MPEG standards. Reference data

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MPEG standards. Reference data

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Compression of digital video signals (they also say - images) has been used for quite a long time and widely. For example, for the transmission of high-quality television programs over satellite channels of digital television, coding is used mainly according to the MPEG-2 standard. Other standards for processing (compression) of video and audio signals and where they are implemented are described in the published material.

The relatively low throughput of terrestrial television channels and the relatively small amount of memory of digital signal carriers impose significant restrictions on the dissemination of high-quality video information. To solve this problem, "compression" of video signals is used - a special encoding that reduces the amount of digital information without a noticeable deterioration in the quality of reproduced images. The encoding standards with the general name MPEG are currently the most widely used.

The applied method of efficient encoding of digital video signals is to remove the spatial and temporal redundancy inherent in video images, and thereby reduce the volume occupied by video signals. Behind a slightly scientific definition of the concept of "compression" of video signals (images) is a relatively simple, at first glance, operation: identifying repeating fragments and redundant information in a video signal and processing them accordingly.

The simplest example. If the video signal does not change for a certain period of time, then it is enough to transmit or record it only once, that is, take a picture, supplementing it with a small explanation: "Remember the picture and repeat the specified number of times." The savings in the volume of recorded or transmitted digital information is obvious.

The next step is processing the changing images. Here the task is more difficult, but also solvable. Two adjacent (in time) images (frames of video signals) can be compared and instead of the second one, not the entire frame can be transmitted, but only what has changed in it in comparison with the first one. It is obvious that information about changes in the image (video signal) occupies a smaller volume than information about a full frame. For not very fast-changing images, the effect is quite noticeable. And such plots prevail in most video films: radical changes in neighboring frames occur only when the plot changes. To this we can add the possibility of synthesizing frames with intermediate states of the plot (which is from the field of computer graphics), and this further expands the possibilities of video signal compression.

Simplicity here, of course, is apparent. The practical implementation of video signal compression goes through the most complex mathematics, specialized ultra-large integrated circuits, etc. But this does not concern users - they only reap the fruits of many years of hard work of specialists from many countries of the world.

The name MPEG comes from the Moving Pictures Experts Group, a working group established by the International Standards Organization in conjunction with the International Electrotechnical Commission to develop video compression standards.

Prior to the advent of the first standard developed by this group - MPEG-1 - in international practice, H.261 and JPEG standards were used to compress video signals. The first one was developed for telecommunications systems and is used mainly for video phones, teleconferencing, etc. The JPEG standard was named from another Working Group - the Joint Photographic Experts Group ("Joint Photographic Experts Group"). Although this standard relates to still picture coding, the solutions used in it were very important for the development of the MPEG-1 standard, as they showed ways to remove redundant spatial information from a video signal without noticeable loss of quality. The JPEG standard is widely used in computers, digital cameras, color faxes, and is also used on the Internet.

The MPEG-1 standard was created to record 74 minutes of video (with accompanying audio) on CDs at bit rates up to 1,5 Mbps. In this standard, ten adjacent frames are processed simultaneously to remove redundancy. The MPEG-1 standard makes it possible to achieve compression of digital video information by 100...150 times.

The next product of the MPEG group was the MPEG-2 standard, which was already created for universal use - for telecommunications, broadcasting and storing information on various media. It supports transfer rates up to 4 Mbps. The quality of the reproduced image provided by this standard allows it to be used even in high-definition television. The MPEG-2 standard has already been adopted for digital television broadcasting (DVB - Digital Video Broadcasting) and universal video discs (DVD - Digital Versatile Disc).

One of the features of this standard is a dynamic change in the speed of information transfer. This allows for consistent high image quality at a relatively low average data rate. With a constant transmission rate, it must either be increased excessively and, consequently, reduce the compression efficiency, or choose not very high and put up with a loss of quality when the plot changes quickly.

Now the MPEG group is busy developing the MP EG-4 standard, which is defined as "intelligent information compression" and is intended for use in telecommunications systems (primarily in mobile and wearable). It will support data transfer rates up to 64 kb/s.

Although the main tasks of MPEG lie in the field of video compression, developers pay great attention to audio compression as well. And this is natural, since in most cases the image and sound are inseparable. For example, the MPEG-1 Audio and MPEG-2 Audio standards are accepted for recording video films on DVD discs, which are replacing CDs.

The MPEG-1 Audio standard was created almost simultaneously with MPEG-2 and allows you to "attach" bits of stereo audio information to video data. At the same time, their transmission speed lies within 128 ... 384 kb / s (2 channels). This standard is widely used for recording video films on CDs, as well as for digital audio broadcasting in Europe and Canada.

The MPEG-2 Audio standard is a "companion" of the MPEG-2 standard. It is compatible with MPEG-1 Audio, but has a number of functional additions necessary for high-quality audio broadcasting and high-definition television.

Another standard developed by the MPEG group is MPEG-2 Digital Surround. It works with 5.1 Digital Surround audio, which has three front channels, two full rear channels and a "subwoofer" (called "0.1 channel", hence the 5.1 designation).

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