? The purpose of modulation of carrier wave of 1 number signal
? The purpose of the encode that believe a source is the efficiency that improves letter source, purify redundancy. The purpose that channel codes basically has at 2 o'clock:
? (1) asks the spectrum character of a row gets used to passageway spectrum character, loss of the energy in making transmit a process thereby is the smallest, increase signal energy and the proportion that sound energy of a confusion of voices measures, reduce the possibility that produces accident, improve transmission efficiency. ?
(2) increases error correction capacity, make even if occurrence accident, also can get corrective.
? The frequency character of general transmission passageway always is finite, have namely on, floor level frequency, exceed this bounds to cannot undertake effective transmission. If digital signal flows frequency character and the frequency character of transmission passageway very not identical, so a lot of energy in signal can be lost, believe comparing of a confusion of voices to be able to be reduced, make be piled up by accident increase, and still can bring very strong interference to adjacent channel. Accordingly, want to undertake some kind of processing to digital signal before transmit, reduce the low frequency heft in digital signal and high frequency weight, make energy is centered to intermediate frequency, perhaps undertake spectrum through process of some kind of modulation move move. These two kinds of processing can be regarded to be to make the spectrum character of the spectrum character of signal and channel matchs. ?
The carrier wave modulation of digital signal is channel encode partially. Cable television broadband is integrated the transmission that the net is the digital signal that is based on imitate environment to fall, image number signal is not base band transmission means is transmitted in radio frequency bandpass however. ?
In to make digital signal is taking communication, transmit, must undertake modulation to carrier wave with digital signal. There also is kind of 3 kinds of basic modulations when transmission number signal: Extent key accuses, frequency moves key accuses and phase shift key accuses, they part correspondence is mixed at the extent with sine wave, frequency phasic will transmit signal of digital base band. A few kinds of several tones of Chinese characters that this chapter gets main introduction applied extensively formulate a method. ? ?
In succession two codes yuan 4 kinds of combination (00, 01, 10, 11) corresponding Yu Zhengxian 4 of wave phasic:
? 
Among them I=1, 2, 3, 4; - 2 ≤ of T ／ ／ of T ≤ T 2; ?
Here
Can be 0, ± π ／ 2, π or ± π ／ 4, 3 π of ± ／ 4, this is 4 photographs PSK(namely QPSK) . Type also can write into on: ?