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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.

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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.

Everything you wanted to know about MPEG-4 / DivX, but were hesitant to ask Part 2

Codecs

A codec (co dermatomyositis dec oder) is an encryption method and application for recording video data in a file, eg AVI, and decrypting the information, respectively, for playback of already recorded AVI files.

The most popular today is the commercial DivX codec. The non-commercial XviD competes with it (respectively, the name is the reverse spelling DivX).

Containers
During the widespread adoption of Windows 3.1x, Microsoft created an AVI container to store synchronous audio / video data. AVI stands for Audio Video Interleave. AVI is a container format that can contain compressed video / audio data using different codec combinations. So if MP3 and JPG files are obtained using only one type of data compression (MPEG Audio Layer 3 and JPEG), then the AVI file can contain several types of compressed data (for example DivX for video and MP3 for audio) . All AVI files look the same “on the outside” (they have the extension .AVI), but “on the inside” they can differ significantly.

A long time has passed since the creation of AVI, and today it is the most popular format for presenting videos. The rise in popularity was largely due to the advent of DivX.

MKV is less popular than the AVI container format. It got its name from the abbreviation of the Russian word “Matryoshka”, which symbolically reflects the principles of his work. The main advantage of MKV is its total openness. MKV has a number of advantages over AVI, however, according to the latest information, DivX Networks, which is a leader in the development and promotion of new formats, plans to use a new container, which will still be built on the basis of AVI.

History of MPEG
MPEG (Moving Pictures Experts Group) is the name of a group of ISO experts that develops standards for encoding and compressing audio and video data. Standards prepared by the committee receive the same name.

The first to appear on the market was the MPEG-1 standard. It was developed in 1992 to compress video to CD-ROM and formed the basis for Video-CD. The quality parameters of the video data processed by MPEG-1 are in many respects similar to ordinary VHS video, therefore this format is mainly used when it is inconvenient or impractical to use standard analog video media. The format has become widespread in Asia and has seriously replaced VHS devices.

In the MPEG-2 standard, compared to MPEG-1, multi-channel sound was added, image resolution was increased, and encoding quality was improved. As a result, MPEG-2 has become widespread in the DVD and satellite broadcasting industry.

MPEG-3 was supposed to be the new standard, but in the end it only slightly expanded the capabilities of MPEG-2.

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

Will MPEG-4 / DivX Hit the Consumer Fixed DVD Player Market?

For its fame and bad reputation, DivX should be grateful to hackers and the Internet. Thanks to this revolutionary format, movies can be distributed over the Web, and a good-quality two-hour video program can easily fit on a standard CD. So when it comes to DivX, publishing publishers are holding on to their hearts and asking journalists, “Just don’t write about how to beat DVDs in DivX, otherwise movie studios and media organizations. human rights start a heartburn attack. ” However, our article is about the fact that DivX is not only used by hackers.

Although the very origins of the format were the story of a hack, today DivX is rightly called “MP3 for video”. And just as MP3 found its place in the mobile entertainment and home electronics industry in its time, DivX is beginning to pave its equally successful path in this direction.

DivX is synonymous with the mobile and desktop digital video applications of the future. The format was recognized by renowned manufacturers, leaders in technology. Now it is being actively introduced into electronic devices, from stationary DVD players to mobile phones, from digital television devices to pocket PCs.

Major consumer electronics companies such as Sony, Philips and Matsushita are working to create a digital cinema standard based on MPEG-4. Several telecommunications operators use the MPEG-4 standard for video transmission over mobile networks. Devices based on specialized chipsets for DivX playback are now available on the market.

Misunderstandings with formats and codecs

As practice shows, there is always a lot of confusion around MPEG-4 and DivX. The most common mistake, Avi, DivX and MPEG-4, is the same. AVI and MKV are containers (file formats), that is, data storage and packaging. Whereas DivX or XviD are codecs, that is, encoding methods and implementation for recording video data to a file and decrypting information, respectively, for playing back already recorded files. MPEG, in turn, describes both the codecs and the storage methods.

Basic principles to reduce
Redundant video data in the MPEG encoding process.

Evolution of digital video compression standards

The MPEG encoding process removes redundant video data in a series of adjacent frames.

Two adjacent frames often contain many of the same picture elements. The information in them differs in a small part from all the information contained in the frame. The video is compressed, in which not all the data from each video frame is used, but the frame dynamics changes, since in most consecutive frames of a video clip the background hardly changes, and clearly noticeable changes occur in the foreground. .

For example, there is a smooth movement of a small object against the background of an unchanging background. In this case, the complete information about the image is stored for the reference images only. For the rest of the frames, only the difference information is digitized: on the position of the object, the direction and magnitude of its displacement, on new background elements that open behind the object as it moves. In addition, this difference information is calculated not only in comparison with the previous images, but also with the later ones (since it is in them, as the object moves, that the previously hidden part of the background is revealed).

The data reduction process is as follows. First, a keyframe (I, Intra frame) is created.

The reference I-frames are used to restore the remaining frames and are placed sequentially every 10-15 frames. Only a few fragments of frames that are between I-frames have time to change, and it is these changes that are recorded during the compression process.

In addition to I-frames, two other types of frames are distinguished in MPEG:

predictable frames (P, Predicted) containing the difference between the current image and the previous I-frame or taking into account the displacements of individual fragments;
Bidirectional predictive frames (B, bidirectionally predictive), containing only references to before or after frames of type I or P, taking into account the offsets of the individual fragments.
I-frames form the basis of an MPEG stream and, through them, random access to a piece of video is performed. The I-frames are slightly compressed to ensure high visual quality.

The P-frames are encoded relative to the previous frames (I or P) and are used as a comparison pattern for an additional sequence of P-frames. In this case, a high level of compression is achieved.

B-frames are highly compressed. To link B-frames to a video sequence, it is necessary to use not only the previous image, but also the next one. B-frames are never used for comparison.

The I, P, B frames are combined into groups (GOP-Group Of Pictures), which represent the minimum repeated set of consecutive frames, for example:

(I0 B1 B2 P3 B4 B5 P6 B7 B8 P9 B10 B11) (I12 B13 B14 P15 B16 B17 P18 …)

Frames are made up of macroblocks, which are small fragments of an image 16 × 16 pixels in size. The MPEG encoder processor analyzes the frames and looks for identical or very close macroblocks by comparing the base and subsequent frames. As a result, only the difference data between frames, called vector motion code, is saved. Macroblocks that do not contain changes are ignored and therefore the amount of data to be transferred is significantly reduced. To reduce the impact of errors during data transmission, sequential macroblocks are combined into independent sections (slices). In turn, each macroblock consists of six blocks, four of which carry information on luminance (Y) and the remaining 2 blocks carry information on color difference signals (U / V). Blocks are basic units

Block diagrams are used 4: 2: 0 or for studio quality (broadcast) 4: 2: 2.

This is an important point that requires a more detailed explanation:

It is useful to know that RGB color data received from a video camera can be represented equivalently as the sum of the luminance signal (Y) and two color difference signals (U and V), called chromatic. The luminance signal Y determines the luminance of the point. The U and V color difference signals, together with the Y signal, can fully restore the original RGB data.

And it is calculated from RGB data using the formula: Y = 0.299R + 0.587G + 0.114B

The U and V signals are calculated as follows: U = R – Y and V = B – Y