Synthetic Control Track Signal Producing Apparatus for Video Tape Machines

This invention relates to a method and apparatus for controlling the speed at which tape is transported in a video tape recording and/or reproducing machine, more particularly, to a method and apparatus for generating a synthetic control track signal from vertical synchronizing information included in a video signal reproduced from the tape for use in servo controlling the transport of the tape.

Magnetic video tape recording and/or reproducing machines generally operate by transporting magnetic recording tape in transducing relation to one or more rotating transducers or heads which scan the tape at a very high rate relative to the speed of transport of the tape. The heads record a plurality of generally parallel tracks on the tape which contain video information, and the same heads or other heads reproduce the recorded video information during playback by scanning the recorded tracks.

During recording, a control track head records control track signal information typically in a track extending longitudinally along one side of the tape, simultaneously as video information is recorded by a rotating video head scanning along the tape at locations spaced from the longitudinally extending control track. The control track signal information typically is a television frame rate square wave. For many helical wrap video recorders which record a field of television or video information along a single track during each revolution of the video head, the frame rate control track signal changes state once for each field of recorded video information. The control track signal information also often includes periodic color framing pulses for identifying specific fields in a multifield sequence forming the color television signal.

The longitudinal transport speed of the tape must be accurately controlled during playback. To achieve such control, many video tape recorders employ a motor driven prime mover, such as a capstan controlled by a capstan servo mechanism in accordance with a tachometer signal and with the timing relationship between a reference vertical sync signal and control track signal information reproduced from the tape during playback.

If for any reason a control track signal is improperly recorded relative to the recorded positions of the video tracks along the tape, the video head will be mispositioned relative to the recorded video track during playback operations, although the capstan servo will cause the tape to be transported at the proper playback speed. If the control track signal is not recorded on the tape, the capstan servo may not be able to move the tape at all, and even if it is able to do so, the tape transport will not be properly synchronized to the controlling reference. For example, control track information may be absent because of not being recorded on the tape during an insert edit or of, malfunctions and the like. Inaccurate control of the transport of tape also occurs if, the control track information is recorded with an improper inverted phase.

Recent developments in helical wrap video tape recording and reproducing machines have included what is commonly referred to as an automatic scan tracking system. In such machines, the reproducing head is mounted at the end of a movable element that is controllable to move the head transversely relative to the longitudinal direction of a recorded track so as to maintain the head on the track. This enables reproduction of a signal of superior quality, even when the tape is being transported at a speed other than normal reproducing speed, such as during operations for generating special motion effects of still frame reproducing, slow motion reproducing and fast motion reproducing. Such systems generate an error signal indicating whether the head is mistracking and the error signal is used in a head positioning servo for moving the movable element to return the head to the center of the recorded track. While the machine is reproducing recorded information, the error signal can be monitored to provide an indication of the tape speed deviation from normal reproduce speed as well as head to tape position deviation. Moreover, this error signal can be used to control the capstan servo to generally maintain proper tape speed and proper head to tape position. However, this error signal is not entirely reliable in providing control of the capstan servo because it does not rely on any well defined timing signals and is subject to mechanical and electrical variations which can affect the accuracy of the control of the capstan servo.

Accordingly, an object of this invention is to provide a new and improved method and apparatus for generating a synthetic control track signal for use in controlling the speed of tape transport in a video tape recording and/or reproducing machine whereby the adverse effects of improperly recorded or missing control track signals are obviated.

Another object is to provide a new and improved synthetic control track signal which is generated from vertical synchronizing signal information included in the video information reproduced from tape.

Yet another object is to provide a new and improved method and apparatus for generating a synthetic control track signal during video playback operations for use in synchronizing the transport of the tape to system reference while simultaneously recording a new control track signal on the tape.

Other objects of the present invention will become apparent from the following detailed description, in conjunction with the accompanying drawings in which:

FIG. 1A is a block diagram of apparatus for generating a synthetic control track signal in accordance with this invention;

FIG. 1B is a block diagram of a common capstan servo arranged to be operated in accordance with this invention.

FIGS. 2(a) through 2(f) are timing diagrams illustrating the relationship between normal and synthetic control track signal information, vertical synchronizing information, and the capstan servo ramp voltage;

FIG. 3 illustrates a detailed schematic of the control track error detector circuit shown in FIG. 1B; and

FIG. 4 is a block diagram of a video signal record and playback system and a control track signal record and playback system arranged so that normal control track record operations can occur selectively during video playback operations.

Broadly stated, the present invention is directed to a method and apparatus for generating a synthetic control track signal during the reproduction of recorded video information for use in a video tape recording and reproducing machine. The synthetic control track signal is utilized to control the capstan servo for moving the tape at the normal reproducing speed when the control track signal recorded on tape is defective or nonexistent so that the tape transport is maintained in synchronism with the system reference. It should be appreciated that when many helical wrap recording machines perform an insert edit operation, i.e., they record new material over a prior recording, the new material is added without recording a new control track signal . If an insert edit recording operation is performed on a portion of previously unrecorded or blank tape, there obviously will not be a control track signal recorded along the blank portion of the tape. If this occurs, then upon subsequent reproducing of the new material, the capstan servo will be unable to operate properly because of the absence of the control track signals in the portion of the tape corresponding to the recorded location of the new material.

In accordance with the present invention a synthetic control track signal is generated from recorded video information signals, which is applied to the capstan servo otherwise operated in a conventional manner. The synthetic control track generating circuitry embodying the present invention is also adapted to insert the color frame field identification pulses in the synthetically generated control track signal. In other words, a field one identification pulse is applied to the control track square wave signal. Another feature of the present invention is that the synthetic control track signal can be used to control the capstan servo to transport the tape at the proper speed during reproducing, and if a control track signal is missing from the control track on the tape, the control track record circuitry can be activated for the purpose of recording a control track signal at the missing location. Once the control track is recorded in the conventional manner, then the tape may be played in the normal fashion by any conventional recording and reproducing machine.

The synthetic control track signal generating circuitry utilizes the field one identification signal from a common color frame circuit included in video tape recording and/or reproducing machines, which in and of itself does not form a part of the present invention. The field one identification signal, together with playback vertical sync are used to generate the synthetic control track signal. It should be appreciated that the relationship of the conventionally recorded control track signal to vertical sync is such that each field rate transition of the control track signal is recorded on the tape approximately half way between the occurrences of successively reproduced vertical sync signals. In the circuitry of the present invention, the transitions of the synthetic control track are generated to occur essentially coincident with the occurrence of vertical sync, which means that they are offset relative to the occurrences of reproduced conventionally recorded control track transitions. However, the offset can be compensated for in the conventional capstan servo with only slight modification.

More particularly, it should be appreciated that the control track signals are recorded and reproduced by a fixed head and that the control track transitions are reproduced in response to the tape being moved relative to the control track head. It should also be appreciated that the speed of transport of the tape is relatively slow compared to the speed of rotation of the head that reproduces the recorded video information from which the synthetic control track signal is derived. The movement of the rotating video head the longitudinal direction of the tape as it reproduces video information is approximately 100 times the speed of movement of the tape past the control track head. In deriving the synthetic control track signal from the reproduced video information, it is necessary to compensate for this significant difference in relative head to tape speeds. In a conventional capstan servo, such as employed in the VPR-20 Video Production Recorder manufactured by Ampex Corporation, the control track error detector includes means for generating a voltage ramp by charging a capacitor and, at a particular time, the value of the ramp is sampled to obtain a control track determined tape speed related error voltage that is applied to the capstan servo for the purpose of controlling the capstan drive and, hence, tape speed. The ramp is reset by a reference frame sync related signal and the ramp is sampled by the occurrence of a reproduced control track transition. Because the movement of the rotating video reproduce head relative to the transported tape is 100 times faster than the movement of the tape past the control track head, it is necessary to increase the slope of the generated ramp by a corresponding factor of 100 without changing the rate of the generation of the ramp signals. When this is done, the relative timing of the sampling of the ramp in accordance with the present invention is slightly different from the timing of the sampling in the conventional capstan servo, but it is approximately the same by virtue of the fact that the synthetic control track signal is essentially synchronized to the reproduced off tape vertical sync extracted from the reproduced video signal. However, there is a small difference. For this reason, when the capstan servo is operated under the control of the synthetic control track signal, the reference vertical sync that is applied to reset the ramp is slightly advanced in time relative to the time of occurrence of the actual reference vertical sync normally used for such purpose.

Stated in other words, the synthetic control track signal has transitions coincident with vertical sync which means that the occurrence of a control track transition is approximately 180.degree. out of phase relative to a conventional control track transition. However, the ramp generator has a ramp slope that is 100 times steeper than the slope of the ramp signal used in a conventional capstan servo, which means that in the capstan servo of the present invention the ramp signal generated in the control track error detector must be sampled much sooner relative to a ramp signal in the conventional capstan servo. The reference vertical sync signal which resets the ramp when a synthetic control track is used is normally delayed in the conventional capstan servo by a full field in circuitry that does not in and of itself form a part of the present invention. By shortening the delay by a small amount, the delayed reference vertical sync will be advanced in time a small amount relative to that found in the conventional capstan servo. This has the effect of providing the final correction to the capstan servo so that the control track related error signal that the capstan servo uses to control the capstan speed is essentially identical, whether the synthetic control track or the conventional control track signal is used to sample the ramp signal.

Turning now to the drawings, the manner in which the synthetic control track signal is generated in accordance with the present invention is shown in FIG. 1A. Vertical sync is extracted from the reproduced video information by conventional circuitry (not shown) found in the aforementioned Ampex manufactured video tape recording and/or reproducing machine as the playback video head scans the various tracks, and is provided at an input 12 and coupled to the clock input of a flip-flop 16. A No. 1 field identification signal (of a four field sequence in a NTSC format color video information signal and of an eight field sequence in a PAL format color video information signal) also conventionally extracted from the reproduced video information signal is provided over line 20 to a delay one shot circuit 28. The delay circuit 28 provides an output signal to the "set" input terminal of the flip-flop 6 to synchronize the same to the occurrence of the field No. 1 identification signal. The time constant of the one shot circuit 28 is set to properly locate the leading edge of the field No. 1 identification signal relative to the proximate control track transition of the synthetic control track signal into which the

identification signal is inserted. When connected as shown, the flip-flop 16 is clocked by play back vertical sync and operates as a divide by two divider and provides a 30 Hertz (Hz) frame rate square wave signal at its output. The period of the square wave signal generated by the flip-flop 16 is therefore equal to that of a normal control track signal and, therefore, the generated synthetic control track signal is a replica of the normal control track signal. However, the 60 Hz rate transitions of the square wave signal are coincident with the 60 Hz vertical sync signal, whereas the transitions of conventional control track signals are offset relative to vertical sync, as is shown by FIGS. 2(a) and 2(c).

The square wave signal is employed to form a synthetic control track signal and, preferably, is combined with a short pulse to provide field identification information. The field identification pulse is added to the synthetic control track square wave signal synthesized by the flip-flop 16. The circuitry shown in FIG. 1A generates a narrow pulse in response to the receipt of the field No. 1 identification signal. It is delayed by the amount of the delay provided by the one shot 28. The delay circuit 28 times out and its Q-not output is coupled to trigger a monostable multivibrator, or one-shot 30. The one-shot 30 responsively produces a narrow pulse at the proper time for adding to the square wave signal as the desired field identification signal and has a controlling time constant that is set to effect the generation of a field No. 1 identification signal of common duration for insertion in the synthetic control track signal. For a NTSC format color video information signal system, the field No. 1 identification signal is inserted in the synthesized square wave signal to occur every two periods of the signal, since the identification signal is at a rate of 15 HZ (or 1/2 the frequency of the 30 HZ square wave signal). For a PAL system, the field No. 1 identification signal is at a rate of 1/8 the frequency of the synthesized square wave signal, hence, is inserted to occur every four periods of the synthesized signal. The output of the one-shot 30 is combined with the square wave of the flip-flop 16 in an NAND gate 18, and a synthetic control track signal having two components, one the narrow pulse or field identification signal and the other the square wave signal or frame related signal, is produced at an output 32.

As previously mentioned, the timing of the synthetic control track is different from that of the conventional control track signal, which signal requires some modification for use in the operation of a capstan servo. When the tape is driven at the proper speed under servo control relative to the conventional control track and capstan tachometer signals, the control track signal transitions occur between vertical sync pulses as shown in FIGS. 2(a) and 2(c). With reference to FIG. 1B, the capstan servo 41 included in the aforementioned Ampex manufactured video production recorder includes a control track error detector 43 that generates a control track related error signal in the form of a tape speed related voltage level that controls a following voltage controlled oscillator 45, which provides an output signal at a frequency that determines the speed of the capstan motor. This output signal is coupled to one input of a capstan tachometer error detector 47 and serves as a reference signal for comparison to the capstan tachometer signal. This error detector 47 provides a capstan servo error signal that is coupled to the capstan motor drive amplifier circuit 49 that responsively controls the drive provided to the capstan motor (not shown) to maintain the transport of the tape at the desired speed.

The control track related error signal is generated by sampling a ramp output of a linear ramp generator, with the sample being taken and held at a particular time, such as at the time of occurrence a control track transition. The ramp is reset by a delayed reference vertical sync signal. Referring to FIGS. 2(b) and 2(c), the servo is controlled by a conventional control track signal as shown. Samples are taken at the occurrences of control track transitions, which nominally occur at the midpoints of the occurrences of the voltage ramp signal when the tape is transported at the correct normal playback speed. Variations in the occurrences of the control track transitions relative to the generation of the ramp signal result in the ramp signal being sampled at points along the slope of the ramp signal which are either higher (tape speed too slow) or lower (tape speed too fast) than the midpoint of the ramp signal.

If a control track signal is generated using vertical sync pulses extracted from the video information signal reproduced by the rotating video playback head, the slope of the ramp must be increased because the head rotates, hence, scans along the tape much faster than the longitudinal speed of transport of the tape past the control track playback head In an interval corresponding to about the duration of 21/2 horizontal lines, or about 0.01 of the duration of a video field, the video head passes a longitudinal length of tape which corresponds to the distance the tape moves relative to the conventional stationary control track playback head during the reproduction of one video field. To compensate for the loss in sensitivity produced by the 100 times increase in the head to tape speed of the video head relative to the head to tape speed of the control track playback head, the slope of the ramp must be increased by the same factor to obtain the same resulting sensitivity.

When the synthetic control track signal is used to control the capstan servo, the slope of the ramp is sharply increased as shown by comparing FIGS. 2(b) with an exaggerated slope shown in FIG. 2(e). The slope is increased by switching switch 42 (FIG. 3) to a position opposite that shown, which effectively removes the larger capacitor 44 from the ramp generating circuit included in the control track error detector 43 (FIG. 1B) and indicated generally at 51. The ramp is reset by switch 52 responsive to the aforementioned delayed reference vertical sync. The delayed reference vertical sync is generated with an amount of delay which is changed depending upon whether regular or synthetic control track signals are used in the control track error detector 43. The delayed vertical sync is normally delayed by an interval corresponding approximately to the time of a full video field and, in the synthetic control track mode, it is delayed by a slightly lesser amount to form an advanced reference vertical signal, as shown by comparing FIGS. 2(a) with 2(d). The ramp is therefore reset slightly earlier by the advanced reference vertical signal than would otherwise occur, as seen by comparing FIGS. 2(b) and 2(c).

An important consideration is that the timing of the ramp signal sampling in the control track error detector 43 is controlled so that the desired nominal zero tape speed error voltage level is identical whether the machine is using actual or synthetic control track signals, as is evident from the sample point elevations shown in FIGS. 2(b) and 2(e). This permits the remainder of the capstan servo 41 to operate normally.

The details of a preferred ramp generating and sampling circuitry included in the control track error detector 43 is shown in FIG. 3. Either the normal control track or the synthetic control track signal is selected for coupling to an input line 38 by an operator controlled switch 53 shown in FIG. 1B. Similarly, the normally delayed or advanced reference vertical signal is selected for coupling to an input line 40 by another operator controlled switch 55 shown in FIG. 1B, the switches 53 and 55 being conjointly operated. During normal control track operation, the switch 42 is in the up position shown, and capacitors 44 and 46 are connected to an input of a buffer 48. The capacitance of the capacitor 44 is about 100 times greater than the capacitance of the capacitor 46, and both capacitors are charged by a current source 50, producing a ramp voltage at the output of the buffer 48. The delayed reference vertical sync signal, which is in the form of a pulse train, received over input line 40 resets the ramp by moving a switch 52 to the illustrated up position whereby the charges on the capacitors 44 and 46 are drained to ground. The switch 52 is activated by the delayed reference vertical sync applied on input line 40 through the switch 55 (FIG. 1B). When the operator initiates operation of the synthetic control track mode by operation of the switches 53 and 55 (FIG. 1B), an active signal is placed on line 92 by the simultaneous closure of switch 55a. This causes switches 42 and a switch 94 to be switched to respective positions opposite those that are shown.

The ramp voltage output of the buffer 48 is sampled by the control track sample pulses corresponding to control track transitions, which occur on input line 38 at the 60 Hz field rate and actuate a switch 54. The switch 54 is closed for a very short time responsive to a control track sample signal being applied thereto via line 38 and simultaneously the supply of current by source 50 is interrupted to maintain a constant level of charge on the capacitors 44 and 46 during the sampling interval. The sampled voltage is stored in a capacitor 56, which provides an input to a unity gain buffer 58. The output of the buffer 58 provides a control signal via amplifier 60 and line 82 to the voltage controlled oscillator 45 included in the capstan servo 41.

During normal control track operation, the switch 42 is in the position shown, and the capacitor 44 is in the circuit and can be charged. If the synthetic control track is to be used, the switch 42 is switched as described herein before to the down position alternate to that shown to isolate the capacitor 44 from the circuit. Since the capacitance of the capacitor 44 is about 100 times greater than that of the capacitor 46, the slope of the ramp will be about 100 times greater when the capacitor 44 is isolated as described .

The output of the buffer 58 is applied to an amplifier 60 which includes a feedback network having four diodes 62, 64, 66 and 68, connected as shown. The diodes 62, 64, 66 and 68 limit the dynamic range of the correction by limiting the maximum speed error signal which may be generated as a result of a possible track jump that can occur with a machine that has an automatic scan tracking system of the type previously described.

Existing video tape recording and/or reproducing machines are arranged to have their control track signal record and playback system and video signal record and playback system in the same record or playback mode, other than such machines arranged to perform confidence recording operations. In machines so arranged, the video signal record system and video signal playback system are simultaneously enabled so that as video information is being recorded by one video head a separate video head reproduces previously recorded video information. During such operation, however, the transport of the tape is not servo controlled with respect to recorded control track. The generation of the synthetic control track signal in accordance with the present invention now makes it possible to synchronously operate a video tape recording and/or reproducing machine so that normal video playback operations and control track record operations can synchronously occur relative to the system reference in the absence of a control track signal or the presence of an improperly recorded control track signal. As described hereinbefore, this is made possible because the synthetic control track signal permits the control of the transport of the tape synchronously with the system reference during normal video playback operations without need of control track signals reproduced from the tape. As explained hereinbefore, the synthetic control track signal is generated from the vertical sync signal extracted from the reproduced video information signal. Therefore, the capstan record/play back signal system and video record/playback signal system can be in opposite operating modes, including the control track signal system in a record mode while the video signal system is in a playback mode. This enables the control track signal system to be operated to record a new control track signal on a tape when the video signal system is reproducing previously recorded video information from the same tape.

The foregoing will be further appreciated upon consideration of FIG. 4, which illustrates a control track signal and video signal record and playback system 91 of a typical video tape recording and/or reproducing machine, such as the aforementioned Ampex manufactured video production recorder. As illustrated in FIG. 4, the control track signal playback system 93 has its control track signal input coupled to a control track playback head (not shown) and the control track signal record system 95 has its control track signal output coupled to a control track record head (not shown). In practice, a single head serves to record and playback control track signals with respect to the tape, that head being switched electronically to the proper part of the control track signal system in accordance with operating mode selected for the system. Similarly, the video signal record system 97 has its video signal output coupled to a video record head (not shown) and the video signal playback system 99 has its video signal input coupled to a video playback head (not shown). In practice, a single video head can be used for both recording and playback operations, with an electronically controlled switch coupling the video head to the proper part of the video signal system in accordance with the operating mode seleted for the system. The aforedescribed control track and video signal systems may be conventional systems found in existing video tape recording and/or playback machines. However, in accordance with the present invention, the operating modes of the control track signal system and video signal system are controlled relative to the operation of the synthetic control track signal generator so that the two systems can be in opposite operating modes, namely, the control track signal system in the record mode while the video signal system is in the playback mode. This is achieved by the generation of two operator initiated commands; one being the control track record command that is coupled to enable the control track record system 95, and the other an inverted form of the insert control track command. The two commands are coupled to separate inputs of an AND gate 101, for selectively disabling the video signal record system 97 while enabling the video signal playback system 99 when an insert control track command is generated coincidentally with the control track record command. More specifically, the AND gate 101 generates logical low level output which is coupled to the video signal record system 97 and the video playback system 99 in the absence of an insert control track command, signified by a logical low level signal at the input to the AND gate. This logical low level signal at the output of the AND gate 101 has no effect on the control of the operative mode of the video signal system. When an insert control track command is generated, however, a logical high level signal is coupled to the input of the AND gate 101 and, if a control track record command is also received at the input of the AND gate, the output of the AND gate is at a logical high level. The logical high level signal disables the video signal record system 97 while enabling the video signal playback system 99, whereby the video signal system can not be placed in a record mode but is able to be operated in the playback mode. The control track record command is also coupled to the control track signal record system 95 and through an inverter 103 to the control track signal playback system 93. The presence of a control track record command enables the control track record system 95 while simultaneously disabling the control track playback system 93. In this manner, new control track signals can be recorded on a tape through the normal operation of the control track signal record system 95 while video information is simultaneously reproduced from the tape through the normal operation of the video signal playback system 99.

By generating a synthetic control track in the manner described, the capstan and tape speed can be properly controlled during reproducing even when there is no recorded control track signal, or when the control track signal has been improperly recorded. Also, control track signals can be recorded so that the tape (or other record media) will have a properly recorded control track signal for future reproduction.

It should be understood that although preferred embodiments of the present invention have been illustrated and described, various modifications thereof will become apparent to those skilled in the art. In helical scan video tape recording and/or reproducing machines, it is possible to generate a synthetic control track signal from the vertical sync signal extracted from the reproduced video information signal because the extracted vertical sync signal is related in time to a properly recorded control track signal and the time bases of the information signal and control track signal are proportional to the speed of transport of the tape. Consequently, any control track signal or other comparable timing signal recorded on tape or other record media can be synthesized in accordance with the present invention from a known timing component included in an information signal if the recorded information signal and timing signal are related in time, and the time bases of the recorded information signal and timing signal are proportional to the relative timing signal head to media speed. Accordingly, the scope of the present invention should be defined only by the appended claims and equivalents thereof.

Various features of the invention are set forth in the following claims.