FIGS. 5 through 10, utilization of input from existing 
hardware such as the multi-pole magnetized disc 46 and reed switch 48 to 
detect aberrant tape conditions and reverse the rotation of the take-up 
reel utilizing an algorithm in conjunction with preexisting microprocessor 
control elements is explained. 
FIG. 5 diagrammatically illustrates variables utilized in the algorithm. 
The radius of the tape pack, designated r, increases as tape winds onto 
the cassette cartridge reel. As the radius increases, the rotational 
frequency of the take-up spindle decreases. The take-up reel rotates in a 
counterclockwise direction. The radius increases from a minimum radius of 
A for an empty take-up reel to a maximum radius of B for a full take-up 
reel. 
##EQU1## 
V=velocity r=radius 
A=minimum radius 
B=maximum radius 
For example, the radius for a C-60 cassette changes approximately 34 
micrometers per revolution and consequently the rotational frequency 
changes approximately 0.002 revolutions per second. 
FIGS. 6A and 6B illustrate the output of the take-up spindle sensor. FIG. 
6A illustrates the take-up spindle sensor output for an empty take-up 
reel. For comparison, FIG. 6B illustrates the take-up spindle sensor 
output for a full take-up reel. The pulse width of the output signal is 
inversely proportional to the rotational frequency of the take-up reel. 
The rotational frequency is related to the radius of the tape pack as 
stated above, and consequently the pulse width of the output signal 
increases as the radius of the tape pack increases and the rotational 
frequency decreases. Similarly, the pulse width of the output signal 
decreases as the rotational frequency of the take-up reel increases. 
Referring to FIG. 7A, the flowchart illustrating the steps of one 
embodiment of the reverse rotation detection algorithm will be described 
beginning with "START". The first step is an initialization routine 
wherein initial X and Y values are determined. This initial step is 
conducted because the diameter of the tape pack at the time the player is 
started could be any size, from full to empty and X and Y values 
previously stored for one tape may not be the correct values for another 
tape. For example, if one tape is ejected and another tape is replaced, 
the diameter of the tape on the take-up reel will almost always be 
different requiring adjustment of the X and Y values. Determination of the 
initial X and Y value is accomplished by measuring the first valid pulse 
width t.sub.1. The microprocessor accomplishes this by determining whether 
the take-up spindle output signal is high or low when the tape player is 
started and then waiting for the first low-to-high or high-to-low 
transition in the output signal before proceeding with the pulse width 
measurement. The system measures both low and high pulses which are both 
compared by the algorithm for better resolution. The microprocessor then 
references the stored values for X and Y in a look-up table which 
correspond to selected pulse width values to be assigned. Once the initial 
X and Y values are assigned, they are periodically updated. The X and Y 
values are updated at predetermined pulse width switch point values. Each 
time a predetermined pulse width switch point value is encountered, X and 
Y values are updated. 
Referring now to FIGS. 8 and 9, under normal operating conditions, a 
cassette cartridge is inserted into the cassette tape player and the drive 
motor is turned on, causing the take-up spindle to rotate and actuate the 
take-up reel sensor. When the microprocessor conducts the initialization 
process as described above, the first measured value t.sub.1 of the 
take-up reel sensor causes the initial X and Y values to be assigned. In 
the next step, the pulse width values t.sub.1 and t.sub.2 are measured and 
stored in memory as reference values for later comparison. The pulse width 
value of t.sub.3 is measured and compared to the value of t.sub.1 +X and 
the value of t.sub.1 -Y. During normal operation, the value of t.sub.3 
meets the t.sub.1 -Y&lt;t.sub.3 &lt;t.sub.1 +X condition and the pulse width 
value t.sub.4 is then measured. The value of t.sub.4 is then compared to 
the value of t.sub.2 +X and the value of t.sub.2 -Y. If the value of 
t.sub.4 meets the t.sub.2 -Y&lt;t.sub.4 &lt;t.sub.2 +X condition for normal 
operation, the algorithm updates the t.sub.1 and t.sub.2 reference values 
with the t.sub.3 and t.sub.4 values respectively. The values of X and Y 
are also adjusted to maintain a constant proportional relationship between 
the measured pulse width value and the X and Y values as the diameter of 
the tape pack increases. X is updated by subtracting t.sub.3 from t.sub.1 
+X. Y is updated by subtracting t.sub.1 -Y from t.sub.3. 
The pulse width value t.sub.5 is then measured and compared to the value of 
t.sub.3 +X and the value of t.sub.3 +Y. If the value of t.sub.5 meets the 
t.sub.3 -Y&lt;t.sub.5 &lt;t.sub.3 +X condition for normal operation the pulse 
width value t.sub.6 is measured. The t.sub.6 value is compared to the 
value of t.sub.4 +X and the value of t.sub.4 -Y. If the value of t.sub.6 
meets the t.sub.4 -Y&lt;t.sub.6 &lt;t.sub.4 +X condition for normal operation, 
the microprocessor updates the t.sub.3 and t.sub.4 reference values, with 
the t.sub.5 and t.sub.6 values respectively. The values of X and Y are 
also adjusted to maintain a constant proportional relationship between the 
measured pulse width value and the X and Y values as the diameter of the 
tape pack increases. 
This process is continued as long as normal operation criteria continues to 
be met. 
Using a single pulse width value to determine the acceptable range of 
operation is simple and easy to implement. However, it may not accurately 
take variations in sensor pulse data into account. These variations can 
occur due to varying forces and operating conditions placed on the 
spindles as well as the mechanics of the spindle itself. The best way to 
compensate for these fluctuations in the sensor pulse data is to use a 
plurality of pulse widths, rather than one pulse width, to choose the X 
and Y values and ultimately determine the range of normal operation at a 
particular point of the tape. 
Referring to FIG. 7B, an alternative and preferred embodiment determines 
the X and Y values by comparing the plurality of pulse widths occurring 
after one rotation of the take-up reel spindle instead of simply measuring 
the first valid pulse width. After storing the plurality of the pulse 
width values in memory, the microprocessor assigns the value t.sub.high to 
the highest value in the set and t.sub.low to the lowest value in the set. 
The X and Y values are then chosen from a look-up table in memory based on 
the values of t.sub.high and t.sub.low. 
During normal operation, the pulse width value t.sub.current(1) meets the 
t.sub.low(1) -Y&lt;t.sub.current(1) &lt;t.sub.high(1) +X condition. If the value 
of t.sub.current(1) meets this condition, the algorithm updates the 
t.sub.low(1) and t.sub.high(1) reference values by comparing a new set of 
pulse widths where the last pulse in the set is t.sub.current(1). The 
algorithm then chooses a new t.sub.high and t.sub.low from this new set 
and consequently a possible new X and Y. The pulse width of 
t.sub.current(2), which is the pulse after t.sub.current(1), is the new 
pulse which is compared with the new range t.sub.low(2) 
-Y&lt;t.sub.current(2) &lt;t.sub.high(2) +X, where t.sub.low(2) and 
t.sub.high(2) are the lowest and highest pulse width values of the new 
group of values stored in memory. The values of X and Y are also adjusted 
to maintain a constant proportional relationship between the measured 
pulse width value and the X and Y values as the diameter of the tape pack 
increases. 
By periodically updating a narrow range of normal operation rather than 
comparing the output signal to a constant predetermined broad range, 
aberrant tape conditions can be detected almost instantaneously before 
excessive tape winds onto the capstan. Each calculated range is customized 
to detect an aberrant tape condition at each particular point of the tape, 
making early detection of aberrant tape conditions possible. 
Referring now to FIG. 10, a graphical representation of the take-up spindle 
output signal during normal operation and three cases of output signals 
during aberrant tape feed conditions are juxtaposed. For simplicity, the 
first embodiment disclosed will be used as an example, but the same 
concepts apply to both embodiments. In the top illustration, normal 
conditions exist and the time t.sub.3 falls within the range t.sub.1 
-Y&lt;t.sub.3 &lt;t.sub.1 +X permitting normal operation to continue. 
In case 1, t.sub.3 is &gt;t.sub.1 +X which falls outside the acceptable 
t.sub.1 -Y&lt;t.sub.3 &lt;t.sub.1 +X range. This occurs when the take-up spindle 
reverses direction after the 50% pulse width value of t.sub.1 +X. When 
this condition occurs, the algorithm controlling the microprocessor sends 
a control signal to the cassette mechanism to either stop or reverse the 
mechanism. 
In case 2, t.sub.3 &lt;t.sub.1 -Y. This occurs when the take-up spindle 
reverses direction before the 50% pulse width value of t.sub.1 -Y. When 
this occurs, the algorithm controlling again sends a control signal to the 
cassette mechanism to either stop or reverse the mechanism, depending upon 
the application. 
Finally, in case 3 the take-up reel reverses direction after the pulse 
width value of t.sub.1 -Y and before the pulse width value of t.sub.1 +X. 
When this condition is detected, the algorithm does not distinguish the 
"tape eating" condition from normal operation. The algorithm is 
ineffective to detect "tape eating" when it occurs within this narrow time 
frame (K.sub.h). 
Referring now to FIGS. 8-10, K.sub.h is defined as the sum of X and Y as 
the output signal goes from high to low. K.sub.L is the same sum as the 
output signal goes from low to high. 
The present invention may be practiced using an Intel 8085 microprocessor 
based SDK-85 development system and the Tanashin Denki model TN-555 
auto-reverse cassette mechanism. The interface between the microprocessor 
and the cassette mechanism included a PNP transistor biased as a saturated 
switch and a 7407 TTL high voltage hex inverter. The transistor was used 
to interface the take-up spindle output signal of the Hall effect sensors 
in the cassette mechanism to the 8355 I/O/ROM input port of the Intel 8085 
microprocessor. The 7407 hex inverter interfaced the 8355 I/O/ROM output 
port to the program direction change input of the cassette mechanism. 
Appendix A details a preferred software program which is written in Intel 
8085 Assembly Language, as implemented in the first embodiment. Appendix B 
details a preferred software program written for National Semiconductor 
COP888E6 assembly language with a Tanashin TN 709 tape deck, as 
implemented in the second embodiment. A brief description of the major 
software routines is as follows: 
______________________________________ 
MAIN PROGRAM: Executes initialization process, 
calls the sub-routines, and 
updates measured pulse width 
values. 
DCHK SAMPLE Takes samples of sensor output. 
SUB-ROUTINE: 
COME Calculates range of pulse widths 
SUB-ROUTINE: for normal operation and compares 
the sensor output with the 
calculated range. 
STOP/REVERSE Sends control signal to the 
CASSETTE cassette mechanism to either stop 
SUB-ROUTINE: or reverse the mechanism. 
______________________________________ 
The above description describes only two preferred embodiments and is to be 
interpreted in an illustrative sense and not in a restrictive sense. There 
are alternate embodiments which have not been specifically mentioned but 
which are obvious and intended to be included within the scope of the 
invention as defined by the following claims. 
##SPC1##