Audio Coding Part 3
flow characteristics
With the development of the Internet, people have put forward requirements to listen to music online, so it is also required that the audio file can be played while reading, without the need to read the entire file and then play it, so that listening is can achieve without downloading. It can also be done while encoding and playing. It is this feature that you can live broadcast online and set up your own digital radio station has become a reality.
Transmission classification code
According to different coding methods, audio coding techniques are divided into three types: waveform coding, parametric coding, and hybrid coding. Generally speaking, waveform coding has high voice quality, but the coding rate is also high; parametric coding has a very low coding rate and the quality of the resulting synthesized speech is not high; hybrid coding uses parametric coding technology and waveform coding technology, coding rate and sound quality among them.
1. Waveform coding
Waveform coding refers to directly transforming the time-domain signal into a digital code without using any parameters of the generated audio signal, so that the reconstructed speech waveform is as consistent as possible with the waveform. waveform of the original speech signal. The basic principle of waveform coding is to sample the analog speech signal at a certain rate on the time axis and then quantize the amplitude samples hierarchically and represent them with codes.
The waveform coding method is simple, easy to implement, strong in adaptability, and good in voice quality. However, because the compression method is simple, it also has some problems: the compression ratio is relatively low, resulting in a higher encoding rate. Generally speaking, the complexity of waveform coding is relatively low and the coding rate is relatively high. Generally, the audio quality is quite high when the encoding rate is higher than 16 kbit/s. When the coding rate is less than 16 kbit/s, the sound quality is drastically reduced.
The simplest waveform coding method is PCM (Pulse Code Modulation), which just samples and quantizes the speech signal. The advantages are that the coding method is simple, the delay time is short, the sound quality is high, and the reconstructed speech signal is almost indistinguishable from the original speech signal. The disadvantage is that the coding rate is relatively high (64 kbit/s) and it is more sensitive to errors in the transmission channel.
2. Parameter coding
Parametric coding consists of extracting the parameters of the generated speech from the speech waveform signal and using these parameters to reconstruct the speech through the speech generation model, so that the reconstructed speech signal can maintain the semantics of the original speech signal as much as possible. . That is, the parameter encoding is based on the digital model generated by the voice signal, and then the model parameters are obtained from the digital model, and then the digital model is restored according to these parameters, and then the talks.
The coding rate of parametric coding is low, which can reach 2.4 kbit/s. The generated speech signal is restored using the established digital model. Therefore, the waveform of the reconstructed speech signal may be quite different from the waveform of the original speech signal. The distortion will be larger AND due to the limitations of the speech generation model, increasing the data rate does not improve the quality of the synthesized speech. However, although the sound quality of the parameter encoding is relatively low, the confidentiality is very good, and it has been used in the military. A typical parameter coding method is LPC (Linear Predictive Coding).
3. Mixed coding
Hybrid encryption refers to the simultaneous use of two or more encryption methods for encryption. This coding method overcomes the weakness of waveform coding and parametric coding, and combines the high quality of waveform coding and the low coding rate of parametric coding, and can achieve better results.
