1. wireless microphone
A 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 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 radio waves 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 broadcast (FM for short) frequency band. VHF 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. The frequency band over 870MHz is 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 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 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 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: refers to the wireless microphone receiver that can receive the signal of the same wireless microphone from two antennas, and select the stronger signal 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 wireless microphones used in large-scale 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 label 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 the promotional materials or manuals are marked with "adjustable frequency" and "user can "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 -101 dBm), the receiver output signal can reach 12dB. Noise ratio. However, 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 to the space, usually expressed in milliwatts (mW), generally between 5-50 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.
In fact, to measure the transmission capacity of a wireless microphone, it depends on 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 longer the effective working distance under the same environment. Combined with whether the receiver is diversity and which diversity method is used, the effective transmission distance of different products can be clearly estimated and compared. Generally speaking, the effective distance of the diversity receiver is larger than that of the non-diversity receiver, and the receiver with medium amplifier diversity is larger than the receiver with antenna diversity.
3. Market product classification
UHF band wireless microphones are currently popular products in the market. These products produced in China have been developed in recent years. Currently, there are not many manufacturers with their own development and production capabilities, but there are many brands on the market, most of which are It is listed as OEM.
In UHF band wireless microphone products, it can be divided into many different types. The following briefly describes the scheme composition and performance characteristics of various U-segment products.
3.1. Double frequency products
The frequency multiplier U-segment microphone is based on the VHF frequency band product, using a frequency multiplier circuit to increase the transmitting and receiving frequencies to 2 to 4 times the original frequency, then the working frequency enters the UHF frequency band, and the product is upgraded accordingly. This type of product is easier to produce, the cost does not increase much, and the sound quality is improved; but after increasing the multiplier, the total efficiency of the transmitting circuit is reduced, which easily leads to excessive power consumption or reduced operating distance, and the spurs of this type of product There is a lot of emission, and the anti-interference ability is not strong. Its operating frequency is unchangeable by the user. This type of product currently mainly has 550MHz frequency band and 750MHz frequency band on the market. This type of product is an entry-level product of U-segment products.
3.2. Frequency doubling diversity products
Frequency doubling diversity products are based on the above-mentioned frequency doubling products and adding antenna diversity circuits to form frequency doubling diversity products. Technically, it can also be designed as an intermediate amplifier diversity product, but in view of the low overall grade of this type of product, it is generally not designed in this way. This antenna diversity method uses two receiving antennas, which are switched to the internal receiving circuit through a high-frequency electronic switch. When the signal is strong enough, one of the antennas is fixedly used. If the internal circuit detects that the signal is weak, it will enter In the squelch state, the switch is automatically switched to another antenna. If the signal of the antenna is sufficient at this time, it can continue to receive; if it is not enough, it will enter the squelch state. This method can reduce the dead point during close operation and expand the effective operating distance. There are often publicity materials that call this antenna diversity method "microcomputer control diversity", "predictive diversity" and so on. Because this type of product adopts antenna diversity, the operating distance is better than ordinary products without diversity function, but it is not as good as the products with medium amplifier diversity. In addition, because the operating frequency is not adjustable, its ease of use and environmental adaptability are still Bad.
3.3. Adjustable multi-channel products
This type of product is developed and produced using phase-locked loop (PLL) technology, microcomputer (MCU) technology, etc., with multiple user adjustable channels, typically 16, 32, etc. When users use this type of product, if they encounter external interference, they can avoid the interference by changing the working channel of the system; in addition, when multiple systems are used on the same occasion, they can use channels that do not interfere with each other to achieve simultaneous use However, the first two types of fixed-frequency products do not have these advantages. In this type of product, the number of adjustable channels is also an important indicator, because sometimes more wireless microphones are required to be used in the same area at the same time, such as a karaoke room, where dozens or even hundreds of rooms may be used simultaneously. Although some products have as many as hundreds or even thousands of adjustable channels, but the channel spacing is too small, usually 25KHz, the number of practical channels will be greatly reduced. Since wireless microphones need to work at a frequency interval of 250KHz-300KHz in order to not interfere with each other, if the channel interval is 25KHz, it needs to be separated by about 10 channels to avoid mutual interference.
Because this type of product does not have the diversity function, the operating distance is not too far, the operating distance in the open area is about 100 meters, and the operating distance in the actual environment is about 20-40 meters, which can meet the needs of most occasions. Due to the relatively complete functions and relatively low price, this type of product is the mainstream product in the current market.
3.4. Adjustable multi-channel antenna diversity product
This type of product has both adjustable multi-channel function and antenna diversity reception function, and naturally has the advantages of these two functions at the same time, and is also more popular in the market.
3.5. Adjustable multi-channel dual tuning diversity product
This type of product has both adjustable multi-channel function and mid-amp diversity receiving function, and its dual-tuning diversity mode has excellent performance. The so-called dual-tuning diversity method refers to a microphone corresponding to two sets of receiving circuits in the receiver, through two antennas, the signal of the microphone is received at the same time, and the internal control circuit detects and compares, and switches between them. A stronger signal, that is, tracking strong signals at any time. This diversity method has better performance than the antenna diversity method in terms of eliminating dead spots and extending the operating distance, but the cost is higher because it corresponds to the same wireless microphone , There are 2 sets of the same receiving circuit. Commonly referred to as "single-transmit and double-receive" and "double-transmit and four-receive" refer to this kind of diversified products.
Most of these products are used in high-demand professional stage performance occasions. Because in large-scale stage performances, the phenomenon of dead spots must not be tolerated. During the rehearsal, tests must be carried out to find and mark the performance area where the dead spots occur, and the actors are required not to enter the area. In this case, the use of dual-tuned true diversity (also called true diversity) products can basically eliminate dead spots and ensure the performance effect.
4. Concluding remarks
The wireless microphone products are rich and varied, with various types and grades, and the cost and price are also very different. The several types of UHF frequency band products introduced above are arranged from low to high according to cost, performance and price. In addition, products of the same category use different production processes, and their costs and performance are also different. Products produced with surface mount technology (SMT) have higher performance indicators, reliability and cost than products produced with ordinary plug-in technology.
With the increasing development of technology, UHF band wireless microphones have begun to use a series of new technologies such as voice code locking, infrared frequency binding, and automatic signal scanning. It can be expected that wireless microphones will develop in the direction of higher performance, better ease of use, and stronger environmental adaptability.