Source Coding
Source coding process is complex source such as a digital signal information text, sound, image, video, etc. expressed. Role of source coding is to convert an analog signal into a digital signal, the economy goal, but the full effective expression of digital source information, different information contents of the different digital coding algorithms.
Waveform coding:
In digital waveform coding is reconstructed sequence encodes a manner as possible to the source waveform. Sampling the analog source according to a certain rate on the time axis, then segment the quantized amplitude sample, and is represented by a sequence of numbers. Decoding which is the reverse of the sequence of received digital signal into an analog recovery.
Encoded speech waveform is the waveform at the source end of the channel to the speech signal is sampled approximation principle, quantizing, coding, decoding end signal to recover the digital speech waveform sequence encoding these.
Waveform coding with good voice quality, anti-interference ability and other characteristics, for applications that require high quality voice environment. But the disadvantage is the large amount of information waveform coding, requires high coding rate, generally between 16 ~ 64kbit / s, more bandwidth resources occupied during transmission. Application: PCM encoding is used in the switch fixed waveform coding.
Parameter encoding:
Extracting coding parameters are analyzed and the source of the necessary information model, but not all of the key features of the parameter, the parameter information through the sampling, quantizing, coding, and then sent out synthesis; value at the receiving end the received parameters coding, restores the information source.
For example, a mobile system is the speech coding parameters to determine the critical parameters that can reproduce a voice from the auditory perspective; re-use these synthesized speech feature parameter information at the receiving end. Parameter encoding a compression ratio, a low coding rate, less transmission bandwidth advantage, generally in 2.4kbit / s or less; disadvantage is computationally intensive, poor voice quality.
Hybrid coding:
Hybrid coding is a combination of the advantages of parametric encoding and waveform coding, to retain the advantages of low bandwidth requirements encoding parameters general, higher quality synthesized speech is obtained in significant part by waveform encoding information, enhancing naturalness of speech. Hybrid coding of speech coding rate is 2.4 ~ 16kbit / s.
Channel coding:
Channel coding is based on both the sending and receiving of information to know the rules add redundant bits, such that the encoded information bits before and after leaving a sequence of numbers relevant regularity. Thus, even if part of the data lost during transmission, the receiving side, a channel decoder using the correlation between the digital sequence regularity will find that there is an error, and correct the error, the missing information back.
modulation
Source information containing a low frequency AC component and the DC component of the baseband signal, can not be directly transferred in a wireless environment. Modulated low-frequency signal is processed into suitable for wireless transmission of radio signals, by varying the amplitude of the RF carrier signal, the phase or the frequency, the low frequency signal to reflect the changes in the process. Demodulator is the inverse process, the useful information is extracted from the radio frequency signal to be received and processed out of the process.
Digital signal transmission over the wireless channel, it must first be converted to a baseband signal on a higher frequency band. Amplitude, frequency and phase of the high-frequency oscillation signal can carry digital information signal.