Method and apparatus for reproducing information at variable speeds from a magnetic tape
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for reproducing information from a magnetic tape, especially a method for a variable speed reproduction by intermittently driving the magnetic tape. 
     In a rotating head helical scan type tape recorder (hereinafter VTR) of the prior art, a synchronizing signal is usually recorded on the magnetic tape during a recording mode, and the recorded synchronizing signal is reproduced during a reproducing mode for controlling a rotating phase of a capstan or a rotating cylinder by a servo circuit so that the rotating heads correctly trace the video tracks on which a video signal is recorded. Generally, for the synchronizing signal, control pulses (hereinafter CTL pulses), which are a series of pulses having a frequency one-half of that of a vertical synchronizing signal, are continuously recorded on one edge of the magnetic tape. 
     In a system using the CTL pulses, even if the magnetic tape is driven at variable speeds as occur with an intermittent drive for a noiseless slow motion picture reproduction, the position of the rotating heads with respect to the video tracks are absolutely determined at the time of detection of the CTL pulses. Therefore, in the system using the CTL pulses, it is easy to reproduce the video signal using an intermittent drive for the noiseless slow motion picture, in which the magnetic tape is stopped at one of the noiseless still states and is normally moved to the next noiseless still state, which is one frame period apart from the former one. By repeating the intermittent drive at a proper time-interval, the video signal is reproduced in noiseless slow motion. Accordingly, in the system using the CTL pulses, the variable speed reproduction by the intermittent drive is simply realized by executing a stop control which contains a reverse braking of a capstan motor after some time delay from the detection of one of the CTL pulses. 
     However, the above-mentioned reproduction cannot be used in other servo systems, for example, a system of the type in which pilot signals are recorded on the video tracks as synchronizing signals as disclosed in U.S. Pat. No. 4,056,832 and U.S. Pat. No. 4,297,733. The aforementioned patents only describe a system using pilot signals having three or four frequencies as a tracking signal for accurately reproducing the video signal at a normal tape speed. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for reproducing information from a magnetic tape at any tape speed in a system using pilot signals as a tracking signal and an apparatus thereof. 
     It is another object of the present invention to provide a method for reproducing a video signal for a noiseless slow motion picture from a video tape by using an intermittent drive in a helical scan type VTR utilizing pilot signals as a tracking signal and an apparatus for carrying out the method. 
     Briefly, to attain the above mentioned objects, according to the present invention, a series of pulses from a frequency generator of a capstan for driving a tape is counted during one intermittent drive in order to detect a timing for the stopping of the feeding of the tape until the count number becomes a set value corresponding to a regular amount of feeding the tape of one intermittent drive, a tracking error signal is produced from the pilot signals reproduced from the tape during the tape stop state of the intermittent drive, and the set value of the count number is adjusted in response to the tracking error signal. 
     Further, in accordance with a feature of the present invention, in order to increase accuracy of the tracking error signal, the tracking error signal is averaged by the integration thereof during at least one field period within the tape stop state of the intermittent drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representation of an apparatus for reproducing a video signal at variable speeds according to an embodiment of the present invention. 
     FIG. 2 illustrates video tracks for explaining the embodiment of FIG. 1. 
     FIG. 3 illustrates waveforms of output signals in accordance with FIG. 1. 
     FIG. 4 represents a characteristic of a sampled tracking error signal in response to a deviation (ΔX). 
     FIGS. 5(I), 5(II) and 5(III) illustrate the relationship between a trace effected by a head, a tracking error signal and control signals (CMRUN and REVERSE). 
     FIG. 6 is a flow chart for explaining a program executed in the microcomputer 20 of FIG. 1. 
     FIG. 7 represents a characteristic waveform of an optimum unit value (ΔFG) in response to the tracking error signal in accordance with another embodiment. 
     FIG. 8 is a flow chart for explaining another program executed in the microcomputer 20 of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the apparatus of the present invention includes a video tape 1 on which a video signal and pilot signals are recorded on tracks thereof, a cylinder motor 2 for rotating magnetic heads 3, a tach head 4 and a capstan motor 5 for rotating a capstan for feeding the tape. As is well known, the tach head 4 generates a series of tach pulses by detecting a rotating phase of the cylinder motor 2. The tach pulses are used for producing a head exchanging signal (SW30) indicative of the exchange of the heads. A processing circuit 6 receives the video and pilot signals picked up by the head 3 and provides an output of the pilot signals to a circuit 7 for generating a tracking error signal from the pilot signals. The tracking error signal generating circuit 7 includes a bandpass filter (hereinafter BPF) 8 for selectively passing the pilot signals having four frequencies, which are described hereafter. 
     The pilot signals passed by BPF 8 are mixed with a selected one of output signals of a local frequency generator 10 at a mixer 9. The output signals of the local frequency generator have the same frequencies as at least selected ones of the pilot signals. The mixer 9 produces output signals having additive and subtractive frequencies between the frequencies of the passed pilot signals and the frequency of the output signal of the local frequency generator 10. BPFs 11 and 13 selectively pass a signal having a particular subtractive frequency, respectively. Detectors 12 and 14 connected to the outputs of BPFs 11 and 13 generate a first error signal and a second error signal, respectively, with the first error signal being directly transferred to a comparator 16, and the second error signal being transferred to the comparator 16 through a variable resistor 15. The comparator 16 generates the tracking error signal (ΔE) as the output of the circuit 7. 
     As shown in FIG. 1, a frequency generator 17 is provided for generating a series of pulses (FG) in response to the rotation of the capstan motor 5 which is controlled by a driver 19 of a servo circuit 18. Further, a controller 20 such as a microcomputer having an analog digital converter 21, a central processing unit (hereinafter CPU) 22 and a bus line 23 is also provided. Variable resistors 24 and 25 are provided for controlling movement of the magnetic tape 1 in the intermittent drive as will be described hereafter. 
     In the present embodiment, pilot signals of four frequencies f1(6.5fH), f2(7.5fH), f3(10.5fH) and f4(9.5fh) are utilized as shown in FIG. 2, wherein fH represents a horizontal synchronizing frequency. Further, the output signals of the mixer 9 of FIG. 1 have frequencies fH, 2fH, 3fH and 4fH and the center frequencies of the BPFs 11 and 13 are selected at frequencies 3fH and fH, respectively, for the reasons described in the aforementioned patents. 
     Referring to FIG. 2, the pilot signals f1, f2, f3 and f4 are sequentially recorded together with the video signal on the tracks at each field of the video signal. The parts illustrated with oblique lines (A), (B), (A) . . . represent trails or traces effected in the direction of the arrow by one of the heads 3 during the tape stop states of the intermittent drive. Further, the labelled arrows (a), (b), . . . represent the tape feed between adjacent tape stop states of the intermittent drive. 
     In this embodiment, the movement of the tape and heads described with respect to the field still mode, in which each of the parts (A), (B), (A) . . . is traced for at least one field by one of the heads 3 in the tape stop states. However, this invention is also applicable to the frame still mode, in which two adjacent tracks are traced by both of the heads in the tape stop states. 
     Next, the operation of the embodiment shown in FIG. 1 is described with reference to FIGS. 4, 5 and 6. At the start in the flow chart shown in FIG. 6, the video signal together with the pilot signals are reproduced by one of the rotating heads in the tape stop state. At first, detection is effected of which part is traced by one of the heads 3. For this purpose, the local frequency generator 10 generates an output signal having frequency f2 or f4. If the output signal having the frequency f2 is suppled to the mixed 9 and the amplitude of the tracking error signal (ΔE) is larger than a predetermined level at the leading or trailing edge of the head exchanging signal (SW30), it is determined that one of the parts (A) is traced by one of the heads 3. Similarly, if the output signal having the frequency f4 is supplied to the mixer 9 and the amplitude of the tracking error signal (ΔE) is larger than a predetermined level at the leading or trailing edge of the head exchanging signal (SW30), it is determined that one of the parts (B) is traced by one of the heads 3. 
     For example, as one of the parts (A) is traced by the rotating head 3 in the direction shown in FIG. 2 and the output signal having the frequency f2 is supplied from the local frequency generator 10 to the mixer 9, the first erro signal, the second error signal and the tracking error signal have a waveform as shown in FIG. 3. The tracking error signal is sampled at a timing (t2) within the tape stop state with the timing (t2) being determined in accordance with the relationship: 
     
         t2-t1=1/2(t3-t1). 
    
     The sampled tracking error signal (ΔE) is supplied to the analog to digital converter 21. 
     The relationship between the tracking error signal (ΔE) and the deviation (ΔX) of the tape 1 is shown in FIG. 4 wherein the abscissa represents the deviation (ΔX) of the tape 1 from a noiseless still position, and the ordinate represents the sample tracking error signal (ΔE). A point Xp indicates the deviation corresponding to a track pitch P. Accordingly, the tracking error signal in response to the deviation (ΔX) is supplied to the microcomputer 20. 
     In the embodiment, the sampled tracking error signal (ΔE) is obtained by sampling the tracking error signal at the timing (t2). However, in order to increase the S/N ratio of the sampled tracking error signal (ΔE), it is better to average the tracking error signals at the timing (t2) of each field during the tape stop state. Further, it is desirable to integrate the values of the tracking error signals during at least one field period during the stop state. In the case of the tape stop state in which one of the parts (B) is traced by one of the heads 3, the operation is the same as described above except the frequency of the output signal of the local frequency generator 10. In this case, the frequency f4 is utilized as shown in FIG. 6. 
     Next, as shown in FIG. 6, according to the polarity of the sampled tracking error signal, i.e., positive (P) or negative (N), an amount of feeding of the tape is controlled. Now, &#34;FGo&#34; is a count number (FGN) of the output pulses (FG) of the frequency generator 17 corresponding to a regular amount of feeding of the tape during one intermittent drive, that is, during one frame period drive. If, however, the sampled tracking error signal (ΔE) is negative, the count number (FGN) is changed from &#34;FGo&#34; to &#34;FGN1=FGo+ΔFG&#34;. in which &#34;ΔFG&#34; is a unit value of the count number (FGN). 
     Referring to FIGS. 5(I), 5(II) and FIG. 6, in order to transfer the tape from the part (A) to the next part (B), the CPU 22 provides a control signal (CMRUN) at a high level (H) to drive the capstan motor 5 and starts to count the output pulses (FG) of the frequency generator 17. At a timing (t4), in which the count number (FGN) becomes &#34;FGN1&#34;, the CPU 22 provides a control signal (REVERSE) at a high level (H) to brake the capstan motor 5 until a timing (t5) and then provides both control signals (CMRUN and REVERSE) at a low level (L) to stop the tape feeding. Here, the time period (t5-t4) is determined by detecting a set value of the variable resistor 24 through the analog digital converter 21. Further, if necessary, the microcomputer 20 adjusts a start timing (0), in which the control signal (CMRUN) becomes a high level. 
     The speed of the noiseless slow reproduction is determined by a time period (ts) of the stop state, which is at least one field period and depends on a set value of the variable resistor 25. In this embodiment, the CPU 22 controls the tape feeding by setting the count number (FGN) from the start timing (0) to the timing (t4). However, it is possible to set the count number (FGN) from the start timing (0) to the timing (t5) by controlling the timing (t4) and the power for braking of the capstan motor 5. 
     In the case of transferring the tape from the part (B) to the part (A) as shown in FIGS. 5(II) and 5(III), as the sampled tracking error signal (ΔE) is still negative, the count number (FGN) is adjusted to &#34;FGN2 (=FGo+ΔFG)&#34;, and the tape is transferred. As the sampled tracking error signal (ΔE) becomes positive as shown in FIG. 5(III) during the next stop state, the count number (FGN) should be set to &#34;FGo-ΔFG&#34;. For example, the microcomputer 20 sets &#34;FGo&#34; to be equal to 40 and &#34;ΔFG&#34; to be equal to 2. In this manner, any deviation between the tracks and the rotating magnetic head during an intermittent drive for noiseless slow motion may be compensated so that the information may be reproduced from the magnetic tape at variable speeds. 
     Next, another embodiment of the present invention will be described with reference to FIG. 7. In this embodiment, the unit value (ΔFG) of the count number (FGN) is varied corresponding to the amplitude of the sampled tracking error signal (ΔE). Accordingly, it is possible to lessen the lead time for accurate tracking. Referring to FIG. 7, the abscissa represents the sampled tracking error signal (ΔE) and the ordinate represents the optimum unit value (ΔFG). The optimum unit value (ΔFG) is varied from 20 to 1 in response to the sampled tracking error signal (ΔE) as shown in dotted line in FIG. 7. By using the optimum unit value (ΔFG), the deviation is compensated at only one intermittent drive. 
     In the cases described above, the count number (FGN) corresponding to the regular amount of feeding of the tape during one intermittent drive is constant, that is, &#34;FGo&#34;. However, an optimum value of the count number may be varied from &#34;FGO&#34; by a time aging. 
     A further embodiment of the present invention will be described with reference to the flow chart of FIG. 8. In this embodiment, the optimum value of the count number (FGN) is automatically compensated by controlling the amount thereof corresponding to a difference between the current sampled tracking error signal (ΔEn) and the previous sampled tracking error signal (ΔEn-1). As shown in FIG. 8, if the sampled tracking error signal (ΔEn) is negative (N) and the result of subtracting the previous sampled tracking error signal (ΔEn-1) from the current sampled tracking error signal (ΔEn) is negative (N), the count number (FGN) is changed to &#34;FGN+1&#34;. On the other hand, if the current sampled tracking error signal (ΔEn) is positive (P) and the result of subtracting the former sampled tracking error signal (ΔEn-1) from the sampled tracking error signal (ΔEn) is positive (P), the count number (FGN) is changed to &#34;FGN-1&#34;. Further, in this embodiment, if the count number (FGN) does not satisfy the following relationship, the count number (FGN) is set back to the optimum value (FGo): 
     
         FGo-4&lt;FGN&lt;FGo+4. 
    
     This step is useful to return the count number (FGN) to the initial value, when the count number (FGN) differs (more than ±4) from the optimum value as, for example, due to a drop out, or the like. As the other steps of the flow chart of FIG. 8 are the same as that of the flow chart of FIG. 6, the explanation thereof is omitted. 
     The count number is changed at each intermittent drive in the above described embodiments. However, for example, it is possible to change the count number at every other frame (every four tracks). In this case, the frequency of the output signal of the local frequency generator 10 is fixed to one of f2 or f4. 
     As described above, the present invention makes it possible to provide variable speed reproduction by an intermittent drive in the system using pilot signals. Further, this invention makes it possible to provide a noiseless variable speed reproduction by averaging the tracking error signal at the microcomputer, and by the other techniques as described. 
     While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.