RF tracking servo for VTR

Abstract: A video signal is recorded in oblique tracks on a magnetic tape. The tracks are successively scanned by rotary heads, and a reproduced radio frequency signal is obtained. The level of the signal as reproduced exhibits a nonlinearity as a function of deviations of the rotary heads from the successively scanned oblique tracks. In accordance with the invention, the running speed of the tape is controlled on the basis of the detected changes in signal level, and the gain of the reproduced radio frequency signal is controlled in such a way as to compensate for the nonlinearity of the reproduced signal.

Description: The present invention relates to apparatus for reproducing a video signal and, more particularly, to a servo circuit of a VTR of the helical scan type. As a tracking control system for use in a helical scan type VTR, a number of different systems are known. In one such system, a control signal or CTL signal is recorded on a track in the longitudinal direction of a magnetic tape. In another such system, four pilot signals having predetermined frequency differences with respect to one another are recorded on the tape. However, in all such conventional systems, the circuitry is quite complicated. This causes manufacturing costs to be higher than is desirable. For example, in systems using a CTL signal, a fixed head for recording and reproducing the CTL signal is necessary. This not only represents additional cost, it also reduces the recording density that can be achieved on the tape, since a significant amount of tape must be devoted to the CTL signal. To solve such problems, there have been proposed tracking control systems in which the recording of a special signal for purposes of tracking control becomes unnecessary. Such tracking control systems are based on special processing of the reproduced RF signal. A so-called dynamic tracking system is representative of tracking control systems based on special processing of the reproduced RF signal. However, even in a dynamic tracking system, a voltage-dependent mechanical transducer such as a piezoelectric bimorph and a drive circuit are needed. Thus the construction is complicated and the manufacturing costs are inordinately increased. Japanese patent application No. 63-241366, assigned to the assignee of the present invention, discloses a magnetic recording and reproducing apparatus in which tracking control is carried out in response to the level of the reproduced RF signal plus information relating to the accelerating or decelerating state of the magnetic tape. However, in the tracking control system as disclosed in that patent application, no compensation is made for the nonlinear characteristics of the level of reproduced RF signal as a function of track deviation (i.e., as a function of deviation between the location of the reproducing head and the location of the previously recorded track). Thus even that system is not ideally suited to achieving stable tracking control. Fig. 1 illustrates the nonlinear characteristics of the reproduced RF signal level as a function of track deviation. In Fig. 1, assuming that the reproduced RF signal level corresponding to a predetermined track deviation is required to be amplified, the gain must be larger at point 50A than, for example, at point 50B. A drive amplifier to drive a motor as one of the elements for setting the gain of a servo system is provided in a servo loop. Since the gain corresponding to point 50A is large, the gain of the drive amplifier can be relatively small. On the other hand, since the gain corresponding to point 50B is small, the gain of the drive amplifier must be relatively large. Therefore, if the gain of the drive amplifier is matched with the level at point 50A, the gain of the servo system may be so small at point 50B that an off-track indication occurs. On the other hand, if the gain of the drive amplifier is matched with the level at point 50B, the gain of the servo system may be so large at point 50A that an oscillation occurs. The servo characteristics thus deteriorate, and it is difficult to carry out the tracking control in a manner that is stable and reliable. An object of the invention is to provide apparatus for reproducing a video signal in such a manner as to carry out stable tracking control. In accordance with one aspect of the invention, there is provided apparatus for reproducing a video signal recorded on a tape in successive oblique tracks, the apparatus comprising: means for advancing the tape; rotary heads for scanning the tracks successively to form a reproduced radio frequency signal; and means for controlling the speed of advance of the tape so as to maximise the level of the reproduced signal; characterised by further comprising signal level detecting means for detecting a change in the level of the reproduced signal and producing an output signal indicative of the change; and gain correcting means having an adjustable gain responsive to the output signal for correcting the level of the reproduced signal to compensate for possible nonlinearity in the reproduced signal level which appears as a function of scanning deviation of the heads from successively scanned tracks which may occur. The invention will be further described by way of non-limitative example with references to the accompanying drawings, in which:- Fig. 1 is a schematic diagram for explaining the operation of a conventional servo circuit; Fig. 2 is a block diagram showing an embodiment of the present invention; Fig. 3 is a flowchart for explaining the operation of the apparatus of Fig. 2. Fig. 4 is a schematic diagram showing typical tracking states of the apparatus of Fig. 2. Fig. 5 is an idealized schematic diagram showing the reproduced RF signal level as a linear function of track deviation; Fig. 6 is a schematic diagram showing two nonlinear components of the idealized linear function of Fig. 5. An embodiment of the present invention will be described hereinbelow with reference to Figs. 2 to 6. In Fig. 2, magnetic heads 1 and 10 are attached to a rotary drum 2 and driven by a motor 3 that is directly coupled thereto. A rotational phase pulse generator (PG) 4 and a frequency signal generator (FG) 5 are coupled to the motor 3. The running speed of a magnetic tape T wound around the rotary drum 2 over a predetermined arc is controlled by a capstan 6. A motor 7 is directly coupled to the capstan 6, and an FG 8 is coupled thereto. A reproduced RF signal is supplied from the magnetic heads 1 and 10 through an amplifier 11 to an envelope detector 12, and an envelope signal S1 is generated. The envelope signal S1 is supplied both to one terminal of a comparator 13 and to a digital servo control circuit 20. Figs. 4 and 5 show an idealized relation between the tracking of the magnetic heads 1 and 10 and the reproduced RF signal level. Fig. 5 shows an ideal linear dependence of the RF signal level on track deviation; in Fig. 6, the RF signal level (shown in a heavy solid line) actually varies nonlinearly as a function of track deviation. The width Wh (Fig. 4) of each of the magnetic heads 1 and 10 is set to be equal to the track width Wtk. As Fig. 4 shows, the relative positions between the magnetic heads 1 and 10 and tracks are represented by the following states. State 41 in which a track Tku on the upstream side having the same azimuth (represented as "+") as that of a target track Tko is partially scanned; State 42 in which only the track having a different azimuth (represented as "-") on the upstream side of the target track Tko is scanned; State 43 in which the track on the upstream side of the target track Tko is partially scanned along with the target track; State 44 in which the entire target track Tko is scanned without any track deviation; State 45 in which the track on the downstream side of the target track Tko is partially scanned along with the target track; State 46 in which only the track having a different azimuth on the downstream side of the target track Tko is scanned; and State 47 in which a downstream side track Tkl having the same azimuth as that of the target track Tko is partially scanned. The level of the reproduced RF signal from the magnetic heads 1 and 10 in each state in Fig. 4 changes as shown in Fig. 5, in which a track deviation to the upstream side corresponds to the positive direction on the horizontal axis. In a tracking control system based on the level of the reproduced RF signal, the running speed Vt (Figs. 4 and 5) of the magnetic tape T is increased or decreased so as to obtain state 44, in which the reproduced RF signal level becomes maximum. That is, in a case where the tracking deviates to the upstream side as shown in state 43, the running speed Vt of the magnetic tape T is decelerated as shown by an arrow Vt- in Fig. 5. On the other hand, in a case where the tracking deviates to the downstream side as shown in state 45, the running speed Vt of the magnetic tape T is accelerated as shown by an arrow Vt+ in Fig. 5. In Fig. 4, Vh denotes a track scanning speed of the magnetic heads 1 and 10. The envelope signal S1 produced as an output by the envelope detector 12 is supplied through an A/D (analog-to-digital) converting circuit 22 to a CPU (central processing unit) 21 of the digital servo control circuit (microcomputer) 20. Outputs of the PG 4 and FG 5 coupled to the rotary drum 2 are supplied to a timer 23 of the digital servo control circuit 20. An output of the timer 23 is supplied to the CPU 21. A tracking control output CX is supplied from the CPU 21 through a D/A (digital-to-analog) converting circuit 24 to the drive amplifier 9 that controls the capstan motor 7. The detection output envelope signal S1 which is supplied from the envelope detector 12 is compared with a reference power source Vref by the comparator 13. When the detection output is larger than the reference power source Vref, a control signal SC of high level is supplied to the drive amplifier 9. When the detection output is smaller than the reference power source Vref, a control signal SC of low level is supplied to the drive amplifier 9. The gain of the drive amplifier 9 is controlled by the control signal SC. For instance, when the reproduced RF signal level shown by the solid line in Fig. 6 has a level below that represented by a broken line 51 in Fig. 6, the gain of the drive amplifier 9 is set to a low value. When the reproduced RF signal level has a level above that represented by the broken line 51 in Fig. 6, the gain of the drive amplifier 9 is set to a high value. In this manner, the gain of the servo system is properly adjusted. In the embodiment of the invention shown in Fig. 2, the gain of the drive amplifier 9 is set to one of two levels respectively corresponding to high and low levels of the reproduced RF signal. Of course, the invention is not limited to such an embodiment; the gain can also be set to one of three levels or one of any number of levels. It is in fact possible to construct the apparatus in a manner such that data to correct for the nonlinear characteristics of the reproduced RF signal level (shown by the solid line in Fig. 6) are stored in a ROM (read-only memory) and the nonlinear characteristics are continuously corrected on the basis of such data. The data used for such a correction are represented in Fig. 6 by a broken line formed by alternating long and short dashes. The tracking control output CX as converted to analog form by the D/A converting circuit 24 is amplified with a predetermined gain by the drive amplifier 9 and supplied to the capstan motor 7. An output of the FG 8 associated with the capstan 6 is supplied to the digital servo control circuit 20, and ordinary capstan servo control is carried out. An output of the digital servo control circuit 20 is supplied to the motor 3 and ordinary drum servo control is carried out. The tracking servo operation will be described hereinbelow with reference to the flowchart of Fig. 3. When a playback button 31 (Fig. 2) is operated and the VTR is set into the reproducing mode, the CPU 21 determines whether or not the magnetic tape T is being accelerated: that is, whether or not the tracking control output CXn in the present field is positive. This determination is made on the basis of the PG pulse from the PG 4, for instance, once every field (step 111). Next, depending on the determination made at step 111, a check is made to see whether or not the reproduced RF signal level in the present field has increased as compared with the reproduced RF signal level in the preceding field: that is, the magnitudes of outputs S1n and S1n-1 in the present and preceding fields of the envelope detector 12 are compared (steps 112 and 113). On the basis of the results of the determinations made in steps 111 to 113, the tracking condition is divided into four states. The tracking control output CX from the CPU 20 corresponding to each state is given below. In the case where the magnetic tape T is being accelerated and the reproduced RF signal level is increasing, the magnetic heads 1 and 10 are known to be in the tracking state 45 shown in Figs. 4 and 5, and they are being controlled in a stabilizing direction. Therefore, a tracking control output CXn+1 > 0 of a positive polarity for acceleration is supplied as an output so that the magnetic heads 1 and 10 are subsequently controlled in the same direction (step 114). In the case where the magnetic tape T is being accelerated and the reproduced RF signal level is decreasing, the magnetic heads 1 and 10 are known to be in the tracking state 43 shown in Figs. 4 and 5, and the magnetic heads 1 and 10 are being controlled in a direction opposite to the stabilizing direction. Therefore, a tracking control output CXn+1 < 0 of a negative polarity for deceleration is supplied as an output so that the magnetic heads 1 and 10 are subsequently controlled in the stabilizing direction (step 115). In the case where in the magnetic tape T is being decelerated and the reproduced RF signal level is increasing, the magnetic heads 1 and 10 are known to be set in the tracking state 43 shown in Figs. 4 and 5, and the magnetic heads 1 and 10 are being controlled in a stabilizing direction. Therefore, the tracking control output CXn+1< 0 of a negative polarity for deceleration is supplied as an output so that the magnetic heads 1 and 10 are subsequently controlled in the same direction (step 116). In the case where the magnetic tape T is being decelerated and the reproduced RF signal level is decreasing, the magnetic heads 1 and 10 are known to be set in the tracking state 45 shown in Figs. 4 and 5, and the magnetic heads 1 and 10 are being controlled in a direction opposite to the stabilizing direction. Therefore, the tracking control output CXn+1 > 0 of a positive polarity for acceleration is supplied as an output so that the magnetic heads 1 and 10 are subsequently controlled in the stabilizing direction (step 117). The error signal generated at step 118 in accordance with the appropriate tracking control outputs CX in steps 114 to 117 is supplied to the drive amplifier 9 for the capstan motor 7. The error signal thus supplied is amplified by the drive amplifier 9 in which the gain has been properly adjusted in accordance with the reproduced RF signal level and supplied to the motor 7. The track deviation is thus corrected. As mentioned above, since the accelerating or decelerating state of the magnetic tape T and the increase or decrease in the reproduced RF signal level are respectively detected and the running speed of the magnetic tape T is controlled on the basis of both of the detected results, a very accurate tracking control can be carried out, even though the apparatus is of simple construction and can be manufactured at low cost. In accordance with the invention, the increase or decrease in the reproduced RF signal level is detected, and the running speed of the magnetic tape T is controlled on the basis of the result of the detection. Further, the nonlinear characteristics of the change in the reproduced RF signal level are corrected, and the gain of the drive amplifier 9 is adjusted accordingly. Therefore, stable tracking control can be carried out. Although the apparatus has been described as reacting only to the polarity of the tracking control output CX, the invention is not limited to such an embodiment. For instance, it is also possible to construct the apparatus in a manner such that the maximum value of the reproduced RF signal level which has previously been stored is employed as a reference value and the amplitude of the tracking control output CX can be determined in accordance with the difference between the reference value and the reproduced RF signal level. Or, the polarity may be made different at a predetermined amplitude, such as +0.1 V and -0.1 V. Moreover, the invention can be similarly applied to a digital audio tape recorder of the helical-scan type. Further, although magnetic heads 1 and 10 each having the same width as the track width Wtk (Fig. 4) are used in the apparatus of Fig. 2, magnetic heads having widths greater than the track width Wtk can optionally be used. In that case, a tracking control system as disclosed in Japanese laid-open patent publication No. 62-222459 may be employed.

Claims: Apparatus for reproducing a video signal recorded on a tape in successive oblique tracks, the apparatus comprising: means (6,7) for advancing the tape (T) ; rotary heads (1,10) for scanning the tracks successively to form a reproduced radio frequency signal (RF); and means (21) for controlling the speed of advance of the tape so as to maximise the level of the reproduced signal; characterised by further comprising signal level detecting means (13) for detecting a change in the level of the reproduced signal and producing an output signal (SC) indicative of the change; and gain correcting means (9) having an adjustable gain responsive to the output signal (SC) for correcting the level of the reproduced signal to compensate for possible nonlinearity in the reproduced signal level which appears as a function of scanning deviation of the heads (1,10) from successively scanned tracks which may occur. Apparatus according to claim 1, wherein the speed controlling means (21) is controlled via a speed controlling signal and further characterised in that said gain correcting means (9) controls the gain of the speed controlling signal for correcting the level of the reproduced signal. Apparatus according to claim 1 or 2, wherein the signal level detecting means (13) comprises means (12) for producing an envelope signal (S1) corresponding to the reproduced signal, means for producing a reference signal (Vref) having a reference level, and comparator means (13) for comparing the envelope signal (S1) with the reference signal (Vref). Apparatus according to any one of the preceding claims, wherein the gain of said gain correcting means (9) may be set to any one of two or more levels.

Inventors: TOMITAKA TADAFUSA, TOMITAKA, TADAFUSA
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B15
IPC codes: G11B15