GB2267011A - Pseudo-vertical sync in VTR output. - Google Patents

Abstract

A helical scan type video tape recorder for reproducing a high quality video signal with no distortions eliminates phase distortion by combining (1000, Fig. 9) a pseudo-vertical synchronizing signal from a circuit 900, which has a specific phase relationship with a video tape phase standard signal at any travelling speed of the video tape, with a reproduced video signal for causing no disturbance in vertical synchronization. A tape phase detector 101 in the pseudo-vertical synchronizing signal generating circuit 900 receives rotary pulses 100 generated at the tape drive capstan and being indicative of the tape speed and tape phase standard pulses 113 which have been detected on the tape and which were recorded with a frame period, the detector being initialised by the pulses 113 and counting the rotary pulses 100. The count is latched by circuit 103 on receipt of a head switching pulse 114 to a processor 105 which generates a ratio of the latched count to the maximum count. This value is input to a circuit 107 which counts down to produce at zero a borrow signal which initiates production of the pseudo-vertical synchronising pulse 112. <IMAGE>

Description

VIDEO TAPE RECORDER This invention relates to a helical scan type video tape recorder (hereinafter called VTR) and, more particularly, to a VTR wherein phase distortion is eliminated from reproduced video signals so as to provide improved picture quality.

Heretofore, sag distortion has been encountered in reproduction of a video signal by a single head VTR. Figure 1 is a block diagram showing a prior art video signal reproducing system and Figure 2 is a plan view of a rotary drum having a single video head to be used therein.

Referring to Figure 1, the prior art video signal reproducing system comprises a rotary transformer 11, an amplifier 12, a signal processing circuit 23, a demodulator circuit 20, a clamp circuit 21, a synchronous separation circuit 22, a clamp circuit 45, an A/D converter 24, a memory unit 26, a memory unit control circuit 31, and a D/A converter 33. In Figure 2, reference numeral 3 denotes a rotary drum, 5 denotes a single video head and 2 denotes a video tape.

In the prior art VTR shown partly in Figure 1, the single video head 5 reproduces an FM signal 13 shown in Figure 3 (d) from the video tape 2 as being switched (switcher is not shown) by a head switching signal 17 shown in Figure 3 (b). The FM signal 13 reproduced by the single video head 5 is demodulated at the demodulator circuit 20 into a video signal 43 having a signal field and a no-signal field alternatively as shown in Figure 3 (e). This video signal 43 is then fed to the clamp circuit 21 before being applied to the synchronous separation circuit 22 for fixing a sync-tip level to a constant level. However, as the time constant of the clamp circuit 21 has normally been selected to be 10 times as much as one interval of horizontal synchronizing signal, sags are generated in a clamped video signal 44 as shown in Figure 3 (f).Therefore, if such clamped video signal 44 is fed to the A/D converter 24 for converting it into a digital signal and further a no-signal field thereof is supplemented with a video signal in the preceding signal field by utilizing the memory unit 26 and the memory unit control circuit 31, a resultant video signal 32 may have a sag in each field, as shown in Figure 3 (g), and be fed to the D/A converter 33 for converting it into a distorted analog signal.

Accordingly, there has been a problem of sag distortion that results in a considerable disturbance of display as it is hard to detect vertical and horizontal synchronizing pulses by the synchronous separation circuit 22 in the duration of the sags.

Skew distortion has also been encountered in the prior art VTR reproducing system such shown by a block diagram in Figure 4.

Referring to Figure 4, an FM signal 13 reproduced from a head 5 is amplified by an amplifier 12 and fed to a switcher 18 which is operated by a switching signal 17. The reproduced FM signal 19 switched by the switcher 18 is then demodulated into a video signal 43 by a demodulator 20 and clamped to a certain level at the tip end of a synchronizing signal thereof in a clamp circuit 21. The camped video signal 44 is fed to an A/D converter 24 and is digitized therein. The digitized video signal 25 is then fed to a memory unit 26.

A memory unit control circuit 31 starts writing the digitized signal 25 into the memory unit 26 from a start point WR (Figure 5) in the signal field and the writing and the reading are repeatedly executed at a sampling rate in the duration of the signal field whilst the video signal written in the memory unit 26 in the duration of the preceding signal field is read out during the no-signal field starting from a start point RR.

The video signal 32 read out from the memory unit 26 is fed to a D/A converter 33 for converting it into an analog signal to attain a reproduced video signal 34 shown in Figure 5 (c) (shown only by a horizontal synchronizing signal).

Since the prior art system has been arranged as described above, if the reproduced video signal performs an interlace scanning, an interval of horizontal synchronizing pulses at the switch over point will be 1.5 H as shown in Figure 5 (c), losing the continuity of the horizontal synchronizing signal and causing the problem of skew distortion in every field.

Moreover, phase distortion has been encountered in a reproduced video signal in accordance with the prior art VTR reproducing system, especially in the case when a recorded video signal is reproduced at a different speed from that of the recording.

Now referring to Figure 6, there is shown a prior art rotary drum assembly in a cross section. The rotary drum assembly comprises a rotary shaft 1, a fixed lower drum 10, an upper rotary drum 3, a head bed 4 secured to the upper rotary drum 3 by screws, video heads 5 mounted at the periphery of the head bed 4 and shown together with a video tape 2, bearing 6, an upper rotary transformer 7 fixed to the upper rotary drum 3 to be rotated therewith, a stationary lower transformer 8, a pedestal 9 secured to the rotary shaft 1 to support the upper rotary drum 3, a magnet 75 mounted on the upper rotary drum 3 and a magnetic detector 76 arranged in the proximity of the magnet 75 for detecting magnetic fluxes therefrom to determine a rotary position of the upper rotary drum 3.

The video heads 5 traverse across the video tape 2 in a slant-wise fashion by the rotation thereof and the travel of video tape 2 for forming parallel tracks as shown in Figure 7. Referring now to Figure 7 (a) to (d), there shown are several tracks formed by the relative movements of the rotary heads 5 and the video tape 2, wherein A2 designates a track of the travelling video tape 2, V1 designates a normal travelling speed of the video tape 2, A5 designates tracks of the rotary heads 5 and V designates a speed of the rotary heads 5. Since the track A2 and the tracks A5 are crossed as it is shown in the drawing, relative tracks to be formed on the travelling video tape 2 by the rotary heads 5 are represented by tracks A in Figure 7 (a).In a still display piay-back mode, the travelling speed of the video tape 2 is 0 and the relative tracks AO conform to A5. In a slow motion dispiay play-back mode, the travelling speed of the video tape 2 is decreased from V1 to Vs and the relative tracks of the rotary heads 5 against the travelling video tape 2 are represented by As in Figure 7 (b).

In a VTR employing rotary drum assembly as described above, a pseudovertical synchronizing signal for a reproduced video signal is generated in a constant phase relationship to a head switching signal in case of playing back a recorded video signal from the video tape 2 other than the normal travelling speed of V1 and combined to the reproduced video signal to form a composite video signal. However, in this type of playback mode, the rotary heads 5 do not trace the relative tracks A or recorded tracks, which are formed on the video tape 2 at normal travelling speed by the rotary heads 5, and a signal output level is decreased considerably, then, over and above noises are generated due to off tracking of the rotary heads 5 Thus, it is hard to attain a clear display from the reproduced video signal because of a poor S/N ratio.

Heretofore, it has been proposed to utilize a rotary drum assembly having an arrangement to displace the rotary heads 5 so as to trace the recorded tracks accurately under any travelling speed of the video tape 2.

The rotary drum assembly of this type is shown in Figure 8 partly in a cross section. A drive unit 40 displaces the rotary heads 5 in the lateral direction of the video tape 2 upon receipt of a control signal from the outside through a slip-ring coupling composed of a contact 52 and a ring electrode 53. The amount of displacement of the rotary heads 5 is so selected, for instance, in the case of slow motion play-back mode with a tape travelling speed of Vs, as to change the tracks A5 of the rotary heads 5 to be A51 within one field play-back period of time in order to make relative tracks coincide with the relative tracks A of the normal tape travelling speed as it is shown clearly in Figure 7 (c).

More generally, it is required to displace the displaceable rotary heads by an amount of P(1 - Vs/v) (where Vs/v is VsN1) in the duration of one field play-back period of time under any slow tape travelling speed of Vs to make the rotary heads trace the recorded tracks without deviation. If the tape travelling speed is decreased from V1 to V1/2, the same track may be traced a plurality of times by the rotary heads. In Figure 7 (d), there is shown an example wherein the video tape 2 is travelling at speed V1/2 and the rotary heads 5 are so displaced in the lateral direction of the video tape 2 as to align relative tracks with the relative tracks A for tracing the track A at the position of LO. In this example, a switching point of the rotary heads is given by So.In the next field, the tracing will be made on a track at the position of L1 where is parted from the previously traced track by an amount of P/2 and a switching point of the rotary heads will be given by S1 provided that the rotary heads are displaced so as to trace the track at L1. Accordingly, a phase difference between reproduced video signals derived from the tracks at the Lo and L1 is given by 1/2. oc H basing on the switching points of So and S1.

As has been described above, in the prior art system, there has been a problem such that if a pseudo-vertical synchronizing signal generator circuit being capable of generating a pseudo-vertical synchronizing signal with a constant phase is employed for the reproduced video signal which has a different phase in every field, there may be vertical vibrations or jitter in the displayed pictures due to the fact that the phase difference between the inserted pseudo-vertical signal and the reproduced video signal varies by an amount of 1/2 oc H in every trace (in the case of slow motion play-back with the tape travelling speed of V1/2).

It is an object of this invention to provide a VTR that can produce a stable image of good picture quality with no distortions regardless of tape travelling speed by eliminating phase distortions with use of a pseudovertical synchronizing signal generator being capable of generating a pseudo-vertical synchronizing signal having no phase shifting from the tape travelling speed.

In accordance with this invention, there provided is an improved video signal reproducer that can reproduce a high quality video signal with no distortions in a VTR.

According to the present invention, the problem of vertical synchronizing phase distortion is solved by inserting a pseudo-vertical synchronizing signal, which has specific phase relationship with a video tape phase standard signal depending on a travelling speed of the video tape, into a reproduced video signal for eliminating the disturbance in the vertical synchronization.More specifically, the video signal reproducer in still another preferred mode of this invention comprises a pseudo-vertical synchronizing signal generator being capable of generating the pseudovertical synchronizing signal, which has a specific phase relationship with the video tape phase standard signal regardless of the travelling speed of the video tape, based on a tape phase standard signal being recorded on the video tape with a frame period, rotation pulses which stand for a rotational speed of a capstan or an amount of travelling of the video tape and a head switching signal.

In the accompanying drawings: Figure 1 is a block diagram of a known video signal reproducer for use in a VTR; Figure 2 is a plan view of a known configuration of a rotary head drum; Figure 3 is a timing diagram showing the waveforms at various parts of in the block diagram of Figure 1; Figure 4 is a block diagram of a known video signal reproducer for use in a VTR; Figure 5 is a timing diagram showing the write/read timing at the memory unit in the block diagram of Figure 4; Figure 6 is a sectional view of a known rotary head drum; Figure 7 is a schematic diagram showing the relationship between the recorded tracks on the video tape and the scanning tracks by the rotary heads; Figure 8 is a sectional view of a rotary head drum of the type having displaceable rotary heads; Figure 9 is a block diagram showing preferred embodiment of this invention;; Figure 10 is a block diagram showing the rotary head drum control circuit in the embodiment of this invention shown in Figure 9; Figure 11 is a block diagram showing the pseudo-vertical synchronizing signal generator circuit in the embodiment of this invention shown in Figure 9; and Figure 12 is a timing diagram showing the pseudo-vertical synchronizing signal timing in the generator circuit of Figure ii.

Referring firstly to Figure 9, there is shown a schematic diagram of this embodiment of the invention in block form, wherein 5a and 5b denote displaceable rotary heads mounted across a rotary drum 3, 700 denotes a rotary drum control circuit, 79 denotes an input for an adjusting signal to adjust a range of the rotary heads displacement, 78 denotes a video signal reproducing circuit for deriving a reproduced video signal 120 from an output terminal 90 thereof, 80 denotes a capstan for driving a magnetic video tape 2 to travel cooperatively with a pinch roller 82 pressed thereto, 81 denotes a capstan motor, 84 denotes a rotary pulse generator arranged in parallel with the capstan motor 81 for deriving rotary pulses 100 from an output terminal 85 thereof and 86 denotes a tape phase standard signal generator arranged in contact with the travelling magnetic video tape 2 for deriving a tape phase standard signal 113, which has been recorded on the magnetic video tape with a frame period, from an output 87 thereof. A pseudo-vertical synchronizing signal generator 900 generates a pseudo-vertical synchronizing signal 112 in such a phrase as being related to the tape phase standard signal of the travelling magnetic tape 2.

A signal composition circuit 1000 combines the reproduced video signal 120 with the pseudo-vertical signal 112 for deriving a composite video signal 121 from an output terminal 91 thereof. Now referring to Figure 10, there is shown a detailed block diagram of the rotary drum control circuit 700 in accordance with the embodiment of the invention shown in Figure 16, wherein 61a and 61b denote periodic signal generators for generating periodic signals which differs 180 degrees in phase under the control of a switching signal 114. The switching signal 114 is generated by a switching signal generator 77 by detecting a mechanical position of the rotary drum 3 with use of a permanent magnet 75 mounted on the rotary drum 3 and a position detecting head 76.Outputs of the periodic signal generators 61a and 62b are fed to drive circuits 600a and 600b each of which is composed of an operational amplifier 64, a differential amplifier 69, a electric current limiter 65, resistors 62, 63, 68 and 70, and a capacitor 71. Outputs of the drive circuits 600a, 600b or control currents for the displaceable rotary heads 5a, 5b are fed to drive coils 42a, 42b of the drive unit 40 through respective contacts 52a, 52b and electrodes 53a, 53b. Non-control side terminals of the drive coils 42a, 42b are connected in common and then connected to a standard potential through a pair of common electrode 53c and contact 52c.

Magnetic fields to be generated by the drive coils 42a, 42b when the control currents flow therethrough displace the displaceable rotary heads 5a, 5b respectively by developing a magnetic repulsion against each cylindrical permanent magnet that constitutes the drive unit 40. An amount of the displacement is controlled in such a manner that head tracing tracks A5 in the playback conform to recorded tracks on the video tape 2. The control currents are also fed to the drive coils 42a, 42b while the rotary heads 5a, Sb are not in contact with the video tape 2.Playback signals by the rotary heads 5a, 5b are fed to head amplifiers 72a, 72b through upper and lower transformers 7, 8 and outputs of the head amplifiers 72a, 72b are fed in turn to the video signal reproducing circuit 78 for deriving a reproduced video signal 120 therefrom after being switched by a switch 73 under the control of a switching signal 114.

Operation of the drive circuits 600a, 600b will now be described more in detail. Since the drive circuit 600a and the drive circuit 600b are the same in structure and operate in the same manner with 180 degrees phase difference, the description will be made only for the drive circuit 600a.

In this embodiment, it is arranged that (resistance of the resistor 63) (resistance of the resistor 70), and (impedance of the capacitor 71) (resistance of the drive coil 42). The drive circuit 600a has two feed back loops, one of which is a current feed back via the differential amplifier 69 which detects and feeds backs a voltage across the resistor 68 (proportional to the drive current that flows through the drive coil 42a) interposed in the control line. The other feed back loop is a voltage feed back loop via the capacitor 71.

When the frequency of the periodic signal is high, an impedance of the capacitor 71 approaches to 0 and the voltage feed back loop gets control over the current feed back loop, resulting in the voltage drive operation On the other hand, when a frequency of the periodic signal is low, an impedance of the capacitor 71 approaches to ooand the current feed back loop gets control over the voltage feed back loop, resulting in the current drive operation. The relationship between a fundamental drive frequency for the rotary head 5a, which rotates simultaneously with the drive coil 42a, and a mechanical resonance frequency of the drive unit 40 is given by an inequality of (fundamental drive frequency) < (mechanical resonance frequency).Hence, the current drive operation is performed at least in the vicinity of the fundamental drive frequency whilst the voltage drive operation is performed for short-circuit control at least in the vicinity of the mechanical resonance frequency.

Further, in.order to eliminate difference in gains at the times of the voltage drive operation and the current drive operation, the following circuit constants are selected in this embodiment of the invention: (gain of the differential amplifier 69) = (resistance of the drive coil 42a) (resistance of the resistor 68) The electric current limiter 65 which comprises of a window comparator 66 and a current limiting element 67 detects a voltage across the resistor 68 (proportional to a current that flows through the drive coil 42a) and limits a control current to make the displacement of the displaceable rotary head Sa to fulfil an inequality of [j required speed - 1 X x track pitch P < displace limit < structural displace limit of the displaceable rotary head 5a].That is, the electric current limiter 65 and the differential amplifier for current feed back are operated simultaneously with a voltage across the resistor 68.

Operation of the pseudo-vertical synchronizing signal generator circuit 900 will be described more specifically with reference to Figure 11. A number of rotary pulses 100 of the capstan motor 81 is proportional to a rotary angle of the capstan 80 and consequently to the travel of the magnetic video tape 2. The rotary pulses 100 and the tape phase standard signal 113 detected from the travelling magnetic video tape 2 through the tape phase standard signal detector 86 are fed to a tape phase detector 101 for counting the rotary pulses 100 by initializing the counting periodically with the tape phase standard signal 113.

Therefore, an output of the tape phase detector 101 or a tape phase data 102 has a value proportional to the phase of the travelling magnetic video tape 2.

The tape phase data 102 is latched at a latch circuit 103 with an edge pulse 116 of the head switching signal 114 being detected by an edge detector 115 and the latched tape phase data 104 is then fed to a processor 105. The processor 105 executes the following process: (tape phase data 104) x 2ccH (maximum value of tape phase data 102) and derives a processed data 106 therefrom.

A delay unit 117 delays the edge pulse 116 and derives a delayed edge pulse 110 after a certain time Tc. A pseudo-vertical synchronizing signal timing generator circuit 107 accepts the processed data 106 from the processor 105 when the delayed edge pulse 110 is fed thereto and counts down the accepted processed data 106 upon receipt of every clock.pulse 119 for deriving a borrow signal 108 therefrom upon reaching 0 in counting. A generation timing of the borrow signal 108 becomes a generation start timing of the pseudo-vertical synchronizing signal 112 to be generated by a pulse generator 109. The borrow signal 108 is assured a pulse width as a vertical signal by the pulse generator 109 and taken out as the pseudo-vertical synchronizing signal 112.

Figure 12 is a timing chart for the waveforms at various parts of the embodiment of the invention shown in Figure 11 in the mode of V1/2 speed slow-motion playback illustrating the relationship amongst the head switching signal 114, the tape phase standard signal 113, the tape phase data 102, the latched tape phase data 104 and the pseudo-vertical synchronizing signal 112. In the case of V1/2 speed slow-motion playback, one period of the tape phase standard signal 113 corresponds to two periods of the head switching signal 114.

Since the tape phase data 102 is attained by counting the rotary pulses 100 from the capstan as being initialized periodically by the tape phase standard signal 113, the wave form of the tape phase data 102 (D/A converted) is given by a solid line in Figure 12(c). Further, the wave form of the latched tape phase data 104 latched by the both edges of the head switching signal 114 is given by a dotted line in Figure 12 (c).

A time T of the pseudo-vertical synchronizing signal 112 from the switch over point of the rotary heads is given by: (tape phase data 104) T = Tc + x 2ccH (maximum value of tape phase data 102) where Tc is the delay time given by the delay unit it 7.

As it has been described above, the pseudo-vertical synchronizing signal is generated with such a phase as being related to the tape speed to be combined with the reproduced video signal. There provided is a VTR which can display a stable image with no jitter at any tape travelling speed that includes not only a slower but a faster travelling speed than the normal tape travelling speed V1 because of the fact that the phase of the pseudo-vertical synchronizing signal can always be maintained constant with respect to that of the reproduced video signal in accordance with this specific embodiment of the invention.

In the foregoing specification, the invention has been described with reference to the specific exemplary embodiments thereof, it will, however, be evident that various modifications and changes may be made thereunto without departing from the scope of the invention as set forth in the appended claims. The specification and the drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

Claims (2)

1. A video tape recorder comprising: a rotary drum having mounted thereon a displaceable rotary head to be driven in the lateral direction of a magnetic video tape for tracing playback tracks that conform to recorded tracks on a magnetic video tape at any travelling speed thereof; a video signal reproducing circuit for reproducing a video signal from playback signals by the displaceable rotary heads; rotary pulse generating means for generating rotary pulses that indicate revolutions of a capstan motor; tape phase standard detecting means for detecting a tape phase standard signal recorded on the magnetic video tape with a frame period; a tape phase detector for counting the rotary pulses being initialized by the tape phase standard signal; a latch circuit for latching counted tape phase data with an edge pulse derived from a head switching signal;; a processor for executing the following process (tape phase data 104) x 2 oc H; (maximum value of tape phase data 102) a pseudo-vertical synchronizing signal timing generator circuit for taking the processed data in when a delayed edge pulse attained by delaying the edge pulse by certain amount of time Tc is fed, counting down the processed data upon receipt of every clock pulse and deriving therefrom a borrow signal upon reaching 0 in counting; a pulse generator circuit for generating a pseudo-vertical synchronizing signal when the borrow signal is fed thereto; and a signal composition circuit for deriving a composite video signal therefrom by combining the pseudo-vertical synchronizing signal with the reproduced video signal from the video signal reproducing circuit.

2. A video tape recorder substantially as hereinbefore described with reference to Figures 9 to 12 of the accompanying drawings.