Capstan servo devices

Abstract: A capstan servo device comprises a frequency generator (2) for detecting the rotation rate of a capstan motor (1) and generating two-phase FG signals mutually having a 90° phase difference; a selector (5) for selecting one of input four-phase signals produced from the two-phase FG signals and having a mutual 90° phase difference; and a sync signal generator (6) including an N-bit counter (8) (where N ≧ 2) to receive a reference frequency signal inputted thereto and generating a sync signal by phase-comparing the two high-order bits of the frequency-divided output of the counter (8) with the two-phase FG signals. In this device, the rotation speed of the capstan motor (1) is controlled in accordance with an error voltage signal selected by the selector (5) in response to the sync signal.

Description: This invention relates to capstan servo devices, for use, for example, in a video tape recorder (VTR). Figure 8 is a block diagram of a previously proposed capstan servo device which includes a capstan motor 1, a frequency generator (FG) 2 for generating a frequency signal proportional to the rotation speed of the capstan motor 1, a speed detector 83 for detecting a speed error e v by frequency-discriminating the FG signal, a phase compensator 85, a mixer 86, and a driver 10. These component elements form a speed loop for controlling the rotation speed of the capstan motor 1 to maintain it constant. Another group of component elements, comprising a frequency divider 87, and a phase detector 84 for detecting a phase error e p between the demultiplied output signal of the frequency divider 87 and a reference signal V-SYNC, and for supplying such phase error e p to the mixer 86, form a phase loop for controlling the rotation phase of the capstan motor 1 and maintaining it constant. The motion of a magnetic tape 82 is maintained at a constant speed by the two control loops. In a recording mode where a switch circuit 88 is connected to a recording (R) side, the capstan motor 1 is rotated in synchronism with the rotation phase of the signal V-SYNC. In a playback mode where the switch circuit 88 is connected to a playback (P) side, the capstan motor 1 is so rotated that a CTL signal, which is read out by a control head 81 from a control track (not shown) on the tape 82 and serves as a comparison signal, is synchronized in phase with the signal V-SYNC. Recent capstan servo devices provide, in addition to a recording/playback mode executed with constant-speed rotation of a capstan motor, other operational modes, such as a mode to reproduce a still picture by controlling the stop position of a magnetic tape by control of the rotation and stopping of the capstan motor, and a mode to perform changed-speed playback by controlling the capstan motor continuously from slow rotation to fast rotation. Such modes call for further precise control. With regard to the still picture playback mode, an exemplary technique is disclosed in Japanese laid-open patent specification 60/39382, which is considered to represent the most relevant state of the art. This document discloses a capstan servo device comprising a frequency generator for detecting the rotation speed of a capstan motor and generating two phase FG signals having a mutual 90° phase difference. Said two-phase FG signal is inverted and not-inverted inorder to obtain a four-phase signal which is amplified, said four-phase signals having also a mutual 90° phase difference. In this capstan servo device the precision of the stop position of a capstan motor is enhanced by additionally connecting a stop control circuit to the aforementioned speed and phase servo loops. Such stop control for a capstan motor will now be described below with reference to the block diagram of Figure 9 and the waveform timing chart of Figure 10. In a state where a switch circuit 96 is connected to its N side in Figure 9, a capstan motor 1 is rotated at a normal constant speed under control of a known speed-phase servo circuit 91. When the switch circuit 96 is changed to its S side representing a stop mode, a signal F a+ of Figure 10A produced from an FG signal f a is supplied as a stop signal to a driver 10. In this case, the capstan motor 1 is so controlled as to stop when the voltage applied to the driver 10 is reduced to zero, and the rotation speed is accelerated by a positive voltage increased from the zero point or is decelerated by a negative voltage decreased therefrom. Accordingly, of zero crossing points P and Q in each of right slope portions and left slope portions as shown in Figure 10A, a stable point for stop control is the point P, and with any point Q, and desired stability is achieved with a shift to either the anterior or posterior point P. Referring back to the circuit configuration of Figure 9, the FG 2 generates two-phase signals f a and f b having a 90° phase difference. Such two-phase signals f a and f b are supplied respectively to non-inverting amplifiers 3a and 3b and inverting amplifiers 4a and 4b of a stop control circuit 92 to become four-phase signals F a+ , F a- and F b+ , F b- shown in Figures 10A to 10D. Simultaneously the two-phase signals f a and f b are also supplied to comparators 93a and 93b to be converted into pulses at the average-level zero crossing points, whereby rectangular signals S a and S b of Figures 10E and 10F are obtained. The four-phase signals F a+ , F a- and F b+ , F b- are supplied to switch circuits 94a and 94b respectively, and are thereby switched in accordance with the rectangular pulses S a and S b , so that the right slope portions alone are extracted to produce signals D a and D b of waveforms shown in Figures 10G and 10H. Thereafter such signals D a and D b are mixed with each other in a mixer 95 to become a signal D s of Figure 10J, where the zero crossing points in the former four-phase waveforms are in the right slope portions alone. If the signal D s thus obtained is fed as a stop control voltage to the driver 10 in place of the aforementioned signal F a+ , then stopping of the capstan motor 1 is controlled at the zero crossing point R1 to R4 shown in Figure 10J, so that the stop position accuracy is made four times higher than the value in the aforementioned case of using the signal F a+ as a stop control voltage. By the employment of such stop control circuit disclosed in the above Japanese patent specification, it becomes possible to control the stop position of a magnetic tape with four-fold enhanced precision without the necessity of increasing the frequency of the FG. In a capstan motor control system where the speed loop based on the FG signal is included in the phase loop based on the signal V-SYNC and the signal CTL as shown in the block diagram of Figure 8, it is possible to raise the rotation phase precision in conformity with an increase in the gain of the phase loop, and also to reduce, with respect to any disturbance to the capstan motor torque, the harmful influence on the rotational variations in conformity with an increase in the gain of the speed loop, and further with an improved frequency characteristic. However, there exists a compromise in the gain balance between the phase loop and the speed loop, and if the gain of either loop is increased, resultant mutual interference thereof causes some problems including instability of the loops and a prolonged pull-in time for synchronization. Furthermore, since the linearity of the speed detector does not extend from a low frequency range to a high frequency range, another problem arises with regard to failure in achieving stable rotation speed control from slow rotation to fast rotation. In the method of controlling the capstan motor by additionally connecting the stop control circuit of the above Japanese patent specification, and switching it selectively in relation to another stop control circuit separate from the closed servo loop, it is difficult to perform rapid and stable switching exactly from the closed servo loop to the stop control circuit. According to the present invention there is provided a capstan servo device comprising: a frequency generator for detecting the rotation speed of a capstan motor and generating two-phase FG signals having a mutual 90° phase difference; a selector for selecting one of input four-phase signals having a mutual 90° phase difference, said four-phase signals being obtained by inverting and not inverting said two-phase FG signals and amplifying the same; and a sync signal generator including an N-bit counter (where N ≥ 2) to receive a reference frequency signal inputted thereto and generating a sync signal by phase-comparing the two high-order bits of the frequency-divided output of said counter with said two-phase FG signals; wherein the rotation speed of said capstan motor is controlled in accordance with an error voltage signal selected by said selector in response to the sync signal. In one embodiment, the present invention may also provide a capstan servo device comprising: a digital-to-analogue converter for converting the (N - 2) low-order bits of the frequency-divided output of said counter into an analogue form; wherein the rotation speed of said capstan motor is controlled in accordance with a difference voltage between the output voltage of said converter and the error voltage signal selected by said selector in response to the sync signal. In another embodiment, the present invention may also provide a capstan servo device comprising: a phase locked loop for generating a reference frequency signal while being phase-locked to an internal reference signal during a recording mode or to a control signal during a playback mode; wherein the rotation speed and the rotation phase of said capstan motor is controlled in accordance with the error voltage signal selected by said selector in response to the sync signal. Speed control is executed by closed-loop phase servo in embodiments of the invention, so that the capstan motor is continuously controllable with a smooth action from stop through slow rotation to fast rotation. Since the phase loop is connected in series to the speed loop, optimum values are settable in each of the loops with regard to the stability and the pull-in time for synchronization. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a block diagram of a first embodiment according to the present invention; Figure 2 is a waveform timing chart for explaining the operation of the first embodiment of Figure 1; Figure 3 is a state transition diagram for explaining the synchronization by a sync signal generator employed in Figure 1; Figure 4 is a block diagram of a second embodiment according to the present invention; Figure 5 is a waveform timing chart for explaining the operation of the second embodiment of Figure 4; Figure 6 graphically shows the characteristics of an FG signal and a reference frequency signal in a changed-speed control operation; Figure 7 is a block diagram of a third embodiment according to the present invention; Figure 8 is a block diagram of a previously proposed capstan servo device; Figure 9 is a block diagram of a previously proposed capstan servo device equipped with a stop control circuit; and Figure 10 is a waveform timing chart for explaining the stop control circuit in Figure 9. In addition to components as described above and denoted by the same reference numerals, the embodiment of Figure 1 comprises a selector 5 for selecting one of four inputs in response to a 2-bit control signal, and outputting the selected input. A sync signal generator 6 comprises pulse shapers 7a and 7b, an N-bit counter 8 and a sync detector 9. Now a detailed description will be given of the operation with reference to a waveform timing chart of Figure 2. FG signals f a and f b (of a frequency f s ) generated with rotation of a capstan motor 1 are fed respectively to non-inverting amplifiers 3a and 3b and inverting amplifiers 4a and 4b to become four-phase signals f1 to f4 having 90° phase differences as shown in Figures 2A to 2D, which are then supplied to input terminals of the selector 5. Simultaneously therewith, the FG signals f a and f b are also fed to the sync signal generator 6 and are processed by the pulse shapers 7a and 7b to become digital signals F a and F b as shown in Figures 2E and 2F. Meanwhile, a reference frequency signal f R serving as a reference for the rotation rate of the capstan motor 1 is supplied to a clock input terminal CK of the N-bit counter 8 in the sync signal generator 6 so as to be frequency-divided or demultiplied by N. The waveforms of Figures 2G and 2H represent the two high-order bits S 0 and S 1 in the N bits of the demultiplied output of the counter 8. Such output bits S 0 and S 1 are supplied to the sync detector 9 to be compared with the digitized FG signals F a and F b , and the input CL of the counter 8 is controlled by an output C 0 obtained as a result of such comparison. In this stage, the FG signals are synchronized with the reference frequency signal, and the demultiplied outputs S 0 and S 1 are fed as sync signals to the selector 5. Meanwhile a rotational frequency signal indicating the forward or backward rotation of the capstan motor 1 is supplied to the U/D input terminal of the counter 8 and also to the sync detector 9. The waveform timing chart of Figure 2 illustrates the phase relation among the signals in the synchronized state of the capstan servo device. In the selector 5: a signal f 1 is selected during a time period T1 where the bits of the sync signals S 0 and S 1 are combined as (0, 0); a signal f 4 is selected during a time period T4 where the bits are (0, 1); a signal f 2 is selected during a time period T2 where the bits are (1, 0); and a signal f 3 is selected during a time period T3 where the bits are (1, 1). Thus, there is extracted a signal D 0 of the waveform shown in Figure 2J which is composed merely of the left slope portions having a ¼ period of the original FG signal. The error voltage signal e 0 of such waveform is supplied to the driver 10 thereby to form a closed loop for controlling the capstan motor 1 at a predetermined rotation rate. The process of synchronization will now be described in further detail. Suppose the rotation of the capstan motor 1 has reached its steady state after a power supply for the capstan servo device has been switched on. In sync unsynchronized condition, the signals F a and F b of Figure 2E and 2F are not stable in the timing relation to the signals S 0 and S 1 of Figures 2G and 2E, and have fluctuations thereto. The states of the signals S 0 and S 1 corresponding to the relationship between the four time periods in Figure 2 and the signals F a and F b are as follows. During the time period T1 where synchronism is attained if the signal F b is "0", there occurs a lag or a lead when (F a , F b ) are (0, 1) or (1, 1) respectively. Also during the time period T2 where synchronism is attained if the signal F b is "1", there occurs a lag or a lead when (F a , F b ) are (0, 1) or (1, 1) respectively. During the time period T3 where synchronism is attained if the signal F a is "1", there occurs a lag or a lead when (F a , F b ) are (0, 1) or (1, 1) respectively. Further, during the time period T4 where synchronism is attained if the signal F b is "0", there occurs a lag or a lead when (F a , F b ) are (0, 1) or (1, 1) respectively. Thus a lag, synchronism and a lead of (F a , F b ) exist in conformity with the four states of the signals (S 0 , S 1 ). Therefore the synchronization can be performed by first detecting such a lag or lead by the sync detector 9 and then presetting, in accordance with the detected lag or lead, the clock input of the counter 8 forcibly to the values of (S 0 , S 1 ) suited for synchronizing with the states of (F a , F b ). More specifically, there occurs a lag during the time period T1 when (F a , F b ) are (0, 1), in the case where (S 0 , S 1 ) are (0, 0). Since a synchronism of (S 0 , S 1 ) conforming to such state is attained with values (0, 1) corresponding to the time period T4, the desired synchronism is achieved by outputting a control signal C 0 to preset the clock input forcibly to such values. The combinations of preset values C 0 and the values of such lag, synchronism and lead of (F a , F b ) with respect to the four states of (S 0 , S 1 ) for synchronization are shown in Figure 3 which illustrates transitions of the synchronized state of the sync signal generator 6. In this diagram, the values of F a and F b for synchronizing the state values of (S 0 , S 1 ) during the time periods enclosed with four ellipses are shown at the heads of double lines, and lag values are shown at the bases of counter-clockwise solid lines, while lead values are shown at the bases of clockwise broken lines respectively with regard to the values of (F a , F b ). This diagram signifies that synchronization can be achieved by outputting a preset control signal C 0 to the state values of (S 0 , S 1 ) within the ellipse at the head of the arrow. In the sync detector 9, a control signal C 0 for presetting the counter 8 is outputted in accordance with the state transition diagram, and sync signals S 0 , S 1 are generated through synchronization with the input FG signals F a , F b thereby to control the selector 5. The operation mentioned above is repeated cyclically to synchronize the servo loop, thereby placing the same in the locked state of Figure 2. The second embodiment, shown in Figure 4, includes, in addition to the aforementioned constitution of Figure 1, a D/A converter 41 for extracting the low-order bits from a counter in a sync signal generator 6 and executing digital-to-analogue conversion of the extracted signal, and a differential amplifier 42 for producing the difference between the output e 1 of the D/A converter 41 and the output e 0 of a selector 5. The operation of the second embodiment will now be described with reference to a waveform timing chart of Figure 5. Figures 5A and 5B denote sync signals S 0 and S 1 corresponding to Figures 2G and 2B respectively, and Figure 5E denotes an error voltage signal e 0 corresponding to Figure 2J. Assuming now that N = 4 in the counter 8, low-order bits S 2 and S 3 of Figures 5C and 5D can be extracted as demultiplied outputs. Such outputs S 2 and S 3 are fed to the D/A converter 41, which then produces an analogue output e 1 of Figure 5F. This output e 1 is supplied to the differential amplifier 42, where the difference between such output e 1 and the output e 0 of the selector 5 is produced as a differential output e C . In the new error voltage signal e C thus obtained, the lower frequency component has been removed from the output e 0 of the selector 5, and the voltage ripple has been reduced to 1 / 2 N-2 . Therefore, when slow rotation control is executed for the capstan motor 1 in the first embodiment by lowering the frequency of the reference frequency signal f R , the problem of electromagnetic noise generated by the ripple current from the capstan motor 1 can be improved if the error voltage output e 0 of the selector 5 is reduced into a range of 100 Hz to several kHz. The following is an exemplary case of rotating the capstan motor 1 at a high speed by raising the reference frequency signal f R . The reference frequency signal f R is expressed as f R = f S · 2 N , where f S is the FG signal frequency, and N is the number of stages of the counter 8. For controlling the capstan motor 1 from stop to fast rotation, the frequency range of the signal f R is so widened as to cover from 0 to f S · 2 N Hz. If the frequency f S of the FG signal generated at the standard rotation of the capstan motor 1 is set to 500 Hz, the reference frequency signal f R comes to have a frequency of 32 kHz with N of the counter 8 being six bits. The frequency becomes 320 kHz at a 10-fold speed, and 1.6 MHz at a 50-fold speed. Figure 6 graphically 5 shows the relationship between the FG frequency f S and the reference frequency signal f R under control at changed speeds. The number N of stages of the counter 8 is selectively switched in such a manner as to become three bits at any frequency higher than the border FG frequency f S = 5 kHz during the operation at a 10-fold speed, whereby the reference frequency signal f R can be changed to 40 kHz at a 10-fold speed or to 200 kHz at a 50-fold speed. It is thus possible to realize linear control of the capstan motor from a stop to fast rotation without widening the frequency range of the reference frequency signal f R . Figure 7 is a block diagram of the third embodiment which comprises a speed servo circuit 71 contrived by modifying the capstan servo device described already in connection with the first embodiment, wherein some internal circuits in Figure 1 are eliminated inclusive of the FG signal input from the FG 2 and the circuit from the reference frequency signal input to the driving output for the capstan motor 1. Further shown components include a reference frequency generator 72, a switch circuit (A) 73, a switch circuit (B) 79, and a phase locked loop (PLL) circuit 74 comprising a phase detector 75, a phase compensator 76, a voltage controlled oscillator (VC0) 77, and a frequency divider 78. In the third embodiment, the phase loop described with regard to the capstan servo device of Figure 8 is additionally incorporated in the first embodiment of the present invention shown in Figure 1, and the speed loop and the phase loop are connected in series to each other. The operation of the third embodiment shown in Figure 7 will now be described. First in a standard recording/playback mode, the switch circuit (A) 73 is selectively connected to its N side. In a recording operation where the switch circuit (B) 79 is connected to its R side, a continuous frequency signal f p phase-locked to either an internal reference signal or a signal V-SYNC is generated from the PLL circuit 74 and is then supplied as a reference frequency signal to the speed servo circuit 7. Meanwhile in a playback operation, the switch circuit (B) 79 is changed to its P side to be connected to the control head 81. As a result, a phase servo is achieved for rotating the capstan motor 1 in such a manner that, during the recording operation, the rotation phase of the capstan motor 1 is synchronized with the internal reference signal similarly to the phase loop employed in the capstan servo device of Figure 8; and during the playback operation, the CTL signal read out by the control head 81 from a control track (not shown) on the tape 82 is synchronized with the phase of the signal V-SYNC. Subsequently, in response to a change of the switch circuit (A) 73 to its V side, the aforementioned phase loop is disconnected from the above state and is then connected to the reference frequency generator 72, so that the speed loop alone functions to execute speed control in accordance with the reference frequency. Since this control is the same as the one described in detail with regard to the first and second embodiments, a repeated explanation is omitted here. It is to be understood that the present invention is not limited merely to the above embodiments representing exemplary cases of speed control for the capstan motor, and it may also be applied to speed control for a linear motor (VCM) or the like. In embodiments of the present invention, as described above in detail, the speed loop for the capstan motor is controlled by the phase servo alone based on the FG signal, so that any loop characteristic variation derived from the rotation speed of the capstan motor can be eliminated consequently to achieve wide range control covering from a stop to fast rotation. Furthermore, since the speed loop and the phase loop are connected in series to each other, any torque disturbance from the capstan motor can be absorbed by the speed loop to attain a great improvement in elimination of rotational variations. In addition, the phase loop serves to control merely the phase relation between the signal V-SYNC and the signal CTL without functioning directly as a loop for control of the capstan motor, so that the loop gain need not be raised beyond the necessity eventually to realize adequate setting for attaining required stability. Due to the provision of such capstan servo device in a VTR, it is possible to enhance the stop position accuracy of a magnetic tape relative to a rotary magnetic head as well as to realize a continuously changeable motion of the magnetic tape, hence obtaining superior still picture quality and a special playback function for smooth and continuous speed changes covering from slow playback to fast playback.

Claims: A capstan servo device comprising: a frequency generator (2) for detecting the rotation speed of a capstan motor (1) and generating two-phase FG signals having a mutual 90° phase difference; a selector (5) for selecting one of input four-phase signals having a mutual 90° phase difference, said four-phase signals being obtained by inverting and not inverting said two-phase FG signals and amplifying the same; and a sync signal generator (6) including an N-bit counter (8) (where N ≥ 2) to receive a reference frequency signal inputted thereto and generating a sync signal by phase-comparing the two high-order bits of the frequency-divided output of said counter (8) with said two-phase FG signals; wherein the rotation speed of said capstan motor (1) is controlled in accordance with an error voltage signal selected by said selector (5) in response to the sync signal. A device according to claim 1 wherein said reference frequency signal is changeable from zero frequency through a low frequency range to a high frequency range so as to control the rotation rate of said capstan motor (1) from stop to a high speed. A capstan servo device according to Claim 1 or Claim 2, comprising: a digital-to-analogue converter (41) for converting the (N - 2) low-order bits of the frequency-divided output of said counter (8) into an analogue form; wherein the rotation speed of said capstan motor (1) is controlled in accordance with a difference voltage between the output voltage of said converter (41) and the error voltage signal selected by said selector (5) in response to the sync signal. A device according to any one of claims 1 to 3 wherein the frequency division ratio of said counter (8) in said sync signal generator (6) is switchable to be lower at fast rotation of said capstan motor (1). A capstan servo device according to any one of Claims 1 to 4, comprising: a phase locked loop (74) for generating a reference frequency signal while being phase-locked to an internal reference signal during a recording mode or to a control signal during a playback mode; wherein the rotation speed and the rotation phase of said capstan motor (1) is controlled in accordance with the error voltage signal selected by said selector (5) in response to the sync signal.

Inventors: SATO TSUGUO, SATO, TSUGUO, Sato, Tsuguo, c/o Patents Division
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G11B15
IPC codes: G11B15, H02P29, H02P23