Abstract:
The tension arm position of a tape recorder is sensed and any change in position is added as a correction factor to the capstan tach or tape timer tach count generated during a reel tach period. To this end, the tension arm position signal is clamped to ground at the start of the capstan count. At the end of the capstan count, the tension arm position is sensed to provides a value that is proportional to the change in the position that has occurred during the capstan count. The value is then added to the capstan count to provide a corrected capstan count that no longer contains a tension arm error and constitutes a precise tape remaining output signal.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to circuits for determining the direction and position of a magnetic tape, and more particularly to a tape remaining circuit which provides a numeric value, in equal increments of the time remaining, corresponding to the amount of tape remaining on a supply reel. 
     Typical of end-of-tape circuits for determining the direction and amount of tape on the supply reel of, for example, a video production recorder, are those systems which determine the tape remaining by comparing the rotation rate of the reel to the rotation rate of the capstan or tape timer tachs. Typically, the number of capstan tach pulses are counted during a selected reel tach period. The value of capstan tach counts then are averaged over several reel tach periods to average out and thus minimize, errors caused, for example, by tension arm movement. The time required to perform the integration is relatively large, i.e., several seconds, and therefore present end-of-tape circuits take a like amount of time to provide a valid tape remaining value. This is particulary disadvantageous when the direction of tape is reversed, particularly towards the end of the tape. That is, the failure to provide a valid tape remaining value during a few reel rotations, can allow the tape to run off the reel. 
     The present invention overcomes the shortcomings of previous mention by providing a tape remaining circuit which minimizes integration time, while compensating for perturbations due to tension arm movement, thereby rapidly providing valid indication of the exact position and direction of a magnetic tape. To this end, the number of capstan tach or tape timer tach pulses are counted during a reel tach period. The tension arm position is sensed and any change of position thereof is added as a correction factor to the capstan tach or tape timer tach count. The combined count provides a corrected capstan or tape timer count which no longer contains a tension arm error. The circuit instantly minimizes errors due to tension arm position and thus reduces the time required for integration or averaging. In a modification, the output is further modified to provide a linearized signal that produces a numeric value equal to a given length of tape remaining, i.e., which compensates for the squaring effects of the radius of the reel. 
     Accordingly, it is an object of the invention to determine rapidly the tape remaining on a supply reel. 
     It is another object to provide a circuit for sensing the direction and position of a tape with a minimum of integration time. 
     It is a further object to sense a change in the position of a tension arm and instantly provide a tape remaining value which compensates for variation in the tension arm position. 
     Another object is to provide a linearized signal as a numeric value equal to a given length of tape remaining on a reel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE is a schematic diagram of an implementation of the invention combination. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the FIGURE a capstan tach input signal corresponding to a train of pulses generated via a conventional capstan tachometer device 9, is provided at input 10. A capstan (or tape) direction input signal also is provided via the device 9 on input 12 as a logic state indicating whether a tape 11 is being pulled in a forward or reverse direction. Supply and takeup reel tach signals are provided on inputs 14, 16 respectively, and are indicative of the rate of rotation of supply and takeup reels 13, 15 respectively, as generated via conventional reel tachometer devices (not shown). 
     Takeup and supply tension arm error signals are supplied to inputs 18, 20 respectively, and comprise analog voltages indicative of the position of takeup and supply tension arms 17, 19 respectively, over a preselected range of position change. A 128 H signal is provided on input 22 and comprises the system master clock, i.e., a clock of the order of 2 MHz. 
     The capstan tach signal on input 10 provides a clock to a capstan counter 24, which thence is coupled to a binary adder 26 via a capstan tach count latch 28. Adder 26 is coupled to a programmable read only memory (PROM) 30 and thence to an output latch 32. Latch 32 supplies a numerical value on an output bus 34 which corresponds to the tape remaining. 
     The 128 H signal on input 22 clocks a counter 36 which, in turn, clocks an A/D converter 38. The output of the converter 38 is supplied to the binary adder 26, along with the capstan tach count from the latch 28. 
     The supply and takeup tach signals on inputs 14, 16 are supplied to respective inputs of switch means 40, and thence to the input of a one-shot multivibrator 42 as well as to the clock input of a validity shift register 43. One-shot 42 is coupled in turn at a not-true output to a one-shot 44. The latter one-shot supplies a true output to reset the capstan counter 24, to clock the capstan tach count latch 28, and to control the status of a clamping circuit 46. The not-true output of one-shot 44 is coupled to the write input of the A/D converter 38. The true output of one-shot 42 is coupled to an AND gate 48 and also controls the switching of a sample/hold (S/H) circuit 50. 
     Thus, one-shot 42 is used to sample and hold the output latch 32 via the AND gate 48, and provide the trigger to the succeeding one-shot 44. One-shot 44 clamps the tension error to a pre-selected zero level via the clamping circuit 46, resets the capstan tach counter 24, latches the capstan tach count in the latch 28 and initiates the A/D converter 38 conversion process. 
     The takeup and supply tension error signals on inputs 18, 20 are supplied via the tension arms 17, 19 to the positive input of an operational amplifier 52 via respective potentiometers 54, 56 and switch means 58. The operational amplifier 52 is coupled, via AC coupling capacitor 59, to the positive input of a second operational amplifier 60 which, in turn, is coupled to the S/H circuit 50. The latter circuit&#39;s output is coupled to the positive input of a further operational amplifier 62 which supplies the sampled and held tension error signal to the A/D converter 38. 
     The capstan direction signal on input 12 controls the status of the switches 40, 58 and is also supplied to an exclusive OR gate 64. The latter is coupled to the reset input of the validity shift register 43. The shift register 43 supplies a data valid output signal on an output line 68, and also is coupled to the second input of the AND gate 48 as well as to a first input of an AND gate 70. AND gate 48 clocks the output latch 32, while AND gate 70 is coupled via an inverter to the disable input of the latch 32. The second input to AND gate 70 is provided by decoder means 72, which decodes a command on a bus 74 from a system microprocessor (not shown) which directs when data is enabled to the output bus 34 via the output latch 32. 
     In operation, a supply reel tach pulse is supplied on either input 14 or 16, as determined by the capstan direction signal on input 12 and switch means 40. The reel tach pulse triggers the one-shot 42. This in turn triggers one-shot 44, which resets the capstan tach counter 24, and also clamps the tape tension arm signal supplied on the corresponding input 18 or 20, to the center of a preselected range via the clamping circuit 46. After the one-shot 42 changes state, the clamp is removed and the capstan tach counter 24 begins counting incoming capstan tach pulses. When the next reel tach pulse occurs, one-shot 42 fires again causing the tension arm error signal that has occurred since it was clamped, to be sampled and held by the S/H circuit 50. One-shot 42 also triggers one-shot 44 again. One-shot 44 then causes the A/D converter 38 to digitize the sampled and held tension value from circuit 50, and further causes the capstan tach count latch 28 to latch the capstan tach count accumulated between reel tach pulses. The capstan tach count and the tension arm error data are then summed in the binary adder 26 and the resulting summation is a corrected capstan or tape timer count that no longer contains a tension arm error. 
     In a further modification, the corrected signal from adder 26 is supplied to the PROM 30. The latter changes the number acquired to linearize the output supplied to the output latch 32, whereby the resulting binary number represents a given amount of tape remaining on the supply reel in equal increments of time remaining. That is, the squaring effect due to the radius of the reel is removed. 
     When the next reel tach pulse is supplied to the one-shot 42 via inputs 14 or 16, the one-shot is again triggered and produces a clock for the output latch 32 which allows the data to be placed on the output bus 34, but only if the output latch 32 is enabled by the decoder means 72. 
     If a direction change occurs, the capstan direction signal on input 12 changes state, and supplies a reset pulse to the validity shift register 43 via the exclusive OR gate 64. This inhibits the output latch 32. The output 68 provides an indication of invalid data, and the validity shift register 43 does not generate a data valid flag until the data at output latch 32 is valid again. In general, the occurrence of two reel tach pulses is required after a direction change to provide a valid data flag. 
     The PROM 30 contains a linearizing map as mentioned previously, for both a 50 Hz PAL/SECAM and a 60 Hz NTSC/PAL-M color television standard, for reels having standard hubs. In addition, the PROM 30 allows for smaller hub diameters, such as those found in spot reels. Thus input signals corresponding to use of a spot reel, and to the 50/60 Hz standard, are supplied to the PROM 30 by inputs 76, 78 respectively. Thus several options are programmed into the PROM 30 to compensate for the variations in color standards and reel sizes. 
     It may be seen that any change of position of the tension arm is added as a correction factor to the capstan tach (or a tape timer tach) count, thereby instantly minimizing the error due to the arm changes and reducing the need for integration or averaging. This, in turn, allows the rapid generation of the tape remaining output signal. 
     The tension arm sensitivity as seen at the output of the A/D converter 38 is preferably the same as the capstan sensitivity. Further, depending upon the tape standard use, the output of the operational amplifier 52 is adjusted for a selected voltage level when the tape is moved, for example, one inch through the capstan, i.e., when the tension arm has changed the tape path length by the selected distance of one inch.