Random _ network and multimedia
Author: Jiang Xuezhe (netsail0@163.net)
Reference: HTML Reference Daquan (Third Edition) Tsinghua University Press [United States] Thomas A.Powell Yang Zhenghua Ge Rankan Li Jinbo Chen Yuxiang ¥: 92
Environment: Windows2000Pro IE6.0
Tint Jiang Computer Program Group (http://chulsoft.xiloo.com) Copyright, please explain it. -------------------------------------------------- ---------------- The family installed 256KB ADSL broadband. The Internet speed sou ~~~~ sou ~~~~ is fast. Even the full set of 3.5GB of the dunk master downloaded. I can't say a happy heart. Download MP3 online, watch movies online, communicate with distant friends, there is absolutely no problem! no problem! ! Casual room found the record of the counter-terrorism elite in China.com. In this way, I really realized what is CS! With the fast-paced music, those masters have entered the gun of the gun, let me get it, I can only use two words to describe - mad! ! ! It's a group of madmen! ! ! One of them actually didn't buy it, just a default pistol, he dared to rush forward! Everything is not afraid of people. How can I have such a cow b? If I see it, I will not believe it. It means death in front of the gangmist master.
I have seen a metamorphosis in the first record of the desert, and I have rushed forward in the Aztoc map. I killed the other six people at once. Is it enough? Let me be stunned! There is a more abnormal kid, there is no weapon! Take a pistol and rush forward! He is a police, and the gun power of 12 hairballs is relatively strong. He used the gun to kill all 6 enemies. Is this more cattle B? Slow slow, there is a more cattle. He is a mess, his pistol is a twenty bullet. Everyone knows that the power of the pistol is very weak. He also saw three police officers in a place, all M4 rifles! ! ! This metamorphosis only uses 12 rounds of bullets to kill them! Scorpio! ! ! Let me feel inferior. CS is a great game! ! His position in my mind is no less than an Empire era and interstellar. Seeing that they play CS is a kind of enjoyment. Very high enjoy!
I started online from 2000. So it's just that the network is like this. Use the browser to watch the web page with image sounds. It is normal. Later I look to a plain text browser Lynx in the process of learning Web development. Lynx does not support image, audio and video. This old antique can only show characters, and there is no beautiful IE. It's really unhappy! From that time I felt that we are so happy now! What I don't understand is that some people are still persistent on plain text browsers. What is it thinking in your mind? Since more excellent new technology is coming, you should accept it! How is this old?
One of the innovations that lead to modern network development is the introduction of images in 1993 (Mosaic is a web browser on the INTERNET), but this is just the first step. Now the network can have a variety of media forms --- including sound, video, and animation to people's browsers.
There is an irreplaceable role in certain occasions. The sound is an important element of a multimedia web page. But how should the sound be used? If you just simply add MIDI to background music repeat, then your web page is equivalent to the electronic piano demonstration in the store. And now the MIDI music in most web pages is excessive.
In the past few years, the support of audio has changed greatly, and its focus has been transferred on a web page to a separate player such as WINAMP, RealPlayer. We have long been accustomed to the "Download -> Play" mode in MP3 format. Until the recent streaming. We should have some understanding of these digital sound technologies. There is a term - "sampling frequency". Sampling frequency, that is, digit the number of sounds in a specific period of time. The unit is kilozep / sec (kHz). The sound of CD is approximately 44.1 kHz, which is 44,100 samples per second. For stereo systems, two channels are required. Each channel 8bits, then each sample is 16bits, so the sound of the CD quality per second needs to generate data of 705600 bits (44100 * 16 = 705600).
Ideally, the use of CD quality sounds should be used on the Internet to meet the high quality music required by the user. However, the bandwidth that most users can enjoy is very limited. There are still many users who drive cats. Download a MP3 takes at least half an hour.
So when you create the digital music published on the web, you can reduce the sampling frequency. Especially for sounds with high sound quality requirements, such as speech recording, etc. Low quality audio requires only 64,000 bits of data, but users still need to wait for download. Today's network bandwidth resources are not very rich.
The same as the graphics file, the audio file can also be compressed, thereby reducing the amount of data that needs to be sent. The server-side software compresses the audio data and then this data will be decompressed and played on the client. This compression / decompression software is referred to as a CODEC (multimedia digital signal codec). The audio compression method is also divided into two types of loss compression and lossless compression. As the name suggests, lossless compression cannot perfectly reproduce the compressed data, but it can be close enough. And thus reduce the size of the file. Data from lossless compression will not have any losses, of course, unable to significantly reduce the size of the file. Compression always requires a balance between the sound quality and the size of the file, because a larger file means longer download time.
Early methods of publishing sounds on the Internet "Download and Play" models. Use file formats like WAV and AU. In this case, the user must first download the full file before starting to play. The MP3 format is my first when downloading music online. Then it is WMA. I have never said that I have never said that I have never said - delete! ! ! Because I am most concerned about the quality of music. The MP3 format can basically meet my requirements. This is the case, MP3 is almost a unique choice for high quality sampling audio playback.
MP3 is developed by MPEG (MOVING PICTURE Experts Group). MP3 is an audio format that is capable of compressing the quality of CD to approximately 10 to 20 times. It considers human hearing characteristics. Minimize the ears of people or not important frequencies and optimize the rest.
"Download and Play" format like MP3 is popular because they can transmit high quality audio files on the Internet, but they also need to let users wait for the completion of the download. For a music, this is necessary. No one is willing to listen to the song online. Do you have to listen to music? This is unrealistic. But there is really such a need, and sometimes people need to have an online instant broadcast broadcast media. Network broadcasts are called "streaming). The most popular flow audio is of course the RealAudio format developed by RealNetworks.
First we have to understand what is "flow". A user who uses a cat online can accept approximately 4K data per second. If you can use this 4k data to express a second of the sound, and the data transmitted to the end user is stable in 4K per second, and the data will effectively "flow", or "play" real-time ". The flow is very meaningful. The user cares about this second when listening to the broadcast. Users may not want to save it to the second, and users are only interested in a small news. Then why do you want to download the entire news and listen? Flow audio does not take up disk space, and it also allows any location of an audio file to be accessed freely. We can see the audio files of the "Download-Play" mode as an alternative to the recording tape, and the streaming audio is a replacement of a radio. However, the flow format also has a serious disadvantage. First, you want to compress the audio file to the flow, you must sacrifice a certain degree of sound quality. Furthermore, the Internet protocol itself does not consider the demand of the stream.
Because Internet is a high-risk area of data blocked and delayed. The TCP / IP protocol used on the Internet is designed for stability and expansion. The Internet is a packet switching network that divides the data into small pieces, send them separately, and then recombine the packet on the receiving end. Since these data packets may be lost during the transfer, or in the sequence of sequences is disrupted, people use the transmission control protocol TCP to ensure the integrity of the data.
The packet switched network allows many users to share a fixed line, however, they have a serious problem. If there is no specific modification, the packet switched network cannot guarantee the transfer time. That is, the data packet issued by the server side may have to be received by the user for a long time. This makes it difficult to implement streaming audio on the network, and "real-time" applications of video are difficult.
Packet switched networks can join a real-time flow protocol (RTSP: Real Time Streaming Protocol). It can distribute data to multiple packets by cache. Even the predetermined bandwidth is required to help improve the possibility of real-time delivery. Although these protocols can enhance real-time data transfer, they cannot change the use of networks. For example, if a reservation of a true bandwidth will result in a problem - how can you limit an endless scheduled, because each user wants to have the entire network bandwidth.
However, this is just a pure idea. Although we have a strong broadband demand while using multimedia on the Internet, the current Internet cannot guarantee that all users have sufficient bandwidth. So I can only make a hypothesis, assume the lowest bandwidth you have. But in fact, many users' bandwidth is much lower than the hypothesis. At this time, users can't listen to audio and video online. Because if it is assumed that it is not settled, the audio or video will lose the signal.
One way to avoid loss of signal is cache data. This approach allows you to download a certain number of data in advance and store them into the cache to prevent data loss. For a long audio clip, the initial 10 or 15 second buffer delay is acceptable. For short audio clips, the cache will bring a reaction.
Many Internet audio solutions are simultaneous use of centralized compression, cache, and a certain assumption to user bandwidth to meet the needs of streams. More complex audio solutions use servers to control. These two streaming audio solutions have their advantages and disadvantages. With the popularity of broadband network, streaming audio will continue to function within a considerable period of time.
