The appearance and application of digital technology have brought far-reaching influence to mankind. People now live in an almost digital world, and digital audio technology is one of the most widely used digital technologies. CD, VCD, etc. It has entered thousands of households, and digital broadcasting is gradually being carried out around the world. It is these products and applications that are closely related to consumers that have become the theme of this article: the driving force for the emergence and development of digital audio compression technology.
1, the emergence and early application of audio compression technology
Audio compression technology refers to the application of appropriate digital signal processing technology to the original digital audio signal stream (pulse code modulation) to reduce (compress) its bit rate without losing useful information or negligible loss, also known as compression coding. It must have a corresponding inverse transformation, called decompression or decoding. Audio signals may introduce a lot of noise and some distortion after passing through the codec system.
The advantages of digital signal are obvious, but it also has its own corresponding disadvantages, that is, the increase of storage capacity requirements and the increase of channel capacity requirements during transmission. Take a CD as an example, the sampling rate is 44. 1KHz, and the quantization accuracy is 16 bits, then the stereo audio signal of 1 minute needs to occupy about 100 bytes of storage capacity, which means that the capacity of a CD turntable is only about 1 hour. Of course, this problem is more prominent in the field of digital video with much higher bandwidth. Are all these bits necessary? It is found that there is great redundancy in direct storage and transmission with PCM code stream. In fact, the sound can be lossless compressed at least 4: 1, that is, only 25% of the digital quantity is used to retain all the information, and the compression ratio can even reach several hundred times in the video field. Therefore, in order to make use of limited resources, compression technology has been widely concerned since its appearance.
The research and application of audio compression technology has a long history. For example, A-law and U-law coding are simple quasi-instantaneous companders, which have been applied to ISDN voice transmission. The research on speech signal developed earlier and was more mature, and it was widely used, such as adaptive differential PCM(ADPCM), linear predictive coding (LPC) and other technologies. In the field of broadcasting, audio compression technology is used in NICAM (near StantaneousCompandedaudiopliplex) and other systems.
2. The main classification and typical representative of audio compression algorithms.
Generally speaking, audio compression technology can be divided into lossless compression and lossy compression, and according to different compression schemes, it can be divided into time domain compression, transform compression, subband compression and mixed compression of various technologies. Different compression technologies have great differences in algorithm complexity (including time complexity and space complexity), audio quality, algorithm efficiency (compression ratio), codec delay and so on. The application of various compression technologies is also different.
(1) Time-domain compression (or waveform coding) technology refers to directly processing the samples of audio PCM code stream, and compressing the code stream by means of mute detection, nonlinear xing quantization and difference. The common characteristics of this kind of compression technology are low algorithm complexity, average sound quality and low compression ratio (CD sound quality >; 400kbps), the encoding and decoding delay is the shortest (compared with other technologies). This compression technique is generally used for speech compression and low bit rate applications (Xiao Yuan signal bandwidth). Time domain compression technologies mainly include G.7 1 1, ADPCM, LPC, CELP, and block companders such as NICAM and subband ADPCM(SB-ADPCM) technologies such as G.72 1, G.722, Apt-X, etc.