1, pulse code modulation
PCM pulse code modulation is short for pulse code modulation. In the previous article, we mentioned the general workflow of PCM. We don't need to care about the calculation method of PCM final coding. We only need to know the advantages and disadvantages of pulse code modulated audio stream. The biggest advantage of pulse code modulation is good sound quality, and the biggest disadvantage is its large size. Our common audio CD uses pulse code modulation, and the capacity of a CD can only accommodate 72 minutes of music information.
Step 2 wave
This is an ancient audio file format developed by Microsoft. WAV is a file format, which conforms to the piff resource exchange file format specification. All wavs have a file header, which is the coding parameter of the audio stream. WAV has no hard and fast rules for the coding of audio streams. Except PCM, almost all codes supporting ACM specification can encode the audio stream of WAV. Many friends don't have this concept. Let's take AVI as an example, because AVI and WAV are very similar in file structure, but AVI has only one video stream. We are exposed to many kinds of AVI, so we often need to install some decoders to watch some AVI. DivX, which we are exposed to, is a kind of video coding. AVI can use DivX coding to compress the video stream, and of course, it can also use other coding to compress it. Similarly, WAV can also use a variety of audio codes to compress its audio stream, but we usually use WAV whose audio stream is pulse code modulated, but this does not mean that WAV can only use pulse code modulation, and MP3 coding can also be used in WAV. Just like AVI, you can enjoy these wavs as long as you install the corresponding decoder.
Under the Windows platform, WAV based on pulse code modulation is the best audio format, and all audio software can support it perfectly. Because it can meet the requirements of high sound quality, WAV is also the preferred format for music editing and creation, which is suitable for saving music materials. Therefore, WAV based on pulse code modulation is used as an intermediate format, which is often used for mutual conversion of other codes, such as MP3 to WMA.
3, MP3 coding
As the most popular audio compression format, MP3 is widely accepted by everyone. Various software products related to MP3 emerge one after another, and more hardware products begin to support MP3. Many VCD/DVD players we can buy can support MP3, as well as more portable MP3 players and so on. Although several musicians are extremely disgusted with this open format, it can't stop the survival and spread of this audio compression format. MP3 has been developed for 10 years. It is the abbreviation of MPEG (Moving Picture Experts Group) Audio Layer -3, and is a derivative coding scheme of MPEG 1 1993 was successfully developed by Hof IIS Institute in Flawn and Thomson Company. MP3 can achieve the amazing compression ratio of 12: 1, and maintain the basic audible quality. In the era when hard disks were expensive, MP3 was quickly accepted by users. With the popularity of the network, MP3 was accepted by hundreds of millions of users. At the beginning of its release, mp3 coding technology was actually very imperfect. Due to the lack of research on sound and human hearing, the early MP3 encoders almost all coded in a rough way, which seriously damaged the sound quality. With the continuous introduction of new technologies, mp3 coding technology has been improved again and again, including two major technical improvements.
VBR: An interesting feature of MP3 files is that they can be played while reading, which is also in line with the most basic characteristics of streaming media. In other words, the player can play anywhere it reads, even if the file is partially damaged, it is not necessary to read all the contents of the file in advance. Although mp3 can have a file header, it is not very important for files in mp3 format. Because of this feature, each segment and each frame of MP3 file can independently average the data rate without special decoding scheme. So a technology called VBR(Variable bitrate) appeared, which can make each segment or even each frame of MP3 file have a separate bit rate. The advantage of this is to limit the file size to the maximum extent on the premise of ensuring sound quality. The advantage of this technique is obvious, but it is really difficult to use, because it requires the encoder to know how to allocate the bit rate to each segment, which is like a dummy for an encoder without waveform analysis. Just like this, VBR technology did not appear dazzling as soon as it appeared.
1, FLC in the silent age
FLC and FLI are a video format developed by Autodesk, which only supports 256 colors, but supports color dithering technology. Therefore, in many cases, true color videos are not very different and do not support audio signals. Now it seems that this format is useless, but it is indeed the best and only choice in the DOS era without true color graphics cards and sound cards. Most importantly, Autodesk's full range of animation software provides support for this format, including the famous 3D Studio X, so this format represents the video coding level of an era. Until today, there are still many video editing software that can read and generate this format. But after all, Lian Po is old, and this format has been ruthlessly eliminated.
2, singing and dancing AVI
Avi-audio and video interleaving, that is, audio and video interleaving format. At the beginning of 1992, Microsoft introduced AVI technology and its application software VFW(Video for Windows). In AVI files, moving images and audio data are stored in a staggered way and are independent of hardware devices. This way of organizing audio and video data in an alternating way can obtain continuous information from the storage medium more effectively when reading the video data stream. The main parameters of AVI file include video parameters, sound parameters and compression parameters. AVI files are in the form of AVI fragments, which are identified by the string "AVI". All AVI files include two necessary list blocks. These blocks define the formats of streams and data streams. AVI files can also include index blocks.
As long as this standard is followed, any video coding scheme can be used in AVI files. This means that AVI has very good scalability. Because this specification was formulated by Microsoft, Microsoft's full range of software, including programming tools VB and VC, provided the most direct support, thus further establishing AVI's video dominance on PC. Because of its openness, AVI has won the support of many coding technology developers. Different coding makes AVI improve continuously. Now almost all general video editing systems running on PC mainly support AVI. The appearance of AVI declared the end of the silent movie era on PC, and the constantly improved AVI format represented the rise of multimedia on PC.
Speaking of AVI, we can't help but mention Intel's Indeo video series coding. Indeo coding technology is a high-performance software-only video compression/decompression solution, which is suitable for PC video. Indeo audio software can provide high-quality compressed audio, which can be used in Internet, Intranet and multimedia applications. Both music and sound can be compressed, and the compression ratio can reach 8: 1 without obvious quality loss. Indeo technology can help you build a multimedia website with richer content. At present, it is widely used in dynamic effect demonstration, game editing animation, nonlinear material preservation and other purposes, and is the most widely used AVI coding technology. Now Indeo coding technology and its related software products have been acquired by Ligos Technology. With the rise of MPEG, Indeo is facing great challenges.
3. MPEG series coding with both capacity and quality.
Contrary to AVI, MPEG is not a simple file format, but an encoding scheme.
MPEG-1(ISO/IEC1172) is formulated at the end of 199 1, which deals with multimedia streams in standard exchange format (SIF) or source input format (SIF). It is an international standard for digital storage media moving images and their audio coding with data transmission rate below 1.5Mbps (MPEG- 1 audio, standard code ISO/IEC 1 172-3), and the audio standard is later derived from today's MP3 coding scheme. MPEG- 1 standardizes the traffic standards in PAL system (352*288, 25 frames per second) and NTSC system (352*240, 30 frames per second), and provides audio and video quality equivalent to VHS. At this time, the video data transmission rate is compressed to 1. 10. Using the compression algorithm of MPEG- 1, a multimedia stream with the length of 120 minutes can be compressed to the size of1.2gb. The common VCD is the masterpiece created by MPEG- 1 coding. MPEG- 1 coding does not have to run according to PAL/NTSC standard. You can freely set the image size and audio and video traffic. With the improvement of the precision of optical pickup, some people increase the information density of optical disc and moderately reduce the audio stream flow, so DVCD appears that one optical disc can only store one movie. DVCD disc is actually an audio-visual product, which has no industry standard, national standard and international standard.
When VCD began to spread to the market, computers had just entered the 486 era. At that time, many friends dreamed of having a hard decompression card to realize their long-cherished wish of watching VCD on PC. Looking back today, I feel really incredible, but at that time, the status quo was that the 486 system could not play VCD smoothly without hard decompression, and the MPEG- 1 that the 486 system could not play smoothly for 10 thousand yuan was marked as a noble. With the release of Pentium, PCs began to Pentium until Windows Media Player directly provided support for MPEG- 1, so the use of MPEG- 1 on PCs was completely barrier-free.
MPEG-2 (standard code IOS/IEC 138 18) released the draft international standard (DIS) in 1994, which is basically the same as MPEG- 1 in video coding algorithm, but there are some minor improvements, such as the addition of interlaced TV coding. It pursues higher quality moving images and its sound effects under large flow. The video quality of MPEG-2 is equivalent to that of PAL or NTSC. In fact, MPEG- 1 can achieve a similar effect, and MPEG-2 has made more improvements in audio coding. At present, the most common MPEG-2 related product is DVD, and SVCD is also encoded by MPEG-2. MPEG-2 also has a more important purpose, which is to make the traditional TV and TV broadcasting system develop in the digital direction.
MPEG-3 was originally developed for HDTV. Due to the rapid development of MPEG-2, MPEG-3 was declared obsolete before it was completely completed.
MPEG-4 was published in 1998. Different from MPEG-2, MPEG-4 pursues not high quality, but high compression ratio and interactive ability suitable for network. MPEG-4 provides a very amazing compression ratio. If the quality of VCD is taken as the standard, MPEG-4 can compress the multimedia stream of120min to 300mm ... MPEG-4 standard is mainly used for videophone, video mail and electronic news. Its transmission rate is low, ranging from 4800 to 64000 bits/second, and its resolution is 176X 144. MPEG-4 uses very narrow bandwidth to compress and transmit data through frame reconstruction technology, so as to obtain the best image quality with the least data.
MJPEG, this is not specially prepared for PC, but for professional and even broadcast video acquisition and playback on the device side. Therefore, MJPEG contains the algorithm of interlaced scanning TV optimized for traditional analog TV. If you play MJPEG encoded files on a PC, the effect will be ugly (if your graphics card does not support MJPEG dynamic compensation), but once it is output to the TV, you will immediately find the benefits of this algorithm.
4. Streaming media belonging to the network
RealNetworks RealVideo adopts Real G2 codec developed by RealNetworks, and has many advanced designs, such as SVT (Scalable Video Technology). 2-2- encoding (similar to VBR). The audio part of RealMedia adopts RealAudio, which can accept a variety of audio coding schemes and realize sound compression of mono and stereo music at different bit rates. The latest RealAudio actually adopts ATRAC3 coding scheme to challenge the increasingly mature MP3.
Windows Media, whose video coding adopts very advanced MPEG-4 video compression technology, is called Microsoft MPEG-4 video codec. Audio coding adopts the coding scheme developed by Microsoft itself, and there is no public technical data at present, which provides satisfactory sound quality and image quality under low traffic. The latest Windows Media coding utility V8.0 pushes streaming media technology to a new height. Our common ASF, WMV and WMA are all Microsoft streaming media files. In fact, our common MPG files also have the biggest feature of streaming media-playing while watching.