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Encoding digital data into a digital signal
(1) Non-return-to-zero coding [NRz]
(3) Differential Manchester coding
(5) Reverse non-return-to-zero encoding [NRZI]
Modulation of digital data to analog signal
Encoding of analog data into a digital signal
Modulation of analog data to analog signal
Baseband and Wideband Signals
Channel : The transmission medium of the signal. It is generally used to represent the medium that transmits information in a certain direction, so a communication line often contains a sending channel and a receiving channel.
baseband signal
The digital signals 1 and 0 are directly represented by two different voltages, and then sent to the digital channel for transmission ( baseband transmission ). The signal from the source , such as the data signal output by the computer representing various text or image files, belongs to the baseband signal. A baseband signal is a signal that directly expresses the information to be transmitted . For example, the sound wave we speak is a baseband signal.
wideband signal
The frequency division multiplexed analog signal formed by modulating the baseband signal is sent to the analog channel for transmission ( broadband transmission ). After the baseband signal is modulated by the carrier , the frequency range of the signal is moved to a higher frequency band for transmission in the channel (that is, only a certain frequency range can pass through the channel).
When the transmission distance is short, the computer network adopts the baseband transmission method (the attenuation is small at close range, so the signal content is not easy to change).
When the transmission distance is long, the computer network adopts the broadband transmission method (the long-distance attenuation is large, even if the signal changes greatly) can finally filter out the baseband signal)
Coding and Modulation
Encoding digital data into a digital signal
(1) Non-return-to-zero coding [NRz]
High 1 low 0 coding is easy to implement, but there is no error detection function, and it is impossible to judge the start and end of a symbol, so that it is difficult for the sender and the receiver to maintain synchronization .
(2) Manchester encoding
Divide a symbol into two equal intervals, the first interval is low and the next interval is high to indicate symbol 1; symbol 0 is just the
opposite. The opposite can also be used. The characteristic of this coding is that level jump occurs in the middle of each symbol, and the jump in the middle of the bit
is used as both a clock signal (which can be used for synchronization) and a data signal, but the frequency bandwidth it occupies is the original baseband width. twice. Each symbol is modulated to two levels, so the data transmission rate is only 1/2 of the modulation rate .
(3) Differential Manchester coding
The same as 1 and 0
is often used for local area network transmission. The rule is: if the symbol is 1, the level of the first half of the symbol is the
same as the level of the second half of the previous symbol, and if it is 0, it is the opposite. The characteristic of this code is that in the middle of each symbol, there is a level jump, which can realize
self-synchronization, and the anti-interference is stronger than that of Manchester code.
(4) Return to zero code [RZ]
This encoding method in which the signal level must be restored to zero within a symbol
(5) Reverse non-return-to-zero encoding [NRZI]
If the signal level flips, it means 0, and if the signal level does not change, it means 1.
(6) 4B/5B encoding
Inserting extra bits into the bit stream to break a series of 0s or 1s is to use 5 bits to encode 4 bits of data and then transmit it to the receiver, so it is called 4B/5B. The encoding efficiency is 80%. Only 16 kinds of 4-bit codes corresponding to 16 different kinds are used, and the other 16 kinds are used as control codes (start and end of frames, status information of lines, etc.) or reserved.
Modulation of digital data to analog signal
The digital data modulation technology converts the digital signal into an analog signal at the sending end, and restores the analog signal to a digital signal at the receiving end, corresponding to the modulation and demodulation process of the modem respectively.
Encoding of analog data into a digital signal
The internal processing of the computer is binary data, and all processing is digital audio , so it is necessary to convert the analog audio into a discrete sequence represented by a limited number of numbers through sampling and quantization (that is, to realize audio digitization).
The most typical example is pulse code modulation ( PCM ) that encodes audio signals. In computer applications, PCM encoding can achieve the highest level of fidelity , which is widely used for material preservation and music appreciation. CD, DVD and our common are used in WAV files. It mainly includes three steps: sampling,
quantization, and encoding.
1. Sampling : Periodically scan the analog signal to change the time-continuous signal into a time-discrete signal.
In order to make the obtained discrete signal represent the sampled analog data without distortion, the sampling
theorem should be used for sampling: f sampling frequency ≥ 2f the highest frequency of
the signal The degree is converted into the corresponding digital value, and
the integer is taken, which converts the continuous level amplitude into a discrete digital quantity.
3. Coding : Convert the quantized result to the corresponding binary code.
Modulation of analog data to analog signal
Higher frequencies may be required for efficient transmission. This modulation method can also use frequency division multiplexing technology to make full use of bandwidth resources. The signal transmitted in the telephone and local exchange adopts the way of analog signal transmission of analog data; the analog sound data is loaded into the analog carrier signal and transmitted.
