Patent Publication Number: US-2007104463-A1

Title: Method and Apparatus for Recording and Playing Back Monitored Video Data

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to methods and apparatuses for recording and playing back video data, more particularly to a method and an apparatus for recording and playing back monitored video data, which will be suitable for recording video data for a long time for the purpose of monitoring and preventing of crimes.  
      There is provided a prior art time lapse VTR used as a video data recording device for recording video data from a video camera, suitable for monitoring and preventing crimes. Many of this type VTR are improved from a home VHS VTR which is very popular at present to enable a long time recording. For example, in such a time lapse VTR, the magnetic tape can be fed at a ⅓ speed of the VHS Extended Play (EP) mode to record only one of the input video  3  fields on the tape, so that the recording time can be extended to 3 times that of the EP mode. Consequently, when a 120-min tape is used, the recording time is extended to 6 hours (120 min×3) in the VHS EP mode. The recording time of the above time lapse VTR can thus be extended to 18 hours, 3 times that of the VHS EP mode. In this case, however, as understood clearly from the recording principle explained above, the number of fields per second is 20, which is ⅓ of the VHS (60 fields). In addition, audio signals can be recorded on audio tracks arranged in the longitudinal direction of the tape just like an ordinary VTR, but the tape speed becomes ⅓ of the normal one. As a result, the sound quality is degraded more than the normal one. In spite of this, the time lapse VTR can have an advantage of 18-hour audio recording time.  
      There is also proposed an intermittent recording system for the above prior art time lapse VTR, in which system, the average tape speed is delayed and the number of recording fields is reduced more to extend the recording time. This system can have the maximum recording time of, for example 720 hours using a normal 120-min tape by, recording video data at a rate of 1 field per 2 seconds (120 fields) per frame (2 hours×3 times×120=720 hours) In such a super-long time recording operation, the average tape feed speed is very slow, so that it becomes difficult to control the continuous feeding of the tape. To avoid this problem, the tape is fed intermittently in most cases and accordingly, it becomes impossible to record audio signals in audio tracks.  
      The conventional time lapse VTR ware opened in, for example, Examined Published Japanese Patent Application No. 3-12380, Examined Published Japanese Patent Application No. 7-63182, etc. The former patent report describes a method for controlling tape feeding so as to obtain the same recording tape pattern as that of the ordinary VTR even at the recording time by feeding the tape intermittently just like in the above mentioned 720-hour recording. The latter patent report (No. 7-63182) describes a method for controlling the rotation speed of the cylinder to prevent generation of skew during the intermittent recording.  
      As explained above, the time lapse VTR needs recording modes for continuous low speed tape feeding and intermittent tape feeding in addition to the normal continuous tape feeding, and a means for precision controlling of the capstan and the cylinder corresponding to those playback modes. In a recording operation, the same playback mode as each of those recording modes is needed to check pictures at the same speed as that of the recording. For this purpose, each of those prior art VTRs must be provided with various precision control means, so their prices become more higher than general home VTRs.  
      The time lapse art VTR cannot load (wind a magnetic tape onto the cylinder to which a magnetic head is attached to prepare for recording or playing back) a magnetic tape (video data recording medium) and unload (separate the tape from the cylinder) the magnetic tape within a short time even for a recording operation in a recording mode for intermittent tape feeding. Therefore, the tape is needed to be kept loaded during recording. Further, since the cylinder has a large moment and takes much time until its rotation speed is stabilized, the cylinder must be rotated almost at a fixed speed continuously during the recording.  
      Therefore, the time lapse VTR must be kept loaded with a tape during recording of monitored video data and the cylinder is rotated continuously. Especially, in the above super-long recording mode (720-hour mode), the VTR has a disadvantage that because the cylinder is kept rotated in contact with the tape for 720 hours, the tape is damaged far more rapidly than used for a home VTR. In the same way, because the head is in contact with the tape, the head is also worn rapidly. The mechanisms such as the motor and belt used for making the cylinder rotate are also degraded rapidly and often damaged, since they are running for a long time respectively.  
      Further, conventional helical scanning VTR also has a following defect; the recording track pattern angle on the tape loaded is decided by both the rotation speed of the cylinder and the feed speed of the tape, so that the track pattern recorded at the above slow speed tape feeding or the tape stops has an angle different from the normal speed tape feeding. Therefore, when playing back the track pattern recorded in such way, a mismatching occurs between the tracing angle of the head and the angle of the track pattern, causing the S/N (signal-noise ratio) to be degraded in a part of the playback signals, with the resultant of lowering the quality of output pictures significantly.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a method and an apparatus for recording and playing back monitored video data, which can store video data on a magnetic tape for a period enough to be used for monitoring and preventing crimes, etc. In addition, in the apparatus, it is no need to record video data while the recording head is put in contact with the magnetic tape for a long time and while the recording head is moved, without special mechanisms for intermittent feeding, etc. In other words, a general home VTR can also be used to store video data on a magnetic tape for a period enough to monitor and prevent crimes, etc.  
      It is another object of the present invention to solve the above-mentioned problems of conventional time lapse VTR and provide an apparatus for recording and playing back video data, which can improve its mechanical reliability and extend the life of the magnetic tape in use by reducing the operating time of the parts that will apt to wear and be damaged, without employing any high precision control means for intermittent tape feeding.  
      It is a further object of the present invention to provide an apparatus for recording and playing back video data, which can output good quality playback images without degrading the playback signals even in a different tape-speed mode from the recording mode.  
      In order to achieve the above objects, according to the present invention, video data compressed on the time axis is stored on a magnetic tape, so that when video signals from a monitoring video camera are to be stored on the magnetic tape continuously for a long time, the magnetic tape recording apparatus is operated for a period shorter than the monitoring period. Thus, it is not required to operate the magnetic tape recording device (VTR) continuously during the monitoring.  
      In other Words, the present invention provides a method for recording and playing back monitored video data for storing video signals, generated continuously for a long time, on a magnetic tape, wherein while continuously generating video signals for a long time and entering such video signals and compressing it on the time axis, then being output as digital video data and recorded on a magnetic tape, so that the digital video signals are stored on the magnetic tape for a period shorter than the period in which the video signals are generated continuously.  
      Furthermore, the present invention provides an apparatus for recording and playing back monitored video data and storing video signals generated continuously for a long time on a magnetic tape, comprising a means of generating video signals continuously for a long time; a means of entering video signals generated by the video signal generating means continuously for a long time and compressing the video signals on the time axis so as to be output as digital signal video data; and a means of recording and playing back digital video data from the time axis compressing means on a magnetic tape, wherein the period in which the magnetic tape recording means records digital video data on the magnetic tape is shorter than the period in which the video signal generating means generates video signals continuously.  
      In order to achieve the above objects, the monitored data recording &amp; playback apparatus provided by the present invention for recording video information for a long time, comprises the first memory means of storing video data temporarily; the first recording &amp; playback means of recording the data output from the first memory means on a recording medium; the second memory means of storing playback data from the first recording &amp; playback means temporarily; the second recording &amp; playback means of recording data output from the second memory means on a magnetic tape by forming oblique tracks on the magnetic tape; and a control means of controlling the write and read operations of the first and second means and recording and playback operations of the first and second recording &amp; playback means. The control means controls so that reading from the first memory means and recording by the first recording &amp; playback means are repeated each time the first specified data amount is reached, as well as controls the first recording &amp; playback means and the second recording &amp; playback means in the first operation mode so that they stop recording and playback operations respectively until the data recorded in the first recording &amp; playback means and not played back yet reaches the second specified data amount, which is greater than the first specified data amount. In addition, the control means controls the first and second recording &amp; playback means so that when the data recorded and not played back yet reaches the second specified data amount, the first recording &amp; playback means time-shares recording and playback of data until the data of the second specified data amount is played back completely. And furthermore, the control means controls so that the second recording &amp; playback means records the data read by the second memory means at a specified transfer rate on a magnetic tape fed continuously while forming oblique tracks on the tape and feeding the tape continuously.  
      Furthermore, in order to achieve the above objects, the recording &amp; playback apparatus provided by the present invention comprises: the first memory means of storing video data temporarily; the first and second recording &amp; playback means of recording the data output from the first memory means on a recording medium; the second memory means of storing playback data from the first and second recording &amp; playback means; the third recording &amp; playback means of recording the data output from the second memory means on a magnetic tape by forming oblique tracks on the tape; and a control means of controlling write and read operations of the first and second memory means and recording and playback operations of the first, second, and third recording &amp; playback means, so that when in recording of video data, the control means controls so that four operation modes are changed appropriately as follows according to the data amount to read and the data amount to play back in the first and second recording &amp; playback means; in the first operation mode, the control means controls so that reading of video data from the first memory means and recording by the first recording &amp; playback means are repeated each time the first specified data amount is reached and recording and playback operations of the second and third recording &amp; playback means are stopped. In the second operation mode, the control means controls so that reading of video data from the first memory means and recording by the second recording &amp; playback means are repeated each time the first specified data amount is reached and the video data read from the first recording &amp; playback means is stored in the second memory means temporarily, and then video data is read from the second memory means at a specified transfer rate while the third recording &amp; playback means is enabled to record transferred data on the magnetic tape by forming oblique tracks on the tape. In the third operation mode, the control means controls so that reading of video data from the first memory means and recording by the second recording &amp; playback means are repeated each time the first specified data amount is reached while recording and playback operations of the first and third recording &amp; playback means are stopped. And, in the fourth operation mode, the control means controls so that reading of video data from the first memory means and recording by the first recording &amp; playback means are repeated each time the first specified data amount is reached, and video data read from the second recording &amp; playback means is stored in the second memory means temporarily, and then video data is read from the second memory means at a specified transfer rate while the third recording &amp; playback means is enabled to record transferred data on the magnetic tape fed continuously by forming oblique tracks on the tape.  
      Furthermore, in order to achieve the above objects, the monitored video data recording &amp; playback apparatus provided by the present invention, which records compressed and encoded video data on a magnetic tape by forming oblique tracks on the tape, is further provided with a means of changing the time order of video data when in a recording operation so that only the first group video data selected from the above video data and compressed and encoded in the m (m=an integer of 2 or over) frame time interval is continued by n (n=an integer of 2 or over) frames; a means of restoring the initial video data time orders of the first group video data and other video data; a means of controlling so as to play back only the first group video data selectively and feed other data fast; and a memory means of storing the played-back first group video data temporarily. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of the monitored video data recording &amp; playback apparatus in the first embodiment of the present invention.  
       FIG. 2  is a block diagram indicating an example of the configuration of the compression encoder &amp; decoder circuit used in the monitored video data recording &amp; playback apparatus shown in  FIG. 1 .  
       FIG. 3  is a block diagram indicating an example of the VTR circuit used in the monitored video data recording &amp; playback apparatus shown in  FIG. 1 .  
       FIG. 4  is a block diagram indicating an example of the control circuit used in the monitored video data recording &amp; playback apparatus shown in  FIG. 1 .  
       FIG. 5  is a timing chart explaining the operation of the monitored video data recording &amp; playback apparatus, especially the operation of the control circuit shown in  FIG. 1 .  
       FIG. 6  is a block diagram indicating an example of the compression encoder &amp; decoder circuit used in the monitored video data recording &amp; playback apparatus in the second embodiment of the present invention.  
       FIG. 7  is a timing chart indicating the operation of the thinning-out circuit in the second embodiment shown in  FIG. 6 .  
       FIG. 8  is a block diagram indicating the configuration of the monitored video data recording &amp; playback apparatus in the third embodiment of the present invention.  
       FIG. 9  is a block diagram indicating an example of the configuration of the control circuit used in the monitored video data recording &amp; playback apparatus in the third embodiment.  
       FIG. 10  is a timing chart indicating the operation of the control circuit used in the monitored video data recording &amp; playback apparatus in the third embodiment.  
       FIG. 11  is a block diagram indicating the configuration of the monitored video data recording &amp; playback apparatus in the fourth embodiment of the present invention.  
       FIG. 12  is a block diagram indicating an example of the configuration of the data order change circuit used in the monitored video data recording &amp; playback apparatus shown in  FIG. 11 .  
       FIG. 13  is a timing chart indicating the operation of the data order change circuit used in the monitored video data recording &amp; playback apparatus shown in  FIG. 11 .  
       FIG. 14  is a timing chart indicating the operation of the monitored video data recording &amp; playback apparatus in the fourth embodiment of the present invention shown in  FIG. 11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereunder, the preferred embodiments of the present invention will be explained with reference to the attached drawings.  
       FIG. 1  is a block diagram of the recording &amp; playback apparatus for monitored video data in the first embodiment of the present invention. In  FIG. 1 , numeral  1  indicates a terminal to input video signals from a video camera (not illustrated),  2  indicates a terminal to output video signals,  3  indicates a compression encoder &amp; decoder circuit,  4  and  8  indicate semiconductor memories ( 4 : the first memory,  8 : the second memory),  5  indicates an SCSI (Small Computer System Interface) controller,  7  indicates a hard disk unit (hereafter, to be abbreviated as HDD) provided with an SCSI interface,  9  indicates a VTR,  10  indicates a control circuit,  11  indicates an operator panel from which the operator instructs various operations,  12  indicates an A/D converter,  13  indicates a D/A converter, and  14  indicates an IEEE1394 interface circuit (hereafter, to be abbreviated as I/F circuit). Of those devices, the semiconductor memories (the first and second memories  4  and  8 ), the SCSI controller  5 , the HDD  7  are combined to form a time axis compression circuit  100  for compressing video data to record on the time axis. Each of those devices forming the circuit  100  will be explained more in detail later.  
      At first, the operation of each device of the recording &amp; playback apparatus for monitored video data in the above configuration will be explained. Receiving a recording instruction from the operator panel  11 , the control circuit  10  generates control signals needed for each corresponding block to record video data. Then, the video signals entered from a video camera (not illustrated) are supplied to the A/D converter  12  via the video input terminal  1 . This A/D converter  12  converts analog video signals to 8-bit digital signals. After this, the compression encoder &amp; decoder circuit  3  encodes the digital video signals supplied from the A/D converter  12  according to the MPEG (Moving Picture Experts Group) standard. Furthermore, the first memory  4 , which is an FIFO (First In First Out) memory being capable of both writing and reading simultaneously, writes and reads MPEG data sequentially according to the control signals from the control circuit  10 .  
      The SCSI controller  5  sends an SCSI command, which writes MPEG data read from the first memory  4  into the HDD  7  according to the control signals from the control circuit  10 , to the HDD  7  The HDD  7  provided with an SCSI interface reads the command received from the SCSI controller  5  and writes the MPEG data entered into itself  7  according to the command. In addition, at a timing to be explained later, the SCSI controller  5  generates an SCSI command to instruct the HDD  7  to play back necessary data according to the control signals from the control circuit  10 . According to this command, the data output from the HDD  7  is sent to the second memory  8 .  
      Just like the first memory  4 , the second memory  8  is also an FIFO memory that can write and read data simultaneously. The second memory  8  thus writes and reads data sent from the SCSI controller  5  sequentially according to the control signals from the control circuit  10 . In addition, the I/F circuit  14  combines the VTR command sent from the control circuit  10  and the MPEG data output from the second memory  8  according to the IEEE1394 standard to generate serial data to be sent to the VTR  9 . The VTR  9  then separates this serial data to commands and data, and writes the separated data on a magnetic tape according to the separated command. In the monitored video data recording &amp; playback apparatus, video data from a video camera is recorded on a magnetic tape loaded in the VTR  9  via the first memory  4 , the, HDD  7 , and the second memory  8  in such way.  
      Subsequently, the compression encoder &amp; decoder circuit  3  will be explained.  FIG. 2  is a block diagram indicating the configuration of the compression encoder &amp; decoder circuit  3 . In  FIG. 2 , numeral  21  indicates a video decoder circuit,  22  indicates a temporal recording circuit,  23  indicates a subtraction circuit,  24  and  34  indicate switches,  25  indicates a DCT (Discrete Cosine Transform) circuit,  26  indicates a quantizer circuit,  27  indicates a variable length encoder circuit,  28  and  35  indicate data buffers,  29  and  37  indicate inverse quantizer circuits,  30  and  38  indicate inverse DCT circuits,  31  and  39  indicate adder circuits,  32  and  41  indicate image memories,  33  indicates a motion compensation prediction circuit,  36  indicates a variable length decoder circuit,  40  indicates a motion compensation circuit,  42  indicates a video encoder circuit,  43  indicates a freeze control circuit, and  44  indicates a switch.  
      In the compression encoder &amp; decoder circuit  3  having such a configuration, video signals digitalized in the A/D converter  12  are converted by the video decoder circuit  21  to luminance signals and color difference signals according to the number of pixels needed for encoding. In the case of the MPEG&#39;s B (Bidirectionally predictive-coded) pictures, video data whose timings are reversed are used for encoding, so input video data are reordered in the temporal reordering circuit  22  according to each picture type of I (Intra-coded), B, and P (Predictive-coded). In addition, input image data and difference data are selected in the switch  24  corresponding to the intra-frame encoding and the inter-frame encoding. Furthermore, selected image data is converted to a spatial frequency region in units of a block comprising 8 pixels×8 lines in the DCT circuits  25 . The data is then quantized in the quantizer circuit  26  through an operation with a quantization matrix. This quantized data is encoded to variable length data in the variable length encoder circuit  27  together with the motion vector from the motion compensation prediction circuit  33  and encoded mode information. The data is then stored in the buffer  28  until it is output as MPEG bit streams.  
      The buffer  28  outputs the amount information of the data stored in the buffer to the control circuit  10 , so that the buffer is prevented from both overflow and underflow. On the other hand, the quantized data is decoded locally by the inverse quantizer circuit  29  and the inverse DCT circuit  30 , then stored in the image memory  32 . The switch  34  is changed over corresponding to the intra-frame encoding and the inter-frame encoding. Furthermore, the decoding motion compensation prediction circuit  33  decides an encoding mode and detects a motion vector according to the image data from the image memory  32  and input image data. The circuit  33  also generates and outputs reference image data needed for inter-frame encoding and inter-frame decoding.  
      On the contrary, when in a playback operation, data is decoded by following the above recording processings in reverse. In other words, each bit stream from the first memory  4  is stored in the buffer  35  first, then the parameters of the bit stream such as encoding mode, motion vector, etc. are separated in the variable length decoding circuit  36 . After this, the data is restored to image data in the inverse quantizer circuit  37  and the inverse DCT circuit  38 . In addition, in the motion compensation prediction mode, block data whose motion compensation is predicted is added to the data in each of the image memory  41 , the motion compensation circuit  40 , and the adder circuit  39  respectively, then composite signals are generated and output from the video encoder circuit  42 . The switch  44  usually selects the a side. When instructed “Freeze” from external, however, the freeze control circuit  43  stops writing into the image memory  41  at a timing at which the compression &amp; decoding time is corrected. At the same time, the switch  44  is pushed down to the b side so that data read from the image memory  41  is entered into the video encoder  42 . Thus, a frozen image can be output.  
      Subsequently, the details of the VTR  9  shown in  FIG. 1  will be explained with reference to Table 1 and  FIG. 3 .  
      This VTR  9  conforms to the D-VHS standard so as to be enabled to record and playback digital signals on the basis of the VHS mechanical system. Table 1 shows the basic technical specifications of this D-VHS standard (STD) mode.  
                   TABLE 1                          Tape   Newly established grade based on S-VHS tape       Cassette   D-VHS cassette       Mechanism   Based on present VHS mechanism       Recording Time/Capacity   Standard . . . DF-300 (5 hours, 31.7 GB)           Max . . . DF-420 (7 hours, 44.4 GB)                         Track Composition   Tape speed   16.67 mm/sec           Head azimuth   +−30 deg.           Drum rotation   1800 rpm           Tracking system   CTL track system       Recording Specification   Main data input rate   14.1 Mbps           Sub data input rate   0.146 Mbps           Recording rate   19.14 Mbps           Track structure   1 sector           Length of sync block   112 bytes           Inner ECC   RS code           Outer ECC   RS code           Code word shuffling   6 tracks           Modulation system   SI-NRZI                     Interface   Based on IEEE1394 digital interface                  
 
      In other words, the speed of this D-VHS tape (refer to the section of track composition) is a half of that of the VHS standard (SP) mode. When a standard tape is used, main data (video and audio) of 14.1 Mbps (bit per second) can be recorded for 5 hours.  
      In the block diagram indicating the configuration of the VTR  9  shown in  FIG. 3 , numeral  71  indicates an IEEE1394 digital interface circuit (hereafter, to be abbreviated as I/F circuit),  72  indicates an error correction encoder,  73  indicates a modulator circuit,  74  indicates a control circuit,  75  indicates a servo circuit,  76  indicates an error correction decoder,  77  indicates a demodulator circuit,  78  indicates a wave equalizer &amp; selector circuit,  79  indicates a magnetic tape,  80  indicates a rotary cylinder,  81  and  82  indicate magnetic heads, and  83  indicates a capstan.  
      In this configuration of the VTR  9 , a recording mode command and data to record are entered into the I/F circuit  71  on the VTR  9  side from the I/F circuit  14  on the HDD  7  side when in a recording operation. Then, the I/F circuit  71  separates the command from the data, and sends the command to the control circuit  74  and the data to the error correction encoder  72  respectively. On the other hand, the control circuit  74  sends necessary control signals to each device to instruct the VTR  9  to execute specified operations according to the entered commands of “RECORD”, “PLAYBACK”, etc. In other words, the error correction encoder  72  shuffles data in units of  6  tracks as shown in Table 1, to execute RS (Read Solomon) encoding. After this, the data is modulated to SI-NRZI data in the modulation circuit  73 , then recorded on the magnetic tape  79  via the magnetic heads  81  and  82  attached to the rotary cylinder  80 . The servo circuit  75  controls the rotation speed and phase of the rotary cylinder  80  and the capstan  83  to form the specified tracks on the magnetic tape  79 .  
      On the contrary, receiving the “PLAYBACK” command via the I/F circuit  71 , the control circuit  74  sends control signals necessary for a playback operation to each device. When in such a playback operation, data on the magnetic tape  79  is played back by the magnetic heads  81  and  82  and entered into the wave form equalizer &amp; selector  78 . This waveform equalizer &amp; selector  78  integrates and equalizes data to compensate the differential characteristics by magnetic recording &amp; playback operation, then decides “1” or “0” by comparing the result with the threshold level. In addition, the demodulator circuit  77  executes a reverse processing of the processing of the modulator circuit  73 . Then, the error correction decoder  76  corrects errors generated in a recording/playback process and to restore the data order through deshuffling. And, the data from the error correction demodulator  76  is output via the I/F circuit  71 .  
      Receiving the “STOP” command via the I/F circuit  71 , the control circuit  74  sends control signals to the servo circuit  75  to stop the rotary cylinder  80  and the capstan  83 , as well as unloads the magnetic tape  79  (separates the magnetic tape  79  from the rotary cylinder  80 ).  
      Subsequently, the operation of the control circuit  10  will be explained, especially focusing on the recording operation, with reference to  FIG. 4 .  FIG. 4  is a block diagram indicating the configuration of the control circuit  10  when in a recording operation. In  FIG. 4 , numeral  51  indicates the first memory write control circuit,  52  indicates the first memory-read control circuit,  53  indicates the first memory data amount monitoring circuit,  54  indicates an HDD recording control circuit,  55  indicates a mode decision circuit,  56  indicates an HDD playback control circuit,  57  indicates an HDD data amount monitoring circuit,  58  is the second memory write control circuit,  59  indicates the second memory read control circuit, and  60  indicates the second memory data amount monitoring circuit.  24   
      In the control circuit  10  having such a configuration, the first memory write control circuit  51  sends control signals to the first memory  4  when data is to be recorded. According to the control signals, the memory  4  writes data in itself  4 . At this time, the first memory write control circuit  51  controls the average write frequency of the first memory  4  according to the data amount information from the buffer  28  of the compression encoder &amp; decoder  3  to prevent the buffer  28  from both overflow and underflow. This operation can be realized by controlling the buffer so that, for example, data is written in the buffer  28  at a frequency higher than the average write frequency and writing is stopped temporarily when the data count in the buffer  28  comes close to zero. In addition, the first memory read control circuit  52  sends control signals to the first memory  4  according to the mode signal from the mode decision circuit  55 , thus reading the data from the first memory.  
      The first memory data amount monitoring circuit  53  in the control circuit  10  incorporates a counter circuit used for monitoring the difference between the amount of the bit stream data stored in the first memory  4  and the amount of the data read from the memory  4 . The circuit  53  generates an MRR1 (Mmeory 1 Read Request) signal according to the count value and sends the signal to the mode decision circuit  55 . This MRR1 signal is used for preventing the first memory  4  from overflow. For example, the first memory  4  is set to the low level until the counter value reaches the specified value (eg., 15.5 Mbits) and to the high level when the value is exceeded, keeping the level high until the count value reaches a value close to zero (eg., 0.5 Mbits) or under. The HDD recording control circuit  54  and the HDD playback control circuit  56  provided in the control circuit  10  respectively generate control signals necessary for recording and playing back data in and from the HDD  7  according to the commands from the mode decision circuit  55  and send the control signals to the SCSI controller  5 . In other words, the HDD recording control circuit  54  increases the write address from zero sequentially and controls so that the maximum value is returned to zero cyclically. The HDD playback circuit  56  also controls write addresses just like the circuit  54 . As a result, the hard disk unit (HDD)  7  works as a means of recording and playing back data cyclically, that is, as an FIFO (First In First Out). The HDD data amount monitoring circuit  57 , when in recording, incorporates a counter used for monitoring the difference between the amount of the recorded data in the hard disk unit (HDD)  7  and the amount of the data read (played back) from the HDD  7 . The HDD data amount monitoring circuit  57  generates an HRR (Hard Disk Read Request) signal according to this counter value and sends the signal to the mode decision circuit  55 . In other words, this HRR signal prevents the HDD  7  from overflow. For example, the HDD  7  is set to the low level until the counter value reaches the second specified amount (eg., 1 GB), and the HDD  7  is set to the high level when the counter value exceeds the second specified amount, keeping the level high until the counter value reaches a value close to zero (eg., 0.5 Mbits) or under.  
      Furthermore, the second memory write control circuit  58  and the second memory read control circuit  59  generate control signals necessary for a write/read operation in/from the second memory according to the commands from the mode decision circuit  55  and sends the signals to the second memory  8 . On the other hand, the second memory data amount monitoring circuit  60  incorporates a counter circuit used for monitoring the difference between the amount of the bit stream data written in the second memory  8  and the amount of the data read from the memory  8 . The second memory data amount monitoring circuit  60  generates an MWR2 (Memory 2 Write Request) signal and an MRR2 (Memory 2 Read Request signal according to the counter value and sends those signals to the mode decision circuit  55 . This MWR2 signal requests that data is to be entered into the second memory  8  when data in the memory  8  becomes less. For example, the second memory  8  is set to the high level when the counter value reaches a ¼ capacity (4M bits) of the full memory capacity (eg., 16M bits), keeping the level high until the counter value reaches ¾ (12M bits) of the second memory capacity (16M bits) or over. When the counter value becomes ¾ or over, the memory  8  is set to the low level again. The MRR2 signal indicates that enough data is written in the second memory  8 , so that data can be read from the memory  8  at a certain rate. For example, when the counter value reaches ½ of the second memory  8  capacity (16M bits) or over, the memory  8  is set to the high level, keeping the level high until the counter value reaches zero. When the counter value reaches zero, the memory  8  is set to the low level.  
      The mode decision circuit  55  provided in the control circuit  10  decides an operation mode according to the MRR1 signal, the HRR signal, the MWR2 signal, and the MRR2 signal and sends commands to the first memory read control circuit  52 , the HDD recording control circuit  54 , the HDD playback control circuit  56 , the second memory read control circuit  59  and the I/F circuit  14  so that those devices operate corresponding to the decided mode. Table 2 shows the relationships among input signals, output commands, and selected modes.  
      In Table  2 , “-” indicates “Don&#39;t Care”. When the “STOP” command is issued, the mode decision circuit  55  sends the “STOP” command to each of the above devices. Consequently, the first memory  4  stops reading, the HDD  7  stops recording/playback, the second memory  8  stops writing/reading, and the VTR  9  stops recording.  
      When the “HDD RECORD” command is output, the mode decision circuit  55  sends the command to both the first memory read control circuit  52  and the HDD recording control circuit  54  to execute the specified operation. As a result, data is read from the first memory and the data is then written in a hard disk provided in the HDD  7  via the SCSI controller  5 .  
      When the “HDD PLAYBACK” command is output, the mode decision circuit  55  sends the command to both the HDD playback control circuit  56  and the second memory write control circuit  58  to execute the specified operation. As a result, data is read from the hard disk provided in the HDD  7  and the data is written in the second memory via the SCSI controller  5 .  
                                       TABLE 2                       Operation Mode   MRR1   HRR   MWR2   MRR2   Command Output   Selected Mode                  1st Mode   L   L   —   —   STOP               H   —   —   —   HDD RECORD           L   H   —   —   STOP   To 2nd mode       2nd Mode   L   H   L   L   STOP           H   —   —   —   HDD RECORD           L   H   H   —   HDD READ           —   —   —   H   VTR RECORD           L   L   L   L   STOP   To 1st mode                  
 
      Furthermore, when the “VTR RECORD” is output, the mode decision circuit  55  sends the command to both the second memory read control circuit  59  and the I/F circuit  14  to execute the specified operation. As a result, data is read from the second memory and the data is then recorded on the magnetic tape loaded in the VTR  9  via the I/F circuit  14 .  
      Subsequently, a long video recording operation of the recording &amp; playback apparatus will be explained sequentially with reference to the timing chart shown in  FIG. 5 . The details of the apparatus configuration is as explained above. (The above recording operation means an operation for compressing video data on the time axis in the time axis compressing circuit  100  mentioned above.)  
      At first, when the apparatus begins a recording operation, data is written almost continuously in the first memory  4 . (In this chart, however, the above-mentioned temporary write stop period is not described to simplify this “first memory write” signal waveform.)  
      After a period of time T 1 , the data amount in the first memory  4  reaches the first specified value (15.5M bits) and the MRR1 signal that requests data reading from the first memory  4  is set to the high level. Consequently, data is read from the first memory  4  and the data is recorded in the HDD  7  only for the next period of time T 2 . In other words, data is kept read from the first memory  4  until the data amount in the memory  4  reaches 0.5M bits. This means that about 15M bits of data from this first memory  4  is recorded in the HDD  7 . After this, reading from the first memory  4  and recording into the HDD  7  are executed intermittently in the same way.  
      Hereunder, the times of both T 1  and T 2  are calculated approximately to understand how the above operations are executed actually.  
      If the average data rate of the bit stream output from the compression encoder/decoder circuit  3  is assumed to be R 1  (eg., R 1 =1.5M bps), the time T 1  required until the data amount in the first memory  4  reaches the first specified value D 1  (D 1 =15.5M bits) from zero (0) is calculated as follows. 
 
 T 1 =D 1 /R 1=15.5M bits/1.5M bps=10.3 sec 
 
      On the other hand, if the average recording/playback data rate of the HDD  7  is assumed to be R 2  (eg., R 2 =30M bps), the time T 2  required for recording the data amount obtained by subtracting 0.5M bits from the first specified value D 1  (D 1  (=15.5M bits)−0.5M bits=15M bits) is calculated as follows. 
 
 T 2=( D 1−0.5)/( R 2 −R 1)=15M bits/(30−1.5)M bps≈0.5 sec 
 
      Consequently, in the above operation mode, that is, in the first operation mode, the HDD  7  stops for about 10 sec after it begins an operation. Then, the HDD  7  records data only for about 0.5 sec. The HDD  7  repeats the operation cyclically.  
      After data transfer from the first memory  4  to the HDD  7  is repeated such way, the data amount in the HDD  7  increases. When the second specified value (eg., 1 GB) is reached, for example, the HRR signal (refer to the  57  shown in  FIG. 4 ) requesting data reading from the HDD  7  is set to the high level. And, after this, the apparatus enters the second operation mode. Since the data amount in the second memory  8  is zero (0) just after this second operation mode is entered, the MWR2 signal (refer to the  60  shown in  FIG. 4 ) requesting the second memory to read data is set still in the high level. Thus, the HDD  7  outputs data and data is written in the second memory  8 . And accordingly, the data in the HDD  7  is written in the second memory  8  via the SCSI controller  5 . The average output data rate of the HDD  7  is assumed to be 30M bps just like in the above recording operation at this time.  
      The data amount in the second memory  8  increases due to this writing from the HDD  7 . After this, when the data amount in the second memory  8  reaches 8M bits, the MRR2 signal requesting reading from the second memory  8  is set to the high level, and accordingly, data is read from the second memory  8  and the VTR 9  records the data. The reading data rate of the second memory  8  and the input data rate of the VTR  9  are set equally (14.1M bps). This fixed rate data is recorded on the magnetic tape  79  loaded in the VTR  9  from the second memory  8  via the I/F circuit  14 . The VTR  9  feeds the magnetic tape  79  continuously at a fixed rate (16.67 mm/sec) at this time. In other words, the VTR  9  is operated in the normal recording mode.  
      When the data amount in the second memory  8  reaches 12M bits, the MWR2 signal requesting writing into the second memory  8  is set to the low level. Thus, the HDD  7  stops the output and the writing into the second memory  8  is also stopped. Reading from the second memory  8  and recording into the VTR  9  are continued on even during this period, however. So, the data amount in this second memory  8  is reduced little by little.  
      After this, when the data amount in this second memory  8  is reduced to 8M bits, the MWR2 signal requesting writing into the second memory  8  is returned to the high level. And accordingly, data outputting from the HDD  7  and writing into the second memory are restarted. The data amount in the second memory  8  begins increasing again. As understood clearly from the waveform shown in the chart(especially, refer to the HDD OUTPUT signal)/writing/reading in/from the HDD  7  is controlled so as to be time-shared.  
      Hereafter, the same operations are repeated, so the data amount in the HDD  7  decreases. When the data amount in the HDD  7  is reduced to 0.5M bits or under, the HRR signal requesting reading from the HDD  7  is set to the low level. After this, however, reading from the second memory  8  and recording into the VTR  9  are continued until the data in the second memory is read completely. When the data amount in the second memory  8  reaches zero (0), the MWR2 signal is set to the high level and the MRR2 signal is set to the low level. Thus, the second operation mode is exited. In other words, the first operation mode is set again.  
      In the monitored video data recording &amp; playback apparatus of the present invention, data is compressed on the time axis and the first and second operation modes are alternated as explained above when in recording and this operation is continued until the magnetic tape recording the data reaches the end of the tape.  
      Subsequently, the video recording time of the monitored video data recording &amp; playback apparatus of the present invention and the VTR recording time to be reduced by the time axis compression will be explained.  
      If the capacity of one magnetic tape is assumed to be D 3  (D 3 =31.7 GB for the standard tape as shown in Table 1), the possible recording time T 3  is represented as follows. 
 
 T 3 =D 3 /R 1=31.7 GB/1.5 M bps=(31.7×8×1000)M bits/1.5M bps≈=169067 sec≈47 hours 
 
      In other words, one magnetic tape can record data for about 47 hours.  
      However, since the total recording time T 4  of the VTR  9  is obtained by dividing the magnetic tape capacity (D 3 ) by the main data input rate (R 3 ) shown in Table 1, the result becomes as shown below. 
 
 T 4 =D 3 /R 3=31.7 GB/14.1M bps=31.7×8×1000)M bits/14.1M bits≈17986≈=300 min=5 hours 
 
      The periods of the first and second operation modes (first operation mode: τ 1 , second operation mode: τ 2 ) when in a recording operation are as shown below.  
      At first, since the first operation mode period τ 1  means a time required until the amount of the image data output from the compression encoder &amp; decoder circuit  3  (bit stream average data rate=1.5M bps) is stored in the HDD  7  and the data amount reaches the second specified value (eg., 1 GB) from the minimum value of 0.5M bits, it is calculated as follows. 
 
τ1=(1 GB−0.5M bits)/ R 1=(8×1000−0.5)M bits/1.5M bps=5333 sec≈89 min≈1 hour 29 min 
 
      On the other hand, since the second operation mode period τ 2  means a time required until the data in the HDD  7  that reaches the second specified value is recorded on the magnetic tape  79  loaded in the VTR  9  (input data rate=14.1M bps), then the data amount reaches the minimum value of 0.5M bits (1 GB−0.5M bits), it is calculated as follows. 
 
τ2=(1 GB−0.5M bits)/ R 2=(8×1000−0.5)M bits/14.1M bps=567 sec≈9 min 27 sec 
 
      Consequently, after stopping for about one and half hours (in the first operation mode) on the standard magnetic tape (=31.7 GB) in the above apparatus due to the compressing operation executed by the time axis compression circuit  100  on the video data time axis, the D-VHS standard VTR  9  records data only for about 10 min (in the second operation mode). Repeating this cycle operation, the VTR  9  can record about 47-hour video data in an operation of about 5 hours. (This means that both the time when the magnetic tape  79  is in contact with the heads  81  and  82  and the time when the magnetic tape  79  is in contact with the rotary cylinder  80  while they are running are 5/47 of those of the conventional time lapse VTR respectively.) The conditions of this operation are not strict so much when compared with those of ordinary home VTRs. Consequently, a general home D-VHS standard VTR can be used as the VTR  9  without taking any special measures for that. The magnetic tape can also be protected from damages, since it is not kept loaded and not in contact with the capstan and the recording head for a long time. In other words, according to the present invention, the D-VHS standard VTR  9  used to record video data on a magnetic tape does not need any long time continuous recording, which is assumed as a special condition for a monitoring purpose, so a low-price home VTR can be used as the VTR  9 . In addition, the failure rate is very low and the magnetic tape used for the apparatus can be protected from damages and wear In the VTR  9  in this embodiment, the magnetic tape  79  is unloaded when the VTR  9  stops. However, the rotary cylinder may be stopped when the CTR  9  stops only to loosen the tension of the magnetic tape. Even in such a case, since the friction among the magnetic tape, the rotary cylinder, and the magnetic head is almost eliminated while the VTR  9  stops, the same effect as the above can be obtained.  
      On the other hand, the playback operation of the apparatus of the present invention will be as follows. The VTR  9  feeds the magnetic tape at the normal continuous feeding rate (16.67 mm/sec) to playback video data. Consequently, the output data rate of the VTR 9  is 14.1M bps. And, when in a playback operation, data flows reversely from the above recording operation; the data played back by the VTR  9  is sent to the compression encoder &amp; decoder circuit  3  via the I/F circuit  14 , the second memory  8 , the SCSI controller, the HDD  7 , and the first memory  4 . After this, the data is MPEG-decoded, then converted by the D/A converter  13  to analog signals and output from the terminal  2 .  
      Even in this playback operation, each memory and the HDD  7  are controlled so as to be prevented from overflow and underflow just like in the recording operation mentioned above. In addition, when in a playback operation, the transfer rate of the data to be entered into the compression encoder &amp; decoder circuit  3  is 1.5M bps just like at the recording time. So, the VTR  9  carries out a normal playback operation only while the specified amount of data is stored in the HDD  7 . Otherwise, the VTR  9  stops. Even while the VTR  9  stops, however, data is read from the HDD  7  and sent to the compression encoder &amp; decoder circuit  3 . The flow of output video data is thus never stopped. The method to control this playback operation is similar to the above recording method, so detailed explanation of the operation timing will be omitted here. Consequently, the operation time of the VTR  9  for playing back 47-hour video data that can be recorded on one standard magnetic tape will also be only about 5 hours in this case.  
      Subsequently, the monitored video data recording &amp; playback apparatus in the second embodiment of the present invention will be explained.  
      In this second embodiment, the input video data time axis is thinned out to record video data at a smaller rate and enable super-long time continuous recording of video data.  
      At first,  FIG. 6  is a block diagram of the compression encoder &amp; decoder circuit (refer to  3  in  FIG. 1 , and  FIG. 2 ) to thin out the above time axis. This compression encoder &amp; decoder circuit differs from the compression encoder &amp; decoder circuit  3  shown in  FIG. 2  in that a thinning-out circuit  45  is added and the speed of the compression encoding main processor (enclosed by a broken line) operation can be changed according to the mode signal from the thinning-out circuit  45 . This thinning-out circuit  45  writes the frame period of video data from the decoder  21  in a memory and reads the data from the memory for a k-frame period at a reading rate of 1/k (k=a natural number of 2 or over).  
       FIG. 7  shows the input and output timings of the thinning-out circuit  45  when the k value is  3 . In this case, the operation speed of the compression encoding main processor is set to ⅓. Consequently, the input signal time axis is thinned out to ⅓, and the compressed and encoded bit stream is output from the buffer at a ⅓ data rate (0.5M bps when other conditions such as quantization, etc. are the same as those of the first embodiment). In addition, one magnetic tape can record data for 141 hours, which is three times that of the first embodiment (47 hours).  
      According to the second embodiment, the output data rate of the compression encoder &amp; decoder circuit  3  becomes 1/k, so one standard magnetic tape can record (monitor) k times (47×k hours) when other conditions such as quantization, etc. are the same as those of the first embodiment. However, when the recording time of the VTR  9  can be calculated with the above expression, the result becomes 5 hours just like in the first embodiment. And, the VTR  9  stops for a 1.5×k hours (in the first operation mode), then records data only for about 10 min (in the second operation mode) at this time. Consequently, the wear and damages of the magnetic tape  79 , the heads  81  and  82 , and the rotary cylinder  80  in the VTR  9 , as well as the degradation of the motor, the belt, and other items can be reduced to 1/k of those of the first embodiment respectively (when the monitoring/recording time is the same).  
      Subsequently, the third embodiment of the present invention will be explained.  FIG. 8  is a block diagram of the monitored video data recording &amp; playback apparatus in the third embodiment of the present invention. In  FIG. 8 , numeral  15  indicates the second SCSI controller (SCSI controller  2 ),  16  indicates the second hard disk unit, and  17  indicates a control circuit. Other items given the same numerals as those in  FIG. 1  are the same as those in  FIG. 1 . In the above configuration, data output from the first memory  4  is recorded in both the first hard disk unit  7  (hereafter, to be abbreviated as HDD  1 ) and in the second hard disk unit (hereafter, to be abbreviated as HDD  2 ) alternately. Data output from the HDD  1  and from the HDD 2  alternately is written into the second memory  8 . Also in this third embodiment, data is written/read into/from HDD in any case via the SCSI controller  5  or  15  just like in the first embodiment.  
       FIG. 9  is a block diagram of the control circuit  17  shown in  FIG. 8  especially in a recording operation. In  FIG. 9 , numeral  63  indicates an HDD 2  recording control circuit,  64  indicates an HDD 1  output control circuit,  65  indicates an HDD 2  output control circuit,  66  indicates an HDD 1  internal data amount monitoring circuit,  67  indicates an HDD 2  data amount monitoring circuit,  68  and  69  indicate switches, and  70  indicates a mode decision circuit. Other items given the same numerals as those in  FIG. 6  are the same as those in  FIG. 4 .  
      Hereunder, the recording operation of this control circuit  17  will be explained. At first, the HDD recording control circuits  62  and  63  function just like the HDD recording control circuit  54  shown in  FIG. 6 . The HDD output control circuits  64  and  65  also function just like the HDD output control circuit  56  shown in  FIG. 6 . In addition, the HDD 1  data amount monitoring circuit  66  functions just like the HDD data amount monitoring circuit  57  shown in  FIG. 4 . In other words, the circuit  66  generates the HRR 1  (Hard Disk  1  Read Request) signal and sends the signal to the mode decision circuit  70 . The HDD data amount monitoring circuit  67  also functions just like the HDD data amount monitoring circuit  57  shown in  FIG. 4 . The circuit  67  generates the HRR 2  (Hard Disk  2  Read Request) signal and sends the signal to the mode decision circuit  70 .  
      The mode decision circuit  70  outputs commands to each device according to the MRR1 signal, the HRR 1  signal, the HRR 2  signal, MWR2 signal, and the MRR2 signal entered into itself and controls the switches  68  and  69 . Table 3 shows the relationships among those input signals, output commands, and switch controls.  
      In other words, the “HDD RECORD” command is sent to the HDD 1  or HDD 2  record control circuit  62  or  63  via the switch  68 . The “HDD OUTPUT” command is sent to the HDD 1  or HDD 2  output control circuit  64  or  65  via the switch  69 .  
      Subsequently, the operation of the monitored video data recording &amp; playback apparatus whose configuration is explained above will be explained with reference to the timing chart shown in  FIG. 10 . The explanation will be made mainly for the differences from the first embodiment, centering on the recording operation.  
      When a recording operation is started, the first operation mode (the first mode in  FIG. 10 ) is set. At this time, the switch  68  selects the HDD 1  and the switch  69  selects the HDD 2  according to the control signals from the mode decision circuit  70 . Consequently, the operation in this first operation mode becomes the same as that in the first operation mode in the first embodiment. Then, the data in the HDD 1  increases. When the HRR 1  signal is set to the high level, the second operation mode (second mode) is set.  
      In this second operation mode, the mode decision circuit  70  makes the switch  68  select the HDD 2 . As a result, the “HDD RECORD” command is entered into the HDD 2  in the second operation mode. And, the data read from the first memory  4  is written into the HDD 2 . On the other hand, the switch  69  is changed over to select the HDD 1 . Then, the “HDD OUTPUT” command is entered into the HDD 1  in the second operation mode. Consequently, the data output from the HDD 1  is written into the second memory  8 . In other words, in this third embodiment, the output from the HDD 1  is always executed according to the MWR2 signal unlike the first embodiment. Recording into the HDD 1  is thus never interrupted by the MRR1 signal. The data amount in the HDD 1  decreases due to this output operation. When the data amount reaches 0.5M bits or under, the HRR 1  signal is set to the low level. After this, the data amount in the second memory  8  reaches zero (0), when the MRR2 signal is set to the low level. As a result, the operation enters the third operation mode (3rd mode).  
                                                   TABLE 3                       Operation                       Command           Mode       Mode   MRR1   HRR1   HRR2   MWR2   MRRR2   Output   SW68   SW69   Change                  1st Mode   L   L   —   —   —   STOP   HDD1   HDD2               H   —   —   —   —   HDD                               RECORD           L   H   —   —   —   STOP           To 2nd                                           mode       2nd Mode   L   H   —   L   L   STOP   HDD2   HDD1           H   H   —   —   —   HDD                               RECORD           —   H   —   H   —   HDD                               OUTPUT           —   —   —   —   H   VTR                               RECORD           —   L   —   —   L   STOP           To 3rd                                           mode       3rd Mode   L   —   L   —   —   STOP   HDD2   HDD1           H   —   —   —   —   HDD                               RECORD           L   —   H   —   —   STOP           To 4th                                           mode       4th Mode   L   —   H   L   L   STOP   HDD1   HDD2           H   —   H   —   —   HDD                               RECORD           —   —   H   H   —   HDD                               OUTPUT           —   —   —   —   H   VTR                               RECORD           —   —   L   —   L   STOP           To 1st                                           mode                  
 
      In this third operation mode, the switches  68  and  69  are set just like in the second operation mode. The data read from the first memory  4  is thus written into the HDD 2 . This HDD 2  does not output any data at this time. So, the data amount in the HDD 2  increases. And, when the HRR 2  signal enters the high level, the fourth operation mode (4th mode) is set.  
      In this fourth operation mode, the switches  68  and  69  are set just like in the first operation mode. In this fourth mode, writing and reading into/from HDD 1  and HDD 2  are alternated by exchanging the HDD 1  and HDD 2 . In other words, when the data amount in the HDD 2  becomes less and the HRR 2  signal enters the low level, and then when the data amount in the second memory  8  becomes zero (0) and the HRR 2  signal enters the low level, the operation mode returns to the first one.  
      In the monitored video data recording &amp; playback apparatus in the third embodiment of the present invention explained above, the output from the HDD  7  or  16  is never interrupted by the MRR1 signal. In other words, recording and output operations are not time-shared. Consequently, the present invention can have an advantage for using a hard disk unit whose recording or output data rate is comparatively low. In other words, when the output rate of the compression encoder &amp; decoder circuit  3  is 1.5M bps, the recording rate should be 1.5M bps or over. When the input data rate of the VTR  9  is 14.1M bps, the playback data rate should be 14.1M bps or over. And, just like in the first embodiment, the wear and degradation of the magnetic tape  79 , the head  81  and  82 , and other mechanical parts can be reduced effectively.  
      Hereunder, another embodiment of the present invention will further be explained.  FIG. 11  is a block diagram of the monitored video data recording &amp; playback apparatus in the fourth embodiment of the present invention. In  FIG. 11 , numeral  18  indicates a data sorting circuit and  19  indicates a control circuit. Other items given the same numerals as those in  FIG. 1  are the same as those in  FIG. 1 .  
       FIG. 12  is a block diagram indicating the configuration of the data sorting circuit  18 . In  FIG. 12 , numeral  101  indicates a type judgement circuit,  102 ,  103 ,  109 , and  110  indicate FIFO memories,  104  and  111  indicate memory control circuits,  105  indicates an order code adding circuit,  106  and  107  indicate FIFO buffers,  108  indicates a code decision circuit,  112  and  114  indicate switch circuits, and  113  indicates an order code deletion circuit.  
      In the configuration of the above data sorting circuit  18 , the type decision circuit  101  detects the sync code (Picture Start Code to be abbreviated as PST) and the video type code (Picture Coding Type to be abbreviated as PCT) for starting a picture layer from the MPEG bit stream output from the compression encoder &amp; decoder circuit  3  when in a recording operation to decide the video type (I picture, P picture, or B picture) and sends the information to the memory control circuit  104  and the order code adding circuit  105 . The order code adding circuit  105  adds an order code to each picture. This order code indicates the order of each picture layer in the MPEG bit stream. For example, the order code is generated in the counter circuit counting up each time a PST is detected. The memory control circuit  104  receives the data amount information of the buffer  28  output from the compression encoder &amp; decoder circuit  3  through the control circuit  19  and generates control signals for writing data into the memories  102  and  103  according to the information (so as to prevent the buffer  28  from overflow and underflow). At this time, P and B picture data are controlled so as to be written into the PB memory  103  according to the video type information output from the type judgement circuit  101 . Other data, that is, various sync codes and the data such as the number of pixels, the number of lines, etc. in the video sequence layer, as well as I picture are written into the I picture memory  102 . Furthermore, the memory control circuit  104  counts the data amount of both P and B pictures using entered video type information. The circuit  104  resets the count value when the specified value is reached (eg., 600 tracks on the magnetic tape). The circuit  104  then generates control signals so that the data having been written into the I memory  102  so far is read and transferred to the buffer  106  via the switch  114 . After this, the circuit  104  generates control signals so that the data written into the PB memory  103  is read during the above same period and transferred to the buffer  106  via the switch  114 .  
       FIG. 13  is a timing chart of the data sorting circuit  18  when in a recording operation. As understood clearly from this chart, writing and reading into/from the I memory  102  and the PB memory  103  are controlled as explained above to obtain the output bit stream as shown at the bottom in  FIG. 13 . In other words, I picture data (including various sync codes and data such as the number of pixels, the number of lines, etc. in the video sequence layer) is output first, then P and B picture data for  600  tracks is output.  FIG. 13  shows data output (P and B pictures for 600 tracks) only by once, but the operation is repeated actually to output a bit stream comprising only an I picture and a bit stream comprising P and B pictures are output alternately.  
      The bit stream shown in  FIG. 13  and output from the data sorting circuit  18  is transferred to the VTR  9  via the first memory  4 , the SCSI controller  5 , the HDD  7 , the second memory  8 , and the I/F circuit  14  just like in the first embodiment, so that the data is recorded on the magnetic tape  79 . As for the track pattern on the magnetic tape  79 , a section recording only an I picture and a section recording P and B pictures are alternated. Thus, 600 tracks are recorded with P and B pictures.  
      In the monitored video data recording &amp; playback apparatus in this fourth embodiment, the data played back by the VTR  9  is entered into the data sorting circuit  18  via the I/F circuit  14 , the second memory  8 , the SCSI controller  5 , the HDD  7 , and the first memory  4  when in a normal playback operation. This entered playback data is then entered into both the order code deletion circuit  113  and the code judgement circuit  108  via the buffer  107 . The code decision circuit  108  detects the PCT (Picture Coding Type) and the order code added in the order code addition circuit  105  when in a recording operation and sends both information items to the memory control circuit  111  and the order code deletion circuit  113 . This order code deletion circuit  113  deletes the order code from the playback data using the order code timing information output from the code judgement circuit  108 . The memory control circuit  111  generates control signals according to the video type (I, P, and B pictures information from the code judgement circuit  108  so that I picture data is written into the I memory  109  and P and B picture data is written into the PB memory  110 . The memory control circuit  111  generates control signals according to the order indicated by the order code output from the code judgement circuit  108  so that data is read from the I memory  109  and the PB memory  110 . This read data is then output via the switch  112  and sent to the compression encoder &amp; decoder circuit  3 . When in a normal playback operation, the switch  112  selects the memory from which data is read. As explained above, when in a normal playback operation, the data order changed when in a recording operation is returned to the initial order and normal MPEG bit streams are sent to the compression encoder &amp; decoder circuit  3 . The compression encoder &amp; decoder circuit  3  executes normal MPEG-decoding to reproduce NTSC signals just like in the first embodiment.  
      Subsequently, the fast playback operation of the monitored video data recording &amp; playback apparatus in the fourth embodiment of the present invention will be explained. When a “fast playback” is instructed from the operator panel, the control circuit  19  sends the “NORMAL PLAYBACK” command to the VTR  9  via the I/F circuit  14 . According to the command, the VTR  9  plays back data from the magnetic tape. Playback data is then sent to the second memory via the I/F circuit  14 , and to the control circuit  19  at the same time. The control circuit  19  detects the PCT (Picture Coding Type) in the playback data to decide the playback data video type. When the video type is decided to be I type, the “normal playback” operation is continued. And, the playback data is entered into the data sorting circuit  18  via the second memory  8 , the SCSI controller  5 , the HDD  7 , and the first memory  4 . The playback data entered into the data sorting circuit  18 , after the order code is deleted from itself in the order code deletion circuit  113  just like when in a normal playback operation, is written into the I memory  109 . Reading from this I memory  109  is executed intermittently as shown in the timing chart in  FIG. 14 . While no data is read from the I memory  109 , the memory control circuit  111  generates the freeze signal. This freeze signal is sent to the compression decoder circuit  3  via the control circuit  19 . The compression decoder circuit  3  delays the freeze signal by a time required to the decoding and stops writing into the image memory  41 . At the same time, the circuit  3  changes over the switch  44  to the output of the image memory  41 . Consequently, compressed and decoded video data and its frozen data are output alternately. In the timing chart shown in  FIG. 14 , one decoded image frame is frozen twice. In this  FIG. 14 , it is assumed that I pictures are encoded at pitches of  15  frames. Each output picture is numbered. As shown in  FIG. 14 , after the first picture is output by  3  frames, the 16th picture is output by  3  frames, and then the 31st picture is output by 3 frames. Consequently, the average change speed of the output pictures becomes 15/3=5 times. In other words, pictures are output at a 5-time speed.  
      When such a normal playback operation is continued, the magnetic tape comes to a region where P and B pictures are recorded. Then, the control circuit  19 , when deciding the playback data picture type to be a P or B picture, sends the “600 TRACKS FAST FEED” command to the VTR  9  via the I/F circuit  14 . The VTR  9  plays back the CTL signals recorded in the control track of the magnetic tape and feeds the magnetic tape fast by counting the CTL. When the CTL count value reaches 600 tracks, the VTR  9  sends the “end of fast feed” information to the control circuit  19  via the I/F circuit  14 . The control circuit  19  sends the “NORMAL PLAYBACK” or “STOP” command to the VTR  9  via the I/F circuit  14  according to the data amount in the HDD. In other words, when the data amount is less, the “NORMAL PLAYBACK” command is sent. When the data amount is much and the HDD might overflow, the “STOP” command is sent.  
      As explained above, according to the fourth embodiment of the present invention, only I pictures can be played back and output selectively. Thus, the playback pictures can be displayed fast. Furthermore, since the operation mode of the VTR  9  in a playback operation of the I pictures is normal playback, the tracing angles of the heads  81  and  82  match with the angle of the recording track pattern on the magnetic tape  79 . This is why perfect pictures can be played back with less S/N degradation. In the above embodiment, since two frames of a picture are frozen, the picture is fed at a 5-time speed. The speed can be selected as follows. If only a frame is frozen, the picture can be fed at a 10-time speed, and furthermore, the picture can be fed at a 15-time if no frame is frozen.  
      As understood clearly from the above detailed explanation, according to the apparatus and method of the present invention for recording and playing back monitored video data, the video data to record is compressed on the time axis, so the period of recording video data becomes shorter than the period in which video signals are generated continuously. Consequently, the apparatus requires no mechanism to feed the magnetic tape very slowly or intermittently, since a recording operation is not continued for a long time. It can be done intermittently unlike the conventional method, which must continue recording of data while the VTR must be in contact with the magnetic tape while the recording head rotates. Any general home VTR can thus be used as the VTR of the present invention, reducing the cost significantly. In addition, the apparatus can record video data for a long time enough for the purpose of monitoring. And, furthermore, the method of the present invention can suppress the degradation of the magnetic tape, the wear of the recording heads, or the degradation of the driving belt and other mechanical items significantly. When the time axis is thinned out, the rate of the operation time of the second recording &amp; playback apparatus becomes lower than the monitoring and recording time. Thus, the above effect increases further more.  
      In addition, according to the monitored data recording &amp; playback apparatus of the present invention, the VTR can record and play back data at a normal tape feed speed, so it does not require controlling of high precision intermittent tape feeding, which is indispensable for the prior art time lapse VTR. The manufacturing cost of the apparatus can thus be reduced significantly, as well as the apparatus can be free of the S/N degradation to be caused by mismatching of the track angle on the magnetic tape with the tracing angle of the recording head, enabling good quality playback pictures to be output.  
      Furthermore, according to the monitored data recording &amp; playback apparatus of the present invention, only the video data compressed and encoded at the m-frame time intervals is recorded in units of n frames on the magnetic tape, so only that portion is usually played back and other portions are fed fast, so that video data can be played back fast with no N/S degradation to be caused by mismatching of the track angle on the magnetic tape with the tracing angle of the recording head.