GB2161976A - Controlling slow replay in a magnetic recording/reproducing system - Google Patents

Abstract

In a system for controlling slow replay in a magnetic recording and reproducing apparatus of a type such as an 8 mm VTR system in which tracking control is performed with a four-frequency tracking technique, the tape is transported through a distance corresponding to a period defined by the time when the rotary head starts to scan the recording medium in the still reproduction mode and the time when the instantaneous level of a tracking level signal, indicative of the difference in the level between two pilot signals obtained from adjacent tracks, becomes equal to a predetermined level. This distance is measured by the rotational frequency of the capstan. <IMAGE>

Description

SPECIFICATION System for controlling slow replay in magnetic recording and reproducing apparatus BACKGROUND OF THE INVENTION The present invention relates to a system for controlling slow replay in a magnetic recording and reproduction apparatus. More particularly, the invention relates to such a system in a magnetic recording and reproduction apparatus that performs tracking control employing a four-frequency tracking technique.

Tracking control with the conventional magnetic recording reproducing apparatus (hereunder referred to as a VTR) is performed by using a control track provided along the edge of the magnetic tape. In the recording mode, pulse signals (hereunder, CTL signals) that indicate the rotational speed of the drum are recorded by a fixed head, and in the reproduction mode, the tape speed is controlled so that the phases of the CTL signals are in agreement with the phases of drum rotation.

With the conventional VTR, the stepping operation in the slow replay mode wherein operations of still reproduction alternate with those of frame feeding, is controlled by timing a braking operation with pulses obtained by delaying the CTL signals reproduced after the start of tape transport.

In this conventional system of controlling slow replay, the amount of tape transport per frame can vary as a result of changes in temperature and load, and the resulting failure of the tape to stop at optimum positions produces noise bars in the reproduced image, resulting in a picture of poor quality.

The recently proposed 8 mm VTR of the helical scanning type uses a four-frequency tracking technique. In accordance with this technique, four pilot signals that have different frequencies related to one another in a predetermined manner are successively superimposed on the information signal in the recording mode, and in the reproduction mode, a pilot signal, read as cross talk from and adjacent track. is used for tracking control purposes. Obviously, this technique precludes the possibility of controlling the slow replay mode by the conventional system which uses CTL signals for timing the braking operation.

SUMMARY OF THE INVENTION In the inventive system of the invention for controlling slow replay in a magnetic recording and reproducing apparatus, cycles of pilot signals produced using the four-frequency tracking technique are counted and the frequency of capstan rotation measured, and the braking operation controlled accordingly, thereby enabling still reproduction at optimum positions.

In accordance with the slow replay mode control system of the present invention, the tape is transported through a distance that corresponds to the period defined by the time when the rotary head starts to scan the recording medium in the still reproduction mode and the time when the instantaneous level of the tracking error signal, which is indicative of the difference in level between two pilot signals obtained from adjacent tracks, becomes equal to a predetermined level. This distance is determined by measuring the rotational frequency of the capstan in the stepping operation.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a control system constructed in accordance with a preferred embodiment of the present invention; Figure 2 illustrates a drum having rotary heads A and B and which is used in the system of Fig. 1; Figure 3 illustrates recording tracks formed on a magnetic medium by the system shown in Fig. 1; Figure 4 is a flowchart showing operations of the system in Fig. 1; Figure 5 shows a path scanned by a rotary head in a still reproduction mode; Figure 6 shows waveforms representing operations of selected parts of the system of Fig. 1; and Figure 7 is a block diagram showing another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a control system constructed in accordance with the present invention are described hereunder with reference to the accompanying drawings.

In Fig. 1, a speed detection signal formed by pulses having a repetition frequency indicative of the rotational speed of a capstan motor 1 is supplied to a speed detector 2, a differential phase detector 3, and a slow replay mode control circuit 4. The speed detection signal is delivered from, for example, a pickup (not shown) that detects magnetic flux from a magnet (not shown) attached to the rotary shaft of the capstan motor 1. The speed detector 2 is composed of, for example, an F/V (frequency/voltage) converter that generates a voltage corresponding to the frequency of the speed detection signal. The output of the F/V converter is used as a speed error signal. The output of the speed detector 2 is fed to one input of an adder 5 through a twoposition switch S, in all modes other than the slow replay mode.The output of the slow replay mode control circuit 4 is fed to the aforementioned one imput of the adder 5 through the twoposition switch S, in the slow replay mode. The sum output of the adder 5 is applied through a driver 6 as a rotation control signal for the capstan motor 1. The differential phase detector 3 is fed with a head switching signal SWP from a reference signal generator circuit 7 that receives an output from a phase generator (not shown) associated with a rotary drum motor. The differential phase detector 3 is designed so as to deliver a phase error signal that indicates the phase difference between the head switching signal SWP and the speed detection signal. The output of the differential phase detector 3 is applied to the other input of the adder 5 through a switch S2 that is turned on in the recording mode.The head switching signal SWP is also supplied to the slow replay mode control circuit 4 and a timing signal generator circuit 8. The timing signal generator circuit 8 is designed so that, in accordance with a replay mode designated by a replay mode determining command from the operating unit (not shown), either a timing signal that is substantially the same as the head switching signal SWP or a timing signal that varies in a predetermined sequence on the basis of the head switching signal SWP is delivered. The timing signal produced by the circuit 8 is supplied to a four-frequency signal generator circuit 9.The four-frequency signal generator circuit 9 successively produces four signals, f1, ~2, f3 and ~4, of frequencies 6.5 fH, 7.5 fH, 10.5 fH and 9.5 fH (fH being the horizontal sync frequency) as the state of the timing signal changes. The output of the four-frequency signal generator circuit 9 is applied as a pilot signal to an adder 10, where the pilot signal is summed with an FM signal containing an information signal such as video information and audio information, and in the recording mode, the sum signal is applied to a pair of rotary heads, A and B, through a two-position switch S3, thus recording the desired information on a magnetic tape T (recording medium).

Assuming that the pilot signals are recorded in the order of f,, ~2, f3 and ~4, the four-frequency signal generator circuit 9 is designed so that, if f, has been recorded just before shifting to the "pause" position, ~2, f3 and f4 will be recorded in this order when there occurs the next shift from the "phase" to "record" position. This arrangement ensures the continuity of the pilot signals for tracking control purposes.

The system shown in Fig. 1 also includes a four-frequency detector 11 employed as an error detector for use in the four-frequency tracking system. The error output from this detector is supplied not only to the slow replay mode control circuit 4, but also to the other input of the adder 5 through a switch S5 that is turned on in the reproduction mode.

The four-frequency error detector 11 is supplied in the reproduction mode with an RF signal through the two-position switch S3 that has been read from the magnetic tape T by rotary heads A and B. In the four-frequency error detector 11, the RF signal is amplified by a preamplifier 12 and passed through a low-pass filter 13 to eliminate the high-frequency components and extract the four pilot signals f, to f4. The reproduced pilot signals are fed through an amplifier 14 to a mixer 15 where they are mixed with the output from the four-frequency signal generator circuit 9. The output of the mixer 15 is passed through handpass filters 16 and 17 to extract the fH and 3fH components, respectively, which are then supplied to detectors 18 and 19.The detectors produce voltages that indicate the signal levels of the fH and 3fH components, and these voltages are respectively applied to the positive and negative input terminals of a differential amplifier 20. The outpt of the differential amplifier 20 is directly applied to one input terminal of a signal switching circuit 21. At the same time, this output is applied to the other input terminal of the signal switching circuit 21 through an inverting amplifier 22. The control input terminal of the signal switching circuit 21 is supplied with the output from the timing signal generator circuit 8. The signal switching circuit 21 alternatively delivers the output of the differential amplifier 20 or the output of the inverting amplifier 22 for use as a tracking error signal from the four-frequency error detector 11.

As illustrated in Fig. 2, the rotary heads A and B are attached to a drum H at positions which are spaced from each other by 1 80' with respect to the axis of rotation of the drum H, and are caused to rotate together with the drum. The drum H is rotated in synchronism with a signal obtained by frequency-driving the vertical sync signal of an input video signal or an internal reference signal (e.g., a 3.58 MHz subcarrier). The magnetic tape T is transported by the capstan motor 1 (Fig. 1) past the rotary heads A and B at a speed in proportion to the rotational speed of the heads. As shown in Fig. 3, a plurality of generally parallel tracks are provided for recording on the magnetic tape T. On each of these tracks is recorded one field of video information. Additionally, frequency-multiplexed pilot signals are recorded on successive tracks in the order of f,, ~2, f3 and f4. In Fig. 3, tracks A correspond to the rotary head A, and tracks B to head B.

The four pilot signals f, to f4 that are frequency-multiplex recorded on these tracks together with the information signals such as video information and audio information have respective frequencies of 6.5fH, 7.5fH, 10'5#H and 9.5fH (fH being the horizontal sync frequency). Therefore, the following relations are established: and (1) f2 - f, = fl - = 3fHw (2) The frequencies of these four pilot signals are within the range of 100 kHz to 160 kHz, and thus they have substantially no azimuth loss. Therefore, nort only are the pilot signals reproduced that were recorded on the track scanned by the rotary head A or B, but also the two pilot signals recorded on the two tracks adjacent the scanned track are reproduced as crosstalk.

The level of this crosstalk is substantially proportional to the width of that part of the rotary head which is in contact with the two adjacent tracks. This means that by detecting the difference in level between the two pilot signals reproduced as crosstalk, the position of either one of the rotary heads relative to the scanned track in a direction vertical to that track can be detected.

The present discussion assumes that the pilot signals were successively recorded in the tracks in the order of f1, ~2, f3 and f4. If the pilot signal recorded on the track being contacted by the rotary head A and B is f1, those pilot signals which were recorded on the two tracks adjacent to that track are f4 and f2. The fH and 3fH components in the output from the mixer 15 correspond to f2 and ~4, respectively. Therefore, if the output from the differential amplifier 20 is at a positive level, the signal level of the f2 component is high, indicating that the rotary head is biased in a direction opposite that of tape transport with respect to the track being scanned.If the output of the differential amplifier 20 is at a negative level, the signal level of the f4 component is high, indicating that the rotary head is biased in a direction parallel to that of tape transport with respect to the scanned track.

If the pilot signal recorded on the track which is in contact with the rotary head is ~2, the pilot signals which were recorded on the two adjacent tracks are f, and ~3, and the fH and 3fH components in the output of the mixer 15 correspond to f, and ~3, respectively. Therefore, in this case, if the output from the differential amplifier 20 is at a positive level, the signal level of the f, component is high, indicating that the rotary head is biased in a direction parallel (not opposite) to that of tape transport with respect to the track being scanned.If, on the other hand, the output of the differential amplifier 20 is at a negative level, the signal level of the f3 component is high, indicating that the rotary head is biased in a direction opposite to that of tape transport with respect to the scanned track. Therefore, by using the output of the timing signal generator circuit 8 in such a manner that the signal switching circuit 21 alternately outputs the output of the differential amplifier 20 and the output of inverting amplifier 22, continuous error signals are obtained for performing accurate tracking control.

The slow replay mode control circuit 4 is composed of a microcomputer having a processor, ROM (read-only memory), RAM (random-access memory), etc., and performs the following control by the processor by operations in accordance with a program stored in the ROM. When the slow replay mode is designated, a timed deceleration command is issued and a drive control signal is generated so as to produce a braking torque on the capstan motor. This drive control signal is then supplied to the adder 5 through the two-position switch S,. The number of speed detection signal pulses (FG) generated in the deceleration period is counted and stored as FGsT.

When the edge of the head switching signal SWP is detected after tape transport has stopped, ''1'' is added to the value stored in a timer register in the process at given intervals (Steps S1 and S2 in the flowcharge of Fig. 4). In the next step, checking is made as to whether the instantaneous level of the tracking error signal has become zero (Step S3 in Fig. 4).When the instantaneous level of the tracking error signal has become zero, data is read from the timer register and the amount of tape transport FGS is calculated by the following formula (Steps S4 and S5 in Fig. 4): NR~NE FGR FGS = FGR + ( (--------~)x ) x , (3) NR 2 where FGr represents the number of pulses generated as a speed detection signal corresponding to the amount of tape transport for one frame, NR is a value corresponding to the period defined by the time when the edge of the head switching signal SWP for the still reproduction mode occurs at an optimum stop position and the time when the instantaneous level of the tracking error signal reaches zero, NE is a value corresponding to the period defined by the time when the edge of the head switching signal SWP for the still reproduction mode occurs and the time when the instantaneous level of the tracking error signal reaches zero, and NF is a value corresponding to the time period of one field.

If the edge of the head switching signal SWP is detected before the instantaneous level of the tracking error signal becomes zero, the value of EG, is set to a predetermined level (Steps S6 and S7 in Fig. 4).

In the next place, data is obtained for setting the deceleration timing for the stepping operation (Step S8 in Fig. 4). Such data can be obtained by subtracting FGsT (the number of pulses of FG during the deceleration period) from FGs (amount of tape transport). Therefore, FGGEN, which represents the number of speed detection signal pulses to be counted befoe shifting to deceleration, is calculated by: FGGEN = FG5 - FGsT. (4) Then, a decision is made as to whether the sequence should proceed to the stepping operation (Step S9 in Fig. 4). If this should not be done, control in the sequence of Steps S1 to S8 is repeated.If shifting to the stepping operation is desired, an acceleration command is issued and a drive control signal is generated so that a positive drive torque is produced on the capstan motor 1, and this drive control signal is applied to the adder 5 through the two-position switch S, (Step S1 in Fig. 4). Thereafter, when the number of pulses generated as the speed detection signal has reached a value corresponding to FGGEN, a deceleration command is issued to bring the tape to a stop (Steps S11 to S14 in Fig. 4). In this case too is counted the number of speed detection signal pulses that have been generated during the deceleration period.

It is assumed that, in accordance with the magnetic recording and reproducing apparatus described above, recording tracks formed as indicated by solid lines in Fig. 5 are scanned by a rotary head in the still reproduction mode, following the path indicated by the dashed lines. The waveform of the head switching signal SWP is shown in Fig. 6A. On the other hand, the level of the reproduced RF signal varies as shown in Fig. 6B. In this case, the zero-crossing point (the point where the instantaneous level of the tracking error signal becomes zero) occurs somewhere between the rising and falling edges of the head switching signal SWP, as shown in Fig. 6C.

The tape stop position on this occasion is the optimum stop position, and the period defined by the time when the edge of the head switching signal occurs and the time when the tracking error signal crosses the zero point is taken as the reference time NR.

It is assumed now that the actual tape stop position is not at the optimum position, producing a tracking error signal having a waveform as shown in Fig. 6D, wherein NET the period defined by the time when the edge of the head switching signal SWP occurs and the time when the tracking error signal crosses the zero point, is longer than the reference time NR. The amount of tape transport FG, to the next optimum stop position is equal to FGR, the amount of tape transport corresponding to one frame, minus a factor associated with the difference between NR and NET and is calculated in accordance with equation (3) above. This enables operation in the still reproduction mode at optimum tape stop positions.

Fig. 7 is a block diagram showing the slow reproduction mode control circuit 4 incorporated in another embodiment of the present invention. The other blocks 1 to 3, 5 to 11, switches S, to S3, and rotary heads A and B are the same as in the embodiment of Fig. 1, and hence omitted. In Fig. 7, a tracking error signal is fed to a zero-crossing detector circuit 20 composed of a comparator, etc., and upon detecting the zero-crossing point of the tracking error signal, the circuit 30 delivers a pulse signal which is fed to a counter 31 as a stop command input. The start command input terminal of the counter 31 is supplied with a head switching signal SWP.

The clock input terminal of the counter 31 is provided with a pulse delivered as a speed detection signal.

The counter 31 starts its counting operation in response to the edge of the head switching signal SWP and ends the counting operation in response to the output pulse from the zero crossing detector circuit 30. Output data from the counter 31 is applied to a subtractor 32, which calculates and holds the difference between the output data from the counter 31 and the reference value NR, and which, at the fall of the control input signal, produces a pulse having a width and polarity that depend on the calculated difference. The output of the subtractor 32 is supplied to one input of an adder 33. To the other input of the adder 33 is supplied a positive pulse of a width that corresponds to one frame of the stepping operation from a pulse generator circuit 34 upon the elapse of a time period equal to the period of the still reproduction mode.

The output of the pulse generator circuit 34 is also supplied to the control input terminal of the subtractor 32. The adder 33 delivers a pulse of width corresponding to the amount of tape transport to the optimum stop position. This pulse is used as a drive control signal for the capstan motor 1.

In the system for controlling slow replay in a magnetic recording and reproducing apparatus in accordance with the present invention, tape is transported in the stepping operation by a distance that corresponds to the period defined by the time when the rotary head starts to scan the recording medium in the still reproduction mode and the time when the instantaneous level of a tracking error signal, which indicates the difference in level between two pilot signals obtained from tracks adjacent to the track being scanned, becomes equal to a predetermined level. This enables still reproduction at optimum stop positions and produces a reproduced picture having fewer noise bars.

Further in accordance with the system of the present invention, the amount of tape transport is monitored by counting the number of speed detection signal pulses, while at the same time, the timing of deceleration is determined by counting the number of speed detection signal pulses generated during the deceleration period for the previous stepping operation. As a result, the system of the present invention can accommodate changes in temperature and load, thus ensuring the reproduction of optimum still and slow pictures.

Claims (3)

1. A system of controlling slow replay in a magnetic recording and reproducing apparatus including at least two rotary heads that rotate in asynchronism and transport means for transporting a recording medium past said at least two rotary heads at a speed corresponding to a designated recording time mode so that said at least two rotary heads alternately scan said recording medium, a plurality of parallel recording tracks being formed on said medium and a plurality of pilot signals that have different frequencies and are related to each other being successively recorded on said plurality of recording tracks in a recording mode, and in a reproduction mode, the position of one of said at least two rotary heads relative to the recording track that is being scanned by that rotary head in a direction vertical to said recording track being controlled by a tracking error signal that has an instantaneous level indicating the difference in level between two of said plurality of pilot signals that are simultaneously obtained from two recording tracks adjacent the track being scanned by said one rotary head, said system comprising: means for stopping the transport of said recording medium by said transport means; means for detecting the period defined by the time when one of said at least two rotary heads start to scan said recording medium and the time when the instantaneous level of that tracking error signal becomes equal to a predetermined level; and means for transporting said recording medium with said transport means through a distance that corresponds to said detected period after said recording medium has been scanned a predetermined number of times by said at least two rotary heads.

2. The system according to claim 1, wherein said transport means comprises a capstan motor, and further comprising means for counting the number of speed detection signal pulses produced in response to a magnet attached to said capstan motor while said recording medium is transported by said transport means through said distance corresponding to said time period.

3. The system according to claim 2, further comprising means for controlling deceleration of said recording medium by said transport means when being transported thereby through said distance corresponding to said time period in accordance with the number of said speed detection signal pulses generated in said time period.