In general, a video signal is recorded on the tracks of a recording medium field by field, or frame by frame. In a reproducing device for still video floppies or video disks, a recorder such as a head or stylus is moved in a predetermined direction of the recording medium (for instance, so as to cross the tracks) to skip over to a desired track or to random-access to a location which includes the desired track. In this operation, when the scanning means moves, the reproduced picture becomes irregular. It is unnecessary to reproduce such an irregular picture, and therefore the reproducing video signal is, in general, muted.
FIG. 3 shows a still picture reproducing device which is one example of a conventional video signal reproducing device.
In FIG. 3, a video floppy magnetic disk 1 is rotated by a spindle motor 2 at a predetermined speed, for instance 36 rpm. A plurality of (for instance fifty) concentric tracks are provided on the reverse side of the disk 1, and a video signal for one field is recorded on each track. A magnetic head 3 is provided below the disk 1 in such a manner as to confront the tracks. As the disk 1 rotates, the head 3 scans one selected track (in the case of a dual head, two adjacent tracks being scanned) to output a voltage signal, i.e., an RF signal S.sub.1. The RF signal S.sub.1 is amplified by an amplifier 4 and supplied to a video signal processing section 5. The video signal processing section 5 conventionally includes an FM demodulator circuit, a deemphasis circuit, a mixer circuit, and a video amplifier circuit, to form a reproducing video signal S.sub.2, for instance of NTSC system format. The reproducing video signal S.sub.2 is applied through an analog switch 6 to, an output terminal 7, and finally to an RF converter (not shown) to a television set. Alternately, the signal can be applied through the analog switch 6 and the output terminal 7 to a monitor display unit or the like.
The center core 1a of the disk 1 has a PG yoke 1b at a position which leads the position of the leading edge of a vertical synchronizing signal recorded on each track by a phase angle corresponding to 7H (where H is the horizontal scanning period). A PG pickup 8 is disposed above the PG yoke 1b. The PG pickup 8 detects the leakage flux of the PG yoke 1b, to output a PG pulse S.sub.3 having a field frequency representative of the rotational phase of the disk 1 (about 60 Hz in the NTSC system for instance). The PG pulse S.sub.3, after being amplified by an amplifier 9, is applied to a delay circuit 10, where it is delayed as much as 7H. The PH pulse S.sub.3 ' thus delayed is applied to a phase comparator 11, where its phase is compared with the phase of a servo reference signal S.sub.4 (about 60 Hz) outputted by a servo reference signal generator 12. The servo reference signal generator 12 includes a 3.5 MHz crystal oscillator, and a frequency divider for subjecting the oscillation frequency of the crystal oscillator to 1/59718 frequency division (about 60 Hz being obtained).
A trapezoid wave formed in the phase comparator 11, by utilizing the servo reference signal S.sub.4 as gated with the delayed PG pulse S.sub.3 '. Thus, when the phase of the delayed PG pulse S.sub.3 ' leads the phase of the servo reference signal S.sub.4, a relatively low error voltage S.sub.5 is outputted, and when the phase of the delayed PG pulse S.sub.3 ' lags the phase of the servo reference signal S.sub.4, a relatively high error voltage S.sub.5 is outputted.
The error voltage S.sub.5 is applied to a phase compensating circuit 13 including a phase lagging circuit or phase leading circuit. An error voltage S.sub.6 provided at the output terminal of the phase compensating circuit 13 is added to a speed control signal S.sub.7 of a speed servo system, and the result of this addition is applied to an amplifier 14.
The speed control signal S.sub.7 is formed as follows. A frequency signal FG representing the speed of a spindle motor 2 is obtained from a frequency generator. The frequency signal FG thus obtained is converted into a DC voltage signal V by a frequency-to-voltage converter 16. The DC voltage signal V is applied to a DC filter 17 to form the speed control signal S.sub.7. The speed control signal S.sub.7 provides a damping effect to suppress any irregular rotation of the spindle motor 2.
The voltage signal (S.sub.6 -S.sub.7) is to the amplifier 14, and after being amplified, is applied as a drive voltage signal S.sub.8, to a motor drive amplifier 18. The motor drive amplifier 18 is adapted to perform switching (commutation) and amplifying operations as required, to supply an exciting current I corresponding to the voltage signal S.sub.5 to the windings of the spindle motor 2, preferably a DC motor.
A servo loop for synchronizing the rotational phase of the magnetic disk with the servo reference signal S.sub.4 is as described above.
The PG pulse S.sub.3 is applied to the video signal processing section 5, as described above to form a frame signal in a one-field/one-track recording operation, or to switch an odd-number field signal and an even-number field signal over to each other or to switch a field signal and a field signal obtained by delaying the field signal by 0.5H, or to switch the heads of a dual head assembly.
An envelope detector 19 and a head moving mechanism 20 are controlled by a control section 21 including a microcomputer. The control section 21 is coupled through a line to the analog switch 6 to control the latter.
For random access, an instruction signal S.sub.5 for shifting the head 3 from the track Ti which is being scanned to a selected track Tj is applied to the head moving mechanism 20 from the control section 21, while an envelope signal S.sub.10 representing the envelope of the RF signal S.sub.1 is applied to the control section 21 from the envelope detector 19. The position of the head 3 is finely adjusted so that, when the head 3 comes near the track Tj, the level of the envelope signal S.sub.10 is maximized, and the head 3 is positioned so that it is able to scan the track Tj. In this operation, the head 3 is moved from the track Ti to the track Tj in a radial direction, perpendicular to a circumferential direction of elongation of the track. Therefore, the reproducing video signal S.sub.2 outputted by the video signal processing section 5 is irregular during this time.
If this irregular signal S.sub.2 is applied to an external picture display unit such as a television set, then the reproduced picture is unacceptable. In order to overcome this difficulty, the control signal S.sub.11, at logic level "0" (hereinafter referred to merely as "0", when applicable) is applied to the switch 6 by the control section 21, as a result of which the armature of the switch 6 is tripped over to the terminal b. Therefore, the transmission of the reproducing video signal S.sub.5 is interrupted during this time of poor video output (i.e., the signal S.sub.5 is muted) and the output terminal 7 is grounded. As a result, the picture temporarily disappears from the screen of the television set. At the instant when the head 3 is ready for scanning the track Tj, the control section 21 applies the control signal S.sub.11 at the logic level "1" (hereinafter referred to merely as "1", when applicable) to the switch, so that the armature of the switch 6 is tripped over to the terminal a. As a result, the reproducing video signal S.sub.2 is transmitted to the television set so that the reproduction picture of the track Tj appears on this screen.