Introduction to the types and properties of wireless microphones

1. wireless microphone
    A wireless microphone, or wireless microphone, is an audio equipment that transmits sound signals. It consists of a transmitter and a receiver, and is usually called a wireless microphone system.
    The transmitter is powered by a battery, and the microphone converts the sound into an audio electric signal. After processing by the internal circuit, it transmits radio waves containing audio information to the surrounding space.
    The receiver is generally powered by the mains, and the receiving antenna receives the radio wave from the transmitter, processes the internal circuit, extracts the audio signal, and sends it to the sound reinforcement system through the output signal line to complete the wireless transmission of the audio signal. A receiver usually contains 1, 2 or 4 sets of receiving circuits, which receive signals from 1, 2 or 4 wireless microphones respectively, which are called "one for one", "one for two" or "One for four" models. Among them, one with two models is the most common.
    The wireless microphone is essentially a one-way wireless communication system.

1.1. UHF band
    The wireless microphone system is a device that transmits sound signals through radio waves. According to the transceiver frequency between the wireless microphone and the receiver, it can be divided into different frequency bands. Generally, FM, VHF and UHF frequency bands are commonly used.
    The FM frequency band refers to the 88-108MHz frequency band and its nearby frequency bands used by public FM broadcasting. Generally, only some simple wireless microphone products use this frequency band.
    The VHF frequency band, divided according to international standards, refers to the 30-300MHz frequency band. The FM frequency band mentioned above is actually included in the VHF frequency band, but it is called the FM frequency band because it is close to the public frequency modulation broadcasting (FM for short) frequency band. VHF frequency band wireless microphones mostly use the 170-260MHz frequency band, which is also often referred to as VHF HIGH BAND.

    UHF frequency band refers to the 300-3000 MHz frequency band. Wireless microphones generally use the 400-870MHz frequency band, and the frequency bands above 870MHz are rarely used. Because the 870-960MHz frequency band has interference from GSM and CDMA mobile phones, the diffraction ability of the frequency band above 960MHz gradually deteriorates , So the most popular UHF frequency band in the world is 800MHz frequency band (740-870MHz).

2. A few terms
    To understand the performance characteristics of wireless microphones, it is necessary to first understand the basic terms of wireless microphones and the specific meaning of the main performance indicators. In addition to the same audio indicators as wired microphones, wireless microphones also have some unique terms and performance indicators, which are introduced one by one below.
    Squelch: When the wireless microphone receiver does not receive a signal or the signal is weak, it will automatically cut off the output signal to avoid output noise. This function is called squelch. If there is no squelch function, or the squelch function is not good, noise will be emitted from the speaker. Noise will affect the sound quality, destroy the atmosphere of the scene, and even damage the sound reinforcement equipment.
    Dead point: also known as dead zone. During the movement of the wireless microphone, the signal received by the receiver will vary in strength due to the difference in distance, relative position, or obstacles. In some locations within the normal use distance, a too weak signal will cause the squelch circuit in the receiver to operate and cut off the output signal; and after leaving this location, it can be received and output normally. This position is called the dead center or dead zone.
    Diversity reception: It means that the wireless microphone receiver can receive the signal of the same wireless microphone from two antennas, and the stronger signal is selected through the internal circuit. In this way, the receiving dead zone can be largely eliminated to avoid muting or generating dead spot noise. There are two ways of diversity reception: antenna diversity and mid-amp diversity.
    In the antenna diversity mode, there are two receiving antennas, a control circuit and a receiving circuit. When the received signal is weak during operation, the control circuit will automatically switch to another antenna.
    In the mid-amp diversity mode, in addition to two antennas and a control circuit, there are two complete receiver circuits that work at the same time, and the control circuit tracks and switches to output a better audio signal. This method is better than the previous method because it tracks strong signals at any time, but the circuit is complicated and the cost is high. This kind of diversity is often called double tuning, true diversity, etc.
    Multi-channel: The carrier frequency of a general wireless microphone is fixed, and the user cannot change it during use. Since wireless microphones transmit sound signals through radio waves, when there are external signals that are the same as or close to their carrier frequency in the working environment, interference will occur, reducing the receiving distance of the receiver, outputting noise, or even failing to receive The signal of the microphone.
    In response to this situation, the manufacturer has developed a multi-channel wireless microphone system. The working frequencies of its transmitter (wireless microphone) and receiver are adjustable, so that users can change the carrier frequency of the system when they encounter external frequency interference to avoid interference signals and work normally; in addition, if they are in the same When multiple wireless microphones are used in the venue, each microphone can be easily adjusted to a different working frequency so that it does not interfere with each other and work in coordination. Most wireless microphones used in large professional stage performances are multi-channel systems, with 8 channels, 16 channels, or even more channels, of which 16 channels are the most common. Multi-channel systems generally use phase-locked loop (PLL) frequency synthesis technology, microcomputer control technology, and other related technologies. Its production technical requirements, equipment requirements, production costs and product performance are much higher than other ordinary models.
    At present, some products on the market are fixed-frequency, but a batch of products of the same model can be produced into products with different frequencies. Users can choose when buying, but cannot adjust their working frequency. Some manufacturers also mark it as "multi-channel" , "32 channels can be chosen arbitrarily", this is inaccurate, or deliberately misleading consumers. This situation requires special attention. There are several ways to distinguish: one is to observe whether there is a switch or button to adjust the channel on the receiver panel; the other is to see whether its promotional materials or manuals are marked with "adjustable frequency" and "user can "Adjust the channel" and other words; the third is actual operation to see if it is adjustable.
    Signal-to-noise ratio: refers to the ratio of the original audio signal to the noise signal in the output signal when the receiver receives a signal of a specified strength (usually 60dBμV), expressed in decibels (dB). The larger the value, the purer the signal and the better the machine performance.
    Receiving sensitivity: In the radio or walkie-talkie, the receiving sensitivity refers to the size of the minimum RF signal that needs to be input when the receiver outputs a signal with a specified signal-to-noise ratio. The smaller the value, the higher the receiving sensitivity of the receiver. In wireless microphones, it should be expressed by the value of the input RF signal when the receiver is critically muted, because when the input signal is lower than the squelch point and the receiver is in the muted state, no signal is output.
    For example, the receiving sensitivity of a product is marked as "-90dBm", which means that when the antenna input signal is lower than -90dBm (ie 7μV), the receiver will enter the squelch state. Such marking can accurately reflect the receiver's receiving ability.
    Some products have sensitivity indicators similar to radios and walkie-talkies, such as "2μV/12dB", which means that when the antenna input signal is 2μV (ie -101 dBm), the receiver output signal can reach 12dB. Noise ratio. The signal-to-noise ratio index of the wireless microphone is much higher than 12dB, so this marking method cannot correctly express the receiver's receiving capability.
    RF output power: refers to the amount of signal energy that the wireless microphone transmitter transmits into space, usually expressed in milliwatts (mW), generally between 5-50 mW.
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