1 Sound is actually an energy wave, so it also has the characteristics of frequency and amplitude, the frequency corresponds to the time axis, the amplitude corresponds to the level axis. The wave is infinitely smooth, and the string line can be regarded as composed of countless points. Because the storage space is relatively limited, the points of the string line must be sampled in the process of digital coding.
2 According to the sampling rate and sampling size, compared with the natural signal, the audio coding can only be infinitely close, at least the current technology can only be so, compared with the natural signal, any digital audio coding scheme is lossy, because it can not be completely restored. In computer applications, the highest level of fidelity is PCM coding, which is widely used for material preservation and music appreciation, CD, DVD and our common WAV files are applied.
3 According to the different encoding methods, audio coding technology is divided into three types: waveform coding, parameter coding and hybrid coding. Generally speaking, waveform coding has high speech quality but high coding rate. The encoding rate of parameter coding is very low, and the sound quality of synthesized speech is not high. Hybrid coding uses parametric coding techniques and waveform coding techniques, with coding rate and sound quality in between.