b. VHF wireless microphone: It is divided into two types, low frequency and high frequency. The former uses the VHF50MHz frequency band. Due to the low frequency, the antenna length is too long, and it is most susceptible to interference from various electrical clutter. Therefore, this type of Products have been replaced by high frequency bands in the 21st century and gradually disappeared from the market. The latter uses the VHF200MHz frequency band. Due to the higher frequency, the antenna is shorter, and it can even be designed as a hidden antenna, which is convenient, safe and beautiful. The clutter interference of the receiver is greatly reduced. The circuit design is extremely mature and the parts are popular at a price. Low cost, so it has become a popular model on the market today.
Obviously, no matter how the sensitivity is expressed, we can convert it to dBμ, provided that the line input is unified to the unit of Pa (Note: here is a supplement: 1 Pa=10μbar. This formula is useful in the following calculations) For example: NEUMANN U89 The sensitivity of the microphone is 8mV/Pa, which can be obtained directly from 20lg[(0.008V/Pa)÷(0.775V/Pa)] and its sensitivity is about
Another example: The sensitivity of the AKG C414 microphone is -60dBV. From 0dBV=1V/μbar=10V/Pa (the formula mentioned earlier is used here: 1 Pa=10μbar), first calculate the output voltage of -60dBV under 1Pa sound pressure. X: 20lg[(XV/Pa)÷(10V/Pa)]=-60
Get X=0.01(V), that is, its sensitivity is 10mV/Pa. From the formula 20lg[(0.01V/Pa)÷(0.775V/Pa)], the sensitivity is about -37dBμ.
The quality of wireless microphones made in Taiwan has been significantly improved due to the improvement of consumer requirements: the price has fallen greatly due to fierce competition from manufacturers. Therefore, some smaller manufacturers, in order to win orders, are not committed to the development of more advanced models, but to produce simpler models with lower prices to attract consumers. As the saying goes: "You get what you pay for." Consumers often choose low-quality products because they are greedy for a little bit cheaper. Some consumers are either too superstitious about famous brands or misled by manufacturers' false advertisements. So that the money spent is not worth the fare. How to choose a model with excellent value for money, first respond to the quality positioning required by yourself, and further understand the principles of how to evaluate the quality of wireless microphones, in order to choose a truly satisfactory product.
Wireless microphone products are products designed by combining audio and wireless communication technology, and are completely different from the technology and equipment for manufacturing wired microphones. Therefore, wired microphone manufacturers that also manufacture "microphones" may not be able to manufacture wireless microphones and wireless communication products. Manufacturers of, may not be able to manufacture good wireless microphones. Only those with long-term professional audio technology and wireless high-frequency technology background can manufacture good wireless microphones. A few years ago, SHURE, a famous manufacturer of wired microphones, often advertised that its wireless microphones could be as good as its wired microphone SM-58. That was a misleading advertisement for consumers! Because the biggest difference between wireless microphone products and other wired products is that the use effect is very closely affected by environmental conditions. Therefore, a high-quality wireless microphone cannot be designed in the laboratory in the short term. It needs to be used for a long time. After actual use in a variety of different environments, the designer will give back the gains and losses to the design engineer, and then through continuous research and development and improvement, in order to create a truly perfect product. Therefore, to choose a good wireless microphone product, you must choose a professional technical background and long-term manufacturing experience.
1. The appearance model has a design conforming to ergonomics and aesthetics:
The tube body of a handheld wireless microphone must have a size suitable for hand-holding and a beautiful shape. Generally, the traditional tube body is in the shape of a bamboo tube or a trapezoid. Not only is there no aesthetic feeling, it is also not suitable for hand-holding, especially for those who tend to sweat The user is less likely to grip and slip off. The most suitable shape for hand grip is the double inner curve shape with a smaller diameter in the middle than the ends. It is like a Chinese Guanyin bottle or a woman's waist. It is not only beautiful in shape, but also easy to hold.
2. Hand-held microphone should use advanced hidden antenna design:
The biggest difference between humans and monkeys is that they don’t have that tail. Earlier wireless microphones had an external antenna at the end. Advanced wireless microphones overcome technical difficulties and no longer use outdated external designs. Instead, they adopt the most perfect hidden antenna design to make wireless microphones easy to use, It has the advantages of safety, beauty and no breakage.
3. To assemble a good sound capsule:
The quality of the sound head determines the first level of the sound quality of wireless microphones. There are two types of capsules, moving coil and capacitive. The moving coil uses a coil loaded on the diaphragm to convert sound energy into electrical energy signals in a high-density magnetic field. There are certain limits on the voice coil characteristics of this type of capsule. But the basic structure is simple, the price is cheap, and it is the most popular model on the market. Condenser microphone is a microphone with a higher technical level in combination with electronics and structure. Its pronunciation is to use the change of the capacitance between the poles to directly convert the sound into an electrical signal with an ultra-thin gold-plated vibrating membrane. The main feature of advanced condenser microphones is that they can show excellent original sound quality, high and low frequency response is very wide and flat, very high sensitivity, large directivity and dynamic range, low distortion rate, light volume and drop resistance, low touch noise, widely used In the recording studio, professional stage, testing equipment and other professional equipment. The only disadvantage is the need to provide a bias voltage (Phantom Power), but because the wireless microphone itself has a power supply, the condenser capsule is the best match for the wireless microphone, allowing all the advantages to be fully utilized in the wireless microphone. Taiwan’s MIPRO wireless microphone is the only product equipped with an advanced condenser capsule.
4. The microphone should have the superior characteristics of low touch noise:
The hand-held wireless microphone has an impact on the normal sound quality due to the frictional touch noise between the hand and the palm during use. Especially the wireless microphone itself has a sensitive preamplifier, which makes the performance of this touch noise more serious and becomes a technical bottleneck. . Generally, wireless microphones do not have professional design experience, and in order to reduce manufacturing costs, they use simple circuits, low-cost capsules, poor suspension design and cheap tube surface treatment, so poor sound quality cannot overcome the remarkable microphone Touch noise degrades the original sound, so when choosing a high-quality wireless microphone, special attention must be paid to the choice of excellent sound quality and ultra-low touch noise.
5. It has the function of eliminating interruption or instability of sound:
The signal emitted by the wireless microphone is attracted and reflected by the surrounding environment, resulting in a blind spot in the signal received by the receiving antenna, which causes the output sound to be interrupted or unstable. This phenomenon is not allowed especially in professional situations. occur. In order to solve this shortcoming, the most perfect result can be obtained only by adopting the most advanced automatic selection receiving system. Generally, the popular low-cost dual-channel receivers on the market do not have this automatic selection receiving function, and the above shortcomings cannot be avoided, so they can only be used in short-distance home karaoke occasions. In professional places or users who value sound quality, they must choose the model of the automatic selection receiving system to meet the sound quality requirements and obtain a perfect performance.
6. It has the function of preventing loud noises caused by interference during standby:
Most receivers generally have a mute control function (Squelch Control). When the power is turned on and there is no microphone signal input or the signal strength is lower than a certain signal-to-noise ratio, the mute control circuit closes the output circuit, and the main receiver is completely muted to prevent noise Output. When the microphone signal is turned on, the receiver immediately turns off the mute circuit and turns on the output circuit, allowing the audio circuit to output the microphone sound. But when the microphone power is turned on and off, or when the microphone signal is turned off, occasionally encounter signal interference that exceeds the mute control strength, and the receiver mute circuit will also be activated by these impact noises and interference noises to output loud noises. In order to solve this deficiency, the so-called "audio lock mute circuit" was installed in advanced models to suppress it. The principle is to add a fixed super audio modulation signal to the transmitter signal of the microphone: at the same time, a discriminator is also installed inside the receiver, so the receiver must receive the microphone signal containing this fixed super audio modulation signal The output circuit can be activated only when it is time to achieve the function of preventing interference from other signals or noise. In order to protect your valuable audio system from being damaged by loud noises, you must choose a model with sound code lock and mute function.
7. With multi-channel non-interference function:
The biggest technical bottleneck in the use of wireless microphones is the problem of signal interference, especially the more frequencies used, the more serious the problem of interference. Therefore, when multiple wireless microphones are used at the same place at the same time, interference should be avoided, except for physical selection. In addition to the frequency that does not interfere with each other and avoiding the interference of adjacent external signals, the receiver must have excellent selectivity, and the emitted and received radiated harmonics must be filtered very cleanly to avoid signal interference. Generally, a receiver in the VHF band can be used for 12 frequencies at the same time. Taiwan’s MIPRO product can use 24 frequencies at the same time without interfering with each other. Even under certain conditions, it is specially designed. And arrangements can achieve more frequency simultaneous use.
8. To solve the problem of simultaneous use of multiple channels and avoid interference, a multi-channel series model with digital lock to change the frequency should be selected:
The traditional wireless microphone system is designed with a fixed frequency locked by quartz. This type of model cannot be arbitrarily changed to the desired frequency when it needs to be used in multiple channels or in the case of strong signal interference. It must be replaced as a whole . In order to solve this deficiency, advanced models adopt a phase-locked frequency synthesis (PLL Synthesized) method. Dozens of frequencies are pre-stored in the transmitter and receiver to allow users to change arbitrarily, although this advanced design is costly , But it provides a very convenient function for distributors and users, and completely solves the above-mentioned deficiencies. Taiwan already has MIPRO energy production of this complete series of products with the most advanced design and the most reasonable price.
9. To avoid frequency "congestion" or signal interference, products with digital locking UHF channel system should be selected:
Due to the excessive use of wireless microphones in the VHF200MHz frequency band, the problem of mutual signal interference and noise interference from various electrical appliances has become more and more serious. Therefore, the frequency of professional-grade wireless microphones has gradually increased to the UHF frequency band of 800MHz, and a PLL phase lock circuit is adopted. , The preset multi-channel design can be switched arbitrarily to avoid interference from other signals and general electrical noise and obtain the best use effect. Because the circuit design of UHF frequency band is more complicated and the high-frequency parts used are more precise, the price is still higher, and there are fewer mass-produced manufacturers. However, UHF models are the best choice for professional quality and will gradually become popular in the future. the trend of. Taiwan still has a full range of advanced models and complete peripheral equipment in mass production from MIPRO.
10. Products that have international quality certification and passed telecommunications regulations:
High-quality wireless microphone products must be manufactured in factories that have passed international quality certification, and they must also be certified by national radio laws and regulations before they can be sold and used legally. Consumers should choose certified products to use, so that the quality can be guaranteed.
The most basic sources of information for purchasing products are catalogs and magazine advertisements provided by manufacturers. However, generally small-scale manufacturers with incomplete equipment cannot provide correct values at all, so consumers cannot understand, analyze and compare from catalogs. Some manufacturers even use catalogs and advertisements to exaggerate facts and mislead consumers' psychology. However, quality is not spoken by the mouth, but is manufactured. Smart consumers can obtain the correct quality evaluation by carefully analyzing the actual products of the manufacturer, actual testing and relative comparison with the advanced information technology.
Choose a set of wireless microphone products that can suit your needs and feel satisfied after use. In addition to understanding the above-mentioned reference principles, the real action is to collect more relevant information by yourself and listen to the actual comparison before you can buy the super A value product. However, technological products are changing with each passing day, and the functions are constantly updated. Since you want to buy a new product, you must choose the most advanced model today. Even if the price is a little bit more expensive, you don’t have to hesitate. The superior advanced product is worth it. You have priority!
1. Only by connecting the negative pole of the 9V battery with the battery case, the transmitting antenna of the wireless microphone can be extended through the battery case, which can increase the transmission distance and prevent frequency interruption and frequency running.
2. Connect the negative terminal of the 9V battery holder of the wireless microphone with an enameled wire wound into a dense spiral, and fix it on the opposite side of the tongue plate of the tail of the wireless microphone. The V section is longer (the diameter is 2.5MM and then it is wound back to the starting point), and the U section is shorter (in the 4MM diameter is rolled to the end and squashed).
3. Those who have the hands-on ability to disassemble wireless microphones. Pull out the tail more than two inches to expose a half-inch multi-circuit board, find the position of the launch tube, solder it to the 1P capacitor (or a few P capacitors and the smallest capacitor) and use enameled wire to wind it into a dense spiral and fix it in the tail slot of the wireless microphone. The outer diameter of the spiral is approximately smaller than the groove width, and it is compressed and fixed in the groove. Note: The wireless microphone with No. 5 battery is not suitable for this side, because the wireless microphone with No. 5 battery has already used the battery shell.
The following is the circuit diagram of the FM wireless microphone (Figure 1). The circuit is very simple and has no redundant components. High frequency transistor V1 and capacitor
FM wireless microphone production PCB board
FM wireless microphone production PCB board 
C3, C5, and C6 form a capacitor three-point oscillator. For beginners, we don't want to ponder the specific working principle of the capacitor three-point oscillator. We only need to know that this circuit structure is a high-frequency oscillator. The load C4 and L of the collector of the triode form a resonator. The resonance frequency is the transmitting frequency of the FM microphone. According to the parameters of the components in the figure, the transmitting frequency can be between 88～108MHz, which just covers the receiving frequency of the FM radio. The value (stretch or compress coil L) can easily change the transmitting frequency and avoid FM radio. The transmitted signal is coupled to the antenna through C4 and then transmitted.
R4 is the base bias resistance of V1, which provides a certain base current to the triode to make V1 work in the amplifying area, and R5 is the DC feedback resistance, which stabilizes the operating point of the triode.
The frequency modulation principle of this FM microphone is achieved by changing the capacitance between the base and emitter of the triode. When the sound voltage signal is applied to the base of the triode, the capacitance between the base and the emitter of the triode will vary. The size of the sound voltage signal changes synchronously, and at the same time the emission frequency of the transistor is changed to realize frequency modulation.
The microphone MIC can collect external sound signals. Here we use a small electret microphone, which has very high sensitivity and can collect faint sounds. At the same time, this microphone must have a DC bias to work. The resistance R3 can provide a certain The greater the resistance of R3, the weaker the sensitivity of the microphone to collect sound. The smaller the resistance, the higher the sensitivity of the microphone. The AC sound signal collected by the microphone is sent to the base of the transistor through C2 coupling and R2 matching. In the circuit, the two diodes D1 and D2 are in anti-parallel connection, which mainly serves as a two-way limiting function. , The conduction voltage of the diode is only 0.7V. If the signal voltage exceeds 0.7V, it will be shunted by the diode. This ensures that the amplitude of the sound signal can be limited between plus and minus 0.7V. Excessive sound signals will cause the transistor to pass Modulation, resulting in sound distortion or even not working properly.
CK is an external signal output socket. External sound signal sources such as TV headphone socket or Walkman headphone socket can be introduced into the FM transmitter through a dedicated cable. The external sound signal is attenuated by R1 and limited by D1 and D2 and then sent to the triode base. The pole is frequency modulated. So this kit can be used not only as a wireless microphone, but also as a TV wireless headset.
The light-emitting diode D3 in the circuit is used to indicate the working state, and it will light up when the FM microphone is energized, and R6 is the current-limiting resistance of the light-emitting diode. C8 and C9 are power supply filter capacitors. Because large capacitors are generally made by winding technology, the equivalent inductance is relatively large. A small capacitor C8 in parallel can reduce the high-frequency internal resistance of the power supply. This circuit is very common.
K1 and K2 in the circuit are actually a switch, it has three
Turn off the power when dialing to the leftmost position. The rightmost position is K1 and K2 connected for FM microphone use, the middle position is K1 connected, K2 disconnected, used as a wireless repeater, because it is used as a wireless repeater The microphone does not work, but the microphone consumes a certain amount of quiescent current, so disconnecting K2 can reduce power consumption and extend battery life.
Some film and television sound workers conducted detailed practical tests on several wireless microphones commonly used in daily work. The sound pickup of wireless microphones involves not only distance issues, but also sound quality issues, including distortion, noise, interference, and stability.
The whole test was carried out in a TV production unit, mainly focusing on the actual needs of TV sound work, such as equipment performance indicators, working distance imitating the actual working environment, sound waveform display, and subjective evaluation of sound quality. This article only selects the distance test of the L group and S group wireless microphones for introduction.
The test method is roughly as follows: In order to place the receivers of the above two sets of wireless microphones on the same point, two sound engineers carry two sets of transmitters of different models, namely transmitter + lavalier microphone head, or external plug-in transmitter Machine + handheld microphone. Both move along the same prescribed route, and continuously provide location descriptions and other voice information during the movement. The receiver groups respectively send the received signals to the two inputs of the mixer, and the mixer is tracked and output to the digital audio workstation for real-time track recording for recording and on-site analysis.
The route taken is shown in Figure 2. The starting point is where the receiver is placed, and the ending point is the north gate.
The tested products are: (1) S version xxxx (RF power 250mW), using S brand lavalier microphone head, receiver is Sanken3xxx type, using the microphone output port of this machine. (2) L brand MM4xx (RF power 100mW, 400 series digital coupling platform), using CountrymanBx microphone head, receiver of the same brand UCR4xx, use the microphone output port of this machine.
In the synchronization process, when S is about 170m, there will be frequency running phenomenon without shielding, and there will be no signal at all if it goes about 5m. See Figure 3 (below): when L is about 212m, it enters the north gate. The shielding is very serious and there is frequency running phenomenon, see Figure 3 (top). The time coordinates of Fig. 3 are the same, and the interrupted part of the waveform is an unreceived signal approved by frequency running, which can be seen. The reception of L is much better than that of S.
Two sets of tests
As shown in Figure 2, the starting point is the location where the receiver is placed, and the ending point is the North Gate Cave. The tested products are: (1) L brand UM2xx (RF power 250mW, 200 series analog RF platform), using CountrymanBx microphone head, receiver is L brand R4xx type, using 200 series compatible receiving mode and line output (+4dBu) port. (2) L brand MM4xx (RF power 100mW, 400 series digital coupling platform), CountrymanBx microphone head, receiver is L brand UCR4xx type, using 400 series digital coupling receiving mode and line output (+4dBu) port.
It should be noted here that the first set of tests used the microphone input of the mixer. This is because in many cases, users are willing to use wireless microphones instead of wired microphones. But we found that for receivers with high sensitivity, the gain must be lowered to match the input indicators of the following equipment. If it is not matched properly, it may limit the amplitude when the signal is large and cause popping sound. In this way, the second set of tests changed the outputs of both sets of receivers to line outputs. Judging from the waveform displayed by the audio workstation, even in the case of individual loud-speaking signals, there is no clipping.
L brand UM2xx and L brand MMxx entered the north gate hole due to severe shielding and frequency running occurred. However, there is still a signal after passing through the Beimen Cave. The distance between them is about 247m. The running frequency is only in the Beimen Cave. The test shows that the working distance of the L brand MM4xx before running frequency is far more than the first set of test data.
Three sets of tests
The route taken is shown in Figure 2. The starting point is where the receiver is placed, and the ending point is the north gate. The tested products are: (1) L brand UH2xx (RF power 100mW, 200 series analog RF platform, external plug-in transmitter), using front-end microphone S brand 4xx dynamic microphone, receiver L brand R4xx type, using 200 series Compatible with receiving mode and line output (+4dBu) port. (2) L brand LMxx (RF power 50mW, 400 series digital coupling platform), using CountrymanBx microphone head, the receiver is L brand UCR4xx type, using 400 series digital coupling receiving mode and line output (+4dBu) port.
The L-brand UH2xx started running at 247m from the starting point to the end, and the L-brand LMxx started running at 199m. It continued to move more and more severely until no signal was received.
The tested products are: (1) S brand xxxx (RF power 250mW), using Sanken lavalier microphone head, receiver is S brand xxxx type, using microphone output port. (2) L brand UMxxx (RF power 250mW, using 200 series analog RF platform), equipped with CountrymanBx microphone head, receiver is LRxxx type, using 200 series compatible receiving mode using line output (+4dBu) port.
Test method: Attach two GSM mobile phones to the receiver under test. When the wireless microphone is working, the phone dials to the rice transplanter to observe the interference of the mobile phone signal to the wireless microphone under test. The other is to close the mobile phone to the receiver under test, and then turn on and off the mobile phone to observe whether the radio frequency signal emitted by the mobile phone during the high-power search for signals within a few seconds after turning on the phone interferes with the wireless microphone reception.
The two groups of tested wireless microphone systems did not respond to the interference of mobile phone signals, and the sound signal output from the receiver was normal. The critical running frequency distance test results are shown in the attached table (Figure 4).
(1) From the actual measurement results, it can be seen that the L brand transmitter with a nominal RF power of 50mW has a critical working distance before running frequency, which greatly exceeds the S brand with a nominal RF power of 250mW. Because the critical working distance of each brand product is proportional to its nominal RF power. For products of the same brand, the distance can be derived from the power. Therefore, the RF power published by different manufacturers may not necessarily reflect the actual safe working distance. Only by doing actual tests can we truly understand the performance of the selected product.
(2) The digital coupling technology (digital audio processing plus analog modulation radio frequency) announced by the L brand wireless microphone. It can indeed combine the advantages of both sound quality and working distance. This is different from the so-called all-digital wireless microphone (digital audio processing plus digital modulation radio frequency) technology, and its working distance ratio is significantly increased.
(3) In the above actual test, the function of the multi-radio frequency mode compatible technology has been verified. For example, in the matching of transmitters and receivers tested in the second and third groups, the L brand 200 series transmitter can be well matched with the 400 series digital coupled receiver, and no problems were found in the test. Analysis of the reasons should be attributed to the use of multi-radio frequency mode compatible technology.
(4) In the test environment, although the nominal RF power of the L-brand UHxxx external plug-in transmitter (so-called "grenade transmitter") is 100mW, in the actual working environment, the critical working distance and nominal RF power before running frequency are 250mW The L-brand UMxxxx bodypack transmitter is almost the same. The main reason is that because the former transmitter is different from the bodypack transmitter, that is, it uses the dipole radio frequency antenna formed when the body shell is held by a person, so that the transmission height is higher than that of the ordinary bodypack transmitter. To.
(5) In the actual measurement, it is found that it is very important to adjust the microphone input gain of the transmitter correctly. This has a significant impact on the dynamic range and signal-to-noise ratio of the output signal at the final receiver. Therefore, in actual use of a wireless microphone, it must first be at the transmitter according to the size of the microphone user’s voice and the degree of concealment of the microphone head. Adjust the microphone input level on the transmitter to a suitable position. Then adjust the output level at the receiver to ensure that the output signal-to-noise ratio reaches the best value.
(6) As mentioned above, if the audio output of the wireless microphone receiver is used instead of the wired microphone plug-in position to input the microphone input of the downstream equipment, you must pay attention to the problem of the amplitude limitation of the downstream equipment caused by the large signal input overload. Although the sound is not distorted after clipping, the dynamics are obviously compressed. The limiting of the mixer or other equipment is to prevent the external input signal level from being overloaded and causing distortion, or in the case of sound reinforcement, it may cause the overload and even burn the speakers. This is dynamic limiting, not clipping, so the sound is not distorted, but the dynamics are compressed. Since the audio output of most receivers only has a XLR socket, the user should adjust the level range of the audio output to the microphone level output (for example, -50dBu) according to the specific situation. ) Or line level output (for example +4dBu) must not be generalized.
(7) This report mainly verifies that users are most concerned about working distance and mobile phone interference. Other indicators, such as battery life, need to be further simulated and tested with the cooperation of the manufacturer.
(8) During the test, it was found that some brands of wireless microphone receiver power adapters are interchangeable, which is good for on-site users in emergencies. It is recommended that manufacturers try to unify the specifications of the products.
The author hopes that through such actual measurement, broadcast TV audio workers and technical engineers have a deeper understanding of the product. Each unit has its own specific situation, but when purchasing products, the true understanding of the selected products is the key to whether they can purchase good products. We believe that only actual measurement is the most convenient and feasible method.
1. 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. 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. 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, 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 because it is close to the public frequency modulation broadcasting (abbreviated 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).
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).
2. Several terminology To understand the performance characteristics of wireless microphones, it is necessary to understand the basic terminology and main performance indicators of wireless microphones. 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 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 weak tone or 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 receiving 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.
Some products on the market are fixed-frequency, but a batch of products of the same model can be produced
Users can choose products with the same frequency when purchasing, but they cannot adjust their working frequency. Some manufacturers also label them as "multi-channel" or "32 channels can be selected at will", which is inaccurate or deliberately misleading consumers of. 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 -101dBm), the receiver output signal can reach 12dB signal 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 to 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.
In fact, to measure the transmission capacity of a wireless microphone, it depends on the transmit power and the receiving sensitivity of 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.