JPH0474791B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a video tape recorder (hereinafter referred to as VTR).
This relates to tracking switching in Normally, in a VTR, it is necessary to ensure that the video head accurately traces the video track on the tape during playback. In other words, you need to do some tracking. For this reason, the rotation of the video head and the running of the tape are set in a predetermined phase relationship. Furthermore, in an apparatus capable of recording and reproducing by changing the tape speed, the loop gain must be changed in accordance with each speed in order to ensure stability of the control system. In other words, in a control system that has a speed control loop and a phase control loop,
Speed control gain Gv when changing speed control frequency
If V is the output voltage and T is the time, then Gv=dV/dω=(dV/dT)*(dT/dω)...(1) Here, dV/dT is the detection sensitivity Kv, and From T=1/f=2κ/ω, dT/dω=−2κ/
ω ² ...(2) Therefore, Gv=-2κ＊Kv/ω ² ...(3) Therefore, when the control speed is halved, the gain in the speed loop becomes 4 times, so the phase It is necessary to reduce this gain in the control loop to stabilize it. At this time, since the gain of the phase comparator is generally switched, an offset occurs in the predetermined phase. FIG. 1 is a block diagram of a servo device based on a digital servo system (Japanese Patent Publication No. 3-136090) invented by the present inventors. Reference numeral 1 indicates a reference signal generating section, 2 indicates a capstan control system, and 3 indicates a drum control system. Furthermore, 4 is a magnetic tape, 5 is a noise rejector, 6 is a monostable multivibrator for tracking (hereinafter abbreviated as tracking MM), and 7
1 is a phase comparator, 8 is a capstan motor, 9 is a drum motor, 10 is a waveform shaping circuit, 11 is a phase comparator, 12 is a reference counter, and 13 is a matrix circuit. The main waveforms of the figure are shown in FIG. 2. Next, the operation will be explained with reference to FIGS. 1 and 2. In the capstan control system 2, the control pulse a reproduced from the magnetic tape 4 is amplified, only the positive pulse portion is waveform-shaped, and is input to a counter-type noise rejector 5. Furthermore, the next-stage tracking MM6 is operated by the shaping control signal b which is this output, and is delayed by half a cycle. This output is input as a comparison signal c to the phase comparator 7 together with the reference signal d, and the phase comparator 7 outputs a voltage commensurate with the phase difference between the two. As a result, the capstan motor 8 is controlled and driven.
The control pulse a and the reference signal d are maintained in a predetermined positional relationship. Here, the standard delay amount Tmm of tracking MM6
is generally set to half of one period. In other words,
It is designed so that when tracking deviates, an equal amount of tracking width can be obtained in both directions of phase lead and lag. On the other hand, in the drum control system 3, the rotation of the drum motor 9 is detected by the tack head. The tack signal g, which is the detection signal, is turned into a tack comparison signal f by the waveform shaping circuit 10, and is input to the next stage phase comparator 11 together with the reference signal e. The drum motor 9 is controlled and driven by the output of the phase comparator 11. Here, the reference signal e is a signal obtained from the reference counter 12 via the matrix circuit 13, and the phase with respect to the REF signal is fixed for each tape speed. This fixed phase is set to a theoretical optimum phase so that tracking is optimum at each tape speed. In other words,
The reference signal e is the control pulse a with the tracking delay amount set to the standard (half of one cycle).
The REF signal is output with an appropriate phase relationship so that the phases of the drum tack signal g and the drum tack signal g match. If you then change the tape speed and record/play back,
Generally, it is necessary to switch the loop gain so that the control system has optimal stability. In other words, as mentioned above, the control speed is changed from standard speed (SP mode) to 1/2 (speed
When switching to LP mode) or 1/3 speed (EP mode),
In order to ensure the stability of the system, the loop gain is reduced to about 1/4,
It is necessary to reduce it to 1/9. This gain reduction, for example, reduces the gain of the speed control loop system to 1/2 (LP mode), 1/3 (EP mode), and the gain of the phase control group system to 1/2 (LP mode), 1/3 ( EP mode)
This is done by making This situation is shown in waveforms in FIG. In the figure, when the tape speed is high, such as during standard recording and reproduction, the phase relationship is similar to that in FIG. 2, as shown in FIGS. 1 to 7. That is, the drum reference signal e is generated at phase A of the REF signal, and the tack signal g is phase synchronized with this. On the other hand, in the capstan system, the reference signal d
The slope period of is short, and the delay end of tracking MM (falling edge of signal c) is phase-locked to the center of this slope. In this case, the delay amount of the tracking MM is a standard value (half of one period), and the positive pulse of the control pulse a has almost the same phase as the drum tracking pulse g. On the other hand, when the tape speed is slower than the above-mentioned tape speed, the reference signal d of the capster control system 2 is activated by the switching signal h to reduce the loop gain of the control system, as shown in FIGS. Make the slope of ′ gentle. Each signal at this time is 7 to 1
It changes like 3. In other words, the capstan-based reference signal d' has a gentle slope in order to reduce the phase comparison gain (FIG. 3 shows the gentlest slope). At this time, the falling edge of the tracking MM, which is the comparison signal c', is phase-locked to approximately the center of the slope, that is, approximately the center of the dynamic range, so that the phase comparison output voltages are approximately equal. (The comparison signal c' is in a state that is To ahead of the comparison signal c.) Here, the driving voltage of the capstan motor 8 changes with respect to the control speed, but the output stage of the phase comparator 7 generally has a high gain. The phase comparison output voltage in the control system constituting the feedback loop changes only slightly. At this time, assuming that the delay amount of the tracking MM is a standard value, the positive pulse of the control pulse a almost coincides with the phase change point of the REF signal. In order to perform tracking in this state, the drum system tack signal g' and tack comparison signal f' must have the phase relationships shown in FIGS. 12 and 13. In other words, it is necessary that the drum system tack signal g' also substantially match the phase of the positive pulse of the control pulse a (to the phase change point of the REF signal). Accordingly, it is necessary to advance the phase of the reference signal e' from A to B in the figure by the time To. That is, when reproducing a tape recorded at a slow tape speed, the matrix circuit 13 switches the drum system reference signal e for tracking. As a result, even when recording is performed on a single tape by continuously changing the tape speed, by switching the reference signal e mentioned above, it is possible to always maintain the optimal tracking state without operating the tracking MM6. can. Generally, the recording tape speed is detected automatically, and accordingly, the tracking is also automatically switched. However, in the above-described method of switching the tracking by automatically detecting the tape speed, lateral wobbling of the reproduced image occurs due to the wobbling of the drum at the time of mode switching. That is, immediately after the start of playback, the tape running has not reached the predetermined speed, and therefore it is difficult to automatically detect the tape speed. Therefore, at the beginning of playback, the tape speed detection output is set to a signal indicating the SP mode. In this state, the drum system is temporarily synchronized with the reference signal e, but if the tape speed detection output after the tape speed rises is a signal indicating an EP mode different from that at the start,
The drum system reference signal is switched from e to e'. At this time, as shown in Figure 3, the reference phase of the drum system is To
As a result, the rotational phase of the drum excessively fluctuates or the phase shifts. For this reason, temporary tracking errors and horizontal shaking of the playback screen may occur. At the same time, there is a drawback that the synchronization pull-in time of the drum control system becomes longer.
Furthermore, if the tape speed is changed and recording is performed continuously, the phase of the reference signal e to the drum system becomes discontinuous during playback, the phase synchronization of the drum system is temporarily lost, and the rotation of the drum fluctuates, causing the above problem. . In the above explanation, the reference signal and phase comparator are shown in analog form, but even if a digital phase comparator is used, its configuration and operation will be the same as in the first example.
The explanation will be made in the same manner as in FIGS. At this time, the reference signal d is each bit output of the reference counter 12, and is 2
It becomes a base number signal. In other words, from the most significant bit (MSB) to the least significant bit (LSB) of the reference counter 12
REF is a numerical representation of the count value up to
The capstan reference signal d is a waveform in which the 3rd and 4th bits from the MSB are the most significant bits and the bits below are expressed numerically, and only one period of this is extracted by a gate. The slope of d is determined by the number of bits to be set as the most significant bit in one period to be gated. For example, in the reference signal d', the MSB is the most significant. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a magnetic recording/reproducing device in which the drum control system does not become unstable even if the speed mode is switched during reproduction or immediately after the start of reproduction. be. The present invention provides magnetic recording and reproduction at a plurality of tape speeds, in which the gain of the phase comparison means is switched according to the tape speed, and the phase of the control pulse reproduced from the tape and the drum tack signal maintain a predetermined relationship. In the recording/reproducing device, the reference signal generator includes means for switching the gain, and generating a reference signal whose slope changes according to the switching of the gain, but whose center of the slope has a linear slope that remains unchanged regardless of the switching of the gain. and phase comparison means for comparing the phases of the reference signal and a comparison signal based on a control pulse reproduced from the tape and sending a voltage commensurate with the phase difference to the capstan drive device, and switching the loop gain of the capstan control system. The feature is that the reference signal for the drum control system is fixed at a predetermined value regardless of the Examples of the present invention will be described in detail below. First, FIG. 4 is a block diagram of an embodiment using the reference signal gate circuit 14, and FIG. 5 shows the main waveforms at the same time. In FIG. 4, parts having the same functions as those in FIG. 1 are given the same reference numerals. A cyclic count signal i of each bit of the reference counter 12 is connected to the reference signal d input to the phase comparator 7 of the capstan control system 2 via a reference signal gate circuit 14. For example, in EP mode, the MSB of reference counter 12
A group of 8-bit count signals is sequentially supplied from the LSB side to the LSB side as a reference signal d via a reference signal gate circuit 14. In addition, in the SP mode, a group of 8-bit count signals is sequentially sent to the LSB side from the MSB of the reference counter 12 to the reference signal gate circuit 14.
is supplied as an equivalent trapezoidal reference signal d. In other words, from the count signal group in FIG. 5,
A method for obtaining an equivalent trapezoidal reference signal d is shown in detail below. FIG. 5 shows a group of count signals below the upper 4 bits of the reference counter 12.
FIG. 3 is a waveform diagram numerically showing a count signal group of lower 8 bits below MSB. This count signal group is not gated and repeats periodically. Now, if the phase switching point of the REF signal in FIG. 8 is taken as the starting point of the reference counter 12, then at the first slope portion of the signal i immediately after that point, all bits from the MSB to the third-order bit are "0". Similarly, in the second slope part, from the MSB to the second bit is "0".
In this case, only the third bit is "1". Thereafter, each bit changes sequentially like a binary counter. As shown in the table,

[Table] It is expressed as follows. In the signal 6d in the same figure, the third
The slope portion is taken out by the reference signal gate circuit 14,
Moreover, before that, it is "L" and after that, it is "H".
To achieve this, the output can be limited using the fourth bit starting from the MSB. That is, first, slope portion numbers 1 and 2 are detected using the MSB and the second bit (an OR gate thereof), and the output of the reference signal gate circuit 14 is forcibly set to "L". Similarly, slope part number 4,
5 using the MSB and the second and third bits,
Force the output of the reference signal gate circuit 14 to “H”
Make it. (This detection can be easily realized using a general matching circuit or a decoding circuit) Note that the output changes from "H" to "L" in the middle of the sixth slope, but this changing point is at the fourth slope. Bits can be similarly easily detected. Further, this variation does not necessarily have to be as shown in FIG. 5 and 6, and may be varied at the periodic point of the third bit. As a result of the above, the comparison signal c is phase-locked to the vicinity of the center of the slope of the reference signal d. (Generally, the phase synchronization point is designed at the center of the control range, so in this embodiment it is near the center of the trapezoidal wave slope.) That is, the reference signal gate circuit 14 is designed as shown in FIGS. 6 and 7. As shown in the figure, the cyclic count signal i, 7 consisting of the lower several bits is gated by the remaining upper bits to output only one specific slope, and all other slopes are set to "H". Level or "L"
Gate to level. As a result, the comparison signal c is
It is set to synchronize with the center of the slope of signal 6. Next, when the tape speed is slow, the cyclic count signal i is switched as shown in FIGS. 9 and 10 to make the slope gentler and to reduce the loop gain. The reference signal gate circuit 14 is arranged so that the center of the slope of the reference signal d', 10 at this time coincides with the center of the slope of the signal 6 when the tape speed is high.
Switch and set. Here, the method of generating the trapezoidal wave signal d' will be briefly explained. This embodiment is an example in which the slope section gain is reduced to 1/3. The explanation will proceed by reproducing the change table for each bit mentioned above. The signal d' extracts a group of count signals from the second bit onwards during the period from the slope part number 2 to the slope part number 4 of the signal i and outputs it via the reference signal gate circuit 14.

[Table] Now, in order to extract slope portion numbers 2 to 4, they can be easily detected using the above-mentioned matching circuit or decoding circuit by using the MSB to the third bit in the change table of each bit. At this time, since the extracted period is shorter than one cycle period of the most significant bit, the output average level is a little higher and the output range is a little narrower (about 3/4). To solve this problem,
Digital offset summation and an amplifier at the output stage are used. Also at signal 12d'' in the same figure,
The same can be realized. At this time, the generation start phase of the slope portion is also switched at the same time, and the amount of timing change is approximately half the difference between the slope portion periods before and after switching. With this setting, there is no change in the synchronization phase due to switching of the loop gain, and therefore the phase of the drum system reference signal e does not need to be switched and can be fixed at a predetermined value. This eliminates tracking errors and screen shakes at recording tape speed switching points or immediately after playback starts, and the synchronization pull-in of the drum control system is stable, and the synchronization pull-in time of the capstan control system is short. can do. In this example, the gain is set to 1/3 (see Fig. 5, 10), which is smaller than 1/2 (LP mode).
d′), 1/5 (Fig. 5, 12d″), which is smaller than 1/3 (EP mode), is set lower than the calculated value, but setting the gain low like this This is because when the motor is operating at low speed, the phase characteristics of the loop gain are delayed due to motor loss, which tends to cause unstable operation. is shown in Fig. 7. In the figure, parts with the same functions as those in Fig. 1 are given the same symbols as in Fig. 1. First, let us consider the case where the tape speed is as fast as in standard recording and playback. To explain, in Fig. 6,
The control pulse a is converted into a shaped control signal b by the noise rejector 5. This signal b becomes a switching delay signal j delayed by To by the delay switching circuit 15 at the next stage. Further tracking MM6
Therefore, the signal is input to the phase comparator 7 with a delay of half a period.
Here, the delay amount To of the delay switching circuit 15 is set equal to the phase change amount of the reference signal e in the conventional example shown in FIG. Next, when the tape speed is slow, the delay switching circuit 15 only performs waveform shaping and outputs a signal without delay.
Output j′. As a result, as shown in FIGS. 9 to 11, the comparison signal c' is synchronized with the center of the slope of the reference signal d'. In other words, by simultaneously changing the comparison signal c by an amount equal to the amount of change To at the center of the slope of the reference signal d, the phase of the drum system reference signal e can be fixed at a predetermined value without switching. At this time, the amount of change To at the center of the slope is the signal 8 in FIG.
This is set to approximately half the difference between the slope period of 9 and 9. Here, the delay switching circuit 15 is generally composed of a monostable multivibrator or the like. Furthermore, when the noise rejector 5 is configured with a group of counters, the delay switching circuit 15 becomes a simple matrix switching circuit, and the configuration is suitable for IC implementation. Further, as can be easily understood from the above explanation, the delay switching circuit 15 may be inserted after the tracking MM6. As described above, according to the first and second embodiments of the present invention, by configuring the center of the slope of the reference signal d of the capstan control system to always coincide regardless of switching of the loop gain, or By simultaneously changing the phase of the control signal equal to the amount of change in the center of the slope, it is possible to always switch to the optimum tracking state while fixing the phase of the drum system reference signal e. As described above, according to the present invention, the reference signal of the drum control system can be fixed at a predetermined value regardless of the switching of the loop gain of the capstan control system, so that tracking at the start of playback is different from that of the prior art. There is no horizontal shaking of the playback screen. Moreover, the synchronization pull-in time of the drum control system can be shortened. Furthermore, by configuring the phase comparator and the like as a counter, it is possible to provide a magnetic recording/reproducing device suitable for IC implementation.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the servo system of a conventional magnetic recording/reproducing device, Figs. 2 1 to 8 are waveform diagrams of main parts for explaining the operation of Fig. 1, and Figs. 3 1 to 13 are tape speed A waveform diagram showing the tracking switching state at the time of switching, FIG. 4 is a block diagram showing an embodiment of the magnetic recording/reproducing apparatus according to the present invention, and FIGS. 5 1 to 12 are
FIG. 4 is a waveform diagram of main parts, FIG. 6 is a block diagram showing another embodiment of the present invention, and FIGS. 7 are 1 to 11.
is a waveform diagram of the main part of FIG. 6. DESCRIPTION OF SYMBOLS 1...Reference signal generation part, 2...Capstan control system, 3...Drum control system, 5...Noise rejector, 6...Tracking MM, 7...Phase comparator, 12...Reference counter, 14...Reference Signal gate circuit, 15...Delay switching circuit.

Claims (1)

[Scope of Claims] 1. A capstan for running a tape, a capstan phase control means for controlling the rotational phase of the capstan, a drum provided with a rotating head, and a drum phase control means for controlling the rotational phase of the drum. , a reference signal generation means for generating reference signals for these phase controls, and a switching means for switching the tape running speed so that recording and playback can be performed at a plurality of tape speeds, and the capstan is In a magnetic recording and reproducing apparatus in which the rotational phase of the drum and the rotational phase of the drum are maintained in a predetermined relationship, the capstan phase control means reproduces the tape at a running speed switched by the switching means. a generation means for generating a comparison sampling signal from a control pulse; a phase comparator for comparing the phases of the comparison sampling signal and the trapezoidal wave-shaped capstan phase control reference signal obtained from the reference signal generation means; a drive means for driving the capstan according to the output of the drum phase control means, and the drum phase control means includes a comparison signal generated based on the phase signal of the drum rotation and a drum phase control reference from the reference signal generation means. a phase comparator that compares the phase of the signal; and a driving means that drives the drum according to the output of the phase comparator, and the reference signal generating means repeatedly counts a predetermined number of bits, a cyclic counter that generates a cyclic signal having a smaller number of bits than the predetermined number of bits as the capstan phase control reference signal, the cyclic period of which is longer as the tape running speed is lower; and a gate that selects one cyclic signal every repetition period for counting and supplies it to the phase comparator of the capstan phase control means, and this gate is opened and closed when the center value of the selected cyclic signal is one of the selected cyclic signals. A magnetic recording and reproducing apparatus characterized in that timing is controlled in conjunction with a signal from the switching means so that tape running speeds are substantially the same. 2. A capstan for running a tape, a capstan phase control means for controlling the rotational phase of this capstan, a drum provided with a rotating head, a drum phase control means for controlling the rotational phase of this drum, and for controlling these phases. and a switching means for switching the tape running speed so that recording and playback can be performed at a plurality of tape speeds. In a magnetic recording and reproducing apparatus, the capstan phase control means performs comparative sampling from the control pulses reproduced at the tape running speed switched by the switching means. a generation means for generating a signal; a phase comparator for comparing the phases of the comparison sampling signal and a trapezoidal wave-shaped capstan phase control reference signal obtained from the reference signal generation means; provided between
The drum phase control means comprises: a delay means whose delay amount for delaying the comparison sampling signal is switched according to the tape running speed; and a drive means which drives the capstan according to the output of the phase comparator. a phase comparator that compares the phase of a comparison signal generated based on a phase signal of drum rotation with a reference signal for drum phase control from the reference signal generating means; and driving the drum in accordance with the output of the phase comparator. and a driving means for repeatedly counting a predetermined number of bits to generate a cyclic signal having a smaller number of bits than the predetermined number of bits, the cyclic period of which is longer as the tape running speed is slower. A cyclic counter generated as a reference signal for capstan phase control, and one cyclic signal selected from among the cyclic signals for each repetition period of counting the predetermined number of bits to compare the phases of the capstan phase control means. the comparison sampling signal is located approximately at the center of the cyclic signal selected by the gate even in the initial stage of operation at the tape running speed after switching by the switching means. A magnetic recording/reproducing apparatus characterized in that the delay means switches the delay time of the comparison sampling signal in conjunction with the switching operation of the switching means.