Abstract:
A video signal recording apparatus is capable of compressing the amount of information of a video signal sequentially containing a plurality of pictures correlated to one another. In this compression, the video signal recording apparatus can selectively assume an intrapicture coding mode for executing compression by using only a correlation within each of pictures of the video signal and an interpicture coding mode for executing compression by using a correlation between a plurality of pictures of the video signal. When a video signal the amount of information of which is compressed is to be recorded on a recording medium, the video signal recording apparatus starts recording with a video signal compressed in the intrapicture coding mode. When a video signal compressed by variable-length coding is to be recorded while sequentially forming a multiplicity of tracks on a recording medium, the video signal recording apparatus begins recording a picture to be recorded immediately after the start of recording, at the forefront of the tracks.

Description:
This application is a continuation of application Ser. No. 08/454,519 filed May 30, 1995 now abandoned, which is a Ser. No. 07/992,851, filed Dec. 16, 1992, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for recording a moving image and, more particularly, to an apparatus capable of compressing the amount of information by using a correlation between pictures or to an apparatus capable of effecting compression so that each picture has a different amount of information. 
     2. Description of the Related Art 
     A digital video tape recorder (VTR) is known as one type of image recording apparatus arranged to digitally compress moving-image information and record the digitally compressed moving-image information on a recording medium such as a magnetic tape. It is also known that there are two types of compression systems: a compression system using a fixed-length coding method and a compression system using a variable-length coding method. 
     FIG. 1 is a block diagram schematically showing the arrangement of a conventional example of an image recording and reproducing apparatus using the fixed-length coding method. An analog image signal indicative of a moving image to be recorded is inputted through an input terminal  10 . An A/D converter  12  digitizes the analog image signal inputted through the input terminal  10 , and a fixed-length coding circuit  14  encodes the output data of the A/D converter  12  by fixed-length coding. A modulating circuit  16  executes suppressed low-frequency modulation of the output of the fixed-length coding circuit  14 . The output of the modulating circuit  16  is amplified to a predetermined level by a recording amplifier  18 . A switch  20  is arranged to be selectively connected to contacts “a” and “b” during recording and reproduction, respectively. The output of the recording amplifier  18  is applied to a magnetic head  22  through the switch  20 , whereby the output is recorded on a magnetic tape  24 . 
     During reproduction, the signal recorded on the magnetic tape  24  is reproduced by the magnetic head  22  and the output of the magnetic head  22  is applied toga demodulating circuit  28  through the switch  20  and a reproducing amplifier  26 . A fixed-length- decoding circuit  30  is a decoding circuit corresponding to the fixed-length coding circuit  14 , and serves to decode the output of the demodulating circuit  28  and output a digital reproduced image signal. The output of the fixed-length decoding circuit  30  is converted into an analog signal by a D/A converter  32 , and the analog signal an analog reproduced image signal, is outputted for external use through an output terminal  34 . 
     The fixed-length coding circuit  14  normally adopts a coding method in which the amount of data after coding is kept constant for one picture (one field or one frame). The fixed-length coding method includes a pulse-code modulation (PCM) method accompanied by no compression as well as a method accompanied by digital compression, such as a so-called sub-sampling method and a differential coding (DPCM) method. Since the former method does not at all compress image information during processing, no substantial degradation occurs in image quality. However, a huge amount of data must be recorded and the rate of recording must be increased with the result that a large number of disadvantages occur in terms of hardware design and the recording density and time of a recording medium. 
     If digital compression is executed by using fixed-length coding, a compression ratio of approximately 1/4 to 1/6 can be achieved. However, if a high-definition television signal such as an HDTV signal is compressed at such a compression ratio, the amount of information to be recorded will still be excessively large. In addition, a considerable degradation in image quality is visually observed. 
     In contrast, the variable-length coding method using, for example, a Huffman code or an arithmetic code, can achieve a compression ratio of as high as approximately 1/10 to 1/20 without excessively degrading image quality. 
     However, the variable-length coding method also has a number of problems. In this coding method, the amount of information recorded per picture is basically inconstant and coding is executed by using a correlation between a plurality of pictures. Therefore, according to the variable-length coding method, it is difficult to realize various reproduction functions such as “edit”, “search” and “special reproduction”. This problem is particularly outstanding in the field of a digital VTR using as a recording medium a magnetic tape which is a sequential access medium. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a recording apparatus in which it is possible to easily realize various reproduction functions such as “edit”, “search” and “special reproduction” and which is capable of effecting recording at a high compression ratio without degrading image quality. 
     To achieve the above-described object, according to one aspect of the present invention, there is provided a video signal recording apparatus which comprises inputting means for inputting a video signal sequentially containing a plurality of pictures correlated to one another, the video signal having an amount of information, compressing means for compressing the amount of information of the video signal, the compressing means being capable of selectively assuming an intrapicture coding mode for executing compression by using only a correlation within each of pictures of the video signal and an interpicture coding mode for executing compression by using a correlation between a plurality of pictures of the video signal, recording means for recording on a recording medium the video signal the amount of information of which is compressed by the compressing means, operating means for commanding the recording means to start a recording, and controlling means responsive to an operation of the operating means for controlling the compressing means so that a video signal for one picture recorded immediately after the recording means has started the recording becomes a video signal compressed in the intrapicture coding mode. 
     According to another aspect of the present invention, there is provided a video signal recording apparatus which comprises inputting means for inputting a video signal sequentially containing a plurality of pictures correlated to one another, the video signal having an amount of information, compressing means for compressing the amount of information of the video signal, the compressing means varying an amount by which information is to be outputted for each picture, operating means for commanding a start of recording of the video signal, and recording means for recording the video signal the amount of information of which is compressed by the compressing means, while sequentially forming a multiplicity of tracks on a recording medium, the recording means being responsive to an operation of the operating means to begin recording a video signal for one picture to be recorded immediately after the start of recording, at the forefront of the tracks. 
     The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematically showing the arrangement of a conventional type of general digital VTR; 
     FIG. 2 is a block diagram schematically showing the arrangement of a digital VTR according to one embodiment of the present invention; 
     FIG. 3 is a block diagram showing a specific example of the arrangement of the variable-length coding circuit shown in FIG. 2; 
     FIG. 4 is a table showing the switching sequence of each switch shown in FIG. 3; 
     FIGS.  5 ( a ) and  5 ( b ) are diagrammatic views aiding in explaining the relations between input images and recorded images in the recording operation of the VTR of FIG. 2; 
     FIG. 6 is a table showing the relations between final images to be recorded by the VTR of FIG.  2  and actually recorded images; 
     FIG. 7 is a view showing one example of a recording track pattern formed by the VTR of FIG. 2; and 
     FIG. 8 is a view showing one example of a track pattern which is formed after tag recording by the VTR of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 2 is a block diagram schematically showing the arrangement of a digital VTR according to one embodiment of the present invention. 
     An analog image signal indicative of a moving image to be recorded is inputted through an input terminal  40 . An A/D converter  42  digitizes the analog image signal inputted through the input terminal  40 . If recording has not yet been started, a variable-length coding circuit  44  encodes the output data of the A/D converter  42  by intraframe coding. If an instruction to start recording is inputted from an operating switch  46 , the variable-length coding circuit  44  starts intraframe and interframe variable-length coding of the output data of the A/D converter  42 . If an instruction to stop recording is inputted from the operating switch  46 , the variable-length coding circuit  44  stops the intraframe and interframe variable-length coding. The variable-length coding circuit  44  will be described in detail later. 
     A modulating circuit  48  executes suppressed low-frequency modulation of the output of the variable-length coding circuit  44 . The output of the modulating circuit  48  is amplified to a predetermined level by a recording amplifier  50 . A switch  52  is arranged to be selectively connected to the contacts “a” and “b” during recording and reproduction, respectively. The output of the recording amplifier  50  is applied to a magnetic head  54  through the switch  52 , whereby the output is recorded on a magnetic tape  56 . 
     During reproduction, the signal recorded on the magnetic tape  56  is reproduced by the magnetic head  54  and the output of the magnetic head  54  is applied to a demodulating circuit  60  through the switch  52  and a reproducing amplifier  58 . A variable-length decoding circuit  62  is a decoding circuit corresponding to the variable-length coding circuit  44 , and serves to decode the output of the demodulating circuit  60  and output a digital reproduced image signal. The output of the variable-length decoding circuit  62  is converted into an analog signal by a D/A converter  64 , and the analog signal, i.e., an analog reproduced image signal, is outputted for external use through an output terminal  66 . 
     FIG. 3 is a block diagram of the arrangement of the variable-length coding circuit  44  which is a feature of the present embodiment, and shows in detail quantizing and predictive coding parts. The shown arrangement includes an input terminal  70  through which the output of the A/D converter  42  is inputted, and a blocking circuit  72  for forming raster-scan image data inputted through the input terminal  70  into blocks each having horizontal i pixels×vertical j pixels. The values of i and j are normally approximately 8 to 16. A delay circuit  74  is provided for delaying the output of the blocking circuit  72  by three frames. 
     The shown arrangement also includes subtracters  76 ,  78  and  80  for calculating a prediction error for predictive differential coding. The subtracter  76  subtracts a local decoded value obtained three frames before from the output of the blocking circuit  72 . The subtracter  78  subtracts from the output of the three-frame delay circuit  74  image data obtained by interpolating and integrating a local decoded value obtained one frame before and a local decoded value obtained four frames before. The subtracter  80  subtracts from the output of the three-frame delay circuit  74  image data obtained by interpolating and integrating a local decoded value obtained two frames before and a local decoded value obtained five frames before. 
     The shown arrangement also includes a switch  82  for selecting the output of the blocking circuit  72  (the contact “a”), the output of the subtracter  76  (the contact “b”), the output of the subtracter  78  (the contact “c”) or the output of the subtracter  80  (the contact “d”). 
     The shown arrangement also includes a DCT circuit  84  for executing discrete cosine transform of data selected by the switch  82 , a quantizing circuit  86  for quantizing the output (frequency coefficient) of the DCT circuit  84  by a different quantum step for each frequency coefficient, and an inverse quantizing circuit  88  for executing inverse quantization of the output of the quantizing circuit  86 . The size of the quantum step used in the quantizing circuit  86  greatly influences the compression ratio of information. The respective characteristics of the quantizing circuit  86  and the inverse quantizing circuit  88  can be altered on the basis of a control variable inputted through an input terminal  90 . Normally, the control variable is determined according to the occupancy of a data buffer provided at a rear stage, and the respective quantizing characteristics of the quantizing circuit  86  and the inverse quantizing circuit  88  are feedback-controlled. 
     The shown arrangement also includes an entropy coding circuit  92  for subjecting the output of the quantizing circuit  86  to entropy coding (for example, Huffman coding) utilizing the statistical nature of continuous-zero data, and an output terminal  94  through which the output of the entropy coding circuit  92  is supplied to the modulating circuit  48  of FIG.  2 . 
     The shown arrangement also includes an inverse DCT circuit  96  for executing inverse discrete cosine transform of the output of the inverse quantizing circuit  88 , an adder  98  for adding zero or a predetermined predicted value to the output of the inverse DCT circuit  96 , a delay circuit  100  for delaying the output of the adder  98  by three frames, a product-sum operation circuit  102  for performing a predetermined weighted product-sum operation on the output of the adder  98  and the output of the three-frame delay circuit  100  and outputting the resultant interpolated and integrated data, a delay circuit  104  for delaying the output of the product-sum operation circuit  102  by one frame, and a delay circuit  106  for delaying the output of the delay circuit  104  by one frame. 
     The shown arrangement also includes a switch  108  for selecting zero (the contact “a”), the output of the delay circuit  100  (the contact “b”), the output of the delay circuit  104  (the contact “c”) or the output of the delay circuit  106  (the contact “d”), and a control circuit  110  for controlling the switches  82  and  108  on the basis of a control signal inputted through an input terminal  112  by the operating switch  46 . Each of the switches  82  and  108  is connected to the associated contact “a” irrespective of each frame before a start of recording. If recording is started, each of the switches  82  and  108  is switched every frame in the switching sequence shown in FIG.  4 . 
     As will be described in detail later, if the switches  82  and  108  are connected to the respective contacts “a”, intraframe coding is executed. If they are connected to the respective contacts “b”, interframe coding of two frames spaced apart by a difference of three frames is executed. If they are connected to the respective contacts “c” or “d”, interframe coding (bidirectional coding) based on an interpolated and integrated value of one frame and a subsequent frame spaced apart by two frames is executed. 
     The operation of the circuit shown in FIG. 3 will be described below with reference to FIGS.  5 ( a ) and  5 ( b ). FIG.  5 ( a ) shows the frame order of image data inputted through the input terminal  70 , while FIG.  5 ( b ) shows the frame order of image data recorded on the magnetic tape  56 . 
     Immediately after a recording start switch of the operating switch  46  has been operated, that is, when a frame # 7  is inputted, each of the switches  82  and  108  is connected to the associated contact “a” as shown in FIG.  4 . The blocking circuit  72  converts raster-scan image data into an array of blocks each consisting of i×j pixels. The output of the blocking circuit  72  is applied to each of the contact “a” of the switch  82 , the subtracter  76  and the delay circuit  74 . At the instant when the blocking circuit  72  outputs image data relative to the frame # 7 , the delay circuit  74  outputs image data relative to the frame # 4  which was inputted three frames before the frame # 7 . 
     The DCT circuit  84  transforms the image data blocked by the blocking circuit  72  into a frequency domain by discrete cosine transform, thereby outputting a conversion coefficient. The quantizing circuit  86  quantizes the output of the DCT circuit  84  by a quantum step having a different size for each conversion coefficient. The size of the quantum step used in the quantizing circuit  86  is controlled by a control coefficient inputted through the input terminal  90 . 
     The entropy coding circuit  92  executes entropy coding of the output of the quantizing circuit  86 , and the output of the entropy coding circuit  92  is supplied to the modulating circuit  48  of FIG.  2  through the output terminal  94 . The image obtained at this time is an intraframe-coded frame (hereinafter referred to as the “I frame”) which is compressed and coded within one frame. 
     The inverse quantizing circuit  88  executes inverse quantization of the output of the quantizing circuit  86 , and the inverse DCT circuit  96  executes inverse quantization of the output of the inverse quantizing circuit  88 . Since the switch  108  is connected to the contact “a”, the adder  98  outputs the output of the inverse DCT circuit  96  as it is inputted. The output of the adder  98  is applied to each of the three-frame delay circuit  100  and the product-sum operation circuit  102 . 
     At this time, the output of the delay circuit  100  is local decoded image data relative to the frame # 4  and the product-sum operation circuit  102  outputs interpolated and integrated image data obtained by performing a weighted product-sum operation on the frames # 7  and # 4 . 
     At the instant when a frame # 8  which is the second frame is inputted, the switches  82  and  108  are connected to the respective contacts “c” as shown in FIG. 4, that is, the switch  82  selects the output of the subtracter  78 . At this time, the delay circuit  74  outputs image data relative to a frame # 5 , the delay circuit  100  outputs local decoded data obtained from intraframe-coded data relative to the frame # 5 , and the delay circuit  104  outputs interpolated and integrated data obtained from the frames # 7  and # 4 . The subtracter  78  subtracts the interpolated and integrated data (bidirectional-predicted image data) obtained from the frames # 7  and # 4 , from the image data relative to the frame # 5 . The output of the subtracter  78  is applied to the DCT circuit  84  through the switch  82 . 
     The output of the subtracter  78  is subjected to discrete cosine transform by the DCT circuit  84 , and the output of the DCT circuit  84  is quantized by the quantizing circuit  86 . The entropy coding circuit  92  executes entropy coding of the output of the quantizing circuit  86 , and the output of the quantizing circuit  86  is supplied to the modulation circuit  48  of FIG.  2  through the output terminal  94 . The image obtained at this time is an image which has been subjected to differential coding based on a predicted value which is an integrated value of the frames # 4  and # 7  which are inputted before and after the frame # 5  of interest. Such an image is hereinafter referred to as the “bidirectional-prediction.interpolation frame (called “B frame”). 
     Bidirectional-predictive-coded data relative to the frame # 5  is inversely transformed through the inverse quantizing circuit  88  and the inverse DCT circuit  96 , and interpolated and integrated data (bidirectional-predicted image data) obtained from the frames # 7  and # 4  is added to the output of the inverse DCT circuit  96  in the adder  98 , whereby the image data indicative of the frame # 5  is decoded. The decoded image data is applied to each of the delay circuit  100  and the product-sum operation circuit  102 . 
     At the instant when a frame # 9  which is the third frame is inputted, the switches  82  and  108  are connected to the respective contacts “d” as shown in FIG. 4, that is, the switch  82  selects the output of the subtracter  80 . At this time, the delay circuit  74  outputs image data relative to a frame # 6 , the delay circuit  100  outputs local decoded data obtained from intraframe-coded data relative to the frame # 6 , and the delay circuit  106  outputs interpolated and integrated data obtained from the frames # 7  and # 4 . The subtracter  80  subtracts the interpolated and integrated data (bidirectional-predicted image data) obtained from the frames # 7  and # 4 , from the image data relative to the frame # 6 . The output of the subtracter  80  is applied to the DCT circuit  84  through the switch  82 . 
     The output of the subtracter  80  is processed similarly to the preceding frame through the DCT circuit  84 , the quantizing circuit  86  and the entropy coding circuit  92 , and the output of the entropy coding circuit  92  is supplied to the modulating circuit  48  of FIG.  2  through the output terminal  94 . The image obtained at this time is an image which has been subjected to differential coding based on a predicted value which is an integrated value of the frames # 4  and # 7  which are inputted before and after the frame # 6  of interest. Accordingly, the image constitutes the bidirectional-prediction.interpolation frame (B frame). 
     Bidirectional-predictive-coded data relative to the frame # 6  is decoded through the inverse quantizing circuit  88 , the inverse DCT circuit  96  and the adder  98 . The decoded image data relative to the frame # 6  is applied to each of the delay circuit  100  and the product-sum operation circuit  102 . 
     At the instant when a frame # 10  which is the fourth frame is inputted, the switches  82  and  108  are connected to the respective contacts “b” as shown in FIG. 4, that is, the switch  82  selects the output of the subtracter  76 . At this time, the delay circuit  100  outputs local decoded data obtained from intraframe-coded data relative to the frame # 7 . The subtracter  76  subtracts the local decoded value (interframe-predicted image data) obtained from the frame # 7 , from image data relative to the frame # 10 . The output of the subtracter  76  is applied to the DCT circuit  84  through the switch  82 . 
     The output of the subtracter  76  is compressed and coded through the DCT circuit  84 , the quantizing circuit  86  and the entropy coding circuit  92 , and the output of the entropy coding circuit  92  is supplied to the modulating circuit  48  of FIG.  2  through the output terminal  94 . The image obtained at this time is an image which has been subjected to differential coding based on a predicted value which is a decoded value of the frame # 7  which is inputted three frames before the frame # 10  of interest. Such an image is hereinafter referred to as the “interframe-coded frame (called “U frame”). 
     The interframe-coded data relative to the frame # 10  is decoded through the inverse quantizing circuit  88 , the inverse DCT circuit  96  and the adder  98 . The decoded image data relative to the frame # 10  is applied to each of the delay circuit  100  and the product-sum operation circuit  102 . 
     Subsequently, two B frames, one U frame and two B frames are formed in that order, and then one I frame is formed. Subsequently, formation of I, U and B frames is repeated in a similar manner. 
     The operation of stopping recording will be described below. In the present embodiment, it may be necessary to record frames inputted after an instruction to stop recording has been inputted, because of the presence of a bidirectional-prediction.interpolation frame. For example, as shown in FIG.  5 ( a ), it is assumed that an instruction to stop recording is inputted from the operating switch  46  between frames # 15  and # 16 . In this case, since the frame # 15  is a bidirectional-prediction-interpolation frame, data relative to the frame # 16  is needed to decode the frame # 15 . For this reason, the frames # 14 , # 15  and # 16  are recorded in the order of # 16 , # 14  and # 15 . FIG. 6 shows the relations of correspondence which are established between frames inputted at the time of stop of recording and recorded frames if recording is started with the frame # 7 . 
     FIG. 7 shows one example of a track pattern formed on the magnetic tape  56  by variable-length coded image data relative to each frame. Since each frame is variable-length coded, each frame has a different amount of recorded data, and data relative to one frame is often recorded over a plurality of tracks. In FIG. 7, frame numbers are added after the respective characters “B”, “U” and “I” indicative of the B, U and I frames, and if one frame is recorded over a plurality of tracks, subnumbers are added after the associated frame numbers. 
     It is assumed that a user desires to record an image after the frame # 15  by tag recording in the pattern shown in FIG.  7 . In this case, in the present embodiment, the operator performs the operation of returning the magnetic head  54  to the first recording track, i.e., a track # 1 , reproducing the frames # 8 , # 9  and # 10  from the frame # 7  and outputting the reproduced image data, and operating the recording start switch of the operating switch  46  at the time when an image indicative of the frame # 15  is reproduced and outputted. According to this operation, variable-length coded data relative to images to be tag-recorded begin to be sequentially recorded at the forefront of a track next to the track on which the image indicative of the frame # 15  is recorded. 
     FIG. 8 shows a recording track pattern formed by the tag recording. In FIG. 8, for ease of understanding, recording of the images to be tag-recorded is assumed to be started with the frame # 7 , and “n” is added to each of the associated frame numbers. For example, “I 7 n” indicates that the frame # 7  which is recorded as one of the tag-recorded images constitutes the I frame. 
     A mark or a signal indicating that the image (the frame # 7 ) is tag-recorded after the frame # 15  which is a previously recorded image is recorded in a predetermined location on the magnetic tape  56 , for example, in a control track extending along the length of the magnetic tape  56 . 
     During reproduction, immediately after an image indicative of the frame # 15  has been reproduced and outputted, an image indicative of the frame # 7 n can be continuously reproduced and outputted on the basis of the mark or signal indicative of the tag recording. 
     Although the above description refers to coding executed on a frame-by-frame basis, coding executed on a field-by-field basis may be adopted. The location and number of intraframe-coded frames, those of interframe-coded frames and those of bidirectional-prediction-interpolation frames are not limited to the above-described example. Of course, the variable-length coding method used in the above-described embodiment is not to be construed as a limiting example. 
     Although the above description refers to the example in which interframe coding, intraframe coding and bidirectional predictive coding are used in combination, the present invention is, of course, applicable to a case where one or two of the three codings is used. 
     Although the above description refers to the example in which the magnetic tape is used as a recording medium, a magnetic disk, an optical disk, an opto-magnetic disk or other recording media may be used without departing from the scope of the present invention. 
     As will be readily understood from the foregoing description, in the VTR according to the aforesaid embodiment, recording of an image is started at a predetermined location of a track (normally, the forefront thereof). Accordingly, even if a tag-recorded image is included in recorded images, it is possible to easily control a reproduction operation so that a reproduced image is not disturbed, whereby a continuous reproduced image can be obtained. 
     Further, since an initial picture recorded after a start of recording is a picture which is coded by using only a correlation within the picture, “edit”, “search” and other similar functions can be easily performed. Further, since pictures each of which is coded by using only a correlation within the picture are present at intervals of predetermined number of pictures, “edit”, “search”, “special reproduction” and other similar functions can be comparatively easily achieved. Further, since an image which is highly compressed by using interpicture coding is located at an adequate position, a comparatively high compression ratio can be achieved as a whole.