Real Time Magnetic Video Digital Recording and Reproducing Apparatus

VIDEO FRAME STORAGE RECORDING AND REPRODUCING APPARATUS, Ser. No. 763,371, filed Jan. 28, 1977, by Joachim P. Diermann and Thomas W. Ritchey, Jr.

PLAYBACK APPARATUS ASSIGNMENT MEANS, Ser. No. 763,462, filed Jan. 28, 1977, by Howard W. Knight and Edwin W. Engberg, now abandoned.

TELEVISION SIGNAL DISC DRIVE RECORDER, Ser. No. 763,795, filed Jan. 28, 1977, by Howard W. Knight and Edwin W. Engberg.

DISC DRIVE RECORDING PROTECTION APPARATUS, Ser. No. 763,761, filed Jan. 28, 1977, by Edwin W. Engberg, now abandoned.

TELEVISION SUBCARRIER PHASE CORRECTION FOR COLOR FIELD SEQUENCING, Ser. No. 763,942, filed Jan. 28, 1977, by Luigi C. Gallo, U.S. Pat. No. 4,145,704.

METHOD AND APPARATUS FOR PROVIDING DC RESTORATION Ser. No. 763,461, filed Jan. 28, 1977, by Luigi C. Gallo, U.S. Pat. No. 4,122,477.

METHOD AND APPARATUS FOR INSERTING SYNCHRONIZING WORDS IN DIGITIZED TELEVISION SIGNAL DATA STREAM, Ser. No. 763,463, filed Jan. 28, 1977, by Luigi C. Gallo.

PRECISION PHASE CONTROLLED CLOCK FOR SAMPLING TELEVISION SIGNALS, Ser. No. 763,453, filed Jan. 28, 1977, by Daniel A. Beaulier and Luigi C. Gallo, U.S. Pat. No. 4,122,487.

DIGITAL TELEVISION SIGNAL PROCESSING SYSTEM, Ser. No. 763,941, filed Jan. 28, 1977, by Luigi C. Gallo, U.S. Pat. No. 4,119,999.

CLOCK SIGNAL GENERATOR PROVIDING NONSYMMETRICAL ALTERNATING PHASE INTERVALS, Ser. No. 763,792, filed Jan. 28, 1977, by Daniel A. Beaulier and Luigi C. Gallo, U.S. Pat. No. 4,122,478.

PHASE LOCK LOOP FOR DATA DECODER CLOCK GENERATOR, Ser. No. 763,793, filed Jan. 28, 1977, by Kenneth Louth and Luigi C. Gallo.

HIGH BIT RATE DATA ENCODER FOR DATA TRANSMISSION SYSTEM, Ser. No. 763,762, filed Jan. 28, 1977, by Luigi C. Gallo.

A CIRCUIT FOR DIGITALLY ENCODING AN ANALOG TELEVISION SIGNAL, Ser. No. 762,901, filed Jan. 26, 1977, by Daniel A. Beaulier, U.S. Pat. No. 4,075,656.

A DIGITAL CHROMINANCE SEPARATING AND PROCESSING SYSTEM AND METHOD, Ser. No. 763,251, filed Jan. 26, 1977, by Robert P. MacKenzie, abandoned in favor of continuation application Ser. No. 765,563, Filed Feb. 4, 1977.

DATA RATE AND TIME BASE CORRECTOR, Ser. No. 763,794, filed Jan. 28, 1977, by Luigi C. Gallo U.S. Pat. No. 4,110,798.

FREQUENCY RESPONSE EQUALIZER, Ser. No. 793,902, filed Jan. 26, 1977, by Jerry W. Miller and Luigi C. Gallo, U.S. Pat. No. 4,110,798.

A CIRCUIT FOR GENERATING A DIGITAL DELETED DATA, BLINKING CROSS SIGNAL WHICH IS STORED IN A DELETED TRACK AND SELECTIVELY DISPLAYED FOR DETECTION, Ser. No. 762,903, filed Jan. 26, 1977, by Luigi C. Gallo and Junaid Sheikh, abandoned in favor of continuation application Ser. No. 765,564, filed Feb. 4, 1977, U.S. Pat. No. 4,130,842.

The present invention generally relates to recording and reproducing apparatus and, more particularly, to apparatus that is adapted to record and reproduce real time television signals, using digital techniques.

Recently, a computerized system for recording and playing back slides and other graphic images has been made available for broadcast station useage. The system provides the high reliability and quality of a digital computer system employing magnetic disc pack storage. Such a system is commercially available from Ampex Corporation of Redwood City, California, as the model ESS-1 system and is disclosed and claimed in the aforecited U.S. patent applications.

This system consists of a video signal processor which utilizes digital techniques to store video images on a computer disc memory and which assigns an address to each image to facilitate rapid access. Two or three computer disc drives are employed under control of a system controller including a microcomputer. Access to the system is through keyboard controls located at the electronics rack, or from remote access stations. As many as eight access stations may be incorporated in the system, each equipped with a keyboard terminal and alphanumeric readout

Up to 1500 slides or other graphic images may be stored on-line in the typical two-disc pack drive system for immediate access. On-line random access time is less than 100 milliseconds

There are two playback channels in the typical system, allowing simultaneous outputs from each on-line disc pack. The system will record color or monochrome images, and operates in four modes: record, play, sequence assembly, and sequence play.

While the aforecited prior art system provided electronic storage and playback of stills, and while it was possible to record and playback video frames at less than real time rates, to obtain a slow motion effect it would be desirable if the system could record and playback television video signals on a real time basis, i.e., 30 frames per second. This capability could have been provided in the prior system by multiplexing the video signals to and from two disc packs, simultaneously. However, this would be costly and would seriously impair the still graphic storage and replay capability of the system.

It would be desirable to employ only one disc pack for the real time television video recording and playback, thereby leaving the second disc drive and disc pack for still graphic storage and replay. However, this was not possible in the prior system because the magnetic recording tracks were circular, each at a predetermined radius from the center of the disc and the magnetic recording/playback transducers of standard computer disc drives could not move from one recording track to the next and maintain and/or achieve time base lock during one vertical interval of television video signal, i.e. 1.3 milliseconds.

Accordingly, it is an object of the present invention to provide an improved recording and playback apparatus that is particularly adapted for recording and reproducing television signals on a real time basis. Another object of the present invention is to provide an apparatus of the foregoing type which requires access to only one computer disc drive for recording and reproducing television signals without impairing the high quality of the reproduced television images.

Other objects and advantages will become apparent upon reading the following detailed description in conjunction with the attached drawings.

FIG. 1 is a perspective view of the apparatus embodying the present invention, illustrating its overall appearance, including the internal access station and two disc drive units;

FIG. 2A is an enlarged perspective view illustrating a representative remote access station that an operator can use to control the operation of the apparatus of the present invention in a still store mode;

FIG. 2B is a plan view of a representative slow motion access station that an operator can use to control the operation of the apparatus of the present invention in the real time or slow motion mode;

FIG. 3 is an enlarged top view of a portion of the internal access station keyboard shown in FIG. 1 particularly illustrating the various keys and bars that an operator uses during operation;

FIG. 4 is a broad functional and simplified block diagram of the entire apparatus of the present invention;

FIG. 5A illustrates a portion of a typical television signal illustrating the vertical interval thereof;

FIG. 5B illustrates a portion of a color television signal, particularly illustrating the horizontal synchronization pulse and color burst signal;

FIG. 6 is a functional block diagram broadly illustrating tbe signal flow path through the apparatus during a record operation;

FIG. 7 is a functional block diagram broadly illustrating the signal flow path through the apparatus during a playback operation;

FIG. 8 is a block diagram illustrating the control system that controls the movement of the magnetic recording/reproducing transducers;

FIG. 9 is a plot of magnetic recording head radial position versus time depicting movement of the head from one recording track to the adjacent recording track.

FIG. 10 is a fragmentary enlarged plan view of three adjacent recording tracks on a magnetic recording disc, and

FIG. 11 is a plot of relative output signal amplitude derived from the playback transducer while transversing from one recording track to the next adjacent track when the system is playing back in the reverse motion mode, as indicated by the doted lines of FIG. 10.

Broadly stated and referring to FIGS. 1-3, the present invention is directed to a recording and reproducing apparatus, indicated generally at 70 in FIG. 1 which includes two bays 71 and 72 containing electrical circuitry associated with the apparatus, together with the various monitoring and control hardware shown specifically in the upper portion of the bay 72. The system also includes a pair of disc drives 73 located adjacent the rightward bay 72 with each of the disc drives 73 having a disc pack 75 mounted thereon. While two disc drive units are specifically illustrated in FIG. 1, it should be understood that there may be additional disc drives used with the system to increase the on-line storage capacity of the apparatus. It should also be appreciated that a single disc drive may be used. Operational control of the apparatus is performed by one or more operators using either one of many remote access stations, such as the remote access station 76 shown in FIG. 2A, or an internal access station 78 which is located in the bay 72. If desired, a video monitor 79, vector and "A" oscillioscopes 80 may be provided as shown in bay 72. Phase control switches 81 are provided above the internal access station 78.

The still store and reproduce modes of the apparatus are controlled by an operator using either the internal access station 78 or a remote access station 76, both types of which have a keyboard with numerical and function keys and bars, a 32 character display 82, which provides a readout of information that is needed to carry out functional operations during use, as well as to display the information concerning the identify of certain stills being addressed and other information. It should be understood that the remote access statlon 76 shown in FIG. 2A is representative of each of the remote access stations and that in the preferred embodiment, up to seven remote access stations can be used to control the apparatus 70. The internal access station keyboard indicated generally at 83 in FIG. 1, as shown in the enlarged fragmentary view of FIG. 3, has more expanded operational capability than the remote access stations, whose keyboards have fewer function keys. As will be explained in detail hereinafter, the keyboard contains a large cluster of keys indicated generally at 84 and a smaller cluster of function keys 85 located on the left side of the keyboard. Additionally, a turn key controlled switch 86 may be provided to switch between "normal" and "delete" operations to safeguard against the possibility of inadvertent or unauthorized erasure of actively used stills.

The slow motion and real time modes of operation are controlled by an operator using a slow motion access station 241 of the type shown in FIG. 2B, containing keys, and a continuously variable speed control 242 which allow control of the record and playback rates of the apparatus, setting of cue points which may be accessed whenever desired, and indicators 243 which display to the operator the time used since passing a cue point, the time left until the next cue point, or an absolute time derived directly from the track address of the disc drive.

Referring to the very simplified block diagram shown in FIG. 4, the apparatus receives a video input signal which is processed by record signal processing circuitry 88 and is then applied to record signal interface circuitry 89 which directs the signal to all of the disc drives 73. Gating circuitry located within a selected disc drive 73 is enabled to allow the signal to be recorded on a selected drive. More than one disc drive 73 can be simultaneously selected for recording the video signal provided by the record signal interface circuitry 89. Switcher circuitry can be substituted for the signal splitter and associated gate circuitry so that the signal provided by the record signal processing circuitry 88 is coupled only to selected disc drives having the disc packs 75 upon which the signal is to be recorded. During playback, a signal originating from one of the disc drives is applied to the playback switching circuitry 90 which directs it to one of the playback channels 91, each of which provides a video output channel. A computer control system 92 is interfaced with the record processing circuitry, signal interfacing and switching circuitry and disc drives for controlling the overall operation of the various components of the apparatus and also interfaces the remote access stations 76 and internal access station 78. As will be hereinafter more fully explained, an operator can select a particular disc pack in which to store a still or television video field or sequence of fields, provided that the disc pack 75 is on-line, i.e., it is physically loaded on one of the disc drives 73. In this regard, it should be understood that the apparatus addresses disc packs 75 rather than disc drlves for the reason that the apparatus is adapted to identify up to 64 separate disc packs, only one of which can be located on a disc drive at any one time. Thus, in the event the apparatus has two disc drives, only two disc packs can be on-line at one time. The operator can use an access station keyboard 83 to enter the address of a disc pack upon which he wishes to record a television video field or sequence of fields and, through the interaction of the computer with the disc drive on which the selected disc pack is loaded, can carry out the recording operation on the selected on-line disc pack. Similarly, an operator can play back a television field or sequence of fields from the disc pack on one of the disc drives and can define the playback channel that he wishes the field or fields to be played through.

The record and play operations will be initially described, while referring to FIGS. 6 and 7 which illustrate somewhat simplified block diagrams of the signal flow paths during recording and playback, respectively, with respect to one of the disc drives 73.

Turning first to the record signal flow block diagram of FIG. 6, the composite video input signal is applied to the input stage circuitry 93 where clamping of the signal takes place and the synchronization and subcarrier components are stripped from the composite video signal. An NTSC type composite color television signal is shown in FIGS. 5A and 5B. The input stage also regenerates the synchronization (hereafter often referred to merely as "sync") and subcarrier signals for later use during reproduction and, accordingly, the regenerated sync and subcarrier signals are directed to a clock generator 94 which also generates reference signals that are used by the downstream elements during operation. The clamped analog video signal with the color burst component is then applied to an analog-to-digital converter (A/D) 95 which provides an output signal at a sample rate of 10.7 megasamples per second, with each of the samples comprising 8 bits of information. The digital video signal is a non-return to zero code (NRZ) which means that it is a binary code defining a ONE as a high level and a ZERO as an equivalent low level. The digitized video signal appears on 8 parallel lines, i.e., one bit per line, which is applied to an encoder and sync word inserter 96 which converts the digitized video into a special recording code (referred to herein as a Miller code or a Miller squared code) that is partlcularly suitable for dlgital magnetic recording in that it minimizes DC content of a data stream. The circuitry also inserts a synchronizing word on alternate television lines with respect to a particular phase angle of the color subcarrier as represented by the color burst sync component. The sync word is used as a reference for correcting time base and skew errors that occur during playback among the eight parallel bits of data that must be combined to define the value represented by each sample. The digital video information in the eight parallel lines is then applied to recording amplifier and head switch circuitry 97 associated with the selected disc drive 73, which switches between two groups of eight recording heads for recording the digitized video signal on the disc drive. The disc drive is servo controlled so that its spindle rotational speed is locked to vertical sync, with the rotational disc speed being 3600 revolutions per minute. By locking the spindle drive to vertical sync, the apparatus records one television field per revolution of the disc pack and simultaneously records the eight data streams on eight disc surfaces. At the completion of recording one field, the head servo circuit more fully described below with regard to FIGS. 8-11, is swtiched to advance the head carriage radially of the disc pack to adjacent recording tracks for simultaneously recording the second field of a television frame on the next adjacent set of eight disc surfaces so that a full televison frame is recorded on two revolutions of the disc drive, using 8 heads and 16 recording tracks. Each disc pack located on a disc drive preferably contains 814 cylinders (circular recording tracks), each of which has 19 recording surfaces used to store 1628 digital television fields. There is one read/write head for each of the 19 disc recording surfaces of a disc pack and all heads are mounted vertically aligned on the common head carriage whose position is controlled by a linear motor. It should be understood that a cylinder (track) is defined to comprise all recording surfaces that are located on the same radius of a disc pack. However, the term track, rather than cylinder, is preferred herein and, accordingly, a track is meant to include all recording surfaces on a same radius, i.e., all surfaces on a cylinder. Thus an addressed track for recording or playing back a video field actually refers to the 19 individual surfaces on the cylinder available at that radius. of the 19 surfaces that are available for recording, one is used to record the address and other housekeeping informatfon, rather than active video information, and it is specifically referred to as the "data track". Two of the 19 surfaces are available for recording a parity bit and 16 surfaces, 8 on the top and 8 on the bottom surface of each disc, are used to record the video data. The television video field signals are recorded consecutively on one side of the discs until the top surface tracks are all filled and then the recording amplifier and head switch circuitry 97 is commanded to activate the bottom set of 8 heads and the head carriage servo circuit switched to reverse the radial direction of the heads for consecutively recording the next 814 video field signals. This format of video field recording sequence allows continuous recording and playback without having to interrupt the recorded signal while the head carriage assembly is returned from the inside track to the outside track or vice versa.

Also, one of the heads, generally referred to as the servo head, travels on the 20th disc pack surface that contains only servo track information prerecorded by the pack manufacturer. The servo tracks carry out two functions, i.e., following a seek command the servo head traverses servo tracks that are counted to determine the instantaneous location of the head and, after completion of a seek phase, the servo head generates an error signal that is used to control the linear motor position to hold the head carriage centered on the appropriate servo track. By using such a feedback system, it is possible to achieve a radial packing density of about 400 tracks per inch or a total of 814 tracks per disc pack.

Since the present apparatus does not record analog video signals because of frequency response limitations of computer disc pack memories, the video signal is digitized for recording. Because the digitized signal is recorded, the video signal to noise ratio of the system is primarily determined by quantization noise rather than recording media and preamplifier noise as is the case with conventional videotape recorders. Thus, the present apparatus delivers a signal to noise ratio of about 58 dB and effects such as "moire" and residual time base error do not exist, the digital random error of the storage channels being typically low enough to make occasional transmission errors virtually invisible.

By recording a digital data stream at a rate of 10.7 megabits per second on each of the eight disc surfaces, the linear packing density of the apparatus is about 6000 bits per inch which is about 60% greater than is used in conventional disc drive usage in data processing.

During Playback and referred to FIG. 7, the heads read, i.e., reproduce the digital video information from the eight surfaces per field and obtain the recorded channel encoded digital video signal from the odd and even flelds. The produced signal is applied to a playback amplifier and head switch circuitry 98 associated with the selected disc drive 73, which amplifies the data streams of digital video information carrier by the eight data bit lines and applies the same to equalizer and data detector circuits 99. The equalizer compensates for phase and amplitude distortion introduced to the signal by the band limiting effects of the record and reproduce processes and insures that the zero crossings of the reproduced signal are distinct and accurately positioned. Following equalization, the channel encoded signals in each data bit line are processed for transmission to the playback circuitry of the signal system over a twisted pair line. The processed channel encoded signals are in the form of a pulse for each zero crossing or signal state transition of the channel encoded signal. The twisted pair lines for the eight data bits of the digital video information apply the processed channel encoded signals to the decoder and time base corrector circuitry 100 of one or more of the playback channels 91 of the apparatus. The decoder and time base corrector circuitry 100 reprocesses the received signals to place them in the channel encoded format, decodes the signal to the non-return to zero digital form and time base corrects the digital signal with respect to station reference to remove inter-data bit line time displacement errors (commonly referred to as skew errors) and timing distortion within each of the data streams carried by the data bit lines. Thus, the time base corrector portion of the circuitry 100 serves to align the eight bits defining a single sample and remove timing distortion in each of the data bit lines relative to station reference. It should be realized that each playback channel 91 is provided with decoder and time base corrector circuitry 100 and within each playback channel each of the eight data bit streams travels through a separate decoder and time base corrector. The output of the circuitry 100 is then applied to a comb filter and chroma inverter 101. The inverter 101 separates the chroma information and selectively inverts and recombines the signal for reconstruction of a four field NTSC sequence. This reconstructed digital signal is applied to a digital-to-analog converter 102 which provides an analog video signal. The new sync and burst are then added by a process amplifier 103 to produce a composite video analog output signal of the playback channel 91 as is desired. Exemplary apparatus for recording and reproducing digitized television signals as described above with reference to FIGS. 1, 2A and 3-7 and which is suited for use in the present invention is described in the above-identified related application, Ser. No. 763,371.

Referring now to FIGS. 8-11 there is shown the improved magnetic recording head servo circuit portion of the present invention which permits continuous recording of consequetive television field signals on a real time basis, i.e., at 30 frames/second. More particularly, the magnetic recording/reproduction heads 201 are carried by a common head carriage assembly 202 driven radially of the disc pack 75 via a linear motor 203. The discs are driven at a constant angular velocity, i.e., 3600 RPM via motor 204 under the control of a disc drive servo controller 205. The servo controller 205 compares the timing of a reference vertical sync input signal from the clock generator 94 with an index input signal derived from the servo recording track on the disc pack 75 to control the angular velocity of the discs. The servo control signal is played back from the servo track by means of a servo control playback head 201'. This signal is decoded by decoder 206 and outputted at 207 to disc drive servo 205.

Also the transducer heads 201 are maintained on the recording tracks by means of signals recorded on the servo track and picked up by servo head 201'. Such a track servo is disclosed in U.S. Pat. No. 3,534,344, issued Oct. 13, 1970. These track control error signals are outputted from the decoder 206 to the servo control loop lock terminal 208 of switch S1. When S1 is in the "lock" position, the error signals are fed to an adder 210 for adding with a velocity feedback signal derived from a linear tachometer 211 coupled to the linear motor 203 for a velocity feedback signal phase lead compensation of the servo loop. The compensated error signal output of adder 210 is amplified by error amplifier 212 and thence by power amplifier 213 to derive the drive signal for the linear motor 203.

When the television field signal reaches a point near the end of the particular field signal being recorded, as sensed from sector signals recorded on the servo track and as picked up by the servo head 201' and inputted to the computer 92 from the decoder 206, the computer 92 commands the head servo circuit of FIG. 8 to start "seek" for the adjacent recording track. If the recording or playback is in the forward direction, i.e., a video field sequence 1, 2, 3, 4, .. n, the computer 92 commands a "forward" input to switch S2 for switching the output of the accelerate one shot 215 to first actuate a current source 216 of a first polarity the output of which is fed to a second adder 217 and thence via switch S1, into the linear motor for accelerating the carriage 202 and transducer heads 201 radially of the discs toward the next adjacent recording tracks. The accelerate one shot 215 provides a pulse output of approximately 680 microseconds duration, and the trailing edge of the "accelerate" pulse triggers firing of a "declerate" one shot 218 which produces an output pulse of approximately the same duration as the "accelerate" pulse. The declerate pulse is applied via switch S2 to energize a current source 219 of opposite sign which is thence fed to the linear motor 203 via the previous path for rapidly decelerating the carriage 202 and transducer heads 201 over the next adjacent recording track. The amplituds of the currents derived from sources 216 and 219 are sufficient to saturate the drive coils of the linear motor 203 so as to obtain the fastest possible acceleration and deceleration of the transducer heads 201 during the track change interval, which occurs during to the vertical interval of the television video field signal. The pulse widths of the one shots 215 and 218 are controlled by RC time constant control circuit 220 and 221, respectively, associated with each of the one shots 215 and 218. By adjusting the time constants of these circuits 220 and 221, the acceleration and deceleration of the heads 201 can be adjusted so the heads 201 are braked to a stop very nearly on the next adjacent track being sought.

In case the heads 201 are to be incremented to a track which is not adjacent their present track, the operator through keyboard 241 causes the computer 92 to preset into counter 223 the number of tracks to be crossed in transiting the heads to the new track. The output of the decoder 206 feeds a track counting input into the counter 223 which decrements the count output to a digital-to-analog converter 224 for producing an analog output voltage proportional to the number of remaining tracks to be crossed. This output is fed via adder 217 and seek switch S1 to the drive circuit of the linear motor 203. When the last track is reached, the output of the D-to-A converter 224 drops to zero amplitude and the computer 92 commands switch S1 to switch to the "lock" position 208, for closed loop servo control lock of the linear drive motor onto the desired track.

Thus, the prior art ESS-1 system has been modified in two ways to provide real time television video field recording and playback from a single disc pack. The first modification was to alter the prior art track format from recording the first field of a video frame on one side of the disc and the second field on the opposite side at the same radial track location to a format which records consecutive fields on consecutive tracks on the same side of the disc until the end of the tracks is reached, whereupon the recording is switched by the computer to the opposite side of the disc with consecutive fields being recorded in the opposite radial direction, i.e., "in on the top, and out on the bottom" format. This permits continuous recording without loss of fields.

The second modification to the ESS-1 prior art system was the provision of the head servo drive circuit which provided both acceleration and deceleration drive for the linear motor 203 which drives the head carriage 2 and transducer heads 201. This permitted change of recording tracks during the vertical interval of the television field signal on a real time basis, i.e., at 30 frames per second.

Referring now to FIGS. 9-11, the transducer head transition between adjacent tracks in the modified system will be described in greater detail. FIG. 9 shows head position versus time for the aforementioned head transition. In the head servo circuit of FIG. 8, up to 500 microinches of mistracking may be permitted recording before errors appears in the video picture. Taking 1/2 of this figure for safety, i.e., 250 microinches, it is found that the actual mistracking time, as depicted by curve 249, is only about 750 microseconds when using the modifications of the present invention. However, in the prior art unmodified system the head motion is depicted by curve 250 and the mistracking time very nearly equals or exceeds the vertical interval of 1.3 microseconds, thus resultin in interruption of the video picture. By starting the "seek" early, before the vertical interval, the mistracking is made to occur entirely within the vertical interval. The sector counter in the disc drive provides a software controlled start of seek position to achieve this result.

The recording/reproduction current to and from the transducer heads 201 is left on continuously, even through the vertical interval. when playing back in the same direction as the recording was made, i.e., forward action, and in real time, there is no discontinuity of the head signal during the vertical interval because the head 201 tracks the same path as it did during the continuous recording. Thus, there is no loss of word sync, i.e., lock up to the video time base signals inserted into every other line of the recorded video signal. However, when playing back still play or worse when playing back reverse action, i.e., field sequence 4, 3, 2, 1, etc., the head crosses an extra track, see FIGS. 10 and 11, before landing on the desired track. The head output at any instant of time during the seek will be of combination of the signals derived from which ever tracks are under the head 201 at that instant, so the relative outputs of the three tracks will be approximately shown in FIG. 11. The signal received by the time base correcting circuit, must be clean enough during the vertical interval to be certain of acquiring word sync lock before the start of the picture data and the active picture area must be error free. In order to meet the second of these conditions, the seek time must fit within the vertical interval of 1.3 milliseconds, which it does, see FIG. 9.

With regard to word sync lock up, the recorded tracks being crossed contain recorded black data within the vertical intervals. As described in the above-identified related application, Ser. No. 763,371, block data is inserted within the vertical interval (other than at locations of the sync words) as binary "0" bits. Black data in Miller .sup.2 code corresponds to a symmetrical sequare wave, which is in reality a sine wave due to system bandwidth limitations. The data on adjacent tracks during the vertical interval will consist of sine waves with unknown but slowly changing phase relationships. Any mixture of two such tracks is also a sine wave of the same frequency, with slowly changing phase and amplitude, i.e., black+black=black. No comparable statement may be made for the region of active video data. Thus, while no adjacent track interference may be allowed during the data period, very large amounts of interference may be permitted during black, subject only to the constraint that the amplitude of the resulting signal not go too low. In practice it is possible to lock the decoder 206 to a sine wave at least 20 dB down with respect to normal amplitude, and this ratio is guaranteed when the final to middle track ratio reaches 55%/45% (or a signal to interference ratio of -1.75 dB). Thus decoder lock is guaranteed just to the right of the middle to final track crossover point of FIG. 11, which leaves adequate margin for word sync lockup prior to active picture data. In fact, about 90% of the time the decoders pass through the entire vertical interval without loosing lock. Lock is lost only if the random phase between two tracks is close to 180 degrees.

The central processing unit is preferably a microprocessor or microcomputer and, in the apparatus described herein, it comprises our LSI-11 system made by the Digital Equipment Corporation of Maynard, Massachusetts. More specifically the apparatus of the present invention incorporates as the CPU a model KDll-F microcomputer type LSI-11 system which contains a microprocessor and a 4 k by 16 bit semiconductor read/write memory.

The slow motion-real time mode access station (keyboard) 241 accesses the CPU and the load module for the slow motion program is as follows: ##SPC1##