What Is Signal Programming?

Signal modulation is a process or processing method that changes certain characteristics of one waveform to another waveform or signal. In radio communications, electromagnetic waves are used as carriers of information. Information is generally a baseband signal (ie, a modulation signal) to be transmitted, which is characterized by a low frequency, a wide frequency band, and overlap with each other. In order to be suitable for a single channel transmission, modulation must be performed. The so-called modulation is the process of loading the baseband signal (modulation signal) to be transmitted onto the high-frequency oscillating signal. The essence is to move the baseband signal to the high-frequency carrier, which is the process of frequency spectrum transfer. The purpose is to transfer the Analog or digital signals are converted into high-frequency signals suitable for channel transmission. ^[1]

The frequency range of sound that human ears can hear is between 20Hz and 20000Hz. This frequency range is usually called audio. Sound waves travel very slowly in the air, about 340m / s, and decay very quickly, and do not travel very far. We know that alternating electromagnetic fields can use antennas to radiate to the sky. However, to achieve effective radiation, the size of the antenna should be 106 to 105 m compared to the wavelength of the electromagnetic wave. It is obviously impossible to make an antenna of equivalent size. Therefore, audio signals cannot be radiated directly into the air. In order to reduce the loss during transmission, people usually perform special processing on the transmission signal before transmitting it. The process of attaching the original signal to be transmitted to high-frequency oscillation is called modulation. The modulation technology is mainly used to convert analog or digital signals into special analog signals.

Signal modulation is to use one signal (modulation signal) to control another signal (carrier signal) as a carrier, and let a certain parameter (amplitude, frequency, phase, pulse width, etc.) of the latter change according to the former value. Signal modulation often uses a high-frequency sinusoidal signal as the carrier signal. ^[2]

There are many types of modulation, and the classification methods are also inconsistent.

According to the form of the modulation signal, it can be divided into analog modulation and digital modulation. Modulation with analog signals is called analog modulation; modulation with data or digital signals is called digital modulation.

According to the type of the modulated signal, it can be divided into pulse modulation, sine wave modulation, and intensity modulation (such as non-coherent light modulation). The modulated carriers are pulse, sine wave, and light wave. Sine wave modulation has three basic modes: amplitude modulation (amplitude modulation ASK), frequency modulation (frequency modulation FSK) and phase modulation (phase modulation PSK). The latter two are collectively called angle modulation. In addition, there are some variant modulations, such as single-sideband AM and residual sideband AM.

Pulse modulation can also be classified in a similar way. In addition, there are composite modulation and multiple modulation.

Different modulation methods have different characteristics and performance.

The three most basic modulation methods of digital signals (amplitude modulation, frequency modulation, and phase modulation) are abbreviated as ASK, FSK, and PSK in English. Various other modulation methods are improvements or combinations of the above methods. For example: QAM is the amplitude modulation and Combination of phase modulation; MSK is an improvement of FSK; GMSK is an improvement of MSK. A Gaussian low-pass pre-modulation filter is inserted before the MSK (Minimum Frequency Shift Keying) modulator, which can improve spectrum utilization and communication. Quality; OFDM can be regarded as a modulation method for multi-carrier.

There are three purposes for modulation:

1) Facilitate wireless transmission and reduce antenna size;

2) Frequency division multiplexing to improve communication capacity;

3) Improve signal anti-interference ability. In order to make full use of the channel capacity and meet the different needs of users, communication signals use different modulation methods. With the rapid development of electronic technology and the continuous improvement of users' requirements for information transmission, the modulation of communication signals has undergone a development process from analog to digital and from simple to complex.