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BOOKS AND ARTICLES Digital video archive for home
Sooner or later, the happy owner of a video camera is faced with the problem of saving the footage. Her obvious solution is to store movies on video cassettes - the simplest, but not always the most efficient. Firstly, the magnetic layer of the film crumbles over time, and as a result, a film about the first steps of a child dear to you may not live up to the last wedding. Secondly, the cost of cassettes, especially the mini-DV format, is very sensitive for the Russian wallet, and I would like to limit their number to the needs of the original shooting itself (three to five pieces). Thirdly, they are not so small, and a certain place will still be needed to store a large number of cassettes. Finally, the last argument has to do with computer editing. The fact is that few people are satisfied with the captured "raw" video material - usually its post-processing is required: "trimming" of extra fragments, rearrangement and smooth gluing of successful plots and building effective transitions between them, overlaying titles, turning on screensavers, etc. All these operations are quite within the power of a modern home computer. It is enough to install an appropriate board and a digital video editing program, for example, from the miroVideo Studio 10plus and AverMedia MV-300 kits. And since the processing was preceded by converting the original video to a digital signal and recording it on a computer hard drive, and since your video has acquired a computer digital form, it is reasonable to store it in the same form, especially if in the future when creating new films you do not expect to time to use the removed materials. In a word, the problem arises of archiving video in digital form on compact, durable and inexpensive media. Of course, we would like to have the video quality as high as possible, but this desire conflicts with the requirement to minimize costs. In addition, the very concept of quality and even more so its assessment is very subjective. In search of a reasonable compromise, we will rely on the classification of video quality provided by consumer cameras of various tape recording formats. It is rather arbitrary to distinguish three levels here: standard video (VHS, C-VHS, Video8), super video (SVHS, C-SVHS, Hi8) and digital video (DV, mini-DV, Digital8). For simplicity, we will refer to them as Video, S-Video and DV. Quantitatively, they are usually characterized by horizontal resolution (the number of elements distinguished in a line - television lines). It is believed that Video provides a resolution of up to 280 lines, S-Video - up to 400, and DV - at least 500 lines. It is important to note that a television frame (hereinafter, the PAL standard) contains 576 active lines (there are 625 in total, but some of them are service lines), and according to the recommendation ITU-R BT.601 of the International Professional Television Community (ITU - International Telecommunications Union ) each line contains 720 independent samples. Therefore, the television frame is a 720 x 576 matrix, and the maximum achievable resolution is limited to 700 lines.
It is known that a television signal is a combination of a luminance signal - Y and two color difference signals - U and V. Variations in their values allow 256 gradations (from 0 to 255 for Y, and from -128 to 127 for U / V), which in binary calculus corresponds to 8 bits, or 1 byte. Theoretically, each frame element has its own YUV values, i.e. requires 3 bytes. This representation, where both luma and chrominance have an equal number of independent values, is commonly referred to as 4:4:4. However, it was found that the human visual system is less sensitive to color spatial changes than to brightness ones. And without a visible loss of quality, the number of color samples in each line can be halved. It is this representation, referred to as 4:2:2, that was adopted in professional television. In this case, the U and V matrices are reduced to 360 x 576, and to transmit the full value of the television signal in each frame sample, 2 bytes are sufficient (alternating independent values of U and V through the sample). But for the purposes of consumer video, it was considered acceptable to halve the vertical color resolution, i.e. switch to 4:2:0 view. This reduces the color matrices to 360 x 288 and the reduced number of bytes per sample to 1,5. It is this representation that was incorporated into the DV format of digital cameras. Thus, taking into account the television frame rate of 25 Hz, we conclude that one second of digital video in 4:2:2 representation requires 20 bytes (736 x 000 xx 25 x 2), i.e. a data stream is 720 MB/s (MBps - MegaByte Per Second), but the 576:21:4 view reduces the stream by 2% to 0 MB/s. Recording such streams is technically feasible, but it is complex, expensive, and inefficient in terms of post-processing. In practice, a significant reduction in flows is required, i.e. we are forced to apply various types of compression. There are many algorithms that perform compression without loss of information, but even the most efficient of them do not provide more than a twofold compression on typical images. Among the algorithms with data loss, one of the most well-known is M-JPEG (Motion-JPEG). It came from digital photography, where, under the name JPEG, it was developed to effectively compress individual frames (JPEG is an abbreviation for the name of the international association that approved it, the Joint Photographic Experts Group). Motion simply reflects its application to a sequence of frames, although each of them is processed completely independently. In this algorithm, the frame is divided into 16 x 16 blocks, each of which is converted into the frequency domain by a discrete cosine transform (DCT). As a result, the distribution of luminance and chrominance signals (4:2:2 representation is used) is converted into the corresponding frequency coefficients, which are then subjected to quantization (rounding of values with a specified interval). DCT itself does not lead to data loss, but coefficient quantization causes image coarsening. The quantization operation is performed with a variable interval - low-frequency information is most accurately transmitted, since the corresponding image distortions are visually most noticeable. At the same time, many high-frequency coefficients responsible for "fine" image details take zero values after it. Thus, JPEG compression leads to a decrease in effective resolution and the possible appearance of minor false details (in particular, at block boundaries), but provides significant compression of the data stream. The trade-off is obvious: the more compression, the lower the quality. It has been established that the Video level corresponds to M-JPEG stream of about 2 MB/s, S-Video - 4 MB/s, and DV - 3,1 MB/s. At first glance, there is a paradox here: S-Video signal with a lower resolution than DV, however, requires more stream. The explanation is simple: in fact, DV encoding is somewhat different from M-JPEG. For example, DV uses a 4:2:0 representation, which is 4% more economical than 2:2:25. And most importantly, the DV conversion algorithm uses a more flexible compression scheme based on the adaptive selection of quantization tables. The compression ratio for different blocks changes according to the image: for blocks with little information (for example, at the edges of the image), it increases, and for blocks with a large number of small details, it decreases relative to the average level in the image. The result is a data reduction of approximately 15% for the same quality. A characteristic feature of the DV-signal is a constant, set by the standard, video data stream - 25 Mbit / s (Mbps - Megabit Per Second), that is, a fixed compression ratio - about 5:1. Further data reduction can be achieved by switching to the MPEG compression algorithm (MPEG - Motion Pictures Experts Group). It is fundamentally focused on the processing of frame sequences and uses a high redundancy of information in images separated by a small time interval. Indeed, only a small part of the scene usually changes between adjacent images, for example, a small object moves smoothly against a fixed background. In this case, the complete information about the scene needs to be saved only selectively - for reference images; for the rest, it is sufficient to transmit only differential information: about the position of the object, the direction and magnitude of the displacement, and new elements of the background (which open up behind the object as it moves). Moreover, these differences can be formed on the basis of comparison not only with previous images, but also with subsequent ones (since it is in them that, as the object moves, a part of the background that was previously hidden behind the object is revealed). Thus, three types of frames are fundamentally formed in MPEG encoding: I (Intra) - acting as reference frames and preserving the full amount of information about the image structure; P (Predictive) - carrying information about changes in the structure of the image compared to the previous frame (types I or P); B (Bi-directional) - retaining only the most significant part of the information about the differences from the previous and subsequent images (only I or P). The concept of subsequent compression of I-frames, as well as differential P- and B-frames, is similar to M-JPEG, but, like in DV, with adaptive adjustment of quantization tables. In particular, this makes it possible to characterize a DV signal as a special case of an MPEG sequence of I-frames with a given fixed stream (compression ratio). Sequences of I-, P-, B-frames are combined into groups of frames fixed in length and structure - GOP (Group of Pictures). Each GOP necessarily begins with I and contains P-frames at regular intervals. Its structure is described as M/N, where M is the total number of frames in the group, and N is the interval between P-frames. For example, a typical Video-CD and DVD IPB group 15/3 has the form IBBBPBBPBBPBBPBB. Here, each B-frame is restored from the P-frames surrounding it (at the beginning and end of the group - from I and P), and in turn, each P-frame from the previous P- (or I-) frame. But I-frames are self-sufficient and can be restored independently of others; they are reference for all P- and even more B-frames of the group. Accordingly, I has the lowest degree of compression, and B has the highest. It has been found that the size of a typical P-frame is 1/3 of I, and B is 1/8. As a result, an IPPP MPEG sequence (GOP 4/1) provides a twofold reduction in the required data stream (with the same quality) compared to a sequence of only I-frames, and the use of GOP 15/3 allows four times compression to be achieved. Summarizing, we come to the evaluation table (see Table 1). For reference, it includes the values of video data streams that characterize the quality of films recorded on Video-CD- and DVD-Video-discs. Discussion of these recording formats is beyond the scope of this material, we plan to consider them in the next issue of the journal. Concluding the story about MPEG, it must be emphasized that this algorithm allows for variation of many other encoding parameters, in particular, spatial resolution. From this point of view, a distinction is made between MPEG-1, which limits the frame size to 352 x 288, and MPEG-2, which allows different levels of resolution (including 352 x 288), but uses 720 x 576 as the main one. Strictly speaking, MPEG-1 is a limited version of MPEG-2. However, the specified Video resolution of 280 lines implies the use of this limitation. It is also important to emphasize that as we move to deeper compression schemes: from M-JPEG and MPEG 422 I-only to MPEG 420 IPB, the process of editing the resulting sequences becomes much more complicated. Currently, it is believed that without additional quality losses, full-fledged editing with frame accuracy is possible only up to the MPEG IPPP 422 level, and then only the simplest operations are allowed (cutting/gluing, and even then up to a group). It follows from the above that for video archiving it is advisable to resort to MPEG-compression of digital data (420 IBP 15/3), and for recording films of the VHS / Video8 level, MPEG-1 with a stream of 2-3 Mbps is sufficient, and for SVHS / Hi8 and DV require MPEG-2 at least 5 Mbps. From a practical point of view, software and hardware implementations of MPEG compression are possible. The most famous and shareware program is XingMPEG Encoder (xingtech.com). It allows you to get MPEG-1 sequences from AVI files (for example, M-JPEG streams captured by one of the non-linear editing boards). But this process, due to the complexity of the compression algorithm, requires significant computational resources. So, on a Pentium II 350 MHz, transcoding every minute of video at a given stream of 3 Mbps requires about 15 minutes of billing. It must be admitted that this ensures high quality of the resulting video. Hardware encoders provide real-time MPEG movies: an analog video signal is fed to their input, and the finished MPEG file is written to the hard drive. A large number of different devices are available today that provide MPEG-1 compression. They can be made as external blocks connected to a computer via an LPT port (AverMedia MPEGWizard, Pinnacle Systems STUDIO MP10, Videonics Python) and internal boards (VITEC Multimedia RT6, Darim MPEGator, Data Translation Broadway). As for MPEG-2 compression devices, their choice is still quite limited. Of the really available, it is worth noting the miroVideo DC1000. Although at the stage of initial digitization it is limited by the compression type 422 IP, which is not the most efficient in terms of compression ratio (but it provides complex video editing with frame accuracy), it allows software and hardware conversion of the resulting sequence to MPEG-2 420 IPB 15/3. The latter, in particular, is the basis for preparing DVD-Video, for example, using Minerva DVD Authoring software (complements DC1000 to miroVideo DVD1000). In table. 2 provides a summary of some of these devices for reference. It's time to figure out what compact media to place the digital video archive on. As follows from Table. 1, one hour of video, even with the most efficient compression method and Video quality, corresponds to a data volume of 1 GB, with S-Video quality - 2 GB, and for DV - about 3 GB. With such values, the choice, in fact, is not great - this is one of the options for recordable CDs. More precisely, it can be a 650 MB CD-R, 2,6 GB DVD-RAM, 3,0 GB DVD-RW or 3,9 GB DVD-R. In addition, a 4,7 GB DVD-R is expected soon, allowing you to write discs that are XNUMX% compatible with DVD-Video. Unfortunately, the cost of the respective recorders is growing substantially faster than (almost exponentially) the amount available. If a CD-R today can be purchased for only $250-300, and the cost of a blank disc is less than $2, then the price of the most promising DVD-R exceeds $6, which is clearly not suitable for home use. In fairness, it should be emphasized that due to the general trends in the development of computer technology, we can expect a revolutionary price reduction in the next year or two. After all, the price of the first CD-R devices was also first measured in thousands. It is impossible not to warn about the incompatibility of different DVD formats. But, fortunately, all of them support CD-R at the read level, as well as DVD-ROM and DVD-Video discs. Thus, under the present conditions, the most reasonable solution to the archiving problem can be based on the use of:
Author: Andrey Ryakhin, based on digitalvideo.ru
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