WO1995027285A1 - Method for recording frame accurate video signals on magnetic tape without frame accurate positioning - Google Patents

Abstract

A method for recording frame accurate video signals onto magnetic tape. The method includes preformatting the tape to allow frame accurate identification, and does not require frame accurate positioning.

Description

  
 



   METHOD FOR RECORDING FRAME ACCURATE VIDEO SIGNALS
 ON MAGNETIC TAPE WITHOUT FRAME ACCURATE POSITIONING
Background of the Invention
 The present invention relates generally to frame accurate recording of video signals on magnetic tape, and more particularly to such recording on magnetic tape using low precision transport or servo motors.



   An increasing fraction of video presentations are computer generated motion pictures and synchronized audio such as those produced with the
QuickTime system of Apple Computer, Inc. (QuickTime is a registered trademark of Apple Computer, Inc.). Such presentations may be used for promotional, instructional or entertainment purposes.



   Generally, computers manage peripheral devices in an interrupt driven fashion and use disk drives whose data output is discontinuous due to the need to move the drive head between tracks and to wait until the appropriate sector passes underneath the drive head. The amount of video information that can be affordably stored on a disk drive is also very limited. Thus in a large percentage of cases of interest, computers cannot generate or even display motion sequences fast enough to provide an uninterrupted video signal, for example, to a monitor. Even if computers are modified to do this, using expensive and specialized hardware, there are still noticeable processing artifacts, such as pixelation, or blocking of pixels, and dropped frames, that are disturbing to the viewer.



   A desirable alternative to providing an uninterrupted video signal directly from a computer is to record small sections or clips of the video signal on a magnetic tape medium. The small size of such sections allows them to be stored in random access memory (RAM), on a fast disk, or at some other suitable staging location. Then, when an entire sequence of small clips is completed, they may be played back on a magnetic playback system and viewed. This
 alternative reduces processing requirements and associated costs, but requires the concatenation of clips initially playable only one at a time. Heretofore, this has been difficult to achieve economically. There are two basic approaches in
 the prior art that attempt to solve this problem.  



   The first approach is to initially provide a digital video signal long enough that a low accuracy transition may be hidden in a program fade. The signal can be recorded on magnetic tape to obtain a recorded section of a presentation. The splicing of a plurality of such sections is then done with low accuracy editing systems. This method constrains the video artist to inserting fades at regular intervals. Also, since the frequency at which program fades can occur is limited, generating video signals of the required duration can be very expensive.



   The second approach is to use frame accurate tape positioning to provide frame accurate recording. Frame accurate recording is the individual recording of correctly positioned adjacent video frames on magnetic tape. The frames are provided without mutual synchronization rather than as synchronized parts of an uninterrupted video signal. Heretofore, frame accurate recording has always required the frame accurate tape positioning system. Such a system can precisely position and hold the magnetic tape at a predetermined position. Each frame may then be recorded individually. This requires a very accurate motor system, some allowance for tape stretching and changes, and some form of computer control. Also, the audio signal must be recorded in a separate pass, raising synchronization problems. These requirements, however, also make the system expensive.



   Accordingly, an object of the present invention is to provide a method for frame accurate recording without frame accurate positioning.



   Another object of the present invention is to provide a method for generating video presentations of arbitrary length, free of processing artifacts and frequent program fades, using commonly available processor speeds, RAM sizes and disk space, using inexpensive low accuracy magnetic tape recorder transports or servo motors such as those of consumer-grade video cassette recorders.



   Additional objects and advantages of the invention will be set forth in the' description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.  



  Summary of the Invention
 The present invention is directed to a method for frame accurate recording of video signals onto magnetic tape. Initially, the magnetic tape is formatted with control track signal and time codes for identifying frames. The magnetic tape is then rewound to a point prior to a frame just prior to the first desired record frame. The servo motors of the tape drive are then activated to provide a constant tape speed, and the time codes are monitored to identify the predetermined frame. After the frame prior to the first desired record frame is identified, a video signal is recorded onto a portion of the magnetic tape while monitoring the control track signal and synchronizing the video signal to the control track signal.



   In this method, arbitrary length video signals can be produced in small clips and then concatenated onto magnetic tape without using a frame accurate positioning system.



  Brief Description of the Drawings
 The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the invention and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.



   Figure 1 is a flow chart of the method of the present invention.



   Figure 2 is a schematic block diagram of a system for the practice of the present invention, comprising a tape drive communicating with an electronic interface.



   Figure 3 is a diagram of the various magnetic recording channels of VHS videotape.



   Figure 4 is a timing diagram of the control signals of the system of Figure 2 during the recording of a clip according to the present invention.  



  Description of the Preferred Embodiments
 The present invention will be described in terms of a preferred embodiment. The preferred embodiment is a method for frame accurate recording using inexpensive low accuracy magnetic tape recorder transports or servo motors such as a consumer-grade VHS or Video 8 videocassette recorder with Hi-Fi audio. The VHS standard is defined in International Electrotechnical
Commission IEC Standard Publication 774,1983. The Video 8 standard is defined in International Electrotechnical Commission International Standard
Publication 843,1987. In the following description, it will be assumed that a plurality of video clips or sections, combined with audio, will be provided by a video source such as a computer system to a tape drive through an interface.



  The clips are to be recorded with frame accuracy, without using an expensive tape positioning system, at predetermined positions on a magnetic tape. As discussed, frame accuracy is the recording of correctly positioned adjacent video frames on magnetic tape.



   A flow chart 10 of a method in accordance with the present invention is shown in Figure 1, and a block diagram of a system 27 for carrying out the method is shown in Figure 2.



   To facilitate the description of the method, system 27 will be described first. The reasons for the choices of system features will become apparent from the description of the method.



   System 27 comprises a tape drive 30 and an electronic interface 28 to a computer or other editing system. Tape drive 30 can use low precision servo motors or transports. It need not have frame accurate positioning capability because the present invention provides frame accurate recording without frame accurate positioning. The tape drive preferably complies with an analog helical scan video recording standard. As is known, tape drive 30 can read and write multiple tracks of a magnetic tape, such as a VHS videotape. As shown in
Figure 3, these tracks may comprise: control track 44, video track 46, stereo audio track 48, and time code track 50.



   Tape drive 30 comprises a signal processing system 52 that drives video write head 54 through switch 55, time code/longitudinal audio read/write head 56 and HiFi audio write head 58. System 52 also outputs TIME CODE signal 42  during the INSERTEDIT mode. Tape drive 30 also comprises a system controller 62 which switches the heads and, through motor controller 66, controls the servo motors 67. Pulse generator 64 is timed by the control track signal and provides SYNC signal 32 during the INSERTEDIT mode. Motor controller 66 and pulse generator 64 are conr. ected to control track read/write head 60.



   As can be seen by referring back to Figure 2, interface 28 and tape drive 30 communicate via a SYNC signal 32, an AUDIO signal 34, a STATUS signal 35, a COMMAND signal 36, a RECORDENABLE signal 38, a VIDEO signal 40, and a TIME CODE signal 42. All of these signals with perhaps the exception of RECORD ENABLE are available in mid-to high-end off the shelf
VHS tape drives. Those skilled in the art will know how to modify the firmware and electronics of such tape drives to provide the function of the
RECORDJENABLE signal, which will be described below together with the
INSERTEDIT mode of tape drive 30.



   COMMAND signal 36 from interface 28 may be a bit stream and is used to place tape drive 30 in one of the following modes: STOP, RECORD,
INSERTEDIT, FASTFORWARD, and REWIND. The provision of additional optional modes such as PAUSE is within the scope of the present invention.



  With the possible exception of INSERT¯EDIT, these modes of tape drive 30 do not differ from the standard modes of consumer-grade videocassette recorders.



  INSERT¯EDIT is similar to the RECORD mode, but instead of recording a control track signal, the control track signal is read from the tape and used to synchronize the scanner with the previously recorded control track.



   STATUS signal 35 is output by tape drive 30 and used by interface 28 to determine the condition tape drive 30 is in. STATUS signal may be used for, but is not limited to, getting diagnostic results from tape drive system tests, determining whether and when the last command was executed, whether a tape is installed, and whether the tape drive is ready for a command.



   SYNC signal 32 is output by tape drive 30 during the INSERT EDIT mode, and it is used to synchronize interface 28 to control track 44 of the magnetic tape. It consists of pulses that identify the vertical sync point, at which the active head switches between the video and the Hi-Fi audio heads.



  SYNC pulses are positionally related to control track pulses. As can be seen, an  example of the control track pulses recorded on control track 44 is given by waveform 33 in Figure 4, and an example of the SYNC pulses is given by waveform 32 in Figure 4. As is known, the control track pulses are used to synchronize the rotation of the tape drive scanner carrying the video heads to' the linear position of the tape.



   AUDIO signal 34 may be a standard stereo audio signal. It is supplied by interface 28 to tape drive 30, and is used to record the sound of the presentation in either the RECORD mode or the INSERTEDIT mode. In the
INSERTEDIT mode, the recording function of tape drive 30 may be disabled by
RECORDENABLE signal 38, as discussed below. The time codes may be recorded on a longitudinal standard audio track such as track 50 in Figure 3 for the VHS standard. Since the present invention uses time codes, the AUDIO signal may be recorded on any remaining longitudinal audio tracks or on the diagonal stereo audio tracks such as tracks 48 in Figure 3.



   VIDEO signal 40 is also supplied by interface 28 to tape drive 30 and may be a standard video signal. This signal has two uses in the present invention. During the RECORD mode, it is used by the tape drive to generate the control track pulses 33 (Figure 4); the specific image content of the VIDEO signal is unimportant at this time. VIDEO signal 40 is also used to provide the image component of the sections or clips to be recorded. This occurs when tape drive 30 is in the INSERTEDIT mode with its recording function enabled, at which time VIDEO signal 40 is recorded on video track 46 (Figure 3).



   TIME CODE signal 42 is supplied by interface 28 to tape drive 30 and recorded on time code track 50 of the tape (Figure 3) during the RECORD mode of the tape drive, to identify, using a serial frame ID, the frames to be recorded.



  As noted above track 50 is the standard longitudinal audio track according to the
VHS standard. Frame identification by suitably marking individual frames with time codes is inexpensive because it is done electrically rather than mechanically. There are several methods of identifying individual frames.'
Among them are Society of Motion Picture and Television Engineers (SMPTE) longitudinal time code (LTC), SMPTE vertical interval time code (VITC), Sony
Corporation Read-Write Consumer time code (RC), and the VASS time code system, which uses control track pulse modulation. They each may be used in the method of the present invention. In Figure 4, TIARE-CODE signal 42 is  represented as a succession of numeric values in boxes rather than the actual waveform, because of the variety of encoding standards available.



   RECORDENABLE signal 38 is supplied by interface 28 to tape drive 30. This signal is used in the INSERTEDIT mode of the tape drive to erable and disable recording with frame accuracy while the servo motors are in continuous motion. In the INSERTEDIT mode, TIME CODE signal 42 is read by tape drive 30 from time code track 50 and supplied to interface 28. As discussed above, SYNC signal 32, synchronized with control track pulses 33 (Figure 4) on control track 44 (Figure 3) is also output during INSERTEDIT mode. In the preferred embodiment of the present invention, the VIDEO and the
AUDIO signals are recorded at the same time, one clip at a time. In an alternate embodiment of the present invention, the audio of the entire presentation is recorded before the desired video information, using the
RECORD mode of the tape drive.

   The desired video information is recorded subsequently using the INSERTEDIT mode. In this case, the audio head of the tape drive must not be activated during the INSERTEDIT mode, in order to avoid erasing the previously recorded audio.



   RECORDENABLE is a binary signal that defeats the recording of audio and video material on the tape. This can be accomplished by putting a semiconductor switch 55 in series with the record head, enabled and disabled by this signal. This interrupts the head current to prevent recording on a precise point. The RECORD ENABLE signal timing by interface 28 and the response of the tape drive must be precise enough to turn recording on or off between two predetermined adjacent frames during the INSERTEDIT mode. The longest tolerable switching time of switch 55 is the period of overlap between frames, which is about 400 microseconds. The switching time is preferably on the order of the 65 microsecond duration of a video line or less,. and most preferably on the order of 5 microseconds or less. It should be noted that RECORD ENABLE signal 38 is externally supplied, by interface 28.



   Interface 28 is capable of providing TIME CODE signal 42, VIDEO signal 40 and also possibly AUDIO signal 34 during the RECORD mode of tape drive 30, in order to cause TIME CODE signal 42 to be recorded on time code track 50 and control track pulses 33 (Figure 4) to be recorded on control track 44 as discussed above. The details of the VIDEO signal provided in this mode are not important, and this video signal may be provided by a pattern generator. In  some embodiments of the present invention, AUDIO signal 34 of the entire presentation is also recorded at this time. The source of this signal may be the same as the source of the video clips or sections to be recorded later. In the present embodiment, this source is a computer system. Techniques ibr" providing an audio signal of any duration, including digitally synthesized audio signals, are well known.



   Interface 28 can also play, one at a time, video clips that are to be concatenated on a recording. The audio signal for the clips may be recorded at the same time, or may have been previously recorded on the magnetic tape as discussed above. The clips are recorded in INSERTEDIT mode, and the
VIDEO signal must be played back synchronized with the preexisting control track 44 on the magnetic tape using SYNC signal 32 provided by tape drive 30.



  Those skilled in the art will be able to provide the required synchronization using either digital or analog techniques. For example, a feedback-controlled variable delay may be used to maintain the synchronization. In the INSERTEDIT mode, the beginning and the end of the actual recording of the clips onto the magnetic tape are timed by the RECORDENABLE signal as discussed above.



  Since the clips are concatenated with frame accuracy, they may be as short as a single frame, and there are no requirements regarding their content at their end points. A wide variety of sources for such clips is known in the art. In particular, such clips may be supplied by a computer using commonly available processor speeds, RAM sizes, disk space, and video interfacing.



   As indicated by box 12 of flow chart 10, the method of the present invention may be carried out by first striping or formatting a magnetic tape with control track pulses 33 derived from an arbitrary VIDEO signal and recorded on control track 44. At the same time, TIME CODE signal 42 for the frames of the entire program is supplied by interface 28 and recorded on time code track 50.



  The AUDIO signal 34 for the entire program or presentation, which can be output in its entirety without interruptions due to its lower bandwidth, may also be recorded at this time. By continuously recording this signal, transitions that' are annoying to the ear may be avoided. If transitions are suppressed, it is possible to record the program audio and video together in box 22. AUDIO signal 34 may be recorded on a monophonic longitudinal audio track 49 of the tape (not shown), or on stereo audio tracks 48 adjacent video tracks 46. The present invention is not limited to any particular standard for the track location, time code encoding, and waveforms for the control and time code signals.  



   Once the magnetic tape is formatted, the clips or sections that constitute the program may be recorded in any sequence. Block or box 14 of flow chart 10 represents the recording of such a clip. In step 16 of Figure 1, the magnetic tape is rewound to a point prior in time code to the point where it is desired to begin recording the new msterial. The timing of the steps that follow is indicated in the timing diagram of Figure 4. At step 17 and time instant t1 in the timing diagram of Figure 4 and as a result of a command presented to tape drive 30 using
COMMAND signal 36, INSERTEDIT mode is entered with the recording function of tape drive 30 disabled. As a result, the tape drive servo motors are activated at time t2 as indicated by waveform 37 of Figure 4, and at time t3 the servo motors and SYNC signal 32 are synchronized to the control track signal or pulses 33.

   The SYNC signal, in turn, may be used to synchronize interface 28 to the formatted magnetic tape.



   During steps 18 and 20, the time codes are output by tape drive 30 and monitored by interface 28 to determine whether they correspond to the frame just prior to the frame where recording is to begin. This test is identified as step 20 in Figure 1. As indicated by NO arrow 19 branching from step 20, steps 18 and 20 are repeated until the time code of the frame just prior to the frame where recording is to begin is received, which occurs between times t4 and t5 in
Figure 4. When the test of step 20 succeeds, YES arrow 21 is followed to step 22. This step is the arming of recording using the RECORDENABLE signal 38 and synchronizing interface 28 to the control track of the magnetic tape, if not synchronized already as noted above.

   The RECORDJENABLE signal is toggled at a time instant t5 between two frames, and at the same time the video signal of the clip to be recorded is supplied by interface 28 to the signal processing system 52 of tape drive 30. The frames of the video clip are output as step 23 is executed repeatedly, until the last frame is output. In the example of Figure 4, the video clip is four frames long, spanning time intervals t5-t6, tg-t7, t7-tg and tg-tg, and time codes 30: 00 to 30: 03. At time tg, when the clip ends, recording is disabled by signal RECORD ENABLE 38. The command to stop the servo motors is given at time t1o and the servo motors stop at time tell. These events are represented by step 24 in Figure 1.



   It is important to note that between time t2 and time tll, the servo motors are simply turned on as shown by waveform 37 in Figure 4, and do not do anything that they do not ordinarily do during playback. Frame accurate positioning is not required. When the recording is turned off, interface 28 can  leisurely inform the mechanism that it is time to stop by issuing a STOP command using COMMAND signal 36. If it is determined, at step 26 of Figure 1, that additional clips remain to be recorded, the steps of block 14 are executed again.



   In summary, an apparatus and method for frame accurate recording on magnetic tape of video signals using low precision transport or servo motors has been described. It should be noted that the present invention is not limited to the
VHS standard. With an appropriate tape drive 30, the present invention may be used with other video tapes, such as S-VHS, HiFi VHS, Video 8 and Hi 8.



   The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.
  

Claims

CLAIMS 1. A tape drive, comprising: a motor control system servo motors controlled by said motor control system for driving a tape; a signal processing system; an externally controllable switch connected to said signal processing system; and a video write head connected to said switch for recording a video signal onto said tape; and wherein said motor control system and said servo motors do not have frame accurate positioning capability; and said switch can switch in at most about 400 microseconds.

2. A method for frame accurate recording of video signals onto magnetic tape, comprising the steps of : (a) formatting a magnetic tape with a control track signal and time codes for identifying frames; (b) rewinding said tape to a point prior to a predetermined frame; (c) activating servo motors of a tape drive for driving said magnetic tape and monitoring said time codes to identify said predetermined frame; and (d) recording a video signal onto said magnetic tape while monitoring said control track signal, said video signal being synchronized to said control track signal.

3. The method of Claim 2, further comprising the steps of : (e) enabling recording between step (c) and (d); and (f) disabling recording at the end of step (d).

4. The method of Claim 3, further comprising a step (g) of stopping the servo motors of said tape drive after step (f).

5. The method of Claim 4, wherein steps (b)- (g) are carried out repeatedly.

6. The method of Claim 5, wherein an audio signal is recorded on a stereo track of said magnetic tape during step (a).

7. The method of Claim 5, wherein an audio signal is recorded on a longitudinal track of said magnetic tape during step (a).

8. The method of Claim 5, wherein an audio signal is recorded on a stereo track cf 3aid magnetic tape during step (d).

9. The method of Claim 5, wherein an audio signal is recorded on a longitudinal track of said magnetic tape during step (d).

10. The method of Claim 2, wherein said time codes are selected from the group consisting of SMPTE longitudinal time codes, SMPTE vertical interval time code, Sony Corporation Read-Write Consumer time codes, and VASS system time codes.

11. The method of Claim 2, wherein said recording is done using an analog helical scan recording system.

12. The method of Claim 2, wherein said magnetic tape is a VHS videotape.