Tracking control circuit of herical scanning magnetic tape apparatus

Abstract: In a digital audio magnetic tape reproducing apparatus of herical scanning using two magnetic heads, a first head reproduces a pilot signal of its own track and two pilot signals of both neighboring tracks of its own track by affection of crosstalk, then a difference value between the values of the reproduced signals of the two pilot signals is calculated and is applied to a tracking servo system as a tracking error signal, moreover, the difference value is controlled by a reproduced signals of a pilot signal reproduced by a second magnetic head in the previous reproducing period.

Description: The present invention relates generally to a helical scanning magnetic tape apparatus with a tracking control circuit, and more particularly to a tracking control circuit in a digital audio tape reproducing apparatus having a rotary head for helical scanning. Recently, a digital recording and reproducing system utilizing a pulse code modulation technology has been introduced into an audio apparatus in order to improve high-fidelity characteristic in reproduction of music or sound. A digital audio tape recorder as one type of a digital audio apparatus comprises plural magnetic heads mounted on a rotary member placed in a rotary cylinder on which a magnetic tape is wound and is transported thereby. and the magnetic heads run on the magnetic tape in a direction which is oblique with respect to the direction along the length of the magnetic tape. FIG.10 is a plan view of an example of a conventional magnetic tape transporting mechanism. A rotary cylinder 1 rotates in the direction of arrow A. A magnetic tape 4 which is drawn from a cassette 5 is wound around the peripheral region of about 90° of central angle of the rotary cylinder 1 by means of two guide pins 6 and 7, and is transported in the direction of arrow B by a driving capstan 9 and a pinch roller 8. Two magnetic heads 2 and 3 mounted on a rotary head holder (not shown) are arranged at the periphery of the rotary cylinder 1 so that each head tip slightly protrudes from the peripheral surface of the cylinder 1. FIG.11 is a plan view of the magnetic tape 4 showing recorded tracks. The tracks are inclined, as shown by an arrow C, relative to the transporting direction D of the magnetic tape 4. Referring to FIG.11, tracks T₂ are formed by the magnetic head 2 and tracks T₃ are formed by the magnetic head 3. In the conventional reprodueing process of the magnetic tape 4, it is required that the magnetic head 2 accurately traces the tracks T₂ and the magnetic head 3 also accurately traces the tracks T₃. In order to realize the accurate tracking of the magnetic heads 2 and 3, tracking servo means is provided in the transporting device of the magnetic tape 4. An automatic track finding method (abbreviated in general as ATF) is employed for the tracking servo means. In the ATF method, when a digitalized signal is recorded on the track, a predetermined ATF signal is recorded on a start region and an end region of each track. Allocation of the ATF signal in the normal track mode is illustrated by FIG.12. The ATF signal is composed of a pilot signal P and a synchronizing signal S which have a lower frequency than the digitalized signal of data. The allocation pattern of the ATF signal is standardized and is repeated every four tracks on the magnetic tape 4. Referring to FIG.12 and FIGs. 13(a), 13(b) and 13(c), the magnetic head 2 traces the track T₂ in the direction of arrow E and detects early pilot signals P A , P B and P C and the synchronizing signal S. A signal as shown by FIG.13(a), FIG.13(b) or FIG.13(c) is obtained by amplifying the detected signal of the pilot signal P A , P B or P C with an amplifier and then passing it through a low-pass filter. When the magnetic head 2 accurately traces the track T₂, a signal shown by FIG.13(a) is output. Referring to FIG.13(a), the wave form P a shows the detected signal of the pilot signal P A , and the waveforms P b and P c show the crosstalks of the pilot signals P B and P C , respectively. Since the center C H of the magnetic head 2 is on the center line C L of the track T₂, the output level P₁ of the wave form P b is equal to that of the wave form P c . On the other hand, when the center C H of the magnetic head 2 deviates from the center line C L of the track T₂ and is deflected downward, the level of the wave form P b is higher than that of the wave form P c as shown in FIG.13(b), because the area of the magnetic head 2 which covers the pilot signal P B is larger than that which covers the pilot signal P c . On the contrary, when the center C H of the magnetic head 2 is deflected upward, as shown in FIG.13(c), the level P₁ of the wave form P b is lower than the level P₂ of the wave form P c in a similar manner. The difference between the levels P₁ and P₂ represents a deflected value of the magnetic head from the track T₂, and a tracking error signal corresponding to the deflected value is obtained. FIG.14 represents a level of a tracking error signal versus the deflected angle value. Referring to FIG.14, when the center C H of the head 2 is on the center ling C L of the track T₂, a deflected angle of the abscissa is zero degree. On the other hand, when the center CH of the head 2 is on a center line of a neighboring track T₃, the deflected angle is +180° or -180°. Then, the ordinate designates the level of the tracking error signal corresponding to the difference between the levels P₁ and P₂. In a similar manner, a tracking error signal of the track T₃ which is traced by the magnetic head 3 is produced. The levels P₁ or P₂ of the pilot signals P B and P C are detected in synchronism with synchronizing signals SP₁ and SP₂, respectively. The synchronizing signals SP₁ and SP₂ are created on the basis of the synchronizing signal S of the ATF signal. FIG.15 is a block diagram of a tracking control circuit for creating the tracking error signal in the prior art. The detected signal of the magnetic head 2 or 3 is amplified by an amplifier 11 and is applied to a low-pass filter 15. The higher frequency component including digitalized signal of data and the synchronizing signal S is eliminated by the low-pass filter 15. Then the output signal of the low-pass filter 15 is applied to an envelope detecting circuit 16, and envelopes of the pilot signals P A , P B and P C are detected. On the other hand, the amplified synchronizing signal S is separated from other signals by a band-pass filter 12, and then the zero level of the synchronizing signal S is detected by a zero-cross comparator 13. A synchronizing signal generator 14 generates the synchronizing signals SP₁ and SP₂ on the basis of the output of the zero cross comparator 13. The synchronizing signal SP₁ is applied to a sample-and-hold circuit 17 and the level P₁ of the pilot signal P B is detected. Then the value of the level P₂ is subtracted from the value of the level P₁ at an adder 18, and a resultant value is held by a sample-and-hold circuit 19 in synchronism with the synchronizing signal SP₂. The resultant value is the value of the tracking error signal. The tracking error signal is applied to a tracking servo means. This is generally understood by the man skilled in the art and therefore is not described in greater details. In the foregoing conventional tracking control circuit, however, a characteristic of the level of the tracking error signal seriously deviates from a normal characteristic shown by a solid line L₂ in FIG.16 due to an unbalance between sensitivities of the respective magnetic heads 2 and 3 or a variation of characteristics of the magnetic tape. Various deviated characteristics of the level of the tracking error signal are represented by a dashed line L₁ or an alternate long and short dash line L₃. Consequently, a tracking servo loop gain must be varied to meet the deviation, and hence stable servo operation can not be maintained. A prior art of the tracking control circuit for improving the above-mentioned difficulty is disclosed in the Japanese published unexamined patent application Sho 62-214543 and is shown in FIG.17. Referring to FIG.17, an automatic gain control circuit 21 (hereinafter abbreviated as AGC circuit) is provided between the amplifier 11 and the low pass filter 15. Moreover, an adder 22 for calculating the sum of the levels P₁, P₂ and P₃ in synchronism with the synchronizing signals SP₁, SP₂ and SP₃ is provided, and the output signal of the adder 22 is applied to the AGC circuit 21. The synchronizing signal SP₃ is created on the basis of the reproduced signal of a synchronizing signal S of a neighboring tack. The gain of the AGC circuit 21 is controlled by the output signal of the adder 22, and fluctuation of the output of the magnetic heads 2 and 3 is compensated. The characteristic of the level of the tracking error signal shown by the solid line L₂ of FIG.16 can be maintained. In digital audio tape recording and reproducing system using a tape cassette, the normal track mode as shown in FIG.18(a) and the wide track mode as shown in FIG.18(b) are standardized. The normal track mode is applicable to recording and reproducing by a consumer. On the other hand, the wide track mode is applied to make prerecorded tapes which are recorded in factory. In the above-mentioned prior art as shown in FIG.17, it is required that the output levels of the magnetic heads 2 and 3 reproducing the pilot signals P A , P B and P C are constant within a range of deflected angle (from +120° to -120°, for example). However, when the magnetic tape recorded by the wide track mode is reproduced by a magnetic tape reproducing apparatus having a characteristic of the level of the tracking error signal which is preferable to the normal track mode as shown by a dotted line L₅ of FIG.19 and which is slightly different from an ideal characteristic represented by a solid ling L₄, the characteristic of the level of the tracking error signal seriously changes as shown by a dotted line L₇ of FIG.20 which is different from the ideal characteristics of the level of the tracking error signal L₆. Hence, stable servo operation cannot be maintained, and compatibility between the normal track mode and the wide track mode cannot be attained. Furthermore, it is premise that the sensitivity of each magnetic head is identical with each other. When the sensitivity of each magnetic head is different from each other, stable tracking servo operation cannot be maintained. An object of the present invention is to provide a helical scanning magnetic tape apparatus with a tracking control circuit in which stable tracking servo operation is maintained. The helical scanning magnetic tape apparatus in accordance with the present invention comprises the features mentioned in claim 1. According to the present invention, respective pilot signals of both neighboring tracks of the actual reproducing track of a magnetic head are detected due to crosstalk effects, and a tracking error signal is created on the basis of a difference between the level of the detected respective pilot signals. Then the level of the tracking error signal is controlled by a signal including a pilot signal which is detected in reproducing step of other track. While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. FIG.1 is a circuit block diagram of a first embodiment of the tracking control circuit in accordance with the present invention; FIGs. 2(a), 2(b), 2(c), 2(d), 2(e), 2(f), 2(g) 2(h) and 2(i) are timing charts in operation of the first embodiment; FIG.3 and FIG.4 are graphs of characteristic of level of tracking error signal; FIG.5 is a circuit block diagram of a second embodiment of the tracking control circuit in accordance with the present invention; FIGs. 6(a), 6(b), 6(c), 6(d), 6(e), 6(f), 6(g), 6(h), 6(i) and 6(j) are timing charts in operation of the second embodiment; FIG.7 is a circuit block diagram of a third embodiment of the tracking control circuit in accordance with the present invention; FIGs. 8(a), 8(b), 8(c), 8(d), 8(e), 8(f), 8(g) and 8(h) are timing charts in operation of the third embodiment; FIG.9 is a circuit block diagram of a fourth embodiment of the tracking control circuit in accordance with the present invention; FIG.10 is a plan view of an example of a magnetic tape transporting mechanism in which the tracking control circuit is applied; FIG.11 is a plan view of a magnetic tape showing recorded tracks in herical scanning; FIG.12 is a plan view of the recorded tracks showing allocation of ATF signal in the normal track mode; FIGs. 13(a), 13(b) and 13(c) are waveforms of an output signal of a magnetic head reproducing the ATF signals; FIG.14 is a graph of characteristic of level of tracking error signal; FIG.15 is the circuit block diagram of the tracking control circuit in the prior art; FIG.16 is the graph of characteristic of level of tracking error signal in the prior art; FIG.17 is the other circuit block diagram of the tracking control circuit in the prior art; FIGs. 18(a) and 18(b) are figures of ATF signal allocation in the normal track mode and the wide track mode, respectively; FIG.19 and FIG.20 are the graphs of characteristic of level of tracking error signal in the normal track mode and the wide track mode, respectively, in the prior art. FIG.1 is a circuit block diagram of a first embodiment of a tracking control circuit in accordance with the present invention. Two magnetic heads 2 and 3 in a magnetic tape transporting mechanism shown in FIG.10 scan a magnetic tape 4. The magnetic head 2 or 3 moves in the direction of arrow E along the magnetic tape 4 which is transported in the direction of arrow D as shown in FIG.12. A reproduced signal S₁ of the magnetic heads 2 and 3 is inputted into an amplifier 31 through an input terminal 30. The reproduced signal S₁ is composed of both the output signals of the magnetic heads 2 and 3 which are combined to one signal in a time sequence. FIG.2(a) shows the reproduced signal S₁. A signal A is reproduced by the magnetic head 2 and a signal B is reproduced by the magnetic head 3. The output of the amplifier 31 is applied to a low-pass filter 32 and a band-pass filter 39. The low-pass filter 32 allows to pass only early pilot signals P A , P B and P C and later pilot signals P D . P E and P F of the ATF signal as shown in FIG.12, and digitalized signals of data and a synchronizing signal S of the ATF signal are eliminated. Referring to FIG.12, when the magnetic head 2 moves in the direction of arrow E on a central track T₂, first, the pilot signal P A is detected. Second, the pilot signal P B is detected, and third, the pilot signal P C is detected. The pilot signals P B and P C are detected due to the effect of crosstalk. A reproduced pilot signal S₂ in FIG.1 is composed of the reproduced signals of the pilot signals P A . P B and P C , and the waveform thereof is shown in FIG.13(a), 13(b) or 13(c). Referring to FIG.1, the reproduced pilot signal S₂ is applied to an envelope detector 33, and an envelope of the reproduced pilot signal S₂ is detected. FIG.2(b) shows the reproduced pilot signal S₂. Referring to FIG.2(b), letters a, f, i and j designate reproduced pilot signals of the actual track of each magnetic head 2 or 3. On the other hand, letters b, c, d, e, g, h, k and ℓ designate pilot signals reproduced from the pilot signals of neighboring tracks which are detected by crosstalk. The reproduced pilot signals a, b, c, g, h and i are reproduced from the early pilot signals of tracks, and the reproduced pilot signals d, e, f, j, k and ℓ are reproduced from the later pilot signals of the tracks. The output of the envelope detector 33 is applied to a first sample-and-hold circuit 34, a third sample-and-hold circuit 42 and an adder 35. The band-pass filter 39 allows to pass only the reproduced synchronizing signal S S of the synchronizing signal S. The synchronizing signal S S is applied to a zero-cross comparator 40, and a zero-cross time point is detected. The output of the zero-cross comparator 40 is applied to a synchronizing signal generator 41, and synchronizing signals SP₁, SP₂ and SP₃ which serve as timing signals are generated on the basis of the zero-cross time points of the synchronizing signal S S . The synchronizing signal SP₁ is generated during the period of reproduction of the pilot signal P B due to crosstalk as shown in FIG.2(c) and FIG.13(a), and is applied to the first sample-and-hold circuit 34. Thus a level P₁ of the reproduced signal of the pilot signal P B is held in the first sample-and-hold circuit 34. The output of the first sample-and-hold circuit 34 is applied to a positive input of an adder 35. On the other hand, the output of the envelope detector 33 is applied to a negative input of the adder 35. In the adder 35, a difference between the value of the level P₁ and the output value of the envelope detector 33 is detected and is held in synchronism with the synchronizing signal SP₂ which is supplied from the synchronizing signal generator 41 with a second sample-and-hold circuit 36. The synchronizing signal SP₂ is generated during the period of reproduction of the pilot signal P C as shown in FIG.2(d) and FIG.13(a). Consequently, the difference between the value of the level P₁ and the value of the level P₂ of the reproduced signal of the pilot signal P C is held in the second sample-and-hold circuit 36 of FIG.1. The difference (P₁-P₂) is designated by "signal S₃", and is applied to an automatic gain control circuit 37 (hereinafter abbreviated as AGC circuit). The level of the signal S₃ is controlled at the AGC circuit 37 on the basis of a signal S₆ which is elucidated hereinafter. FIG.2(f) is a time chart of the signal S₃. A representation "b-c" represents the difference (P₁-P₂) between reproduced signals of the pilot signals P B and P C , and a representation "d-e" represents the difference (P₁-P₂) between reproduced signals of the pilot signals P D and P E , for example. The output of the envelope detector 33 is also applied to the third sample-and-hold circuit 42 which is controlled by the synchronizing signal SP₃. The synchronizing signal SP₃ is generated during the period reproducing the pilot signal P A of the actual track of the magnetic head 2 as shown in FIG.2(e) and FIG.13(a). Consequently, the level of the output signal S₄ of the third sample-and-hold circuit 42 is equal to the level P₃ of the reproduced signal of the pilot signal P A . Then the signal S₄ is applied to a negative input of an adder 44. The time chart of the signal S₄ is shown in FIG.2(g). On the other hand, the signal S₄ is also applied to a delay circuit 43 which is controlled by the synchronizing signal SP₃. The delay circuit 43 causes the signal 4 to be delayed by a time period between two consecutive reproduced pilot signals. A delayed signal S₅, as shown in FIG.2(h), is output from the delay circuit 43. In the track T₂ in FIG.12, for example, the delay time is equal to a time period between the pilot signals P A and P F . Moreover, when reproduction is proceeding across the tracks T₂ and T₃, the delay time is equal to a time period between the pilot signals P F and P B . The signal S₅ is applied to a positive input of the adder 44. Consequently, a difference between the level of the reproduced signal of the pilot signal P A and the level of the reproduced signal of the pilot signal P F in the track T₂ is obtained. The signal representing the difference between the two values is designated as "S₆" in FIG.2(i). The signal S₆ is applied to the AGC circuit 37, and the level of the signal S₃ is controlled by the signal S₆ and is output from an output terminal 38. The output of the AGC circuit 37 is a tracking error signal which is applicable to the servo control system in a manner that will be familiar to one skilled in the art. In operation of the above-mentioned tracking control circuit, as shown in FIG.2(f), during the time period in which the signal S₃ of the tracking error signal is issued on the basis of the difference of representation "g-h", the AGC circuit 37 is controlled by the signal S₆ which is issued on the basis of the difference of representation "f-i" shown in FIG.2(i). The representation "f-i" represents a difference between a reproduced signal level of the signal "f" of the actual track of the magnetic head 2 and a reproduced signal level of the signal "i" of the actual track of the magnetic head 3 as shown in FIGs. 2(b) and 2(i). According to the first embodiment of the present invention, for example in FIG.2(a), when a recorded level of the signal A is lower than that of the signal B due to nonuniformity in the recording process of the magnetic tape, the reproduced signal levels of the signals g, h and f are lower than the reproduced signal levels of the signals d, e and i, respectively, and thus the difference "f-i" becomes a negative value. The AGC circuit 37 is made to increase its gain by applying a negative control signal. Thus, the gain of the AGC circuit 37 is increased by applying the negative signal according to the difference "f-i", and the output level of the AGC circuit 37 which is represented by the difference "g-h" (tracking error signal) increases. As mentioned above, according to the first embodiment of the present invention, fluctuation of the level of the tracking error signal due to ununiformity of the recorded level of the respective tracks is prevented, and hence, a preferable tracking error signal is output in both the normal track mode and the wide track mode. As shown in FIG.3 (normal track mode) or FIG.4 (wide track mode), the improved characteristic of the level of the tracking error signal shown in dotted lines meets an ideal characteristic shown in solid lines in the range of deflected angle from +120° to -120° . Thus, stable servo control is realized without requiring any change of the gain of the tracking servo system. FIG.5 is a circuit block diagram of a second embodiment of the present invention. Referring to FIG.5, arrangement and operation of the respective circuits with the exception of adders 45 and 46 are identical with that of the first embodiment. The output of the envelope detector 33 is inputted into the adder 45 which is controlled by the synchronizing signals SP₁ and SP₂. The output of the adder 45 is applied to the positive input of the adder 46, and the output of the delay circuit 43 is applied to the other positive input thereof. The output of the adder 46 is applied to the AGC circuit 37. The adder 45 holds the output of the envelope detector 33 in synchronism with the synchronizing signals SP₁ and SP₂, and calculates a sum of levels P₁ and P₂. Then a signal S₇ having the level of the sum is created as shown in FIG.6(g). The signal S₇ is added to the signal S₅ output from the delay circuit 43 by means of the adder 46, and a resultant signal S₈ is created as shown in FIG.6(j). The signal S₈ is applied to the AGC circuit 37 to control the gain. According to the second embodiment, the tracking error signal S₃ is controlled by a sum of the level of the reproduced signal of the later pilot signal of a track detected during previous reproducing period, and the levels of the signals of the early pilot signals of both neighboring tracks of the next successive track which is presently in reproduction. Namely, the tracking error signal S₃ represented by the difference "g-h" is controlled on the basis of the signal S₈ represented by the sum "f + g + h". The AGC circuit 37 in this embodiment is made to vary its gain in inverse proportion to a level of a control signal applied thereto. Therefore the level of the tracking error signal S₃ is inversely proportional to the level of the sum "f + g + h". For example, when the recorded level of the signal A is lower than that of the signal B, the level of the sum "f + g + h" is lowered. Consequently, the gain of the AGC circuit 37 increases, and the level of the tracking error signal S₃ increases. FIG.7 is a circuit block diagram of a third embodiment of the present invention. In the embodiment, an AGC circuit 47 is placed between the amplifier 31 and the low pass filter 32, and the output of the second sample-and-hold circuit 36 is the tracking error signal. The output of the delay circuit 43 is applied to the AGC circuit 47, and the remaining circuit is identical with that of the first embodiment. In the embodiment, as shown in FIGs. 8(g) and 8(h), the signal S₄ based on a later pilot signal is delayed by the delay circuit 43, and a control signal S₅ is created. The control signal S₅ is applied to the AGC circuit 47 during reproduction of the subsequent track. According to the third embodiment, therefore, the signal S₅ controls the AGC circuit 47 in a direction to reduce a difference between the respective reproduced levels of neighboring two tracks. FIG.9 is a circuit block diagram of a fourth embodiment of the present invention. In the fourth embodiment, in a manner similar to that illustrated in FIG.1, the AGC circuit 37 receives the output of the second sample-and-hold circuit 36, and is controlled by the output of the delay circuit 43. Difference of the fourth embodiment with respect to the first embodiment shown in FIG.1 is that the control signal S₅ is directly inputted to the AGC circuit 37 from the delay circuit 43. Furthermore, since a signal reproduced due to crosstalk of pilot signals of neighboring tracks is not included in the control signal S₅ which is applied to the AGC circuit, serious variation of the level of the tracking error signal can be prevented, even if the level of the tracking error signal S₃ varies due to unstable contact between the head chip and the tape surface. In the fourth embodiment, in a manner similar to the third embodiment, since the tracking error signal created depending on crosstalk of pilot signals of the neighboring tracks is controlled by a reproduced signal of the own track of the head, a serious variation of the tracking error signal is prevented. Moreover, the third and fourth embodiment is simplified in configuration in comparison with the first or second embodiment.

Claims: A helical scanning magnetic tape apparatus with a tracking control circuit comprising: at least one magnetic head (2, 3) for reproducing a recorded signal from slanted tracks of a magnetic tape including, in each track, at least one short pilot signal (P A to P F ), and a short synchronizing signal (S) being arranged in a predetermined relationship with respect to said at least one pilot signal, first filter means (32) for passing said pilot signals of said reproduced signal, envelope detecting means (33) for detecting the level of an evelope of said pilot signals passed through said first filter means (32), second filter means (39) for passing said synchronizing signal (S) of said reproduced signal, a synchronizing signal generator (41) for creating timing signals (SP₁, SP₂, SP₃) on the basis of said synchronizing signal passed through said second filter means (39), calculating means (35) for calculating for a present track traced by said magnetic head a difference level between the respective levels of said pilot signals recorded on both neighboring tracks, said respective levels detected in synchronism with said timing signals (SP₁, SP₂) by said envelope detecting means from crosstalk components in the reproduced signal from said present track, a sample-and-hold circuit (36) for holding said difference level in synchronism with said timing signal (SP₂), the apparatus being characterized by: a delay circuit (43) for delaying the level of the pilot signal of said present track detected by said envelope detecting means in synchronism with said timing signal (SP₃), and an automatic gain control circuit (37) for controlling said difference level output by said calculating means on the basis of at least said delayed level of the pilot signal of the track traced previous to said present track. An apparatus according to claim 1, wherein two magnetic heads (2, 3) are provided for alternately tracing succeeding tracks. An apparatus according to claim 1 or 2, wherein said automatic gain control circuit (37) controls said difference level on the basis of the sum of levels of the sum of said respective levels of said pilot signals recorded on both neighboring tracks, and said delayed level of the pilot signal.

Inventors: MIYAZAKI HIDETO SANDA SEISAKUS, MIYAZAKI, HIDETO SANDA SEISAKUSHO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B15
IPC codes: G11B15, G11B5