Tracking error detection system

Abstract: Disclosed is a tracking error detection system for a digital magnetic recording/reproducing apparatus for recording/reproducing a digital signal, in which a digital signal recorded by means of a magnetic head (1;2;22, 23;82;112, 113) having an azimuth is reproduced to thereby produce a reproduction clock signal and the phase or period of the reproduction clock signal is measured to thereby detect a tracking error. Since the tracking error can be known every time the phase or period of the reproduction clock signal is measured, it is possible to detect the tracking error including the linearity of a recording track and it is possible to improve the accuracy in tracking error detection. Further, since no pilot signal or the like is frequency-­ multiplexed on a low frequency, the recording digital is not affected.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking error detection system for detecting tracking errors in a magnetic recording/reproducing apparatus for recording/reproducing digital signals. 2. Description of the Prior Art Conventionally, in a tracking error detection system used in home-use VTR sets, a control track is provided in the longitudinal direction on a magnetic tape, separately from a video track on which recording/reproducing of video signals are effected, so that pulses indicating a rotational phase of a head drum are recorded as a control signal on the control track, whereby, in reproduction, the phase of a reproduced control signal is compared with that of the pulses indicating rotation of the head drum to thereby detect a tracking error. (For example, reference is made to "VTR Technique", supervised by Ryo Takahashi, Japan Broadcasting Corporation (NHK)). On the other hand, in a tracking error detection system used in 8 mm videos, four low-frequency pilot signals different in frequency from one another are established and those four low-frequency pilot signals are recorded one by one on every video track so that every pilot signal is frequency-multiplexed with a video signal on the track. In reproduction, two low-frequency pilot signals recorded on two adjacent video tracks are detected as cross-talk from adjacent video tracks and a power difference between the two detected low-frequency pilot signals are compared with each other to thereby detect a tracking error. In such a conventional method using a control track, however, it is impossible to detect the linearity of a recording track and it is very difficult to improve the accuracy in tracking error detection. In the tracking error detection system used in 8 mm videos, on the other hand, it is possible to detect the linearity of the video track, however, the frequency-multiplexing of a low-frequency pilot signal with a digital video signal causes a problem that the pilot signal acts as a disturbance against the digital signal because such a digital signal used in a digital magnetic image recording/reproducing apparatus includes signal components even in a low frequency band. EP-A-0040066, delimiting the preamble of claim 1, discloses a tracking system which compares the phase of synchronisation signals from adjacent tracks having equal and opposite azimuth angles, thereby enabling the position of the head along the perpendicular direction to the track to be detected. EP-A-0355721, which was published after the priority date of this invention, discloses a similar system but includes a frequency dividing circuit to increase the dynamic range of the tracking error detection. WO-A-86/02190 discloses a tracking system which adjusts the direction of the rotational axis of the rotary head drum in response to differences in the phases of signals read from the tracks on the tape. SUMMARY OF THE INVENTION It is therefore an object of the present invention to eliminate the disadvantages in the above mentioned conventional art. It is another object of the present invention to provide a tracking error detection system in which the linearity of a recording track can be detected, the accuracy in tracking error detection can be improved, and no disturbance is given against the digital signal. In order to attain the above objects, according to the present invention there is provided a tracking error detection system for a digital magnetic recording/reproducing apparatus for recording/reproducing digital signals, comprising: a magnetic head having an azimuth; a PLL circuit for generating a reproduction clock signal on the basis of a digital signal outputted from by the magnetic head; a frequency dividing circuit for dividing the frequency of the reproduction clock signal; and characterised by a period measuring circuit for measuring the period of an output signal from the frequency dividing circuit; and an integrating circuit for integrating a difference between a period measured by the period measuring circuit and a period in normal tracking to provide a signal indicative of the difference. Preferably, the period measuring circuit includes a counter for measuring the period of an output signal from the frequency-dividing circuit, and a clock generator circuit for generating a free oscillation clock signal for the counter. With this construction, the period of the reproduction clock signal varies when the magnetic head comes out of the recording track. Accordingly, a tracking error is detected by measuring the period of the reproduction clock signal or the period of a signal obtained by dividing the frequency of the reproduction clock signal. Accordingly, the linearity of a recording track can be detected simultaneously with the tracking error detection, so that it is possible to improve the accuracy in tracking error detection. Further, the digital signal is not affected by a low frequency signal component because no low-frequency signal component because no low-frequency pilot signal is frequency-multiplexed. Advantageously the tracking error detection system comprises a multi-head magnetic head having two magnetic heads disposed in a multiple configuration and having azimuths opposite to each other, a PLL circuit for generating a reproduction clock signal on the basis of a digital video signal outputted from one of the two magnetic heads, a frequency-dividing circuit for dividing the frequency of the reproduction clock signal, and a period measuring circuit for measuring the period of an output signal from the frequency-dividing circuit. Preferably, the period measuring circuit includes a counter for measuring the period of an output signal from the frequency-dividing circuit, a second PLL circuit for generating a reproduction clock signal on the basis of a digital video signal outputted from the other one of the two magnetic heads, and a frequency multiplying circuit for multiplying the reproduction clock signal from the second PLL circuit so as to generate a clock signal for actuating the counter to operate. In this advantageous construction according to the present invention, the following effect can be obtained in addition to the effects obtained in the construction according to the second aspect of the present invention. That is, since a tracking error is detected on the basis of a ratio between the periods of the two reproduction clock signals reproduced by the multiple magnetic heads having azimuths opposite to each other, the accuracy in tracking error detection is higher than that in the construction according to the second aspect of the present invention, and since no influence is given by jitter, it is possible to perform tracking error detection with higher accuracy. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings, wherein: Fig. 1 is a block diagram illustrating main part of a first embodiment of the tracking error detection system having a first configuration according to the present invention; Fig. 2 is a diagram illustrating a recording pattern on a magnetic tape according to the present invention; Fig. 3 is a block diagram illustrating main part of the configuration of the PLL circuit according to the present invention; Fig. 4 is a waveform diagram for explaining the method of phase comparison according to the present invention; Fig. 5 is a block diagram illustrating the main part of a modification of the second embodiment of the tracking error detection system having a first configuration according to the present invention; Fig. 6A is a diagram illustrating the synchronizing pattern recording timing according to the present invention; Fig. 6B is a diagram illustrating the recording pattern on a magnetic tape according to the present invention; Fig. 7 is waveform diagram showing the phase timing of the frequency dividing circuit according to the present invention; Fig. 8 is a block diagram illustrating the main part of an embodiment of the tracking error detection system having a second configuration according to the present invention; Fig. 9 is a block diagram illustrating the main part of the tracking error detection circuit 88; Figs. 10A through 10C are diagrams showing the displacement in tracking of the magnetic heads; Fig. 11 is a block diagram illustrating the main part of an embodiment of the tracking error detection system having a third configuration according to the present invention; Fig. 12 is a block diagram illustrating the main part of the tracking error detection circuit 124; Figs. 13A through 13C are diagrams showing the displacement in tracking of the magnetic heads; and Fig. 14A and 14B are diagrams showing the tracking under existence of jitter. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the accompanying drawings hereunder. Fig. 1 is a block diagram showing main portions of an embodiment of the tracking error detection system having a first configuration according to the present invention, and Fig. 2 is a diagram showing a recording pattern on a magnetic tape. In Fig. 2, the reference numerals 1 and 2 designate multiple magnetic heads disposed on a single base. Representing the value of an azimuth by ϑ, the azimuths of the respective magnetic heads 1 and 2 opposite to each other take values of ϑ and -ϑ respectively. If tracking is displaced left in Fig. 2, the power center position of a track T₁ to be reproduced by one 1 of the magnetic heads is displaced left downward in the drawing, so that the time base varies to generate a delay. The power center position of a track T₂ to be reproduced by the other 2 of the magnetic heads, on the contrary, is displaced left upward in the drawing, so that the time base varies to generate an advance. Further, variations in time base are caused by expansion/contraction of a magnetic tape and unevenness in rotation of a rotary cylinder in reproducing operation. However, the variations in time base are the same with each other between the magnetic heads 1 and 2 because the magnetic heads 1 and 2 are disposed on a single base, and therefore cause no problem. Next, returning to Fig. 1, description will be made as to the construction of the tracking error detection system. A digital signal recorded on a magnetic tape 3 is reproduced by the magnetic heads 1 and 2, and the outputs of the magnetic heads 1 and 2 are supplied to PLL (phase-locked loop) circuits 4 and 5 respectively. The PPL circuits 4 and 5 have the same in configuration. Fig. 3 shows the configuration of each PLL circuit in detail. Each PLL circuit is constituted by an input terminal 10, an equalizer circuit 11, an integrating circuit 12, a phase comparator 13, a low-pass filter (LPF) 14, a voltage-controlled oscillation circuit (VCO) 15, and an output terminal 16. The reproduced signal, that is, the output signal of the corresponding magnetic head, is supplied to the equalizer circuit 11 through the input terminal 10. The equalizer circuit 11 is constituted by a reproduction amplifier circuit and a transversal filter so as to amplify the received digital signal and then correct the frequency characteristic of the amplified digital signal to make the frequency characteristic be a differential characteristic. The thus obtained digital signal, that is, the output of the equalizer circuit 11 is supplied to the integrating circuit 12. The integrating circuit 12 integrates the input signal to convert it so as to make it have the same cosine roll-off waveform as that of the original digital signal. The phase comparator 13, the low-pass filter 14, and the voltage-controlled oscillation circuit 15 constitute a so-called phase-locked loop for generating a reproduction clock phase-synchronized with the reproduced digital signal, the reproduction clock being put out through the output terminal 16. The reproduction clock signals produced from the respective PLL circuits 4 and 5 are supplied to a phase comparator 6. In an embodiment of the phase comparator 6 in which the two reproduction clock signals are subject to exclusive-ORing, the respective phases of the reproduction clock signals produced from the PLL circuits 4 and 5 as shown in the diagrams (a) and (b) in Fig. 4 are compared with each other to thereby obtain an output as shown in the diagram (c) of Fig. 4. Next, another embodiment of the tracking error detection system shown on Fig. 5 will be described. In the foregoing embodiment of Fig. 1, if the displacement of tracking becomes larger than the period of the reproduction clock signal, the phase width of a time base variation exceeds the edge of the reproduction clock signal, so that the outputs after phase comparison do not have one-to-one correspondency. That is, the dynamic range is narrow. In the embodiment of Fig. 5, therefore, the problem concerning the dynamic range is solved. That is, generally, synchronizing patterns each showing a time base reference used in a recording format of a digital magnetic recording system are arranged so as to have a fixed mutual relationship (T may be set to zero) with respect to the time base between channels 1 (ch1) and 2 (ch2) as shown in Fig. 6A, and recording is made with the recording patterns with such azimuths as shown in Fig. 6B so as to make the signal patterns be pilot signals for enlarging the dynamic range. Next, referring to Fig. 5, the tracking error detection system will be described. At the reproduction side, a magnetic tape 21, multiple magnetic heads 22 and 23, and PLL circuits 24 and 25 are the same in configuration basically as those in the foregoing embodiment of Fig. 1. A reproduction clock signal of the channel 1 which is the output of a VCO 34 is supplied to a frequency dividing circuit 38, and a reproduction clock signal of the channel 2 which is the output of a VCO 35 is supplied to a frequency dividing circuit 39, so that the reproduction clock signals are frequency-divided in order to enlarge the dynamic range. If the frequency division is performed, however, the phase to be taken increases by a value corresponding to the frequency dividing ratio, and therefore it is impossible to determine the phase with no ambiguity. Accordingly, the foregoing synchronizing patterns are detected by synchronization detection circuits 36 and 37 which generate pulses when they find patterns coincident with the synchronizing patterns in data trains from integrating circuits 28 and 29 respectively. The detected synchronizing patterns are sent to the frequency dividing circuits 38 and 39 as the time base references so that the phases are determined with no ambiguity. Fig. 7 shows the process in which the phase is determined. That is, the diagrams A, B, and C show the synchronizing pattern of the channel 1, the output of the frequency dividing circuit 38, and the reproduction clock signal, respectively, and the diagrams D, E, and F show the synchronizing pattern of the channel 2, the output of the frequency dividing circuit 39, and the reproduction clock signal respectively. That is, the phases of the signals B and E are determined on the basis of the time base positions of the signals A and D. Finally, the outputs of the frequency dividing circuits 38 and 39 are supplied to a phase comparator 40 so that the phases are compared with each other in the same manner as in the case of the embodiment of Fig. 1, and the quantity and direction of a tracking error are produced. As described above, by use of the embodiment of Fig. 5, the dynamic range can be enlarged. Further, in the case of considering the interchangeability of the digital magnetic recording system, the period T of Fig. 7 is sometimes different from the period T in recording shown in Fig. 6A because of errors in secure positions of the multiple magnetic heads 1 and 2. However, no problem is caused when the dynamic range is sufficiently larger than the attachment position errors. Although description has been made as to the configuration in which the reproduction clock signals are frequency-divided in the embodiment of the present invention shown in Fig. 5, the same effects as those of the embodiment of Fig. 5 can be obtained also when the phases of the pulses of the channels 1 and 2 showing the time bases of the synchronizing patterns are compared with each other. Fig. 8 is a block diagram showing main portions of an embodiment of the tracking error detection system having a second configuration according to the present invention. In the drawing, the system includes: a magnetic tape 81; a magnetic head having an azimuth 82, an equalizer circuit (EQ) 83 constituted by a reproduction amplifier circuit and a transversal filter so as to amplify the digital signal reproduced by the magnetic head and then correct the frequency characteristic of the amplified digital signal to make the frequency characteristic be a differential characteristic; an integrating circuit 84 for integrating the output of the equalizer circuit 83 so as to convert it to make it have the same cosine roll-off waveform as that of the original digital signal; a phase comparator (PC) 85; a low-pass filter (LPF) 86; a voltage-controlled oscillation circuit (VCO) 87; a tracking error detection circuit 88 for detecting a tracking error from the reproduction clock signal; and an output terminal 89. The phase comparator 85, the low-pass filter 86, and the voltage-controlled oscillation circuit 87 constitute a usual phase-locked loop (PLL) which generates a reproduction clock phase-synchronized with the reproduced digital signal. Fig. 9 is a block diagram showing the configuration of the tracking error detection circuit 88 of Fig. 8. In Fig. 9, the tracking error detection circuit 88 includes: a frequency dividing circuit 91 for dividing the frequency of a reproduction clock signal; a differential circuit 92 for generating a one-shot pulse at the leading edge of the output of the frequency dividing circuit 91; a counter 93 for measuring the period of a pulse which is the output of the differential circuit 92; a free oscillation clock generation circuit 94 for making the period measuring counter 93 operate; and an integrating circuit 95 for integrating a difference between the period actually measured by the period measuring counter 93 and a reference value, that is the period in normal tracking operation. The operation of the tracking error detection circuit 88 will be described hereunder with reference to Figs. 8 and 9. Figs. 10A through 10C show displacement of tracking of the magnetic head 82. Recording/reproducing is made on a track by the magnetic head 82 with an azimuth ϑ. When tracking is performed normally as shown in Fig. 10A, the period of the reproduction clock signal takes a fixed value T. If the tracking is displaced as shown in Fig. 10B, the period t′ of the reproduction clock signal becomes smaller than T (t′ < T). If the tracking is displaced as shown in Fig. 10C, on the contrary, the period t″ of the reproduction clock signal becomes larger than T (t″ > T). In the tracking error detection circuit 88 shown in Fig. 9, therefore, the reproduction clock signal is subject to frequency-division by n (n being a positive integer). Next, the period of the output signal of the differential circuit 92 is measured by the period measuring counter 93 which is actuated to operate by a free oscillation clock signal and which is reset by a pulse generated from the differential circuit 92. Thus, the period of the output of the differential circuit 92 is to be n · T in normal tracking. Accordingly, an error in period, that is, a value obtained by subtracting n · T from the period of the output of the differential circuit 92 represents displacement of tracking in this period. That is, when the detected error in period is negative, tracking is displaced in such a manner as shown in Fig. 10B, and when the detected error in period is positive, tracking is displaced in such a manner as shown in Fig. 10C. Therefore, the integration of the error in period, that is, the deviation of the period of the output of the differential circuit 92 from the reference period n · T is used as a tracking error output. In short, it will do to adjust tracking so as to make the tracking error output be zero. In this method, however, there is a possibility that an influence of jitter may cause an erroneous operation. Then, referring to the drawings, description will be made as to an embodiment in which jitter is taken into consideration. Fig. 11 is a block diagram showing main portions of an embodiment of the tracking error detection system having a third configuration according to the present invetnion. The system of the embodiment includes: a magnetic tape 111; a multi-head magnetic head having two magnetic heads 112 and 113 disposed on a single base respectively with azimuths opposite to each other; equalizer circuits (EQ) 114 and 115 each constituted by a reproduction amplifier circuit and a transversal filter so as to amplify the digital signal reproduced by the corresponding magnetic head and then correct the frequency characteristic of the amplified digital signal to make the frequency characteristic be a differential characteristic; integrating circuits 116 and 117 for integrating the respective outputs of the equalizer circuits 114 and 115 for integrating the outputs of the equalizer circuits 114 and 115 so as to convert those outputs to make them have the same cosine roll-off waveforms as those of the original digital signals respectively; phase comparators (PC) 118 and 119; low-pass filters (LPF) 120 and 121; and voltage-controlled oscillation circuits (VCO) 122 and 123. The phase comparators 118 and 119, the low-pass filters 120 and 121, and the voltage-controlled oscillation circuits 122 and 123 constitute usual phase-locked loops (PLLs) which generate reproduction clock signals 125 and 126 phase-synchronized with the reproduced digital signals respectively. The system further includes: a tracking error detection circuit 124 for detecting a tracking error on the bases of the reproduction clock signals 125 and 126; and an output terminal 127. Fig. 12 is a block diagram showing the configuration of the tracking error detection circuit 124. In the drawing, the tracking error detection circuit 124 includes a frequency dividing circuit 131 for dividing the frequency of the reproduction clock signal 125; a differential circuit 132 for generating a one-shot pulse at the leading edge of the output of the frequency dividing circuit 131; a counter 133 for measuring a period of a pulse which is the output of the differential circuit 132; an integrating circuit 134 for integrating a difference between the period actually measured by the period measuring counter 133 and a reference value, that is, the period in normal tracking; a phase comparator 135; a low-pass filter 136; a voltage-controlled oscillator 137; and a frequency-dividing circuit 138. The phase comparator 135, the low-pass filter 136, the voltage-controlled oscillator 137, and the frequency-dividing circuit 138 constitute a phase-locked loop acting as a frequency multiplying circuit which generates a clock signal for the counter, the clock signal being phase-synchronized with the reproduction clock signal 126 and obtained by multiplying the reproduction clock signal 126. The operation of the tracking error detection circuit 124 having such a configuration will be described hereunder. The tracking error detection system of this embodiment having such a configuration as described above is substantially the same as that of the embodiment of the second configuration, except that the former is different from the latter in that the former is provided with the multi-head magnetic head having two magnetic heads 112 and 113 so that a tracking error is detected on the basis of the ratio between the two reproduction clock signals reproduced by the magnetic heads 112 and 113. Figs. 13A through 13c show displacement of tracking in the magnetic heads 112 and 113. Recording/reproducing is made on two tracks by the respective magnetic heads 112 and 113 with azimuths ± ϑ opposite to each other. When tracking is performed normally as shown in Fig. 13a, both the periods of the reproduction clock signals 125 and 126 take the same fixed values T. Further, the period A of an output signal of the frequency dividing circuit 131, which is obtained by dividing the frequency of the reproduction clock signal 125 by n₁ (n₁ being a positive integer), is n₁ · T (A = n₁ · T). The period B of an output signal of the frequency multiplying circuit constituted by the phase comparator 135, the low-pass filter 136, the voltage-controlled oscillator 137, and the frequency-dividing circuit 138, which is obtained by multiplying the reproduction clock signal 126 by n₂ (n₂ being a positive integer), is T/n₂ (B = T/n₂). The counter value N of the period measuring counter 133 is therefore n₁ · n₂ (N = n₁ · n₂). If tracking is displaced as shown in Fig. 13B, the period t₁′ of the reproduction clock signal 125 becomes smaller than T, and the period t₂′ of the reproduction clock signal 126, on the contrary, becomes larger than T. Since the period A = n₁ · t₁, and the period B = t₂′/n₂ at this time, the count value N′ of the period measuring counter 133 becomes as follows: N′ = n₁ · n₂ · (t₁′/t₂′) < N If the tracking is displaced as shown in Fig. 13C, on the contrary, the period t₁″ of the reproduction clock signal 125 becomes larger than T, and the period t₂″ of the reproduction clock signal 126 becomes smaller than T. Since the period A = n₁ · t₁″ and the period B = t₂″/n₂ similarly to the foregoing case, the count value N″ of the period measuring counter 133 becomes as follows: N″ = n₁ · n₂ · (t₁″/t₂″) > N Therefore, it is possible to detect the tracking error in the same manner as in the embodiment of the second configuration. Further, although the tracking error is detected on the basis of the period of only one reproduction clock signal in the embodiment of the second configuration, the tracking error is detected on the basis of the ratio between two reproduction clock signals reproduced by the magnetic heads with azimuths opposite to each other in the embodiment of the third configuration. In this case, the periods of the two reproduction clock signals change reversely to each other in response to displacement of tracking. Accordingly, the detection sensitivity in the embodiment of the third configuration is higher than that in the embodiment of the second configuration. Next, the influence of jitter will be described. Figs. 14A and 14B show tracking under the condition of occurrence of jitter. Fig. 14A shows the case of the embodiment of the second configuration. If the period of the reproduction clock signal is made short by jitter on the way, the period measured by the period measuring counter becomes short so that the head is displaced upward in the drawing. Fig. 14B shows the case of the embodiment of the third configuration. The periods t₁″ and t₂″ are made to change by the influence of jitter as follows. t₁‴ = t₂‴ = t‴ < T. Since the period A = n₁ · t‴ and the period B = t‴/n₂, however, the value N‴ of the period measuring counter 133 becomes N‴ = n₁ · n₂ = N. Accordingly, no change is generated in the count value of the counter. It is therefore found that tracking can be performed normally without being affected by jitter.

Claims: A tracking error detection system for a digital magnetic recording/reproducing apparatus for recording/reproducing digital signals, comprising: a magnetic head (82) having an azimuth; a PLL circuit (85, 86, 87) for generating a reproduction clock signal on the basis of a digital signal outputted from by the magnetic head; a frequency dividing circuit (91) for dividing the frequency of the reproduction clock signal; and characterised by a period measuring circuit (93, 94) for measuring the period of an output signal from the frequency dividing circuit; and an integrating circuit (95) for integrating a difference between a period measured by the period measuring circuit and a period in normal tracking to provide a signal indicative of the difference. A tracking error detection system according to claim 1, wherein the period measuring circuit includes a counter, and the system further comprises a clock signal generator circuit for generating a free oscillation clock signal for the counter. A tracking error detection system according to claim 1, further comprising: a second magnetic head having an azimuth opposite to the magnetic head of claim 1, wherein the respective magnetic heads (112, 113) are disposed in multiple configuration on a single base. A tracking error detection system according to claim 3, wherein the period measuring circuit includes a counter (133), and the system further comprises a second PLL circuit (119, 121, 123) for generating a second reproduction clock signal on the basis of a digital video signal outputted from the other one of the two heads of the multi-head magnetic head (113), and a frequency multiplying circuit (135, 136, 137, 138) for multiplying the second reproduction clock signal so as to generate a dependent clock signal for the counter; the arrangement being such that the period of the dependent clock signal increases or decreases whereas the period of the first reproduction clock signal measured by the counter changes in the opposite manner when the direction of tracking deviates from the direction of the track, thus enhancing the range of the count produced by the counter and hence the accuracy of the tracking response.

Inventors: FUJIOKA SOUICHIRO, MIZUSHIMA TETSUYA, OKAMOTO HIROSHI, SHIMOTASHIRO MASAFUMI, FUJIOKA, SOUICHIRO, MIZUSHIMA, TETSUYA, OKAMOTO, HIROSHI, SHIMOTASHIRO, MASAFUMI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G11B5
IPC codes: G11B15, G11B5