CA1102914A - Method and apparatus for recording a single video frame - Google Patents

Abstract

METHOD AND APPARATUS FOR RECORDING A SINGLE VIDEO FRAME

Abstract of the Disclosure A disc recorder having a single magnetic pick-up and record head and having a magnetic disc, rotating at the frame rate of a video signal, records and provides for display of a single frame from a video signal consisting of a series of video frames. Motion jitter is eliminated by recording on the disc recorder one of the two fields which make up a chosen video frame and then reproducing this field.
The reproduced field signal is then stored and rerecorded onto the magnetic disc at a position relative to the original field recording corresponding to the delay between two inter-laced fields. A memory having less capacity than that re-quired for storing a complete field signal is used with only a portion of a field being reproduced, stored and rerecorded during each rotation of the magnetic disc. A complete video frame is thus recorded, ready for display, in which the two interlaced fields are identical.

Description

Docket 6398 ~ 4 Background of the Invention This invention relates to magnetic recorders and recording methods and especially to such recorders adapted to be used to record and reproduce individual video frames for stop-action or slow motion video display. The standard for video signals currently used in the United States requires that a scanning pattern of 525 horizontal scanning lines per frame be utilized. A complete frame consists of two inter-laced fields, the first field including the even numbered lines and the second field including the odd numbered lines in the picture scan. All of the lines in the first field are scanned in succession before the lines in the second , ~
field are scanned.
Magnetic disc recoxders have in the past been used to produce high resolution images of single video frames.
An individual _rame may be reproduced repeatedly and supplied to a monitor so that a stop-action effect is created.
Successive ~ideo frames each may be supplied to a video moni-tor a number of times to create a slow motion effect. If the subject portrayed in the video frame is in rapid motion, however, the first and second fields may differ significantly and interlaced display of these fields may therefore cause significant distortion. An effect known as motion jitter may occur due to the spatial displacement of the subject during the elapsed time between the two interlaced fields~
, Docket 6398 One approach to this problem has been to record only one field and to display this field repeatedly at the field rate. A frame thus would consist of only one field which is displayed twice in interlaced fashion. As shown in U.S. Patent No. 3,636,255 to Netani et al, issued October 23, 1969, this may be accomplished by using a field recorder in which one field is recorded during one full rotation of the magnetic disc. A frame is developed by reproducing the field signal twice and by displacing the second reproduced field by one-half horizontal line scan time from the first reproduced field to create the proper conditions for interlace. Since the first and second fields are identical, however, vertical resolution is needlessly sacrificed in displaying still subjects or subjects which are moving relatively slowly.
To allow for high vertical resolution in the case of still subjects and jitter-free operation in the case of - rapidly moving subjectsl two methods have previously been utilized. The first method is to xequire the disc recorder to operate at two rotational speeds. If a rapidly moving subject is to be displayed, the recorder is operated at the field rate with one field repeatedly displayed in interlaced fashion. If a still or slowly moving subject is displayed, however, the disc recorder is operated at the frame rate with both fields of a frame being recorded and displayed.
This method of operation is dificult to achieve and relatively expensive. The disc drive servo-mechanism for two speed operation is complicated and sophistica~ed controls are .

Docket 6398 ~ Z ~ 4 required. Further, differences in dynamic performance of the recorder at the t~70 speeds must be considered, both in the case of flying head hard disc recorders and in the case of flexible disc recorder~.
A second method shown in U.S. Patent No.
3,518,366 to Phan, issued June 30, 1972, involves rerecording one field of a frame twice on a frame recorder. To accomplish this, a frame recorder is ]0 modified by providing a second recording head precisely displaced from the first head. The two magnetic heads then record the single field simultaneously and one head is used for playback of the frame. If the disc is a multi-track disc, an additional stepper mechanism is required for the extra head. This method may also be costly and complicated and may cause excessiYe disc wear.

Summary of the Invention In accordance with the present invention, a stop action recorder is provided for recording a frame of video information consisting of interlaced video fields of first and second field type and further including horizontal and vertical sync signals, and for providing a video frame consisting of two interlaced fields to a monitor at a frame rate without motion jitter. The recorder includes a video source means for providing a signal to be recorded, a disc means, including a magnetic disc and a magnetic pick-up and record head adjacent the magnetic discp for recording a video signal, and means for rotating the disc at the rame rate o one revolution per frame. The recorder further includes a memory for storing a video signal applied thereto, a switch for applying a video signal to the disc means to be recorded on the magnetic disc, and ,;~ ,~ ,.
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Docket 6398 ~ ~

means, connected to the head, for reproducing the video signals recorded on the magnetic disc and for applying the reproduced signal to the memory, and means, connected to the memory, for reapplying the reproduced signal to the switch for re-recording the signal on the disc. A control means is provided, including a field discriminator providing a field type output indicating the type of field being reproduced, for selectively enabling the memory and the switch such that a portion of a field of one of the field types is stored in the memory during playback and reapplied through the switch as a corresponding portion of a field o the other of the field types.
The control means may comprise a first counter incremented once during each rotation of the disc means and a second counter responsive to the horizontal sync si~nals in the reproduced video signal. The control means further includes a comparator for comparing the count in the first counter with the count in the second counter and for applying an enabling signal to the switcha The discriminator may comprise means for discriminating between fields of the first field type and fields of the second field type in dependence on the timing between horizontal sync pulses and vertical ~ync pulses at the beginning of each vertical sync interval.
Other objects and advantages of the : 30 invention will be apparent f rom the following description, the accompanying drawings and the appended claims.

Brief Description of the Drawings ~ig. 1 is a representation of the standard interlaced video scanning pattern;

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Docket 639B ~ 4 Figs. 2A and 2B are representations of portions of a video signal;
Fig. 3 is a simplified diagrammatic representation of the preferred embodiment of the invention;
Fig. 4 is a schematic representation of the circuit arrangement of the present invention;
Fig. 5 is a schematic representation of the circuit for the odd/even field discriminator of Fiug. 4; and Figs. 6 and 7 are timing diagrams useful in explaining the operation of the device shown in Fig.
5.

Descrl~tion of the Preferred Embodiment Referring now to Fig. 1, there is shown in diagrammatic form the standard interlaced scanning pattern utili~ed for video signals in the United States. The video signal consists of a succession of video pictures or frames which are presented at the rate of 30 per second. Each frame is made up of a number of horizontal scan lines with the United States standard being 525 lines per frame. As shown in Fig. 1, each borizontal line is slightly inclined to the right. This occurs because during the scanning process a vertical deflection signal is generated which moves the scanning beam from the top of the picture to the bottom of the picture at a constant rate. The horizontal deflection is much more rapid and acts to move the scanning beam from left to right to scan a line and then very rapidly to return the beam to the left edge of the picture to begin the scan of the next succeeding line in the fiel~.
As seen in Fig, 1, each frame consists of two fields. Each odd numbered field begins at point Docket 6398 ~ 2~1~

20 and ends at point 25. After an odd nu~bered field is scanned, the scanning beam is returned by a vertical deflection signal to point 30 where an even numbered field scan is begun. The even numbered field scan ends at point 35 and the scanning beam is then returned to point 20 for the initiation of the scan of the next odd numbered field.
Referring now to Figs. 2A and 2B, two portions of a standard video signal are shown. Fig.
2A illustrates the end of an even field, the beginning of an odd field, and the vertical blanking ! between the fields. Fig. 2B is similar to Fig. 2A
except that the end of an odd field and the beginning of an even field are shown. As seen in ~ig. 2A, the scanning beam intensity for each line is provided by a wave form such as that indicated at 40. A level 45 is reached by the signal when a white portion of the line is scanned and a level 50 is reached by the signal when a black portion of the line is scanned. Timing information is provided on the video signal by a series of pulses, the amplitudes of which are not within the range used to specify the intensity of the beam.
Specifically, horizontal sync pulses, such as pulse 55~ are provided to synchronize the horizontal beam deflection circuit in the video monitor with the video information supplied to the video monitor. Each of these pulses is outside the limits of the video beam intensity signal and, therefore, the picture displayed by the monitor is unaffected.
Between each field is a vertical sync pulse~ shown at 60O The vertical sync pulse is serrated at 61, 62, 63, 64, 65 and 66 so that the horizontal deflection circuit is provided with information allowing it to remain synchronized with Docket 6398 ~ ~ ~9 ~ ~

the incoming video signal. During the time period between fields, the scanning beam is returned to the top of the picture in preparation for scanning the next field. The vertical sync pulses are repeated at the field frequency of 60 per second and like the horizontal sync pulses, are of an amplitude which will leave the picture unaffected. Before and after the vertical sync pulse 6~, six equalization pulses are provided in the equalization pulse intervals.
The equalization pulses and serration pulses 61-66 appear at twice the horizontal line rate.
A comparison of Figs. 2A and 2B indicates that diffrences exist in signal timing between even and odd fields. Specifically~ when compared with the timing of each vertical sync pulse, the hori20ntal sync pulses used during an odd field are out of phase with the horiæontal pulses used during an even field by an amount equal to one-half of a line scan. Since the initial line scan of an even numbered field is only one-halÇ of a line and sinGe, as seen in Fig. 1, the even and odd fields start at the same vertical position, this half line phase difference will cause each of the even field lines to be positioned exactly midway between two adjacent odd field scan lines.
Referring now to Fig~ 3~ there is shown the preferred embodiment of the invention. Disc recording means 70 includes a magnetic disc 71 and a magnetic pick-up and record head 75. Record head 75 may be positioned by a device such as that disclosed in U~S. Patent No. 3,814,441, issued June 4, 1974, to Craggs. The recording disc may be mountPd as shown in U.S. Patent No. 3,840,897, issued October 8, 1974, to Kelley et al or UOS. Patent No.
3,737,880, issued June 5, 1973~ to Kelley. Magnetic disc 71 is rotated with respect to pick-up head 75 ,~, ...

Docket 6398 ~ ~ 2~

by a means for rotating the magnetic disc at the frame rate of 30 rps. Such means may include the disc drive devices disclosed in U.S. Patent No.
3,814~844, issued June 4, 1974, to Waldspurger et al and U.S. Patent No. 3,583,0g0, issued May 13, 1975, to Hall. A source of video siynals 78 supplies a video signal, consisting of a succession of video frames, to record electronics 80 which are of standard design. Switches 85 and 86 comprise a switching means which is initially set with both switches in their A positions. Although illustrated for the sake of simplicity as mechanical switches, switches 85 and 86 will be implemented by means of semiconductor switching arrangements or equivalent high speed switching devices.
When the preerred embodiment of the present invention is operating/ a rame comprised of two interlocked fields, will be recorded on disc 71. When it is desired to record and display a single frame, comprised of two identical fields, from the video signal, the timing logic 88 is actuated and causes switches 85 and 86 to switch into their respective B positions. One of the two interlocked fields recorded on disc 71 is then reproduced by pick-up head 75 and standard reproduce electronics 90. The reproduced field signal is supplied via line 91 to a delay or buffer memory means 95 which stores a portion o~ the signal.
Memory 95 may be in the form Qf a charge transport device æuch as a charge coupled device, a buck~t brigade~ charge injection device~ or a MOS diode array. Alternatively, the memory may be digital with an analog-to-digital converter at the input and a digital-to analog converter at the output.
After a portion of the field has been stored in the buff~r memory 95, switch 86 is _g_ Docket 6398 ~ 14 returned to its A position and the stored field information is then reapplied to record electronics 80 via line 98. The stored portion of the 'ield is applied through switch 86 to the record head 75 and recorded by disc recorder 70. A portion of the field signal is then rerecorded during each of a plurality of successive rotations oE the disc recorder until the entire field signal has been rerecorded. The timing of this rerecording is such that the rerecorded field is recorded directly over the second of the two interlaced fields previously recorded. The recording in FM erases the previously recorded signals. Thus the same field signal will be recorded on disc 71 as both the first and the second field of a frame. Switch 86 may then be switched into its B position and the recorded frame supplied to monitor 100 where the frame may be displayed. Since the first and sacond fields of the ~rame are identical, no motion jitter will occur.
The storage capacity of the memory determines the speed of the rerecording operation.
If the memory has a capacity of N lines, N lines may be rerecorded during each rotation of the disc.
Since there are, at most, 245 active video lines per field, the number of di~c rotations required to rerecord a Eield would be 245 divided by N (rounded off to the next larger integer).
As mentioned previously, rerecording of a single fi~ld signal takes place directly over a previously recorded field signal to time precisely the rerecording process. The timing logic B8 and sync stripper 105 may monitor the horizontal sync of the field to be replaced. Further these sync signals of the origional video signal may be left unmodified on disc 71 with only the active line signal of the field to be rerecorded being stored in --10~
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Docket 6398 ~ ~2~ ~ ~

memory. The logic will thus cause the stored lines to be rerecorded onto the disc, replacing one for one the lines complementary to this field.
The choice of fields for rerecording may be made on either a first-come-first-serve basis or may be previously determined. The timing logic may rerecord only even or odd numbered fields into complementary positions. Operating in a mode where the field to be rerecorded is selected arbitrarily may require that the playback rerecord sequence be controlled by one of two separate control circuits after it is determined whether an even numbered field or an odd numbered field is to be rerecorded.
As previously mentioned, there is a one-half horizontal line displacement between odd and even fields and this may serve as a method of detecting which of the two fields has been chosen for rerecording~
If it is desired to increase the speed of operation, the storage capacity of buffer memory 95 may be ;ncreased so that an entire field signal is stored simultaneously. Such change would of course materially increase the cost of the recording apparatus. Wi~h such a memory, however, the apparatus of Fig. 3 could also function as a ~ield recorder having an expanded capacity. This would be accomplished by only recording alternate fields on recording disc 71. Since rotation is at the frame rate, two non related fields would be stored on each circular track on the disc~ When one of the recorded ~ields is ~o be viewed on monitor 100, transducer 75 is moved ~o the appropriate track and switch 86 is switched into its B position. The desired field signal is then supplied to monitor 100 via reproduce electronics 90. This same field signal is simultaneously fed via line 91 to buffer ' Docket 6398 ~ 14 memory 95. The output o~ buffer memory 9S may t'nen be applied by a switching arrangement (not shown) to monitor 100. The second application of the field signal is timed so as to create the necessary conditions for interlace. Thus by storing only alternate fields on the disc recorder, twice as many frames would be available for reproduction with acceptable definition.
To prevent moire effects that may occur as one field is rerecorded over another field, the disc 71 may be rotated initially with one field being erased prior to initiating of the rerecording process. The erasure may extend into the horizontal sync pulses so that the switching transients which lS occur as the FM modulator is turned on and off will be removed during sync processing. The sync must not be completely erased.
Reference is now made to Fig. 4 in which a schematic representation of the circuit of the preferred embodiment of the invention is shown. The disc recorder 70 includes a single transducer head 75 which cooperates to record on magnetic disc 71 in the manner previously discussed. Transducer head 75 is used for recording and playback on the disc. ~n FM modulated video signal to be recorded is supplied to input A on line 110 which provides an input for standard record driver electronics 117. The siynal on line 110 will be continuously recorded on the disc 71 by transducer head 75. Playback circuit 119 controls the playback of the video signals which have been recorded on disc 71, Playback circuit includes an FM demodulator and amplifiers and is gated off by a signal on line 121.
When it is desired to rerecord a video field on the disc 71 to produce a video frame having identical fields~ the signal on input A is removed , " , ...
, -Docket 6398 ~ 9 ~ 4 and a record pulse applied by appropriate circuitry to line 123. This resets counter 125 and sets flip-flop 127, enabling circuit operation in a manner to be described below~ ~he playback circuit 113 wlll also be enabled such that the frame which has been recorded will be played back by electronics 119 and ~upplied to line 123.
Sync stripper 131 separates the vertical, horizontal and composite sync signals from the video signals and applies them to lines 133, 135, 136, respectively. The vertical ~ync pulses on lines 133 will reset counter 137, divide~by-~ counter 139, and divide-by-12 divider 141. The horiziontal sync pulses on line 135 are supplied to gate Gl via divide-by-N counter 139 and via divide-by-12 divider 141. The number N is set to equal the number of video lines which are to be rerecorded during each revolution of the recorder. Divider 141 supplies a high signal on its output after receiving 12 input pulses. The horizontal sync pulses are also supplied to inverter 145 which disables AND gates G2 and G3 upon receipt oE each horizontal sync pulse.
Phase locked loop oscillator 147 with feedback divider 149 provides a pulse train at the rate of Q times the horizontal sync requency to gata G2. Gate G2, when e~abled, supplies this high frequency output to a buffex memory 151 and counter 152. Bufer memory 151 may be a charge transport memory or, alternatively, a digital memory with an analog-to-digital converter at its input and a digital-to-analog converter at its output. The ~: pulse train output from gate G~ controls the rate at which the video signal information is shifted through the buffer memory 151~ A conventional ~M
modulatox 153 receives the output from buff~r memory ,, . i ~

Docket 6398 ~ 4 151 and supplies it to gate ~4. Counter 152 counts the pulse train and supplies an output signal on line 154 after Q pulses have been counted and the active portion of the video line reproduced. The output from inverter 145 resets counter 152 prior to each video line.
The sync stripper 131 provides a composite sync signal on line 136 which includes both the vertical and horizontal sync information as well as the equalization pulses and the serration pulses which ocaur during the vertical pulse interval.
This composite signal is applied to odd/eYen vertical interval discriminator 157 which also receives the vertical sync signal on line 159.
Discriminator 157 provides output pulses on lines 159 and 161 which indicatP when the odd and even fields of the recorded material are being reproduced. Output 159 will remain high for the duration of each odd field and go low for the duration of each even field. Similarly, output 161 will go high during each even field and low during each odd field. Switch 163 is provided to permit the operator to choose whether the odd field or the even field on the disc 71 is to be replaced with the opposite field information.
The output of the odd~even vertical interval discriminator 157 is supplied on line 165 to gates G4 and G5 and, on line 167, to the count input of binary counter 125. The parallel 3~ outputs of counters 125 and 137 are provided to digital comparator 169. When comparator 169 senses that the count in the two counters is equal, it provides an output signal on line 171. The count in counter 125, additionally, is sensed by decoder 173 which provides a reset pulse to flip-flop 127 when counter 125 reaches a count state equalling 242 divided by N.

Docket 6398 ~29~4 The operation of the circuit of Fig. 4 is as follows. Initially, when an operator determines that a frame is to be recorded, a single frame signal will be applied to input A and, via line 110, to the disc recorder 70. Since playback circuit 119 will not at this time be enabled, it will provide no signal to line 129. The disc recorder 70 will now have recorded on a single track a frame signal consisting of two interlaced field signals. If the operator should determine that it is desirable to compose a video frame consisting of two interlaced, identical fields, the operator will actuate appropriate circuitry which will apply a pulse to line 123, thus resetting counter 125, while setting flip-flop 127. The Q output of flip-flop 127 will then go high. Playback circuit 119 will also receive a signal on its enabling input 175. Such an enabling signal may conveniently be obtained from the Q output of flip-flop 127~ A single video frame consisting of two interlaced fields of non-identical video information will have been recorded on the disc 71. Switch 163 will have been set to either the sdd or even switch position, thus determining which of the two fields is to be replaced.
Assuming that an even field is then reproduced, the signal on line 165 will ~e low and playb~ck circuit 119 will not be inhibited by AND
gate GS. Circuit 119 will produce video signals : stored by th~ recorder and apply this signal to line 129. Playback circuit 119 includes the appropriat~
standard FM demodulator and amplifiers. The video signal which is played back will be applied by line 129 to sync stripper 131 and also to buffer memory 151. The line termination marked B at the input of sync stripper 131 is connected to the line marked B
on the input of buffer memory 151 The buffer Docket 6398 memory 151 will begin to store the video signal.
Sync stripper 131 will separate the vertical, horizontal, and composite sync pulses from the video sign~l and apply them to lines 133, 135, and 136, respectively. When the first vertical interval is reached, a vertical sync pulse will be applied to line 133. The trailing edge of the vertical sync pulse will reset counter 137, divide-by-N counter 139, and divide-by-12 counter 141. After the vertical blanking interval, the horizontal sync pulses applied to line 135 will be counted by divider 141 and counter 139. After divider 141 has counted 1~ horiiontal sync pulses, and the divide-by-N counter 139 has counted N
horizontal sync pulses, gate Gl will be enabled.
Counter 137 will have been initially reset to a zero count. Counter 125 will have also been reset but will have received a pulse on line 167 ~uch that it will be at a "one" count. Only the first set of N active lines of video will be clocked into the buffer memory 151, thereEore. When the 8 bit counter 137 receives a second pulse from Gl, line 171 from comparator 169 will be switched low and gate~ G2 and G3 disabled. Thus only the active lines o video information will be stored in the buf~er memory and the first tw~lve inactive lines ignored.
Asæuming that the field from which these video horiziontal lines were taken was an even field, gate G4 will be enabled by the odd/even vertical interval discriminator 157 during the next vertical interval, just prior to the succeeding odd field. Counter 137 will be reset by the vertical sync signal to a zero count and thus digital comparator 169 will provide a high output signal which will enabl~ AND gates G2 and G3 only after ~16-, . . .

Docket 6398 the recorder has reproduced the desired number of horizontal lines and the counts in counter 137 and 125 are again equal. Inverter 145 insures that only active portions of the video ~ignal are stored in memory 15 and rerecorded onto the disc. The stored lines of video information will then be gated through gate G4 and recorded onto the disc recorder. The high signal on line 165 will be transmitted to inhibit playback circuit 119 during the rerecording process except when inhibited by counter 152. Counter 152 will provide an output on line 154 after each active line of video information; the previously recorded sync signals will therefore be applied to the line 129, The counter 137 will thereafter increase its count by one, disabling comparator 171 and gates G3 and G2 .
During the next revolution counter 125 will have been indexed such that its count is increased by one. Counters 137 and 125 will be equal in count twice during this revolution but during the next successive group of N horizontal video lines. Thus counter 125 will count the revolutions oE the disc recorder and counter 137 and digital comparator 169 will insure that during each revolution a group of N
horizontal lines is stored in the buffer memory 151 and then rerecorded onto the disc.
The decoder 173 is responsive to the count in counter 125 to provide a reset pulse to flip-flop lZ7 when counter 125 reaches a count equal to 242 ~the number of active video lines) divided by N, rounded to the next larger integer. This will, by definition, occur after all the groups of N
horizon~al lines have been properly rerecorded onto the disc at the appropriate locations.

Docket 6398 Referring now to Fig. 5, the odd/even vertical interval discriminator 157 is shown in greater detail. Figs. 6 and 7 are pulse diagrams useful in explaining the manner in which this circuit operates. Vertical sync signals are applied to line 159 while the composite sync signals are applied to line 136. Monostable multivibrator 179 is triggered by the leading edge of the vertical sync pulse. The multivibrator is non~retriggerable provides provides a pulse on its output 181 which is equal in duration to 75~ of the horizontal line period. The output of monostable multivibrator 179 is represented in FigsO 6 and 7 as pulses 183.
Monostable multivibrator 185 receives the composite sync signal on line 136 and provides on its output a series of pulses in phase with the horizontal sync signal and at the horizontal sync frequency but having a duration of 75~ of the horizontal line period. These pulses are represented in dashed lines in Figs. 6 and 7 as pulses 187. The solid line pulses 189 illustrate the relative width oE the horizontal sync pulses.
The composite sync signals 191 are shown in Figs. 6 and 7. It should be noticed that the vertical sync portion of the composite signal is broken by a plurality of serration pulses 193.
Fig. 6 indicates the relationship of the three pulse trains during the vertical interval just prior to an even field while Fig. 7 shows such a time relationship during the vertical interval just prior to an odd field~ A careful comparison of the relationship among the pulse trains illustrated in Figs. 6 and 7 will reveal that pulses 183', 193', and 187' in Fig. 6 will all be high simultaneously.
In Figs. 7, however, pulse 193' will not be coincident with any of the pulses 187.

~ocket 6398 If the three pulse trains are applied to an AND gate, such as AND gate 195 in Fig. 5, the AND
gate will supply pulse output at the beginning of each even field. The gate will, however, not provide such an output at the beginning of each odd field. The output of gate 195 is supplied to a flip Elop 1~7 which changes state in dependence upon the signal applied to its D input at the time it receives a cloclc input. AND gate 199 combines the vertical sync pulse 183 with the composite sync signal 191 such that it provides a high output at the first serration pulse of each vertical interval. IE an even field is about to occur, AND
gate 195 will also provide an output, thus causing ~lip-flop 197 to provide a high signal on its Q
output. If, on the other hand, an odd field is about to occur, the flip-flop 197 will be clocked as AND gate 195 provides a low output. Flip-flop 1~7 therefore provide a high output on its Q output.
Thus flip-flip ]97 will provide complementary pulse trains in phase with the alternation between Eields by the video disc recorder.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

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Claims (3)

WHAT IS CLAIMED IS:

1. A stop action recorder for recording a frame of video information consisting of interlaced video fields of first and second field type and further including horizontal and vertical sync singals and for providing a video frame consisting of two interlaced fields to a monitor at a frame rate without motion jitter, comprising:
video source means for providing a signal to be recorded, disc means, including a magnetic disc and a magnetic pick-up and record head adjacent said magnetic disc, for recording a video signal, means for rotating said disc at the frame rate of one revolution per frame, memory means for storing a video signal applied thereto, switch means for applying a video signal to said disc means to be recorded on said magnetic disc, means, connected to said head, for reproducing the video signals recorded on said magnetic disc and for applying the reproduced signal to said memory means, means, connected to said memory means, for reapplying said reproduced signal to said switch means for re-recording said signal on said disc, and control means, including a field discriminator providing a field type output indicating the type of field being reproduced, for selectively enabling said memory means and said switch means such that a portion of a field of one of said field types is stored in said memory means during playback and reapplied through said switch means as a corresponding portion of a field of the other of said field types.

2. The recorder of claim 1 in which said control means comprises first counter means incremented once during each rotation of said disc means, second counter means responsive to the hori-zontal sync signals in said reproduced video signal, and comparator means for comparing the count in said first counter means with the count in said second counter means and for applying an enabling signal to said switch means, whereby successive portions of said reproduced video signal are applied to said memory means during successive rotations of said disc means such that a plurality of segments of said reproduced video signal are recorded onto said disc means during a plurality of rotations of said disc means.

3. The stop action recorder of claim 1 in which said discriminator comprises means for discriminating between fields of said first field type and fields of said second field type in dependence on the timing between horizontal sync pulses and vertical sync pulses at the beginning of each vertical sync interval.