To send a signal far away, FM means frequency modulation, and physical mud means to add a carrier to the signal and send it out.
It's much more efficient to go around the bay.
However, you may not want to know how far or how high it is. In fact, the old radio station will use AM to send signals, and the new radio station will use FM to send signals. What is the difference? It means that AM will receive a poor D.
If you tie a foot of the mountain, you may not get FM, but it should be much better. What about abroad ... this is not relevant.
Amplitude modulation (AM) refers to a modulation method that makes the amplitude change of higher frequency signal have a certain functional relationship with the modulation signal at the modulation end and carries out inverse modulation at the demodulation end. In fact, the so-called functional relationship is generally the relationship between correction and ratio. The biggest advantage of this modulation method is that modulation and demodulation are very simple, and only one diode and one capacitor are needed. Of course, the biggest disadvantage is that the distortion is relatively large and it is sensitive to interference. This is a relatively old technology. However, this ancient technology does not mean that it is not widely used. It is still used in many fields, such as radio and aviation radio. At the same time, there are some improved amplitude modulation techniques, such as single sideband modulation (SSB).
Single sideband (also known as single sideband), vestigial sideband modulation (vestigial sideband)
VSB), and variations of amplitude modulation, such as multi-amplitude digital modulation widely used in mobile communication at present. Schematic diagram The simple schematic diagram of amplitude modulation is as follows: Picture reference: upload.wikimedia/ * */ZH/6/6F/AM _ AM No.4 AM signal is obtained from No.65438+No.0+No.2) * No.3, and a general example considers a modulation signal m(t), which may include all situations. The modulated mathematical formula is written as follows: picture reference: upload.wikimedia/math/f/8/2/f82d1b00cf5e43534f0d037529cf3a8. Or use the plural form: picture reference: upload.wikimedia/math/d/4/b/d4b689E20c5cd61fb62a1406ffe71b 91for Fourier transform: picture reference: upload.wikimedia/math/d/. c/ 1/. DC 16 1e 49d 18e 74 f 367689 e 4 1bb 469 1fb
δ(x) is Dirac δ function-unit pulse on x-capital letter function represents Fourier transform. After Fourier transform, there are two components: one at the positive frequency (centered on +ωc) and the other at the negative frequency (with? Ω c). Although there is no negative frequency in real life, it is not wrong to get negative frequency mathematically, and negative frequency must be considered here-otherwise one side of the sideband will disappear. Frequency modulation (FM for short) is a modulation method to express information through the instantaneous frequency change of carrier. The corresponding amplitude modulation method is to express information through the change of carrier amplitude, while its frequency remains unchanged. ) In analog applications, the change of carrier frequency is directly proportional to the amplitude of the input signal. In the field of digital application, the frequency of the carrier wave jumps discretely according to the value of the data sequence, which is called frequency keying. FM technology is usually used in VHF radio to transmit high fidelity radio music and sound in VHF band. The audio signal of ordinary (analog) TV is also transmitted by FM. Narrowband FM broadcasting (N-FM) is limited to commercial voice communication and amateur radio, and the FM technology used in broadcasting is generally called broadband FM (W-FM). Most analog video recorders, including VHS, a home video system, also use FM technology to record brightness (black and white) information of video signals, but the medium frequency band is used. Frequency modulation is the only modulation technology that will not cause large signal aliasing when recording video tapes, because the frequency spectrum of video information is very wide, from a few hertz to tens of megahertz, and it is difficult to keep the noise information below -60 decibels when using an equalizer. FM mode also saturates the tape and plays a role in noise reduction. At the same time, the FM capture effect at the receiving end basically eliminates the phenomena such as through printing and pre-echo. If continuous pilot tones are added to the signal, such as V2000 and many high-band formats, the mechanical jitter can be effectively controlled, thus contributing to time-base correction. Fm technology is also used in audio synthesis, so-called FM synthesis, which is widely used in early digital synthesizers and has become the standard symbol of several generations of personal computer sound cards.
Reference: zh. *** /w/index？ Title =% E8% B0% 83% E9% A2% 91&; Variant =zh-