FM radio is a device that transmits radio by means of wireless transmission. It has the characteristics of no need for pole wiring, wide coverage, unlimited expansion, convenient installation and maintenance, low investment, and beautiful and clear sound quality. It completely solves the problems of traditional cable broadcasting wiring difficulties, complicated installation, poor expansion, damage to the wall and the campus environment. For large-scale and wide-area schools, FM broadcasting has incomparable advantages over traditional cable broadcasting.
Introduction
FM radio is a medium for sound transmission. Through FM radio, many contents can be updated in time. At the same time, the sound quality of FM radio is also very beautiful, and the reception is also very convenient, so FM radio has received people’s love, especially For many middle-aged and elderly people, listening to the radio has become a way of life for them. However, in traditional wireless broadcasting, there are serious problems of poor coverage and waste of broadcasting frequency resources. In order to make the coverage of wireless broadcasting wider and reduce the waste of broadcasting frequency resources, wireless broadcasting introduces FM synchronization technology, through the introduction of FM synchronization technology, makes the broadcast coverage wider, effectively reducing the waste of resources.
Transmission method
FM broadcasting transmits audio signals by means of frequency modulation. The carrier of FM waves changes on both sides of the carrier center frequency (the center frequency before modulation) with the change of the audio modulation signal. The number of frequency deviation changes per second and the audio signal The modulation frequency is the same, if the frequency of the audio signal is 1KHZ, the frequency deviation of the carrier wave is also changed 1K times per second. The size of the frequency offset is determined by the amplitude of the audio signal.
FM broadcasting is a broadcasting technology in which the high frequency oscillation frequency changes with the amplitude of the audio signal. Strong anti-interference ability, small distortion, high equipment utilization, but occupying frequency bandwidth, so it often works in very high frequency band.
In the FM transmitter, the maximum frequency deviation is allowed to be limited to 75KHZ. my country’s FM frequency regulation range is 87–108MHZ. The school generally stipulates that it is between 70–87MHZ, and the general FM radio cannot receive the signal.
my country’s FM broadcasting has developed rapidly and has replaced the original cable broadcasting. Although TV technology has developed rapidly, it cannot replace broadcasting after all. Because of the flexibility of broadcasting, the listening equipment is small, the investment is small, and the effect is fast. It is irreplaceable for TV equipment. of.
Transmission characteristics
Frequency Modulation is called frequency modulation
Advantages: Because the anti-interference performance of the FM system is stronger than that of the amplitude modulation system, and the generation and reception methods of the FM signal are not complicated, the FM system is widely used. The transmission bandwidth of FM signal is much wider than that of amplitude modulation (AM), so the anti-noise performance of FM system is better than that of AM system.
Disadvantage: The frequency bandwidth of the FM system is much wider than that of the amplitude modulation, so the effectiveness of the system is poor. The transmission distance is short.
Operation method
Automatic expansion machine
FM radio automatic receiver is a new product that appeared in 2015. It can automatically turn on the power amplifier after receiving the RF signal. Because it integrates receiving, power amplifier and automatic power-on, it is convenient and flexible to use, which brings great convenience to users. . However, some users have raised some problems for some similar products, such as the problem of anti-interference, the problem of frequency drift, and the problem of low sensitivity. Focusing on the above problems, Xingbo Electronics Research Institute has taken the lead in doing a lot of work, such as frequency synthesis technology, infrared remote control technology, selective call, group call open, shutdown technology, and pilot frequency technology. A number of scientific research achievements have been obtained in these areas. Make the FM broadcast automatic receiver and expansion machine technology more mature.
How FM radio is automatically turned on and off
There are three ways to automatically start the automatic expansion machine,
One is the automatic startup method of squelch
The advantage of this method is that the circuit is simple, but the disadvantage is that it is easy to clutter and cause the wrong startup. Some people call this wrong startup (chicken crowing in the middle of the night).
The second method is the pilot boot method
The national standard for the pilot frequency of the FM broadcast stereo encoder is 19KHz, and the decoding frequency of the FM broadcast automatic receiver and expansion machine must also be 19KHz before it can be turned on. Use the stereo coded pilot to turn on, so as to avoid the interference of the same-frequency non-pilot carrier signal. The disadvantage is that the transmitter with the same frequency and the same pilot frequency can also be turned on.
The third boot method is to code boot
It uses the DTMF encoding and decoding method to add an encoder to the audio signal input end of the transmitter and a decoder to the receiver to realize multi-digit code group call on and off, and optional call on and off. The disadvantage of this method is that the circuit is complicated and the cost is high.
Tuning method of FM broadcast automatic receiver
The FM broadcast automatic receiver and expansion machine is generally fixed frequency, and sometimes the frequency must be changed when it encounters interference. The requirement for tuning frequency is that stability is the overriding factor. It doesn’t matter to the indoor unit if the frequency is unstable, just adjust it. But it is troublesome for the outdoor unit. There are four tuning methods:
(1) The variable capacitance tuning method has a simple circuit and is easy to adjust, but the stability is poor. It must be adjusted every once in a while, otherwise the frequency will drift, and the signal will not be received or distorted.
(2) The varactor voltage tuning method is a big step forward compared with the variable capacitor tuning method, but the problem of frequency drift still cannot be fundamentally solved.
(3) Quartz crystal oscillation mode, which fundamentally solves the problem of frequency drift. It has been used for many years and has good stability. The disadvantage of this method is that the crystal is generally multiplied multiple times, and its harmonic components are many, the anti-interference is poor, the sensitivity is difficult to control, the frequency is fixed, and the crystal must be replaced when adjusting the frequency.
(4) The frequency synthesis method is an ideal method, which has good frequency stability and easy adjustment. Frequency and volume can be controlled remotely using a handheld infrared remote. 1
Technical Improvement
1. Long-wave broadcasting: It is rarely used in the long-wave band in our country, because it requires large power and the antenna is troublesome to erect. It is generally used for power line carrier. With the advent of high-power components, this long-forgotten band has been repurposed. Because it is ground wave transmission, there is no obstruction by obstacles. Therefore, it is of great use value in mountainous areas and rural areas. Over the years, many devices in this area have appeared and are widely used.
2. Computer broadcasting system: English listening in schools is generally played through tape recorders, and then broadcast through amplifiers or transmitters. Some users suggested that the recorder has many disadvantages:
(1) The jitter of the tape is easy to cause distortion.
(2) After the tape is used for many times, especially after repeated copying, the distortion is serious.
(3) The operation is troublesome, and it is not easy to connect to the Internet.
In response to the above problems, we have developed a set of software and used the computer to broadcast, and the effect is very good. The program source is generally in MP3 format, and tapes and VCD discs can be input into the computer and converted into MP3 for storage. In this way, the transmission and use are very convenient, and there is no distortion problem caused by sound transmission or long-term wear and tear. Timely automatic broadcast, timed broadcast, loop broadcast. You can also control the transmitter or amplifier to automatically turn on. It can be operated remotely via the Internet. General computers can be fully utilized.
3. Digital Broadcasting: Digital Broadcasting (DAB) for short, transmits audio signals and other additional information digitally, such as song lyrics, weather forecast, traffic information and other character information.
Features of digital broadcasting:
1. It adopts advanced digital compression and decompression method, which can transmit hundreds of radio stations at the same time, and select your favorite programs to store and play at any time.
2. In weak field strength areas, it exerts a strong error correction ability, so that it cannot hear any noise.
3. Other data services can be easily carried out. 2
Application field
In order to allow students to practice English listening, some campuses have installed small FM transmitters, and each student is equipped with wireless listening equipment. High-power FM radio speakers can also be installed on campus. For a period of time, Beijing used FM radio to conduct a unified listening test for the college entrance examination (cancelled from 2014).
Some factories and mines use the FM broadcast transmission method of one-point transmission and multi-point automatic reception in order to convey instructions in a timely manner.
Some towns and villages have installed FM radio transmitters in natural villages in order to smooth government orders and do a good job in publicity. 3
