The codec of audio is mainly determined by several major technical organizations according to the application scenarios, namely ITU-T, 3GPP, and MPEG. Of course, there are also companies or companies that are co-organized, such as Microsoft's WMA. They not only developed the specification of the codec, but also provided the reference code of the software implementation, which facilitates the popularization of the use of codec. This article first talk about these codecs, and then how to optimize the codec according to the reference code (mainly reduce the CPU load). 1) ITU-T The ITU-T has developed the codec standard for wired voice, namely the G series, mainly including G.711, G.722, G.726, G.728, G.729, and so on. The sampling rate is 8KHz and the bandwidth is 16KHz. The code rate ranges from 64 kbps to 8 kbps. The table below lists the specific sample rates and code rates. 2) 3GPP The 3GPP has developed a codec standard for mobile voice, mainly AMR (adapTIve mulTI-rate) series, which can adaptively adjust the code rate according to network conditions. The sampling rate is 8KHz and the bandwidth is 16KHz. In recent years, in order to cope with the competition of the Internet (Internet companies have proposed OPUS codec covering voice and music), 3GPP has issued an EVS (enhanced voice service) audio codec specification. EVS also covers voice and music, allowing for flexible switching between the two, supporting multiple sample rates and bit rates. The details are as follows. 3) MPEG The main part of MPEG is the codec specification for music, mainly MP3, AAC, etc. MP3 is very familiar to everyone. It is the most important format for listening to music in the past 20 years. AAC is the successor of MP3, the most important music codec specification for the next generation. The sampling rate in music is generally 44100HZ, and some use 48000HZ. The code rate is in a range, and the higher the code rate, the better the sound quality. 4) Company or company consortium Some companies or companies have developed audio codec specifications as needed, such as Microsoft's WMA, Skype's SILK, GIPS (GIPS was acquired by Google in 2011, Google's GIPS-based audio and video solutions launched webRTC and open source, affecting Huge) ILBC and so on. Another thing I have to mention is OPUS, which was developed by the non-profit Xiph.org Foundation, Skype and Mozilla. It supports full range (8kHZ to 48kHZ) and supports voice and music (SILK for voice, music) CELT) has been accepted by the IETF as a sound codec standard on the network (RFC6716). I have used codec from voice to music respectively G.711/G.722/G.726/G.728/G.729/AMR-NB/AMR-WB/ILBC/OPUS/MP3/AAC/WMA/APE /Vorbis/ALAC/FLAC, etc. The optimization mentioned here mainly refers to the optimization of CPU load, that is, running codec after optimization takes up less CPU, and runs more smoothly on a specific hardware platform. Optimization depends on the extent to which this depends on the demand. If it is optimized for the project, it depends on how long the project gives you optimization and the optimized CPU load that the project can accept. Under normal circumstances, the project can run smoothly in the most complicated scene after using the optimized codec. It does not affect other functions, because the project has to free up other people to do other things, after all, project schedule and quality are the most important. If it is optimized and sold as a library to customers, it should be optimized to the extreme, because this is an important indicator for the company to choose which company library to use, is a selling point, in this case there will be more optimization methods and techniques. The optimizations I have done are for the project, not for the library, so the skills are not particularly high. (1) Preparation before optimization a) Read through the codec code to be optimized, try to understand, even if you don't understand, you need to figure out what the function does, which is good for later optimization. b) Prepare the profiling tool. The profiling tool measures how many clocks are spent running a function. It is best to have a ready-made profiling tool. If not, do it yourself according to the specific OS and hardware platform (ARM/MIPS, etc.). c) Prepare the test vector, the source of the test, which is usually provided by the official codec, usually multiple vectors, corresponding to different scenes. The principle of optimization is that the algorithm operation result is not changed while reducing the CPU load, so every optimization should be done with the test vector when doing optimization, to see if the result has changed, if it changes, it is necessary to return to the previous version. . When I do optimization, I keep at least one version every day, sometimes two or three, in order to get back to the problem, and find out where the optimization is wrong as soon as possible. (2) Optimization steps and methods a) Change the compiler's optimization options from -o0 to -o3 b) add inline to the short function in the code that is often called Usually, after a, b, the load will go down a lot, like a squeeze of foam, it will squeeze a large part. c) There are many functions of basic operation (addition, subtraction, multiplication, division and division) in ITU-T or 3GPP codec reference code. These functions are particularly rigorous, and the frequency of calling is very high, which increases the computational complexity. Some of these functions can be simplified on the premise of ensuring correctness (such as some anti-saturation can not be), so that the load will be reduced after processing. d) Use the profiling tool to step by step to see which function spends more load, understand what this function is doing, and then analyze the specific problem and see how to optimize. e) Some functions are a small algorithm, the reference code is more complicated to write, and the calling frequency is higher. If you want to find out if there is a simple implementation, you can replace it. If you replace the load, it will drop down. For example, in codec, there is often a calculation for finding the square root. The reference code is usually written more complicated. We know that using the Newton iteration method can also find the square root, you can use Newton iteration to replace the load. f) Optimized with assembly. Do not use assembly if you can solve the problem at the C level. Each processor has its own set of assembly instructions that you need to learn and master the ideas and techniques. Usually, the function that uses higher frequency and compares load is written by assembly, that is, mixed programming with C and assembly. Assembly optimization takes a relatively long time. Of course, there are some small tricks such as expanding the for loop, replacing the array with pointers, etc., not to mention them here. 3.5mm Wire To Board Connectors 3.5mm Wire To Board Connectors.Standards 3.5mm Wire To Board Connectors ShenZhen Antenk Electronics Co,Ltd , https://www.antenkconn.com
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