Amplitude modulation and frequency modulation are two modulation methods commonly used in communication systems and broadcasting and television systems. Amplitude modulation means that the amplitude of the carrier changes with the modulating signal. It is abbreviated as AM. In the broadcasting system, it is divided into long wave (LW), medium wave (MW) and short wave (SW) frequency bands. The carrier frequency ranges from more than 100 KHz to Tens of MHz, characterized by relatively low frequency and long transmission distance, especially in the short-wave band. The installation distance can reach thousands of kilometers due to the reflection of the atmospheric ionosphere. When satellite and optical fiber communications are underdeveloped, short-wave communications have played an important role. The role of. In the mid-wave band, the ionosphere emits weakly during the day, and can only receive signals within a few tens of kilometers, but the ionospheric reflection is strengthened at night, and many remote signals can also be received. When I was young, there was no other way of entertainment. Listening to the radio on the radio was the only thing I could enjoy, especially on the short-wave band. You can receive many overseas broadcasts at night. The carrier frequency band of AM broadcasting is relatively narrow, so the sound quality is average, especially when listening to short wave, because the signal is unstable, the sound quality is fluctuating, but this feature has become the feature of medium and short wave, many middle-aged and elderly people I like this feeling.
FM is the frequency modulation that allows the carrier frequency to change with the modulated signal. Abbreviated as FM, the frequency range is 87~108MHz in the broadcast segment. In fact, TV audio also uses FM mode, so some radios with a receiving range as low as 76MHz can also receive part TV sound. FM broadcasting has a high frequency and a small signal coverage, about tens of kilometers, and generally only some local radio stations. If you use an outdoor antenna, you can receive more, but there are exceptions. Under the influence of a specific time or event (such as a meteor shower track), it will also spread far. In the past, there were some FM long-distance receivers and received many foreign radio stations. Instance. FM sound quality is better than amplitude modulation, and stereo broadcasting can be achieved, but the threshold effect is significant, and the signal is weak to a certain extent and the signal-to-noise ratio drops sharply.
The above-mentioned frequency modulation and amplitude modulation are all analog signal modulation methods. At present, with the popularization of digitalization, digital broadcasting forms have appeared in some areas, such as DAB broadcasting in Beijing, and CMMB terrestrial digital TV broadcasting in the past few years. , The emergence of these forms allows broadcasting not only to transmit sound signals, but also to transmit video and other data signals. The sound quality is significantly improved, but the digital signal has a threshold effect, and it will be unavailable below the threshold. Unlike an analog signal, it can be heard even if it deteriorates, and it will not disappear suddenly. However, the digitalized terminal promotion market has never been recognized, so these new forms have a small market share and have never been popularized and developed. With the popularity of the Internet, WIFI, and smart phones, one-way broadcasting is becoming more and more niche, but due to the need for emergency broadcasting, it will not die out in a short time, but more and more media no longer use broadcasting Formal communication has become an indisputable fact.
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