Digital image recording

Abstract: A method for recording images in a digital video cassette recorder, which utilizes high compressibility which is an advantage in an interframe coding method, regularly and repeatedly records low-frequency data 120 of an intraframe corresponding to the outline of a picture in a group of pictures (GOP) and provides a remaining data recording area 130 in an end area of each GOP to resolve a problem of data quantity fixation and in order to perform trick play operations. Further, the method may group sub-images of a picture regularly or irregularly and record the low-frequency data of all of the sub-images which belong to a same group in a low-frequency data recording area 120 of each sub-image, thereby to raise picture quality during trick play.

Description: The present invention relates to the digital recording of images and is concerned particularly, although not exclusively, with digital video cassette recorders (DVCR). Due to the advanced development of digital video technology in the 1980s, a digital video compression technology came to be used in various electronics applications and in communication fields such as video conferencing, digital broadcast codec and video telephones. Particularly, a signal band compression method for DVCRs which has been under development for the past several years has mainly adopted an intraframe coding method in which the redundancy of inter-pictures is not considered. However, since the data compression using the intraframe coding method eradicates redundancy within a digital image, the compression efficiency of the data is not high. Thus, attempts are now being made to use an interframe coding method as a motion video compression method for eradicating temporal redundancy, in DVCR data compression. Since the interframe coding method can eradicate redundancy of inter-pictures, its signal compression rate is higher than that of the intraframe coding method. For example, if an NTSC signal is sampled by a sampling ratio of 4:2:2, data of about 158Mbps is generated. For recording such data on tape, the data should be compressed to about 25Mbps. According to experimental results, when like amounts of the compressed result data are compared, the compression result of the interframe coding method shows uncomparably better performance than that of the intraframe coding method. Thus, a conclusion can be reached that the interframe coding method should be used for the data compression of a DVCR. However, even though the interframe coding method has such an advantage, it has not been used in DVCRs due to specific characteristics of the tape, that is, the recording medium. In other words, when the data is recorded on the recording medium such as the tape, although a constant data rate or a fixed data rate is necessary, such a constant data rate is difficult to obtain. An encoder of the Motion Picture Experts Group (MPEG) is a representative means for coding an interframe video signal, which has been proposed to solve the constant data rate problem. Figure 1 of the accompanying diagrammatic drawings is a block diagram of one example of a general MPEG motion video encoder. A brief explanation of the operation of the MPEG motion video encoder is as follows. In the encoder shown in Figure 1, frame-recorder 10 receives a source input video signal and performs colour coordinate conversion, sub-sampling and block division. Motion estimator 12 estimates similar portions between a preceding picture and the following picture, thereby to detect a position movement result as vector data and output video signal data, vector data and mode data I, B and P. Here, mode data I, B and P represent classes of the picture. The I-picture is an intra-coded picture which is obtained by eradicating only spacial redundancy. The P-picture, or predicted picture, is predicted from the I-picture and is obtained by encoding only the differences. Thus, the P-picture can be said to eradicate similar portions between the I-picture and the P-picture. On the other hand, the P-picture has motion vector data generated during the prediction, as additional data. The B-picture is located between the I-picture and the P-picture. By bidirectionally predicting from the I-picture and the P-picture, a B-picture is obtained by encoding prediction errors. The amount of I-picture data is less than that of the P-picture data which is less than that of the B-picture data. Since the amount of each picture's data (I, P and B) is not uniform, a buffer 32 is provided to maintain a constant data transmission velocity in a transmission path, thereby to adjust the amount of data. As described above, motion estimator 12 generates mode data I, B and P, and according to the mode data, generates both the video signal data and the motion vector data. First adder 14 adds the video signal data output from frame storage/predictor 28 to the video signal data output from motion estimator 12. Discrete cosine transform (DCT) circuit 16 performs energy compression of the video signal data supplied from first adder 14, to perform a discrete-cosine-transform operation to the generally distributed video signal and have the thus-operated video signal distributed densely in a certain area. Quantizer 18 quantizes the energy compressed video signal output from DCT circuit 16 into a predetermined quantization level (Q-level or step-size), and performs run-length-coding of the quantized video signal. A variable-length-coding (VLC) circuit 20 compresses the quantized and run-length-coded video signal supplied from quantizer 18. That is, among signals having 256 levels represented as eight bits, higher frequency data is represented with fewer bits, while lower frequency data is represented with more bits. Accordingly, the total number of the bits representing the video signal is reduced. On the other hand, inverse-quantizer 22 receives the quantized video signal output from quantizer 18, and restores the received signal into the signal prior to quantization. Inverse-DCT (DCT -1 ) circuit 24 receives the output from inverse-quantizer 22, and restores the received signal into the signal prior to a discrete-cosine-transform operation being performed. Second adder 26 adds the output of inverse-DCT circuit 24 to the output of frame storage/predictor 28. Frame storage/predictor 28 compensates the movement location of the preceding picture which is stored after being output from second adder 26 according to the motion vector data and the mode data output from motion estimator 12. Multiplexer 30 selectively outputs the video signal data compressed in and generated from VLC circuit 20, the motion vector data and the mode data I, B and P generated from motion estimator 12. Buffer 32 temporarily stores the data since the data length from multiplexer 30 is not uniform, to then output the stored data at a constant velocity. Here, buffer 32 determines a quantization level according to the fullness of the data and controls quantizer 18 via regulator 34 based on the quantization level. If the data fullness of buffer 32 is high, the quantization level is increased to reduce the amount of data. Meanwhile, if the data fullness is low, the quantization level is reduced to increase the amount of data. As described above, a video signal whose data output quantity is constantly adjusted is finally recorded on the recording medium (tape). Also, for recording on tape, one frame of video data is divided and recorded on four video tracks, according to the particular characteristics of the DVCR. However, since the video signal is divided and recorded on multiple tracks in the above conventional recording method, image regeneration is impossible during trick play operations such as high speed searching. Considering the above problem, to enable high speed searching in such a digital VCR, a separate method for recording one frame of a video signal on the expected running track of a head during trick play operations has been proposed. However, since this method should accurately estimate the running track and accurately follow the running track during trick play, various technical difficulties have been encountered and certain problems in utilization are present. Also, since the constant data rate is not completely accomplished. the data amount of inter-pictures is not uniform. On the other hand, as the compressed and coded data is different from the analog signal, the data quantity of a given picture area does not match a predetermined size of the original picture. Hence, during trick play, the reconstruction performance of the original picture may not be higher than that of the analog signal. Since an unreconstructed area in the picture may remain if the compressed digital data is restored during trick play operation, a proper signal should be added in such an area, to prevent lowering of the picture quality. EP-A-0 562 845 (Toshiba) discloses a variable length code recording and playback apparatus for a video recorder having a data rearranger for recording the data in areas according to the playback speed and a corresponding decoder for restoring the original arrangement of data for each playback speed. Further background information concerning trick play for a digital VCR can be found in IEEE Transactions On Consumer Electronics Volume 37 (1991) August, number 3, "A Study on Trick Plays for Digital VCR" by Yamamitsu et al. Preferred embodiments of the present invention aim to provide a method of recording images in a digital video cassette recorder capable of improving the quality of a reproduced picture by maintaining a constant data rate for an amount of data to be recorded on recording media and reducing data loss during trick play operation while utilizing the high compressibility advantage of an interframe coding method. It is another aim to provide a method for recording images in a digital video cassette recorder by adding certain picture data on a tape to avoid any empty portion in a picture by effectively filling the picture with compressed digital data during trick play operation. According to the present invention, there is provided a method of recording images in a digital video recorder in which a compressed digital video signal of each sub-image of a picture divided into a predetermined number of sub-images is divided into low-frequency data and high-frequency data, so that low-frequency data is recorded for a certain distance defining a recording area on tracks of a length of tape and high-frequency data is sequentially recorded on the remaining area of the tracks, wherein the predetermined number of sub-images is divided into a plurality of groups in accordance with a predetermined method and then the low-frequency data of all of the sub-images which belong to the same group is recorded together on the low-frequency data recording area of each sub-image belonging to the same group. When dividing said predetermined number of sub-images into said plurality of groups, said sub-images evenly distributed over the whole picture may be classified so as to be included in the same group. When dividing said predetermined number of sub-images into said plurality of groups, and when said predetermined number of sub-images are serially numbered from left to right and from top to bottom and each serial sub-image is divided by a divisor of a predetermined magnitude, the sub-image whose serially assigned number has the same remainder may be classified into the same group. Said low-frequency data may comprise a direct current component only, or alternatively said low-frequency data may comprise a direct current component and low-frequency data portions of an alternating current component. In a preferred embodiment of the method, the images are recorded on tape in a digital video recorder in GOP units, groups of pictures units, constituted by randomly combining an I-picture coded using only information of the corresponding picture itself, a B-picture coded using a motion-compensated prediction from a past and/or future reference picture and a P-picture coded using the motion-compensated prediction from the past reference picture, wherein low-frequency data of each sync block of the I-picture is recorded for a certain distance on each track for the I-picture, the B-picture and the P-picture, high-frequency data of each sync block of the I-picture is sequentially recorded on a non-recorded area of the tracks for the I-picture, data of the P-picture is sequentially recorded on the area which remains after low-frequency data of the I-picture is recorded on the tracks for the P-picture, and the data of the B-picture is sequentially recorded on the area which remains after low-frequency data of the I-picture is recorded on the tracks for the B-picture. Preferably, said data is recorded in a data recording area of a predetermined size separately provided on a track in a picture's end recording area of each said GOP unit, for fixing the data quantity of each said GOP unit. Preferably, said predetermined size is a tape recording area corresponding to the data storage size of an output buffer for controlling an output bit rate of compressed and coded data. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is, as mentioned above, a block diagram of a general motion picture encoder; Figure 2 shows one example of a tape recording format according to the present invention; Figure 3 shows an example of another tape recording format according to the present invention; Figures 4A through 4C show a picture composed of a plurality of sub-images; and Figure 5 shows a type of sync block for performing trick play operations. An example of the present invention will be described below with reference to the accompanying drawings Figures 1 through 3. Figure 2 shows one example of a tape format in which data is recorded by an image recording method in a DVCR according to the present invention. In Figure 2, one frame of digital video data is recorded per four tracks on a recording medium 100. As for pictures obtained by the motion picture compressing method, one case is composed of an I-picture coded using only information of the corresponding picture itself as an intraframe, a B-picture coded using a motion compensated prediction bidirectionally, that is, from a past and/or future reference picture and a P-picture coded using a motion compensated prediction from the past reference picture, that is, the I-picture. The other case is composed of merely the I-picture and the P-picture. This embodiment takes the former as an example. When pictures to be recorded consist of the I-picture, the B-picture and the P-picture, even though there can be many different orders of pictures to be recorded on the tracks of recording medium 100, the case of being recorded in the order of [I, B, B, P, B, B, P ..., I] will be explained as an example in this description. Of course, the digital data of the I-picture, the B-picture and the P-picture are respectively recorded on the four tracks of recording medium 100. Here, the video signal recording method will be described below first referring to the Figure 2. In Figure 2, low-frequency data at the I-picture which represents an outline of a picture is recorded in low-frequency data recording areas 120 periodically formed on the four tracks constituting the I-picture. Also, the low-frequency data of the I-picture is regularly recorded in low-frequency data recording areas 120 periodically formed on the tracks of the B-picture and the P-picture. The high-frequency data of the I-picture is recorded in high-frequency data recording areas 110 periodically formed on the four tracks constituting the I-picture. The B-picture and P-picture data are recorded in the portion where the area in which the low-frequency data of the I-picture is not recorded. Furthermore, in the method for recording video signals in a DVCR, the fixing of data quantity is accomplished by grouping N pictures into one group. Here, N is any integer greater than one. The group is called a group of pictures (GOP). A track length (as long as the buffer size or as long as the maximum size to be occupied by the buffered data) is prepared as remaining data recording area 130, for every GOP. Thus, the data remaining in the buffer is recorded on remaining data recording area 130 of recording medium 100, every time a GOP encoding operation is concluded. By doing so, data quantity can be fixed. In fact, the quantity of lost data is reduced because, during trick play, a sufficiently large area on the recording medium is utilized as the remaining data recording area. As described above, low-frequency data of the I-picture is regularly recorded again on other tracks. Thus, the loss of data is reduced in any multiple speed mode. Of course, due to the utilization of only low-frequency data, picture quality is lowered. However, if picture quality is only slightly lowered, the human eye cannot recognize well this phenomenon in a random multiple speed mode. Accordingly, there is no problem in actual use. If data of about 15Mbps is utilized in encoding by the motion picture method, data of about 5Mbps can be sufficiently used for trick play operation. That is, if the total data quantity is less than 20Mbps, this is a quite acceptable data rate for recording media such as tape. As described above, in Figure 2, low-frequency data of the intraframe is recorded as regular and repetitive arrangements in a GOP. Therefore, a lost quantity of data is small although the recording medium runs in a random multiple speed mode, thereby to obtain a regenerated picture. Next, another embodiment of this invention will be described below. Figure 3 shows a tape format according to the other embodiment of this invention. Since the tape format shown in Figure 3 is the same as that shown in Figure 2 except for the following descriptions, an explanation of the common parts will be omitted. The embodiment shown in Figure 3 is different from the tape format shown in Figure 2 in that the remaining data recording area of the buffer is not provided in the last portion of the GOP. Generally, for encoding and decoding, the encoding process is independently carried out in GOP units. Therefore, remaining data recording area 130 corresponding to the buffer in Figure 2 cannot be provided by properly adjusting the bit rate of the buffer. Next, a video signal recording method will be described below referring to Figures 4A through 4C and Figure 5. As described above, these embodiments of the invention propose to reinforce the function of trick play reproduction when compressing a motion picture and recording it on tape, thereby more accurately reconstituting the picture in fast-forward and fast-backward search modes. Generally, it is difficult to perfectly constitute the picture due to the characteristic of compressed digital data, even though one sync block is made to represent a certain area of the picture. Considering this problem, preferred embodiments of this invention propose a new recording method wherein the data quantity read by a magnetic head is increased to obtain advantageous image reconstruction. That is, based on the above recording method which divides one sync block into low-frequency and high-frequency data and records the divided frequency data, this method records not only the low-frequency data of a certain part of a picture but also the low-frequency data of various parts of the picture together in low-frequency data recording area 120. By doing so, as the information quantity read by the magnetic head increases and the information is uniformly distributed throughout the whole picture, the quality of the trick-play-reproduced image can be improved. For this purpose, a picture is firstly divided into a plurality of sub-images which are then regrouped using a predetermined method. That is, one picture is constituted by a plurality of groups, and each group consists of a plurality of sub-images. There are two methods to group sub-images. One is to group in accordance with a rule and the other is to group randomly. Here the former will be described first. One picture can be divided into a plurality of sub-images (or macroblocks) as shown in Figure 4A in the case of a motion picture of thirty frames per second. There are L sub-images per picture, as can be seen below in equation (1) which assumes that a picture has H pixels horizontally and V pixels vertically and a sub-image has h pixels horizontally and v pixels vertically. Here, a symbol [β] represents the maximum integer not exceeding β. The sub-images are grouped as follows by using the concept of a remainder set, in the case of a picture being divided into L sub-images. Namely, as shown in Figures 4A, 4B and 4C, when sub-images are serially numbered from left to right and from top to bottom and the serial sub-images are divided by a divisor of a predetermined magnitude, the number having the same remainder, that is, the sub-image corresponding to the same remainder set, is classified into the same group. According to the above method, one picture is divided into a number of groups, thereby recording both the low-frequency data of other sub-images in the same group as well as the low-frequency data of the corresponding sub-image itself in low-frequency data recording area 120 of each sub-image belong to the same group. The sub-image has a sync block representing the sub-image. Accordingly, the groups of data which are identical to each other are recorded in low-frequency data recording area 120 of the sync block of each sub-image belonging to the same remainder set, while the original data of each sub-image, that is, the groups of data which are different from each other, are recorded in the high-frequency data recording area 110. In other words, not only low-frequency data of itself but also the low-frequency data of other sub-images which are involved in a remainder set including the sub-image itself is recorded in low-frequency data recording area 120 of each sub-image belonging to a certain remainder set. At this time, in high-frequency data recording area 110 of each sub-image, only the high-frequency data of the corresponding sub-image itself is recorded. A more detailed description will now be given, referring to Figures 4B and 5. It is assumed that there is a picture consisting of sixteen sub-images in total as shown in Figure 4B. Figure 4B assumes that dividend L, i.e., the number of sub-images, is sixteen. The low-frequency data of only itself is recorded in low-frequency data recording area 120 of each sync block in the case of MOD(L,16). In the case of MOD(L,8), the data of low-frequency data recording area 120 of sync block 1 is equal to that of sync block 9. In the case of MOD(L,4), low-frequency data recording areas 120 of sync blocks 1, 5, 9 and 13 have the same data. If the above example is generally represented and given that MOD(L,n)=α, the same data is recorded in low-frequency data recording area 120 of the sync blocks of the sub-images whose serially assigned numbers are α, n+α, 2n+α..., mn+α (where mm≤L and m, n and α are positive integers). This is shown in Figure 5 where one sync block is composed of a sync area, an identification area, low-frequency data recording area 120, high-frequency data recording area 110 and a parity area. Here, a sync pattern is recorded in the sync area, an identification number of a certain sub-image is recorded in the identification area, and an error correction code is recorded in an parity area. Furthermore, while the high-frequency data of a certain sub-image itself is recorded in high-frequency data recording area 110, not only the low-frequency data of a certain sub-image itself but also the low-frequency data of other sub-images classified as the same sub-image group are recorded together in low-frequency data recording area 120. In the above description, the low-frequency data can comprise either the direct current component only or the direct current component together with the low-frequency data component of the alternating current component. On the other hand, a mapping table can be used as a method of randomly grouping sub-images as shown in Figure 4C, without relying on the method using the remainder set concept of MOD(L,n) explained above. The method using the random mapping table can be much more advantageous and flexible in raising the quality of a reproduced picture in that the sub-image can be irregularly or uniformly mixed. That is, if the low-frequency data recorded together is made to be a signal of the sub-image distributed evenly over the whole picture, the quality of the trick-play-reproduced image will be improved accordingly. Also, an effect of such a recording method will be different in accordance with the recording density of the tape. If the recording density becomes sufficiently high due to further technical development, much more of the sub-images' low-frequency data can be recorded in low-frequency data recording area 120 of one sub-image, thereby greatly improving the quality of the trick-play-reproduced image. As described above, the interframe coding method can be effectively used in a DVCR since the DVCR image recording method according to preferred embodiments of this invention can fix the bit rate of data to be recorded on tape. Also, preferred embodiments of this invention enable trick play reproduction since, even though the recording medium passes at a random multiple speed, if one sync block is restored, the data of other sub-images classified into the same group as that of the sync block are restored at the same time. Furthermore, preferred embodiments of this invention can upgrade the quality of a trick-play-reproduced image by share-recording the low-frequency data of each sub-image which belongs to the same group. Although preferred embodiments of the invention relate to DVCR's, other embodiments may relate to digital video recorders that employ tape in non-cassette form, and methods of the invention may be adapted to recording media other than tape. The invention may be applied to systems other than NTSC systems - such as, for example, the PAL and SECAM systems used in Europe and elsewhere.

Claims: A method of recording images in a digital video recorder in which a compressed digital video signal of each sub-image of a picture divided into a predetermined number of sub-images is divided into low-frequency data and high-frequency data, wherein: said low-frequency data is recorded for a certain distance defining a recording area (120) on tracks of a length of tape and said high-frequency data is sequentially recorded on the remaining area (130) of the tracks; and the predetermined number of sub-images is divided into a plurality of groups in accordance with a predetermined method and the low-frequency data of all of the sub-images which belong to the same group is recorded together on the low-frequency data recording area (120) of each sub-image belonging to the same group. A method according to claim 1, wherein, when dividing said predetermined number of sub-images into said plurality of groups, said sub-images evenly distributed over the whole picture are classified so as to be included in the same group. A method according to claim 1, wherein, when dividing said predetermined number of sub-images into said plurality of groups, and when said predetermined number of sub-images are serially numbered from left to right and from top to bottom and each serial sub-image is divided by a divisor of a predetermined magnitude, the sub-image whose serially assigned number has the same remainder is classified into the same group. A method according to claim 1, 2 or 3, wherein said low-frequency data comprises a direct current component only. A method according to claim 1, 2 or 3, wherein said low-frequency data comprises a direct current component and low-frequency data portions of an alternating current component. A method according to any of claims 1 to 5, wherein said images are recorded on tape in the digital video recorder in GOP units, groups of pictures units, constituted by randomly combining an I-picture coded using only information of the corresponding picture itself, a B-picture coded using a motion-compensated prediction from a past and/or future reference picture and a P-picture coded using the motion-compensated prediction from the past reference picture; and low-frequency data of each sync block of the I-picture is recorded for said certain distance on each track for the I-picture, the B-picture and the P-picture, high-frequency data of each sync block of the I-picture is sequentially recorded on a non-recorded area of the tracks for the I-picture, data of the P-picture is sequentially recorded on said area which remains after low-frequency data of the I-picture is recorded on the tracks for the P-picture, and the data of the B-picture is sequentially recorded on the area which remains after low-frequency data of the I-picture is recorded on the tracks for the B-picture. An image recording method according to claim 6, wherein said data is recorded in a data recording area of a predetermined size separately provided on a track in a picture's end recording area of each said GOP unit, for fixing the data quantity of each said GOP unit. An image recording method according to claim 7, wherein said predetermined size is a tape recording area corresponding to the data storage size of an output buffer for controlling an output bit rate of compressed and coded data.

Inventors: PARK KU-MAN, PARK, KU-MAN, PARK, KU-MAN, 4-106, DONGA APT.
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1993
Main classification: H04N5
IPC codes: G11B20, H04N5