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MPEG format: specifications and capabilities Part 5

Frame sequence can have any frame arrangement

I, P and B. In industrial practice, it is common to have a fixed sequence (such as IBBPBBPBBPBBPBB), however more powerful encoders can optimize frame type selection based on the context and global characteristics of the footage.
Each type of framing has its own advantages depending on the characteristics of the image (movement activity, temporary masking effects, …).
For example, if the sequence of images changes little from frame to frame, it makes sense to encode more B-frames than P. Since B-frames are not used in the subsequent decoding process, they can be further compressed without affecting the overall video. . quality.
Specific application requirements also affect the choice of frame type: keyframes, channel switching, program indexing, error recovery, etc.
The following statistical characteristics are used for video compression:
1. Spatial correlation: 8 × 8 point discrete cosine transform.

MPEG format

2. Characteristics of human vision – immunity to high frequency components: scalar quantification of DCT coefficients with loss of quality.

MPEG format

3. Large spatial correlation of the entire image: prediction of the first low-frequency transformation coefficient in an 8×8 block (mean value of the entire block).

4. Statistics of occurrence of syntactic elements in the most probable coding stream: optimal coding of motion vectors, DCT coefficients, types of macroblocks, and so on.

5. Sparse matrix of quantized DCT coefficients: encoding of repeated zero elements with the designation of the end of the block.

6. Spatial masking: the degree of quantization of the macroblock.

7. Coding of areas taking into account the content of the scene: the degree of quantization of the macroblock.

8. Adaptation to the characteristics of the local image: block coding, macroblock type, adaptive quantization.

9. Constant adaptive quantization step size: The new quantization degree is set only by a special macroblock type and is not transmitted by default.

10. Temporal redundancy: 16×16 pixel macroblock level forward and backward motion vectors.

11. Coding of the macroblock prediction error taking into account perception: adaptive quantization and quantization of transformation coefficients.

12. Small prediction error: no error can be reported for a macroblock.

13. Macroblock level prediction error fine encoding: Each of the blocks within the macroblock can be encoded or omitted.

14. Motion vectors: slow motion of a fragment of an image with a complex pattern: predicting motion vectors.

15. Appearances and disappearances: forward and backward prediction in B-frames.

16. Interprediction precision: bilinear interpolated (filtered) block differences. In the real world, the movement of objects from one frame to another rarely falls within the bounds of the points. Interpolation allows you to find out the actual position of the object, which often increases the compression efficiency by 1 dB.

MPEG format

17. Motion activity limited in P-frames: macroblocks skipped. When the motion vector and the prediction error are zero. Skipped macroblocks are very desirable in the codestream, as they take no bits except in the header of the next macroblock.

18. Coplanar movement in B-frames: macroblocks skipped. When the motion vector is the same and the prediction error is zero.

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MPEG format: specifications and capabilities Part 4

MPEG audio compression algorithm

Audio compression uses well-designed psychoacoustic models, derived from experimentation with the most demanding listeners, to deliver sounds that are not audible to the human ear. This is what is called “masking”, for example, a large component at a certain frequency does not allow to hear components with a lower coefficient at nearby frequencies, where the relationship between the energies of the frequencies that are masked is described by some curve empirical. There are similar temporal masking effects, as well as more complex interactions where the temporal effect can emphasize the frequency, or vice versa.

The sound is divided into spectral blocks using a hybrid scheme that combines sine and band transformations, and a psychoacoustic model described in the language of these blocks. Anything that can be trimmed or clipped is trimmed and trimmed, with the rest sent to the outflow. Actually, things seem a bit more complicated, as the bits have to be distributed between the strips. And of course, everything that is sent is encrypted with redundancy reduction.

With the advent of the MPEG-2 specification, the most popular combinations have been merged into levels and profiles. The most common are:

Source Input Format (SIF), 352 dots x 240 lines x 30 fps, also known as Low Level (LL), and
“CCIR 601” (for example, 720 dots / line x 480 lines x 30 fps) o Main level: the main level.
Motion compensation replaces macroblocks with macroblocks from older images.
Macroblock predictions are generated from corresponding 16×16 dot blocks (16×8 in MPEG-2) from previous reconstructed frames. There are no restrictions on the position of the macroblock in the image above, except for its edges.

MPEG format

The original (reference) frames (from which predictions are formed) are displayed regardless of their encoded form. Once the frame is decoded, it does not become a set of blocks, but an ordinary flat digital image of dots.

MPEG format

In MPEG, the displayed image size and frame rate may differ from those encoded in the stream. For example, before encoding, a subset of frames can be omitted from the original sequence, and then each frame is filtered and processed. When restoring, dithered to restore original size and frame rate. In fact, the three fundamental phases (original, encoded and displayed frequency) can differ in parameters. The MPEG syntax describes the rate encoded and displayed through headers, and the original frame rate and size are known only to the encoder. That is why MPEG-2 headers include elements that describe the size of the screen to display the footage.
In an I-frame, macroblocks must be coded internal, without reference to before or after, unless scalable modes are used. However, the macroblocks in a P frame can be internal or referenced from previous frames. Macroblocks in a B-frame can be internal or refer to the previous frame, the next frame, or both. Each macroblock has an element in the header that defines its type.

MPEG format: specifications and capabilities Part 3

Due to the fact that MPEG is developed in an organization as accredited as ISO and is a fairly universal compression method (it can be used in video recording, television broadcasting, home video editing, multimedia programs (educational, games), teleconferencing , creating videos for Internet presentations), has become the dominant standard for digital video compression, eliminating the need to use many incompatible video compression methods that existed before.

How MPEG Video Works
A digital color image of the compressed sequence is converted to the YUV (YCbCr) color space. The Y component represents intensity and the U and V components represent chromaticity. Since the human eye is less sensitive to chromaticity than intensity, the resolution of color components can be reduced 2 times vertically, or both vertically and horizontally. For high-quality studio video and animation, downsampling is not applied to preserve quality, and for consumer applications, where streams are lower and equipment is cheaper, this action does not lead to a Noticeable loss in visual perception, while saving valuable bits of data. .

MPEG format
MPEG format
MPEG format

The basic idea of ​​the whole scheme is to predict the movement from one frame to another and then apply a discrete cosine transform (DCT) to redistribute the redundancy in space. DCT is carried out in blocks of 8×8 points, the prediction of movement is carried out in the intensity channel (Y) in blocks of 16×16 points or, depending on the characteristics of the original image sequence (interlaced, content), in blocks of 16×8 points. In other words, a given 16×16 dot block is searched in the current frame in the corresponding larger area in the previous or subsequent frames. The DCT coefficients (original data or the difference of this block and its corresponding) are quantized, that is, they are divided by a certain number to discard insignificant bits. Many coefficients after such an operation turn out to be zero.

Frame rates and types
There are three types of encoded frames. I-frames are frames encoded as still images, without reference to the next or the next. They are used as starting points. P-frames are predicted frames from previous I or P-frames. Each macroblock in a P frame can come with a vector and the DCT coefficient difference of the corresponding block from the last decoded I or P, or can be encoded as in I, if the corresponding block was not found.

And finally, there are B-frames that are predicted from the two closest I or P-frames, one before and one after. The corresponding blocks are searched in these frames and the best of them is selected. The forward vector is searched, then the inverse, and the average between the corresponding macroblocks in the past and the future is calculated. If this doesn’t work, then the block can be encoded as an I-frame.

MPEG format: specifications and capabilities Part 2

HOW COMPRESSION HAPPENS

The basic object of encodig in the MPEG standard is a television frame. Since the background of the image remains fairly stable throughout most of the image, and the action takes place only in the foreground, compression begins with the creation of the original frame. The original (Intra) frames are encoded only by intra-frame compression using algorithms similar to those used in JPEG. The frame is divided into 8 × 8 pixel blocks. A discrete cosine transform (DCT) is performed on each block, followed by the quantization of the coefficients obtained. Due to the high spatial correlation of brightness between adjacent pixels in the image, DCT leads to a concentration of the signal in the low-frequency part of the spectrum, which, after quantization, is effectively compressed using variable-length encoding. Predicted frames are processed using direct prediction of previous original or predictable frames. The frame is divided into 16 × 16 pixel macroblocks, each macroblock is assigned the most similar image area of ​​the reference frame, shifted by the displacement vector. This procedure is called motion analysis and compensation. The allowed compression ratio for predictable frames is 3 times that of the original frames. Depending on the nature of the video image, bidirectional interpolated frames are encoded in one of four ways: direct prediction; inverse prediction with motion compensation: used when new image objects appear in the coded frame; motion compensated bidirectional prediction; Intra-frame prediction: when there is a sudden change of scene or a high speed of movement of the elements of the picture. Bidirectional frames are associated with the deepest compression of video data, but since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. Since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. Since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. cause image distortion. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. lead to image distortion. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions.

MPEG format: specifications and capabilities

Today, every video lover has the opportunity to create their own personal computer-based home video studio ….

It is known that when working with video files, it becomes necessary to process and store very large amounts of information, for example, one minute of digital video signal with SIF resolution (comparable to VHS) and true color (millions of colors) will take ( 288 x 358) pixels x 24 bits x 25 frames / sec x 60 sec = 442 MB, that is, on media used in modern PCs, such as a CD (CD-ROM, approximately 650 MB) or a hard disk (several dozen gigabytes), an error will occur when saving a full-time video recorded in this format. By using MPEG compression, the amount of video information can be significantly reduced without noticeable image degradation. How MPEG works and what other applications it has will be discussed further.

MPEG
MPEG is an abbreviation for Moving Picture Expert Group, the name of a group of ISO experts working to develop standards for encoding and compressing audio and video data. The official name of the group is ISO / IEC JTC1 SC29 WG11. The abbreviation MPEG is often used to refer to the standards developed by this group. To date, the following are known:

MPEG-1 is designed to record synchronized video (usually in SIF format, 288 x 358) and audio on CD-ROM with a maximum read speed of approximately 1.5 Mbps. The quality parameters of the processed video data by MPEG-1 are in many ways similar to ordinary VHS video, so this format is mainly used when it is inconvenient or impractical to use standard analog video media.
MPEG-2 is designed to process television-quality video images with a data transmission system capacity ranging from 3 to 15 Mbit / s, and professional equipment uses transmission speeds of up to 50 Mbit / s. Many television channels are switching to technologies based on MPEG-2, the signal compressed according to this standard is transmitted through television satellites and is used to archive large volumes of video material.
MPEG-3 – Designed for use in High Definition Television (HDTV) systems with a data rate of 20 to 40 Mbps, but later became part of the MPEG-2 standard and is no longer mentioned separately. By the way, the MP3 format, which is sometimes confused with MPEG-3, is designed only to compress audio information and the full name of MP3 sounds like MPEG-Audio Layer-3.
MPEG-4: defines the principles for working with the digital representation of multimedia data in three areas: interactive multimedia (including products distributed on optical discs and on the Web), graphic applications (synthetic content) and digital television.

Everything you wanted to know about MPEG-4 / DivX, but was hesitant to ask last part

Resume

DivX is a progressive implementation of the MPEG-4 format. It offers a significantly higher video compression ratio and brings a number of groundbreaking innovations.

Applications for MPEG-4 include multimedia applications for mobile use and the Internet, digital television, home video, and mobile communications. With the advent of specialized processors, it was possible to create inexpensive universal fixed and mobile players that, in addition to DVD movies and popular formats, also play files based on the MPEG-4 format. Today, the price, performance, and format compatibility of players have reached a point where a purchase makes sense for all home video enthusiasts.

Editor Comments Very Evil:

I, like many of my colleagues, have the opinion that DivX was not appreciated in time by DVD player manufacturers and as a result of its slowness and wait-and-see attitude, it was slow to enter the mass market and, according to In the current situation, it completely lost the battle DVD format to inexpensive entry-level DVD players ($ 80-100).

Currently, a two-disc MPEG4 movie with 5.1 sound costs the same retail price in Russia as a single-layer DVD-Video movie ($ 5-6). At the same time, DVDs have no problem changing disc while viewing, player firmware raw, codec version incompatibility, braking, and artifacts while viewing.

My summary is as follows: consumer MPEG4 / DivX players and discs with movies in this format will undoubtedly strengthen their position and occupy their niche in the market, but they will be as widespread in the consumer sphere as DVD-Video players. And consumer discs (based on MPEG-2 video, plus DolbyDigital 5.1 sound) is a very important question, which only time can answer.

Everything you wanted to know about MPEG-4 / DivX, but were hesitant to ask Part 7

 

DVD players with MPEG-4 DivX function

We are talking about DVD players widely available for less than $ 250 that can be found on the shelves of electronics stores today and in the usual supermarkets of Western Europe. The ability to play MP3 and JPEG files on these players is standard. But it was not always like this. The first DVD players only worked with DVD, VCD and audio CD formats. We have every reason to believe that next year, DivX playback will become standard on all mainstream players on the market.

What’s up today? The first MPEG-4 compatible DVD players appeared in early 2003 from KISS Technologies at a price of around $ 500. This price could not attract a wide range of users, so they did not cause much enthusiasm in the market. The situation changed radically, a few months after the second wave of cheaper players appeared, in the $ 200-250 category. Thus, for example, at the CeBIT-2003 exhibition, the German company MAS Elektronik AG announced the product Xoro HSD 400 (support for 3.11 and higher, integrated 5.1 audio decoder, VGA output) for only $ 250.

The market turned to DivX. The press has published many articles on the prospects for MPEG-4 and DivX in home appliances. Heated discussions broke out. As a result, today there are DVD / MPEG players on the market from various manufacturers, the demand for them is so high that they are in short supply both in Europe and in Russia.

Currently, three manufacturers offer chipsets for the production of DivX players: Mediatek, ESS Technology and Sigma Design. Sigma has historically been the first and has been skimming the cream for about a year. However, today ESS is the undisputed leader in this market. ESS Vibratto offers a powerful and affordable solution to play all popular formats and codecs (including the old 3.11). Also, on the basis of an ESS chip, unlike Sigma, 6-channel Dolby Digital audio decoding can be implemented.

Modern popular players implemented in ESS microcontrollers with DivX support
Philips 737 is a stylish player (6-channel built-in audio decoder is not installed)
Xoro HSD 311 – Popular Affordable DivX Player with 6-Channel Audio Support
Xoro HSD 400 Plus – 311 modification in elegant design with karaoke and VGA (PAL / NTSC progressive scan) output, it is a successor to Xoro HSD 400 and Xoro HSD 410
The JVC XV NP1 DVD player is often mentioned in the forums. However, unfortunately, this player only plays MPEG-4 movies on memory cards.

Everything you wanted to know about MPEG-4 / DivX, but were hesitant to ask Part 5

The only real “threat” to MP3 today comes from the new Ogg Vorbis format.

This development is completely open source and free from the start and has great potential. Compared to MP3, Ogg Vorbis has better sound quality for the same file size. Its popularity is evidenced by the fact that Ogg Vorbis, like MP3, has gone beyond the computer and is compatible with some consumer audio systems.

Devices for playing MPEG-4
The undisputed leader was and continues to be the computer. The downsides associated with this way of watching movies are obvious. You cannot comfortably sit on the sofa with the remote control in your hands and enjoy watching. You have to wait for the download, the field of view is limited by the screen, 5.1 sound is not available to everyone, etc. Everyone can find many reasons in their settings why it would be nice to have a dedicated device for them.

A reasonable question arises: why, for example, a normal DVD player, is not equipped with the ability to play DivX? However, as you can see from the history of DivX, there is no unity in formats and solutions. This is the main challenge and the main obstacle for DivX to go beyond the computer industry. PCs are second to none when it comes to flexibility. The PC is limited only by the power of the processor and the perfection of the settings in the ability to find a way to play any combination of video and audio formats, as well as for any encoding settings, find a solution in the form of a codec or viewfinder settings.

Users of mobile applications or stationary solutions based on specialized chips do not have this luxury, they have to rely on the sagacity of the manufacturers and the ability to update software for new versions of codecs. Naturally, until now there is no single solution outside the PC that can read absolutely all formats.

Therefore, those who wish to view a “pirate” collection on the television screen using a stationary player will hardly find an ideal solution. The answer of the manufacturer will be: use one or another setting of the encoding program to obtain an acceptable result. As a general rule, this is a rejection of advanced MPEG4 functions such as QPEL, GMC when recording movies, as well as some restrictions on audio formats.

Either way, this shouldn’t be an obstacle when it comes to choosing a DVD player for home video enthusiasts as well as those looking for cutting edge technology with maximum capabilities.

DivX movies are not inferior in quality to DVDs, but they are much cheaper and more compact in production. Many modern camcorders and digital cameras offer the ability to create DivX video, and there are a large number of programs to process and record video in this format.

Now mobile devices can be found to play DivX. For example, a compact hard drive-based mobile player with a 3.8-inch screen from Archos, the review of which is available at iXBT.com. Or an MP3 player with a built-in Thomson Lyra display. Xoro also plans to introduce a similar player with a 5.8-inch widescreen.

But the locomotive of the market will remain stationary DVD players with improved functionality.

Everything you wanted to know about MPEG-4 / DivX, but were hesitant to ask Part 4

DivX: 3.22

In January 2000, the following version of DivX was created: 3.22 or 3.11 VKI (Variable Keyframe Interval). It has improved decoding algorithms and reliability. In May, DivX Networks was born, founded by Gej with former MP3.COM director Jordan Greenhall and Joe Bezdek. In July, DivX Networks launched the Majo project, which launched OpenDivX in January 2001. OpenDivX is incompatible with 3.11 and has nothing to do with it except name. In August 2001, DivX Networks released their own DivX 4.0 and they froze open source. Before version 4.12 of the codec, all previous versions were worse than DivX ;-).

In March 2002, the first commercial version of DivX 5.0x appeared. The free base option creates a video stream that conforms to the ISO certified version of the MPEG-4 format. While the commercial Pro version fully supports global motion compensation, B-Frames (encoded frames with two-frame support, B = bidirectional) and QPel (motion recovery method designation, Q = 1/4).

In the summer of 2003, DivX Networks announced the release of the 5.1 codec which, thanks to the technology implemented to evaluate visual frames, allows you to obtain better results with lower bit rates than with previous versions. This codec will improve the quality of movie playback on “weak” computers and consumer devices.

In 2004, the release of the DivX Q version is expected, which, according to the developers, should be twice as efficient as the current 5.1x.

In addition, DivX Networks plans to create a solution from a single source. It is planned to use our own development as a container for video and audio tracks (main reasons for incompatibility). According to DivX Networks, the AVI format provides the best storage option, but requires minor modifications. Licenses and audio format are being negotiated.

In parallel, an independent team based on OpenDivX created the less popular XviD format.

Audio formats in MPEG-4
The audio track in MPEG-4 can be mono, stereo, and even multi-channel AC3, which is used similarly on DVD. Supported sound formats: MP3, Ogg Vorbis, WMA, ACC, VGF, AC3 and others.

The most popular is MP3 (Very Evil Editor comment: it would be more correct to say MPEG-1 Layer3, but we must admit that the established name is still MP3). Currently, the pure MP3 is compatible with acoustic systems and all modern DVD players, the production of portable devices and car radio recorders that play MP3 has been launched. Thus, MP3 became the first massively recognized audio storage format after Audio-CD. Despite the fact that MP3 appeared a long time ago, the newer formats that claim their place have not yet been able to reach the same level of popularity. This also applies to the development of Microsoft’s WMA (Windows Media Audio) and VGF, a joint creation of Japanese firms NNT and Yamaha, and MPEG-2 AAC (Advanced Audio Coding – Advanced Audio Coding), one of the attempts to improve MP3.

Everything you wanted to know about MPEG-4 / DivX, but were hesitant to ask Part 3

Mpeg-4

Truly revolutionary is MPEG-4, which was officially completed in 1998. But since MPEG-4 is, in fact, a set of tools, they extend and complement each other. The last such additions to the standard were made in May of this year. It has become an extension of MPEG-1 and MPEG-2 and offers many innovative solutions, not all of which have found application in multimedia content and devices.

MPEG-4
The MPEG-4 standard was conceived as a way of transmitting transmission media, mainly video, over low-bandwidth channels. Unexpectedly, it gained popularity among users, thanks to the fact that it allowed to place feature films of one and a half hours to two in good quality on a single CD and to exchange video files on the Internet.

The MPEG-4 standard defines the principles for working with the digital representation of multimedia data in three areas: interactive multimedia itself (including products distributed on optical discs and on the Web), graphic applications (synthetic content) and digital television – DTV. . In fact, the format sets the rules for organizing the environment and the environment is object-oriented. It not only deals with streams and media matrices, but also with media objects.

With the same bit rate and certain encoding conditions, the image quality of a movie in MPEG-4 can be comparable or even better than when using MPEG-1 or MPEG-2. The video compression algorithm in MPEG-4 works in the same way as in previous formats. When encoding a source image, the codec saves keyframes and, instead of storing intermediate ones, predicts and stores only information about changes in the current frame relative to the previous one. The information obtained in this way is saved in a file. Sound compression is most often done in MP3, Ogg Vorbis, WMA formats. However, it is possible to use any codec, up to the six-channel AC-3 used on DVD.

The story behind DivX
The original DivX codec 😉 3.11 Alpha is a cracked version of MPEG-4 Version 3 (MP43c32.dll). The authors of the patch, known by the nicknames MaxMorice and Gej, published it in September 1999 on their website. It is no coincidence that the emoticon appeared in the title. At that time, the American company Circuit City was trying to commercialize the Digital Video Express (DIVX) Pay-per-View (Pay-per-View) video system. The business idea was to sell movies on CD, which would have been cheap but paid to see it again. Back then, hacking into the Digital Video Express security system was the target of many American hackers. Before this could be done, DivX disappeared from the market for financial reasons, and French programmers MaxMorice and Jerome “Gej” Rota immortalized it in the name of the new format.