JPH0547905B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field of the invention] This invention relates to a videotape recorder (hereinafter referred to as VTR).
When switching from playback mode to recording mode, the servo circuit locks in a short period of time, allowing the tape to run stably, especially when performing continuous shooting or editing. This article relates to a capstan servo device. [Technical background of the invention and its problems] Capstan servo devices for VTRs do not always work properly due to reasons such as the need to switch the tape speed.
An AFC (Automatic Frequency Control) circuit is included, and of course an APC (Automatic Phase Control) circuit is added to match the tape phase with the video head phase. A conventional capstan servo device is shown in FIG. In FIG. 1, reference numeral 11 denotes a capstan motor, and rotational force is applied to the capstan 12 from the capstan motor 11 via a belt 13. Magnetic plate 14 coaxially with capstan 12
The magnet plate 14 and the FG head 15 constitute an FG pulse generator. In the figure, reference numeral 16 denotes a tape, which is pressed against the capstan 12 by a pinch roller (not shown) and runs.
The FG pulse output from the FG head 15 naturally has a frequency proportional to the rotational speed of the capstan. The FG pulse is amplified by the FG pulse amplifier 17, and is then amplified by the I/N frequency divider 1 according to the tape speed.
After being frequency-divided by an 8 or I/n frequency divider 19, the signal is guided to an AFC detector 20. The AFC detector 20 detects the frequency of the FG pulse, and the detection signal is converted into direct current by the low-pass filter 21 and guided to the adder 22. The output of the adder 22 is supplied to a motor driver 25 via a low-pass filter 23 and an operational amplifier 24. AFC with above loop
The circuit is configured. Note that two frequency dividers 18 and 19
is selectively switched by switch 29 in response to a control signal from tape speed switch 26 and incorporated into the loop. In the recording mode, the FG pulse that has passed through the frequency divider 18 or 19 is further divided by the I/NP frequency divider 27, converted to the same frequency as the reference trapezoidal wave, and then supplied to the APC detector 28 as a comparison signal. . The reference signal to be compared is a signal obtained from a reference oscillator 35, converted into a trapezoidal wave by a trapezoidal wave generator 30, and further delayed appropriately by a trapezoidal wave shift circuit 31. Note that in the recording mode, the reference oscillator 35 is synchronized by the vertical synchronization signal of the recording television signal. Further, the vertical synchronizing signal is frequency-divided by the 1/2 frequency divider 29 and recorded by the control head 33 as a control signal. The output of the APC detector 28 is converted to a DC voltage by a low-pass filter 32, and then converted to a DC voltage by a mixer 22.
The motor driver 2 is mixed with the AFC control voltage and passed through a low-pass filter 23 and an operational amplifier 24.
5. With the above loop, in recording mode
APC circuit is configured. In the reproduction mode, the reproduction control pulse reproduced by the control head 33 is switched to the one amplified and shaped by the control pulse amplifier 34 as the comparison signal of the APC detector 28.
As a reference signal, the output of a reference oscillator 35 is applied via a trapezoidal wave generator 30 and a trapezoidal wave shift circuit 31. The trapezoidal wave shift circuit 31 adjusts the generation timing of the trapezoidal wave, and is composed of a monostable multivibrator or the like. In recording mode, the timing is fixed, and in playback mode, tracking is adjusted by a variable resistor. In principle, the comparison period in an APC circuit is 1/
(fυ/2) (fυ: field frequency), and the lock time usually takes about 0.8 seconds. Capstan servo systems are lighter inertia than drum servo systems.
The pull-in time of the AFC circuit is about 0.1 to 0.2 seconds, which is quite fast. Therefore, during playback, the tape speed reaches approximately a predetermined value within 0.1 to 0.2 seconds from the start of playback. Further, as long as the phase between the tape and the video head, that is, the phase between the playback control signal and the video head are not opposite, no noise bands will appear in the reproduced picture and will not cause much of a problem. However, in this case, the frequency of the audio will be slightly distorted, so it will be muted. However, in recording mode, if the tape and video head are not in phase, satisfactory recording cannot be achieved, and the long pull-in time of the APC circuit becomes a major problem. For example, in a so-called splicing operation in which recording is stopped for one position and then recorded again from that position, the tape is rewound a small amount after recording is stopped, and the capstan servo system is replayed. mode, synchronizes the playback control pulse with the video head, and ensures continuity between the track position on the tape to be newly recorded and the phase of the video head. In the capstan servo device shown in FIG. 1, when the reproduction mode is switched to the recording mode, the comparison signal of the APC circuit is switched from the reproduction control pulse to the frequency-divided FG pulse.
Therefore, the servo may be disturbed at that moment. That is, as shown in FIG. 2, in the playback mode, the playback control signal b is
Assuming that the recording mode is switched to the recording mode at time _t0 with _{the phase of}
= 3, N _P = 4, the pulses e, f, and g, which are frequency-divided by 1/8 or 1/12, do not arrive at a fixed timing, and the comparison signal (the falling edge of pulses e, f, and g) timing comparison) does not necessarily occur in the slope portion of the reference trapezoidal wave a. If the pulses e and f occur outside the slope, the servo will be greatly disturbed. The rate at which the first comparison signal after switching enters the slope of the reference trapezoidal wave is 2/fv×te (te: slope time width of the reference trapezoidal wave), and the probability of phase locking with the first comparison signal is low. As a result, the recording tape speed does not reach a predetermined level until the phase is locked, and when that portion is played back, the tape pattern is incorrect, causing beats to appear on the screen, or in the worst case, noise bars to appear. Furthermore, it has the disadvantage that the audio frequency is played back out of order. [Object of the Invention] The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a capstan servo device in which the phase of the APC circuit is almost instantaneously locked when switching from play mode to record mode. With the goal. [Summary of the Invention] The present invention provides that when switching from playback mode to recording mode, a reference signal is supplied to the D terminal and the FG
The above object is achieved by using a D-type flip-flop whose pulses are supplied as clock pulses so that the FG pulse located approximately at the center of the slope of the reference trapezoidal wave is extracted as a comparison signal. [Embodiments of the Invention] The apparatus of the present invention will be described in detail below with reference to FIGS. 3 and 4. In FIG. 3, the same parts as in FIG. 1 are denoted by the same reference numerals, and detailed explanations are omitted. In FIG. 3, the control pulse amplifier 3
The output of 4 is connected to the clear terminal CL of the D-type flip-flop 41 via a resistor 42 and also to the preset terminal PR via a diode 43. Furthermore, this preset terminal PR
A switch 47 selectively connects the power supply +B or the reference potential (earth) to the switch 47. Further, the output of the FG pulse amplifier 17 is supplied to the clock terminal CP of the D-type flip-flop 41 via a multiplier 44, and the head switching pulse equivalent to the reference signal is supplied to the D terminal D from the input terminal 46 to the inverter 45.
It has been added via. The output terminal of the D-type flip-flop 41 is connected to the APC detector 28. The main parts of the device according to the present invention are constructed as described above. Next, the operation will be explained. The AFC circuit operates in the same manner as in FIG. 1 in both playback mode and recording mode. In the reproduction mode, the APC circuit connects the power supply +B to the preset terminal PR of the D-type flip-flop 41.
is supplied and is at a high level, so the level of its output terminal Q, changes only by the control pulse supplied to the clear terminal CL. That is, when the clear terminal CL input is at a high level, the output is at a high level, and when the clear terminal CL input is at a low level, the output is at a low level. Therefore, the output is the same as the control pulse, and the APC circuit works in the same way as in Figure 1. In the recording mode, both the preset terminal PR clear terminal CL of the D-type flip-flop 41 are at a low level, and the D-type flip-flop 41 outputs the signal level supplied to the D terminal D in synchronization with the clock supplied to the clock terminal CP. It works to transfer to Q. In other words, the output of the D-type flip-flop is the head switching pulse delayed in synchronization with the FG pulse. The table shows the truth values of the D-type flip-flop.

〔Effect of the invention〕

As described above, according to the capstan servo device of the present invention, when switching from playback mode to recording mode, FG pulses can be selected almost in synchronization with the rise and fall of the reference trapezoidal wave. Since the FG pulse closest to the reproduction control pulse can be selected, the APC circuit can be phase-locked with almost no servo disturbance. Therefore, when a so-called seam recorded in this manner is reproduced, there is no fluctuation in the period of the control signal, and the reproduced image is good. Furthermore, by using a D-type flip-flop, a frequency divider 27 that divides the frequency of the FG pulse, which was conventionally required, to have the same frequency as the reference signal, and the output of this frequency divider 27 and the control pulse amplifier 34 are integrated. This eliminates the need for a switch for switching the signal and guiding it to the APC detector 28, simplifying the circuit configuration.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing a conventional capstan servo device, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, and FIG. 3 is a block diagram showing a capstan servo device according to the present invention. The circuit diagram and FIG. 4 are waveform diagrams for explaining the operation of FIG. 3. 11... Capstan motor, 12... Capstan, 15... FG head, 35... Reference oscillator, 41
...D type flip-flop.

Claims (1)

[Claims] 1. An AFC circuit that controls the rotation speed of the capstan so that the frequency of pulses related to the rotation of the capstan is constant, and a reference signal is compared with a comparison signal obtained by delaying this reference signal in synchronization with the pulse. A capstan servo device for a video tape recorder, which is equipped with an APC circuit that controls the rotational phase of the capstan so that the phases of both are constant, wherein the comparison signal is supplied to the D terminal while the reference signal is supplied to the D terminal. A capstan servo device for a video tape recorder, characterized in that said pulse is obtained as an output of a D-type flip-flop supplied as a clock.