The transmission from the transmission mode can be basically divided into three categories: constant pressure broadcast, FM radio, and network broadcast.
1. Constant pressure broadcast
The constant voltage transmission broadcast is to directly amplify the audio signal and transmit it based on the power signal. In order to reduce the transmission loss of the line, the 4~160 matching impedance is converted to 100V constant voltage mode by the step-up transformer, and transmitted to the terminal and then stepped down to the speaker of 4~16 ports. The general transmission distance is tens of meters. A few hundred meters.
advantage:
The technology is mature, the structure is simple, the performance is stable, the maintenance is easy, and the terminal is cheap. It is currently widely used in stations, terminals, schools, commercial and residential buildings.
Disadvantages:
1) The constant voltage transmission is affected by the transformer's bandwidth, speaker size, cable diameter and other factors. The frequency response range is from 200Hz to 12kHz, and the distortion is ≤10%. Stereo transmission cannot be realized.
2) The program capacity is small, and it is not possible to address control. Only one program can be transmitted on one line.
3) The sound source is basically an analog sound source, which cannot play digital format audio files, and cannot realize automatic playback and automatic control. Constant voltage broadcasts are designed according to the principle of power matching and impedance matching. The capacity of the system expansion is very limited.
2, FM radio
The FM transmission broadcast adopts the method of frequency modulation modulation to transmit the audio signal to the high frequency carrier, and describes the change of the audio signal by the frequency variation of the high frequency carrier. Different carrier frequencies can carry different audio programs at the same time. China divides 87-108MHz into FM radio frequency bands. At this stage, China's urban broadcasting and closed-circuit broadcasting use FM radio broadcasting. FM radio can be transmitted with cable TV.
advantage
1) The audio range of FM radio is 30Hz~7kHz, and the distortion is ≤0.7%. FM radio has the advantages of wide frequency response, rich treble, strong anti-interference ability and small distortion, and can transmit stereo.
2) The technology is mature, the program capacity is large, and the price of supporting equipment is very low. The bandwidth of FM broadcasting is 16 kHz. In this frequency band, more than 60 sets of FM radio programs can be transmitted at the same time, which can meet the requirements of simultaneous broadcasting of multiple partitions.
3) Compatibility and scalability.
Disadvantages FM radio transmission is based on weak signal transmission. Each receiving device must be an active device, that is, each speaker and terminal must be connected to a 220V power supply.
3. Webcast
Digital audio network broadcasts must be transmitted and played on the Internet to enable broadcast functions and requirements.
The network audio broadcast is sent by an IP network broadcast control host, a set of broadcast software or server software, and the audio files are sent to the remote network terminal by IP flow. Each terminal should have a fixed IP address and network module. A professional digital audio decoding device (software or hardware), power amplifier control unit. At present, manufacturers have begun to develop network audio broadcasting, and have launched products.
advantage
1) The network broadcast is digital and networked from the production to the transmission. The system has a high signal-to-noise ratio, and can obtain better sound quality and stereo transmission.
2) It is easier to implement intelligent broadcasting. Ethernet itself is a two-way network. The timing, addressing, grouping and other functions of intelligent broadcasting can be easily realized by software on the Ethernet.
3) Multi-channel broadcasting can be achieved when network speed allows.
4) Implement interactive mode broadcast.
5) Convenient management.
Disadvantages:
1) For the decompression of digital audio files, it must be PC+ special software, or a dedicated decompression chip, and each terminal must have its own IP address, and the technical content is relatively high.
2) It is expensive. At present, there is no professional chip-level developer in China, and it can only be decoded by embedded human-machine microcomputer dedicated decompression chip, which leads to higher product price.
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