operating principle
pcm
Pulse code modulation is to transform a time-continuous analog signal into a time-discrete digital signal and transmit it in the channel. Pulse code modulation is the process of sampling the analog signal first, quantizing the amplitude of the sample and coding it.
Sampling, is to periodically scan the analog signal, the time continuous signal into time discrete signal. The analog signal should also contain all the information in the original signal after sampling, that is, the original analog signal can be restored without distortion. The lower limit of its sampling rate is determined by the sampling theorem. The sampling rate is 8Kbit\/s.
Quantization is to discretize the amplitude of the instantaneous value obtained through sampling, that is, to use a set of specified levels to express the instantaneous sampling value with the closest level value.
After sampling quantization of an analog signal, the quantized pulse amplitude modulation signal is obtained, which has only a finite number of values.
Encoding is to use a set of binary codes to represent each quantized value with a fixed level. However, in fact, the quantization is done simultaneously in the coding process, so the coding process is also called the analog\/digital transformation, which can be denoted as A\/D.
pcm
The voice signal is first sampled by anti-aliasing low-pass filter, pulse sampling, into a sampling signal of 8KHz repetition frequency (that is, a discrete pulse amplitude modulation PAM signal), and then the PAM signal with continuous amplitude is quantized by \"rounding\" method into a finite amplitude value signal, and then encoded into binary code. For telephones, CCITT stipulates that the sampling rate is 8KHz, and each sampling value is coded with 8 bits, that is, a total of 28=256 quantized values, so the standard digital rate after PCM encoding per voice channel is 64kb\/s. In order to solve the problem of large quantization error and poor sound quality of small signal in uniform quantization, the nonlinear quantization method with uneven quantization interval is adopted in practice, that is, the quantization characteristics are densely layered and the quantization interval is small in small signal, while the quantization characteristics are sparse and the quantization interval is large in large signal.