Video tape recorder speed control

Abstract: Video tape recorder speed control apparatus for controlling a video tape recorder (1) to cause it to reproduce a recorded video signal at a predetermined speed, such as a fraction 1/n of the normal speed, the apparatus comprising the video tape recorder (1), first and second counters (25,26), the first and second counters (25, 26) being incremented by a signal locked in frequency to the field repetition frequency of the video signal at the normal playback speed thereof, the first counter (25) resetting periodically at intervals determined by the predetermined speed, the second counter (26) being reset with a signal locked in frequency to the field repetition frequency of the video signal being reproduced by the video tape recorder (1), a comparator (27) for periodically comparing the counts of the first and second counters (25, 26) thereby to derive a phase error signal, and a control signal generator (28, 29, 30) to derive in dependence on the phase error signal a signal to control the reproduction speed of the video tape recorder (1).

Description: This invention relates to methods of and apparatus for video tape recorder speed control, and in particular to methods of and apparatus for controlling a video tape recorder to reproduce at less than the normal playback speed. Such methods and apparatus have application, but not exclusive application, in the conversion of video signals to photographic film images. Video tape recorders (VTRs) are available which can reproduce a recorded video signal at less than the normal playback speed. One example of such a machine is the Sony Corporation high definition (HD) digital VTR type HDD 1000. However, this digital VTR, in common with other such machines, has the limitation that reproduction at less than the normal playback speed is possible only at a number of preset fractions of the normal playback speed. This is inconvenient where the need arises for constant and stable reproduction at a speed different from the preset speeds available. According to the present invention there is provided a method of controlling the reproduction speed of a video tape recorder to cause it to reproduce a recorded video signal at a predetermined speed, the method comprising: incrementing first and second counters with a signal locked in frequency to the field frequency of said video signal at the normal playback speed of said video signal; periodically resetting said first counter at intervals dependent on said predetermined speed; periodically resetting said second counter with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder; comparing the counts of said first and second counters, thereby to derive a phase error signal; and controlling the tape speed of said video tape recorder in dependence on said phase error signal. According to the present invention there is also provided video tape recorder speed control apparatus for controlling the reproduction speed of a video tape recorder to cause it to reproduce a recorded video signal at a predetermined speed, the apparatus comprising: a video tape recorder; first and second counters; means to increment said first and second counters with a signal locked in frequency to the field frequency of said video signal at the normal playback speed of said video signal; means to reset said first counter periodically at intervals dependent on said predetermined speed; means to reset said second counter with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder; a comparator for periodically comparing the counts of said first and second counters, thereby to derive a phase error signal: and means to control the tape speed of said video tape recorder in dependence on said phase error signal. According to the present invention there is also provided video tape recorder speed control apparatus for controlling the reproduction speed of a video tape recorder to cause it to reproduce a recorded video signal at a predetermined speed, the apparatus comprising: a video tape recorder; counter means; means to increment said counter means with a signal locked in frequency to the required line frequency of said video signal; means to reset said counter means with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder; means to derive a phase error signal in dependence on said field reproduction frequency signal and the output of said counter means; and means to control the tape speed of said video tape recorder in dependence on said phase error signal. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figures 1 and 2 are block diagrams of respective parts of a conversion apparatus for video signal to photographic film conversion which incorporates an embodiment of the present invention; Figure 3 shown time charts for describing speed control of a VTR; Figure 4 is a block diagram of a first embodiment of VTR speed control apparatus according to the present invention; Figure 5 is a block diagram showing part of the first embodiment in more detail; and Figure 6 is a block diagram of a second embodiment of VTR speed control apparatus according to the present invention. The three embodiments of VTR speed control apparatus to be described are used as part of apparatus for video signal to photographic film conversion, but it is to be understood that the invention is not limited to such applications, and that embodiments of the invention can be used in a large range of equipments where it is required to control a VTR to reproduce at a constant and stable speed, generally less than the normal playback speed, for example, for supply to video signal processing apparatus at less than real time speed. The conversion apparatus for video signal to photographic film conversion to be described is particularly intended for use in the conversion of a high definition video signal (HDVS) having 1125 lines per frame, 60 fields per second, to 24 frames per second 35mm film. However, it will be understood that the conversion apparatus can readily be adapted to effect conversion from other input video signals. The conversion apparatus can conveniently be considered in two parts; the first part, shown in Figure 1, effects the conversion of the input HDVS to a progressive scan digital video signal corresponding to 24 frames per second which is recorded on a VTR; and the second part, shown in Figure 2, reproduces the recorded video signal and transfers it to photographic film. The part of the apparatus shown in Figure 1 comprises a high definition digital VTR 1, a television standards converter 2, a frame recorder 3, a second high definition digital VTR 4, a system controller 5 having associated with it a tracker ball control 6, a keyboard 7 and a graphics display 8, and television monitors 9, 10 and 11, interconnected as shown, and operating as will be described below. The digital VTRs 1 and 4 can be Sony Corporation digital VTRs type HDD 1000, and the frame recorder 3 can be a Sony Corporation frame recorder type HDDF 500. The system controller 5 can be an IBM compatible personal computer, and the standards converter 2 is of non-standard form so is described in detail below. The other elements of the apparatus are standard items, and it is noted that the television monitor 10 can conveniently be a black and white monitor. The second part of the apparatus, shown in Figure 2, comprises a high definition digital VTR 15, a digital interface (I/F) unit 16, a gamma corrector 17, a digital-to-analogue converter 18, an electron beam recorder 19, a television monitor 20 and a switch 21, interconnected as shown, and operating as will be described below. Referring again to Figure 1, the video signal connections marked D are digital connections, that is carrying Y, U/V signals including video sync signals; the video signal connections marked A are analogue connections carrying Y, U, V signals; the connections marked S are serial connections (RS422); the video signal connection marked Y is an analogue connection carrying the Y signal only; and the connection marked REF carries the reference video sync signals (a complete set excluding only picture information) derived from the digital VTR 1 and having a field frequency of 60 Hz. In other embodiments, the digital VTR1 may have a different field frequency, in which case the reference video sync signal REF has that frequency. Alternatively, the whole apparatus may be locked to an external sync generator. The input video signal which is to be transferred to film, and which may have been derived from a high definition video camera, is recorded on a magnetic tape reproduced by the digital VTR 1. The digital VTR 1 is controlled as will be described below to reproduce the recorded video signal at 1/8 of the normal playback speed, as this is a convenient speed of operation for the subsequence circuitry, and in particular the standards converter 2. The elements 1 to 4, 9, 10 and 11 are under control of the system controller 5, the system controller 5 being in turn controllable by inputs from the tracker ball control 6 and the keyboard 7, and having associated with it the graphics display 8 on which is displayed information relating to the progress of the conversion. A portion of the input HDVS is reproduced from the digital VTR 1 and supplied to the standards converter 2. This operates, as described in detail below, to derive from the input video signal, which is a 60 fields per second interlace scanned video signal, firstly, a motion adapted progressive scan digital video signal at 60 frames per second, and then from this a motion compensated progressive scan digital video signal corresponding to 24 frames per second, but not necessarily at that rate. This video signal is recorded by the frame recorder 3, for subsequent recording by the digital VTR 4. Thus the operation is intermittent. Such intermittent operation is generally required for video signal to film conversion, because of the need to check at frequent intervals that the conversion is proceeding satisfactorily. Thus depending on the content of the video signal to be converted, adjustment of the parameters, in particular those of the standards converter 2, may need to be made, and the results evaluated before proceeding. The monitors 9 to 11 are provided as further means for checking the video signal at respective points in the conversion apparatus. In the second part of the conversion apparatus, shown in Figure 2, the motion compensated progressive scan digital video signal recorded by the digital VTR 4 (Figure 1) is reproduced by the digital VTR 15 and passed by way of the digital I/F unit 16 to the gamma corrector 17, the purpose of which is to match the gamma characteristics of the video signal to the gamma characteristics of the film being used. The separated operation permitted by the recording of the motion compensated progressive scan digital video signal by the digital VTR 4 (Figure 1), for subsequent reproduction by the digital VTR 15, enables the gamma correction to be set accurately by the gamma corrector 17. The gamma corrected digital video signal is then converted to an analogue signal by the digital-to-analogue converter 18 and supplied to the electron beam recorder 19 to be recorded on photographic film. This recording may, for example, be in the form of three monochrome frames for each frame of the video signal, the three frames corresponding respectively to red, green and blue. The further television monitor 20 can be selectively connected by way of the switch 21 to the output of the digital VTR 15 or to the output of the digital-to-analogue converter 18, or alternatively of course two separate television monitors can be provided. The characteristics of the conversion apparatus are such that it produces sharp, clear pictures with good motion portrayal on the film, and in particular it produces pictures without motion blur and without introducing any additional judder components. The separated operation permitted by the recording of the motion compensated progressive scan digital video signal on the digital VTR 4 (Figure 1) permits easy and frequent checking of the parameters of the conversion apparatus, to ensure the quality of the pictures obtained on the film. Iterative operation is perfectly possible, so that the results can rapidly be evaluated and conversion repeated with any flaws corrected by adjustment of the parameters. To obtain higher speed operation, it is of course possible for the first part of the conversion apparatus, that is the part shown in Figure 1 to be replicated a number of times, to provide additional inputs to the digital VTR 15, so permitting a more intensive use of the part of the apparatus shown in Figure 2, and hence a higher overall conversion speed. Referring to Figure 1, the first embodiment of VTR speed control apparatus which controls the digital VTR 1 to reproduce at 1/8 of the normal playback speed, and which is comprised in the suitably programmed system controller 5, or alternatively may be embodied in other hardware, will now be described. Basically the speed control apparatus operates by relaxing the speed constraint of the digital VTR 1. This is done by putting it into a closed loop control system, and sampling the video output within a given window. The speed control apparatus also synchronizes the elements downstream from the digital VTR 1. Thus the system controller 5 monitors and adjusts the playback speed of the digital VTR 1 to be 1/8 of the normal playback speed, via the RS422 link, so that the digital VTR 1 and the standards converter 2 are locked, and the system is locked to the reference video sync signal REF. To implement the slow motion, the digital VTR 1 repeatedly reproduces fields (or in an alternative embodiment it could be frames) of the recorded video signal, such that the output still conforms to the video transmission standard, except that the average number of new frames per second is 1/8 of the normal playback number. One of the fields in each group of repeated fields is then grabbed by the television standards converter 2. Since all the fields in each group of repeated fields are identical, it is permissible to grab any one of them. Figure 3(A) shows the output of the digital VTR 1 (Figure 1) at 1/8 of the normal playback speed, these being sixteen fields corresponding to each frame of the recorded source material. O n represents the video signal originating from the odd field of source material frame n, and E n represents the video signal originating from the even field of source material frame n. If the digital VTR 1 (Figure 1) plays slightly too fast, occasionally there will be less than sixteen fields per frame. Similarly, if it plays slightly too slowly, occasionally there will be more than sixteen fields per frame. Figure 3(B) shows the ideal sampling positions used to grab the required video signal, these positions being at the third and eleventh fields in the frame. Figure 3(C) shows the output value of a reference counter referred to below in connection with Figure 4. Figure 3(D) shows the corresponding source material frame numbers, and Figure 3(E) shows the corresponding times in milliseconds. Reference will now be made to Figures 4 and 5. Figure 4 shows the first embodiment of speed control apparatus for the digital VTR 1 (Figure 1) is block form, and Figure 5 relates the speed control apparatus functionally to elements in the part of the conversion apparatus shown in Figure 1. The speed control apparatus comprises counters 25 and 26, a subtracter 27, an amplifier 28, an adder 29, a base speed control 30 and a timecode reader 31 interconnected as shown, and operating as will now be described. The counter 25 is a system reference counter which increments modulo-16 at field rate intervals (Figures 3(C)) under control of the reference video sync signal REF, which is also supplied to the counter 26 which is a VTR phase counter. The system reference counter 25 substitutes as an ideal VTR running at precisely 1/8 of the normal playback speed. When the system reference counter 25 is between counts of 0 and 7, the ideal VTR would supply odd fields, and when it is between counts of 8 and 15, the ideal VTR would supply even fields. Thus the count value represents the phase of the video signal with respect to the cycle of repeating fields. The standards converter 2 receives a pulse reference signal from the system reference counter 25 such that it is synchronized with the system reference counter 25 and it grabs the odd and even fields when the count thereof is 3 and 11 respectively (Figure 2(B)). Feedback is provided to lock the digital VTR 1 to the system reference counter 25. This is done by synthesizing the phase of the digital VTR 1 by the VTR phase counter 26 which increments at the field rate in response to the reference video sync signal REF, and is reset to zero in response to the video timecode which changes frame by frame, and which is read by the timecode reader 31. The subtracter 27 thus receives a count signal from the system reference counter 25 representing the ideal phase, and a count signal from the VTR phase counter 26 representing the actual phase of the digital VTR 1. By subtracting one count from the other, the subtracter 27 derives a phase error signal which after amplification by the amplifier 28 which sets the loop gain, forms a VTR speed adjustment signal. The subtraction is done modulo -8, so that the phase error is kept is the range -7 to +8 fields. Thus, for example, an error of 15 fields is converted to -1 field. The VTR speed adjustment signal is supplied to the adder 29 where it is added to a signal from the base speed control 30 to derive a new VTR speed signal for supply to the transport system of the digital VTR 1. Since both of the counters 25 and 26 count with the same clock (the reference video sync signal REF), the phase error can only change when they reset, and therefore the new VTR speed signal need only be sent at these times. So long as the phase error between the system reference counter 25 and the digital VTR 1 is maintained at less than four fields, the standards converter 2 will always sample or grab the correct field, as if the digital VTR 1 were running at a true 1/8 of the normal playback speed. If the timecode supplied from the digital VTR 1 to the timecode reader 31 is read from a longitudinal track on the tape, then there may be errors in reading the timecode from the tape at low speeds of the tape relative to the timecode reading head. Such difficulties do not arise when the timecode is recorded with the video signal in oblique tracks on the tape. Referring now to Figure 6, the second embodiment of VTR speed control apparatus overcomes such problems associated with errors in reading the timecode. This second embodiment differs from the first embodiment shown in Figure 4, in that it additionally includes a filter 35, a base speed adjuster 36 and a limiter 37, connected as shown. The filter 35 is a 2-tap low-pass filter which counteracts anomalies in the timecode. Whilst the first embodiment ensures that the phase error between the system reference counter 25 and the VTR phase counter 26 remains approximately constant, there is the problem that if this error is close to +8 or -7 fields then the lock becomes unstable, as these are the maximum possible representable phase errors. It would therefore be preferable to lock to a phase error of zero, and in the second embodiment the base speed adjuster 36 operates to adjust the base speed of the digital VTR 1 such that the phase error tends to zero. The base speed adjuster 36 monitors the speed of the digital VTR 1 by recording the count of the VTR phase counter 26 just before it is reset. The base speed of the digital VTR 1 is then only changed in the direction that given sixteen repeat fields per frame. This ensures that the control is stable, and also increases the capture range of the system. The limiter 37 is optional, but may be included to prevent excessive speed commands being sent, as may occur if there is a system failure. In other respects the form and operation of the second embodiment of VTR speed control apparatus are the same as these of the first embodiment. The third embodiment of VTR speed control apparatus comprises a counter which counts line sync pulses derived from the connection REF from the VTR 1 (Figure 1), and is periodically reset by a control track pulse which is derived from a longitudinal track on the tape being reproduced and has the field repetition rate. The control track pulses are also supplied to a VTR phase error store in which the phase error is accumulated, and which supplies an output to control the VTR speed. As in the second embodiment, the speed control output may be supplied by way of a limiter to prevent excessive speed commands being sent. Feedback is provided by supplying the outputs of the counter and of the VTR phase error store to an adder, the output of which is supplied to one input of a subtracter, to the other input of which is supplied a signal representing the expected inter-field phase change. The output of the subtracter is supplied to another input of the VTR phase error store. Various modifications are of course possible. Thus the 1/8 speed is more generally 1/n of the normal playback speed where n is preferably an integer equal to two or more. Also, the invention is readily applicable to cases where frames are repeated instead of the fields which are repeated in the above-described embodiments. A detailed description of the structure and operation of the television standards converter 2 (Figure 1), which receives at its input the grabbed video signal, is not considered necessary for an understanding of the present invention. Such a description is, however, given in EP-A-0,474,398, which is the publication of the present application, and is incorporated herein by reference.

Claims: A method of controlling the reproduction speed of a video tape recorder (1) to cause it to reproduce a recorded video signal at a predetermined speed, the method comprising: incrementing first and second counters (25, 26) with a signal (REF) locked in frequency to the field frequency of said video signal at the normal playback speed of said video signal; periodically resetting said first counter (25) at intervals dependent on said predetermined speed; periodically resetting said second counter (26) with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder; comparing the counts of said first and second counters (25, 26), thereby to derive a phase error signal; and controlling the tape speed of said video tape recorder (1) in dependence on said phase error signal. A method according to claim 1 wherein said predetermined speed is 1/n of said normal playback speed, where n is an integer greater than or equal to two, and wherein: said first counter (25) is a modulo - 2n counter which resets to zero at every 2n th count; and said second counter (26) is reset at every 2n th field in said video signal being reproduced by said video tape recorder. A method according to claim 2 in wherein: said first counter (25) supplies a first output signal at a first predetermined count thereof in the range 0 to n-1, and supplies a second output signal at a second predetermined count thereof in the range n to 2n-1; said first and second output signals are supplied to an output device (2); and said output device (2) grabs a field of said video signal being reproduced by said video tape recorder (1), in response to each said first and second output signal. A method according to claim 3 wherein said output device (2) is a television standards converter (2). A method according to claim 3 wherein n is eight. A method according to claim 1 wherein said predetermined speed is 1/n of said normal playback speed, where n is an integer greater than or equal to two, and wherein: said first counter (25) is a modulo - n counter which resets to zero at every n th count; and said second counter (26) is reset at every n th frame in said video signal being reproduced by said video tape recorder (1). A method according to claim 6 wherein: said first counter (25) supplies a first output signal at a predetermined count thereof in the range 0 to n-1; said first output signal is supplied to an output device (2); and said output device (2) grabs a frame of said video signal being reproduced by said video tape recorder (1), in response to each said first output signal. A method according to claim 7 wherein said output device (2) is a television standards converter (2). A method according to claim 7 wherein said n is eight. Video tape recorder speed control apparatus for controlling the reproduction speed of a video tape recorder (1) to cause it to reproduce a recorded video signal at a predetermined speed, the apparatus comprising: a video tape recorder (1); first and second counters (25, 26); means (1) to increment said first and second counters (25, 26) with a signal (REF) locked in frequency to the field frequency of said video signal at the normal playback speed of said video signal; means (1) to reset said first counter periodically at intervals dependent on said predetermined speed; means (31) to reset said second counter with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder (1); a comparator (27 to 30) for periodically comparing the counts of said first and second counters, thereby to derive a phase error signal: and means to control the tape speed of said video tape recorder (1) in dependence on said phase error signal. Apparatus according to claim 10 wherein said comparator (27 to 30) compares the counts of said first and second counters (25, 26) each time said second counter (26) is reset. Apparatus according to claim 10 wherein said predetermined speed is 1/n of said normal playback speed, where n is an integer greater than or equal to two, and wherein: said first counter (25) is a modulo - 2n counter which resets to zero at every 2n th count; and said second counter (26) is reset at every 2n th field in said video signal being reproduced by said video tape recorder (1). Apparatus according to claim 12 wherein: said first counter (25) supplies a first output signal at a first predetermined count thereof in the range 0 to n-1, and supplies a second output signal at a second predetermined count thereof in the range n to 2n-1; said first and second output signals are supplied to an output device (2); and said output device (2) grabs a field of said video signal being reproduced by said video tape recorder (1), in response to each said first and second output signal. Apparatus according to claim 13 wherein said output device (2) is a television standards converter (2). Apparatus according to claim 13 wherein n is eight. Apparatus according to claim 10 wherein said predetermined speed is 1/n of said normal playback speed, where n is an integer greater than or equal to two, and wherein: said first counter (25) is a modulo - n counter which resets to zero at every n th count; and said second counter (26) is reset at every n th frame in said video signal being reproduced by said video tape recorder (1). Apparatus according to claim 16 wherein: said first counter (25) supplies a first output signal at a predetermined count thereof in the range 0 to n-1, said first output signal is supplied to an output device (2); and said output device (2) grabs a frame of said video signal being reproduced by said video tape recorder (1), in response to each said first output signal. Apparatus according to claim 17 wherein said output device (2) is a television standards converter (2). Apparatus according to claim 17 wherein n is eight. Apparatus according to claim 10 comprising a low-pass filter (35) through which said phase error signal is passed, and wherein a control signal to control the reproduction speed of said video tape recorder (1) is derived from the filtered said phase error signal. Apparatus according to claim 20 further comprising a limiter (37) to limit the maximum value of said control signal. Video tape recorder speed control apparatus for controlling the reproduction speed of a video tape recorder (1) to cause it to reproduce a recorded video signal at a predetermined speed, the apparatus comprising: a video tape recorder (1); counter means; means (1) to increment said counter means with a signal locked in frequency to the required line frequency of said video signal; means (1) to reset said counter means with a signal locked in frequency to the reproduction frequency of new fields of said recorded video signal as reproduced by said video tape recorder (1); means to derive a phase error signal in dependence on said field reproduction frequency signal and the output of said counter means; and means to control the tape speed of said video tape recorder (1) in dependence on said phase error signal.

Inventors: DORRICOTT MARTIN REX, RICHARDS JOHN WILLIAM, DORRICOTT, MARTIN REX, RICHARDS, JOHN WILLIAM
Applicants: SONY UK LTD, SONY UNITED KINGDOM LIMITED
Application year: 1991
Main classification: G11B15
IPC codes: H04N7, H04N5, G11B15