This article analyzes the types, technical solutions, and performance characteristics of UHF band wireless microphone products, which can be used as a reference for users to choose such products.
1. Wireless microphone, also known as wireless microphone, is a kind of audio equipment that transmits sound signals. It consists of a transmitter and a receiver. It is called a wireless microphone system in professional audio.
The transmitter is powered by a battery, and the microphone converts the sound into an audio electrical signal. After being processed by the internal circuit, the radio wave containing the audio information is emitted into the surrounding space.
The receiver is generally powered by the mains, the receiving antenna receives the wireless on-demand of the transmitter, and then through the internal circuit processing, extracts the audio signal, and sends it to the sound reinforcement system through the output signal line to complete the audio signal Wireless transmission. . A receiver can usually contain 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. Frequency band 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, several frequency bands such as FM, VHF and UHF are commonly used.
The FM frequency band refers to the 88~108MHz frequency band used in public FM broadcasting and its nearby frequency bands. Generally, only some simple wireless microphone products use this frequency band.
UHF frequency band refers to the 300~3000MHz frequency band. Wireless microphones generally use the 400~830MHz frequency band, and the frequency band exceeding 830MHz is rarely used. Because the 830~960MHz frequency band has interference from GSM and CDMA mobile phones, the diffraction ability of the frequency band above 960MHz gradually deteriorates. Therefore, the most popular UHF frequency band in the world is the 800MHz frequency band (740~830MHz).
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 because it is close to the public frequency modulation broadcasting (FM) frequency band, it is called the FM frequency band. VHF frequency band wireless microphones mostly use the 170~260MHz frequency band, which is also often called VHF HIGHBAND (VHFHIGHBAND).
2. Several terminology To understand the performance characteristics of wireless microphones, it is necessary to understand the basic terminology of wireless microphones and the specific meaning of the main performance indicators. In addition to wireless microphones having the same audio indicators as wired microphones, there are also some unique terms and performance indicators, which will be 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 in order 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 spot: 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 act 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. This method can greatly eliminate the receiving dead zone and avoid muting or producing 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 the strong signal at any time, but the circuit is complicated and the cost is high. This kind of diversity is often referred to as dual tuning, true diversity, and so on.
Multi-channel: The carrier frequency of a general wireless microphone is fixed, and the user cannot change it during use. Because 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, which will reduce the receiving distance of the receiver, output noise, or even fail to receive it. The signal of the microphone.
In response to this situation, the manufacturer has developed a multi-channel wireless microphone system. The working frequency 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 they do not interfere with each other and work in coordination. Most of the 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; "Adjust the channel" and other words; the third is the 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 a 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 a wireless microphone, 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 muting 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 -101dBm), the receiver output signal can reach 12dB signal noise ratio. and
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 size of the signal energy transmitted by the wireless microphone transmitter to the space, usually expressed in milliwatts (mW), generally between 5 and 50 mW.
Effective working distance: refers to the maximum distance that the wireless microphone can normally transmit signals. Most of the parameters marked on the product indicate that it is in an open area or under ideal conditions. Because the actual transmission distance of the wireless microphone is affected by the actual environment, it cannot be accurately marked. Only indicators in open areas or under ideal conditions can provide reference and can be compared with each other.
To measure the transmission capacity of a wireless microphone, it depends on two indicators, the transmit power and the receiving sensitivity at critical squelch. After these two indicators are converted into the same unit, the greater the difference, the greater the effective working distance under the same environment. far. Then, in combination with whether the receiver is diversity and which type of diversity, it is possible to clearly estimate and compare the limited transmission distance of different microphones. Generally, the effective distance of the diversity receiver is greater than that of the non-diversity receiver. , The receiver of mid-amp diversity is larger than the receiver of antenna diversity.