Analyzed the advantages and disadvantages of each mainstream codec and gave advice on the choice of codec
The first month of 2018 is not calm for the editing code standard ecology. Apple joined the AOM Alliance to support AV1. The domestic AVS2 announced open source. MPEG founder and chairman Leonardo Chiariglione issued a message saying that "the MPEG business model has broken down." Although AV1 and AVS2 will take some time to mature, at least give users more choices. Fu Deliang, senior research and development manager of Hulu, wrote an analysis of the advantages and disadvantages of mainstream codecs and gave his own suggestions for the choice of codecs.
Text / Fu Deliang
Time flies, and in the twinkling of an eye it has come to 2018. In the past few years, the entire audio and video industry has developed rapidly with the tide of the Internet, and has enabled users around the world to connect with the entire world through this high-density information carrier of video.
However, what the whole industry has not expected is that the "power transfer" of the video codec standard, which is an important cornerstone of the existence and development of the online audio and video industry, remains confusing today. 2018 is the fifteenth year of H.264 release, and HEVC, the official successor of H.264, has been officially released for five years. However, although the current industry needs for the next generation of audio and video codec standards to increase the compression efficiency is very urgent, but which standard, which codec is the true dragon of the H.264 world, It is still unknown.
H.264: Unification of rivers and lakes
H.264 / AVC is by far the most successful audio and video codec standard in the industry. In the past decade, the H.264 standard has appeared in various video application scenarios. On the encoder side, the open source codec x264 based on the H.264 standard is almost standard for streaming media service companies of all sizes; on the decoder side, H.264 decoding support covers all kinds of dedicated and general-purpose devices, such as Desktop browsers, various mobile phones, computers, camera equipment, game consoles, smart TVs, various boxes, and more.
Uniform coding standards allow companies involved in all aspects of the video industry to perform their own roles, focusing on their own focused business and modules. As long as you can ensure that your products can meet the H.264 standard, you can go unimpeded in the video industry without worrying about the direct connection and cooperation with upstream and downstream. This unified situation has enabled the entire industry to avoid a lot of repetitive development and debugging, and significantly improved the development speed of the entire video industry objectively.
The most important reason why H.264 can achieve unprecedented success is its advanced technology, and the compression efficiency is significantly higher than the previous generation codec standard. On the other hand, H.264's relatively open patent licensing mechanism and rates are also a big boost. In particular, H.264 is free for online video production and transmission, greatly improving the adoption rate of H.264 on the Internet.
HEVC: Weight-bearing successor
The official successor of H.264 is H.265 / HEVC jointly proposed and released by ITU-T and MPEG. As a next-generation standard with the same roots as H.264, HEVC has been aiming to replace H.264 one day. In order to achieve this goal, ITU-T and MPEG established a joint working group in the early stages of standard setting, and assembled the best video experts in the world to carry out hard work. On this basis, the working group also learned from the experience of H.264 standard in industrialization and practice in the process of developing HEVC: HEVC standard fully considers the complexity of decoder side and the complexity of hardware implementation, making the whole standard The implementation cost is reasonably controlled.
It can be said that HEVC is technically a codec standard that is a generation of Dacheng. However, like the sons of the great kings of the past, HEVC also took extra pressure from its birth. On the one hand, in order to achieve the highest possible performance, in the choice of coding tools, HEVC adopts various technologies based on performance and speed. On the other hand, the great success of H.264 has also made the companies in the industry aware of the importance and significant potential of audio and video coding standards, and therefore hope to achieve higher economic returns by participating in standardization work. These factors have combined to create a complex situation in the current HEVC patent licensing, and the potential high patent fees are currently the biggest obstacle to the full application of HEVC.
Today, HEVC is still a strong contender for the next generation of video codec standards in the industry with its first-mover advantage, excellent technical performance and deep influence in academics and standardization organizations. However, its further industrialization is currently significantly affected by patent licensing fees and remains uncertain in the future.
VP9: princes that cannot be ignored
As early as the H.264 era, the VP series of standards is a diligent chaser on the market. After being acquired by Google, open sourced and developed for several years, the latest standard VP9 of the VP series has grown into a non-negligible competitor of HEVC. VP9's completely open source and free policy is an important advantage, and good decoding support in browsers and Android systems also lays a good foundation for its application. At present, Google's Youtube uses VP9 as the video codec standard. Netflix also uses VP9 on the mobile side. Although VP9 is not famous, but with the influence of Google, it does not fall behind HEVC in the OTT market.
The problem with VP9 is that it is not popular enough, without rich and efficient encoder selection and extensive decoder support, which casts a shadow over the prospects for its widespread application. In addition, the performance of VP9 has not been widely recognized by the industry, and the industry still has reservations about whether its compression efficiency can reach the level equivalent to HEVC. As Google shifts its focus to AV1, VP9 plays a more fundamental role in the next generation of codec standards. However, if HEVC's patent cost problem continues to ferment and AV1 is too late to be widely deployed, then VP9 still hopes to stage the following flops.
AV1: Upstart Challenge
In response to the patent issue of HEVC, a number of companies led by Google established AOM (Alliance of Open Media, http://aomedia.org/) to develop related alternative technologies. AOM's goal is to develop a free open video codec standard that replaces HEVC, which is named AV1 (AOMedia Video 1). AV1 was developed based on Google's VP10 and incorporates Xia's/Mozilla's Daala and Cisco's Thor encoder/standard related technology.
Currently AOM members include browser-related major vendors such as Google, Mozilla and Microsoft; content providers such as Google / Youtube, Netflix, Amazon and hulu; hardware vendors such as Intel, AMD, ARM and Nvidia. This lineup has basically covered the entire industry chain of online video. It can be seen that although AV1 started late, the layout of the entire industry has been basically mature. Recently, Apple's joining has injected a strong force into AV1. AV1 stands for the vibrant Internet industry, which has abundant funds and talents, and has a more open and inclusive mindset, and dares to propose new standards for free and free. The emergence of AV1 will definitely have a strong impact on the entire video standardization industry.
However, AV1 is not perfect. On the one hand, the AOM organization is still very young compared to ITU-T and MPEG. There is still a certain gap in the coordination organization of standardization work, which will affect the progress of the entire standard development to a certain extent; on the other hand, in order to achieve open source free The purpose of AV1 is limited in the choice of technology, which affects the final performance; finally, AV1 started later than HEVC, in hardware decoding, support for various video packages, and corresponding optimization research in the academic field. Time will be relatively backward. Therefore, although AV1 has a brighter future, it is quite difficult to completely defeat HEVC.
Other options
In addition to HEVC and AV1, there are other video coding and decoding standards in the industry that may become the mainstream in the future. For example, H.266 / FVC (Future Video Coding) proposed by ITU and MPEG on the basis of HEVC, AVS2 and AVS3 of domestic proprietary intellectual property. Regardless of the standard, if you want to achieve widespread adoption in the future, you must have support for compression efficiency, licensing fees and structure, hardware and software decoding support coverage, support for new features, and the availability of available codecs. Degree and other aspects achieved a better overall performance.
Choice of coding standards
As the next video coding standard battles and the winners are not yet a foregone conclusion, the new features of online video and the growing demand for high-definition video from growing users today, how to choose between various standards and encoders Important issues of concern to various streaming service providers. Choosing the right technology solution plays an important role in controlling costs, improving system stability, and ensuring user experience. It is worthy of in-depth research. When examining each standard, the main aspects to be considered are as follows:
Decoder support
The support of the decoder is the first thing to consider when choosing a video coding standard. The video decoder has a complicated structure, a long development cycle, and high performance requirements. More mature decoder support can significantly reduce the research and development costs of OTT online video companies and shorten the service online cycle. Choosing a wide range of decoders, decoder support for mature video coding standards is the consensus of industry companies.
In the next-generation video coding standard, HEVC is relatively good in hardware decoder support due to its early start and support from many companies. Most of the multimedia devices introduced in the past two years have built-in HEVC hardware decoders. In 2017, Apple devices also joined the HEVC decoding support camp, so HEVC has completed the full coverage of the living room equipment, set-top boxes and mobile devices. VP9's hardware decoding support is relatively small compared to HEVC, but it has also covered a considerable number of Android devices and set-top boxes. Since the AV1 standard has not yet been finalized, it will take time for the hardware decoding to be widely promoted.
The decoding support on the browser side is quite different. HEVC's support on the desktop browser is very limited, and currently only Safari and Edge browsers support HEVC decoding. VP9 decoding has been supported on all major browsers except IE and Safari. Although the AV1 standard has not been finalized, from the perspective of the company lineup that has joined the AOM organization, AV1's future support for browsers is worth looking forward to.
Encoder maturity
The quality and efficiency of the encoder largely determine the efficiency of video coding. The rich and powerful encoder lineup will greatly ensure the production deployment speed and online compression efficiency of the relevant standards. Therefore, the maturity and diversity of the encoders on the market are also important considerations when selecting coding standards.
Encoders can be divided into hardware encoders and software encoders in terms of implementation architecture. From the commercial model, they can be divided into open source encoders, commercial encoders and private encoders. Among the next-generation coding standards, HEVC is currently the most abundant choice of encoders, such as open source software encoders such as x265; a variety of commercial software decoders; hardware encoders from Intel, Nvidia and other companies. HEVC's relatively complete and standardized documentation also lays a good foundation for the development of private encoders based on standards. VP9 is relatively poor at encoder support. The lack of documentation hinders the development of encoders. The only common VP9 encoders on the market today are the open source software encoder libvpx and a few commercial encoders. The AV1 standard plans to provide a reference encoder at the same time as the release. Unlike HEVC, H.264 and many other standard reference software, AV1's reference encoder is dedicated to commercial applications, so its performance is now widely used in the industry. attention. In addition, some encoder manufacturers have already started the research and development of AV1 encoders intensively. It is believed that a variety of AV1 encoders will be available on the market in the near future for manufacturers to choose.
Support for new features
Today, as the video industry is changing rapidly, support for new features also needs to be considered when selecting coding standards. Due to different launch times and sources, each standard has different levels of support for new features and the entire video transmission standard chain. Consideration should be given to potential new feature support when selecting video coding standards to avoid duplication and wasted investment.
Some typical new features include HDR video, 360-degree and VR video. Taking HDR as an example, the current mainstream HDR formats in the industry are based on HEVC encoding. Therefore, if there is a demand for HDR feature support in the near future, the coding capability of HEVC is indispensable.
Company and business needs
In the choice of video coding standards, the most important factors in the final analysis are the company's situation and business needs. Only by fully considering the needs of the company and the business can we make the right choices in coding standards.
The first thing to consider is whether you need to upgrade to the next generation of video coding standards. H.264 is very mature in all aspects. If the support for compression efficiency and new features is not high, you can choose to continue using the current H.264 system. Before the decoding of each new standard supports the entire platform, companies still need to use H.264 to serve platforms that do not support next-generation encoding. Therefore, for video encoding computing and video storage, the next-generation standard will be used in a certain period of time. Bring additional costs. The benefits of the new standard are bandwidth savings, support for new features (such as 4K, HDR), playback fluency, and improved user experience. Companies should make choices based on their own specific situation, rather than blindly following up on new standards and technologies.
Second, you need to consider the types of platforms you need to support. The standards are supported differently on different devices, so there should be a global and forward-looking consideration in the selection of standards. For example, if the company's business focus is on the browser's web page and does not develop its own decoder, then the VP series or AV1 standard should be considered more; if the company is targeting mobile devices, HEVC has become a relatively mature option. In addition to the client type, you should also consider the business model when examining the platform type. For example, when the service needs to be coded in real time on the side, priority should be given to the standard with a more mature hardware encoder.
On the basis of the platform type investigation, the company should also fully understand the upstream and downstream control of the codec and the investment in codec development. Companies with sufficient upstream and downstream control of codecs and more investment in codec development can have more space and autonomy in the selection of coding standards; smaller companies should be more cautious in standard selection. Try to choose a standard that is more mature in codec.
Finally, considering the current patent rights issue of HEVC disputes, potential legal risks need to be evaluated when selecting video coding standards. In the next generation of video coding standards, HEVC's patent rights issue is at the forefront, and the use of HEVC means taking the corresponding risks and licensing fees. VP9 and AV1 have a strong advantage in this respect, and its free and open features are guaranteed by giants like Google, which solves a lot of worries for the market.
Comparison and selection of video encoders
Based on the selected video standard, the next problem to be solved is to choose a video encoder. The comparison of video encoders is a fairly complex problem in itself, and it is very challenging to make a very objective, fair and comprehensive encoder comparison. One of the authors of X264, Dark Shikari, wrote a blog that explained the difficulties and common misconceptions of fair and equivial encoder comparisons.
For online video companies, in the encoder comparison, the following dimensions need to be considered:
Evaluation index
The first consideration in encoder comparison and evaluation is which evaluation criteria are used. In video coding, the evaluation indicators mainly include two types of code rate indicators and quality indicators. The code rate indicator is relatively simple and straightforward, and the actual code rate can be directly used; and the quality index is much more complicated.
Traditionally, video quality is measured by the objective index represented by PSNR. However, a large number of studies in recent years have shown that the matching between such indicators and the subjective feelings of the human eye is insufficient, and in some specific scenarios, there is even a conflict with subjective quality. Case. Therefore, in the industry, it has become less and less independent to use objective indicators for encoder evaluation.
Corresponding to objective indicators is the use of subjective evaluation to compare encoders. Subjective evaluation encodes the video with a different encoder and asks the real person to score the video quality. The accuracy of subjective evaluation is high, and the main problem is the high threshold and high cost. If you want to use the subjective evaluation method for encoder comparison, you should do hardware and software support, technical support and human and time resources.
In recent years, the academic community and the industry have launched a series of objective video quality evaluation standards based on subjective quality. The use of these standards enables the achievement of performance similar to subjective evaluation without subjective quality evaluation. At present, the problem of such algorithms is that the industry lacks recognized indicators of better quality. Therefore, such indicators are rarely used in open large-scale tests, mainly relying on large-scale objective indicators + small-scale subjective experiments.
For online streaming companies, it is necessary to select appropriate indicators as the quality evaluation criteria according to their actual situation and needs. The selection of objective evaluation indicators based on subjective quality is a major focus, and it is worth spending more energy and resources.
Coding speed requirement
In video coding, the encoding speed has an essential effect on the coding efficiency. The same encoder of the same standard can have a performance difference of more than 20% at different speeds, and the performance difference of different encoders at different speeds will be more obvious. The performance of the comparison between the real-time encoder and the standard reference software is meaningless because their application scenarios are completely different.
In the choice of encoders, online streaming companies should make reasonable demands on the encoding speed according to the characteristics of their own business. For example, for live broadcast services, real-time or even ultra-real-time encoders are necessary; for on-demand services, the coding speed can vary greatly depending on the specific form of the service. In the encoder comparison, each of the alternative encoders should be adjusted to operate within the selected required coding speed range and performance comparisons are made on this basis.
Common code rate range
On the basis of the coding speed, the range of common code rates should also be considered. Unlike the standard-setting phase, the range of code rates that are commonly used for each resolution is generally not large for streaming service providers. For different markets and application scenarios, the highest and lowest transmission code rates at the same bit rate can be doubled or even doubled. This huge difference makes the performance comparison of each encoder essentially different. For example, some of the better encoders optimized by the optimization and fast algorithms perform well at the low bit rate side, while the standard features support more comprehensive encoders that may perform better at the high bit rate side.
Online streaming companies should analyze the application scenarios in depth, summarize the common code rate ranges, and compare and select the performance of each encoder in this range.
Video content features
Video content characteristics are another major factor affecting video encoding efficiency. Different encoders will behave differently on different content. For example, some encoders have special optimizations for heavily noisy scenarios, which are of added value to companies with more user-uploaded content; companies that focus on secondary content should consider more adequate coding for animated content. Device. It is often difficult and unnecessary to select a universal global optimal encoder. It is recommended that companies summarize and use the video content with their own characteristics to evaluate and select the encoder.
Feature support
Feature support is also an important consideration in encoder selection. For example, whether it supports 10bit encoding, whether it supports the rate control mode of CBR encoding, whether it has various parameters that are easy to use, whether it is easy to integrate with various platforms, and whether it supports secondary development. Just having excellent coding efficiency can make an encoder known as excellent, but it is difficult to make it called success. Streaming media companies should select encoders with sufficient features to serve users based on their business needs and prospects for the future.
Flexible architecture: smart solutions
At present, the entire video coding standard is still in the stage of melee. In the end, it is difficult to judge who wins. The choice of the encoder is even more foggy. The results of various evaluations are endless but do not converge. Select a certain standard and select one. Encoders for long-term use are quite difficult. On the other hand, the urgent needs of users and the pressure of bandwidth make it necessary for all companies in the industry to deploy next-generation video coding standards in the near future, so it is not reliable to continue to wait and see.
In this situation, designing and implementing a more flexible video coding architecture is a wise choice. This kind of flexible architecture requires the coexistence and flexible replacement of multiple coding standards and encoders on the one hand, and the re-encoding and on-line of all-site video in a certain period of time on the other hand. This kind of flexible coding architecture enables the company to have higher flexibility and adjust the coding standard and encoder selection strategy in real time according to the development of the industry and the standard competition status. In the era of H.264's unification, this kind of architecture is not necessary, but this is probably the price that the industry must pay for not having a unified standard for the time being.
Conclusion
This is the best era, this is the worst era. Today's video coding, a long-established and long-lasting technical direction, will usher in a new era of the Warring States. The collision between the development of technology and the needs of users will inevitably spark a fierce spark and bring about changes in the entire industry. As witnesses and participants in all of this, we are undoubtedly lucky. In the next few years, whether HEVC is holding on to the foot, or AV1 is coming up, or other standards are born, let us wait and see.
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