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Test method of main performance of wireless microphone

Author:admin   AddTime:2021-06-24

Although operating a wireless microphone system is sometimes complicated, we can perform some simple tests (without special test equipment) to understand the main performance of the wireless microphone, which is very practical. The wireless microphone is still a "microphone" by definition. Its sole design goal is to generate accurate audio signals for various applications. The "wireless" microphone means that it can be used without an attached connecting wire.

Before you decide to buy or rent a system, it is recommended to use the following tests to help you assess the quality of a special wireless microphone system. Each test will check the system's specific types of performance and problems. In order to get an overall assessment of the quality of the system, it is best to conduct as many such tests as possible (but not all of them), because you will find that some designs perform well in some areas, but will be poor in another area. . Only one or two tests are not enough to get a better overall assessment.

There is no essential difference in the principle of wireless microphones from various manufacturers. Many products have good technical indicators, but they are not satisfactory in actual use. In practice, the microphone technical index test is a difficult problem, because many use unit cities do not have the full set of measuring instruments of the radio laboratory, and it is impossible to test the product index at all. This has led to a "gathering" or one-sided choice of wireless microphones. If you use it, I will use it. Few people have delved into whether it is suitable for your application. If the usage conditions are not so harsh or critical, no major problems will generally occur, especially when the channel is selected properly. However, in actual use, especially in non-fixed installations, such as shooting TV programs and other situations with high mobility, the surrounding conditions are difficult to predict, and the receiving distance often changes, and sometimes, it does not exceed the wireless microphone technology used. The range of indicators, but the reception situation has deteriorated. As a result, the film crew was forced to stop work for adjustment, or reconsider the pickup plan, or even change equipment.

In response to these situations, recently some film and television sound workers have 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.

test

The entire test was carried out in a TV production unit, and mainly focused 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 at the same point, two sound engineers carry two sets of transmitter sets of different models, namely the transmitter lavalier microphone head, or an external plug-in transmitter. Hand-held 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 first set of tests

The route taken is shown in Figure 1. The starting point is where the receiver is placed, and the ending point is the North Gate Cave.

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, the receiver is the UCR4xx model of the same brand, using the microphone output port of this machine.

Test method of main performance of wireless microphone

The route taken is shown in Figure 1. The starting point is 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 radio frequency 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), using CountrymanBx microphone head, receiver is L brand UCR4xx type, using 400 series digital coupling receiving mode and line output (4dBu) port.

What I want to explain here is 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 specifications of the following equipment. If the matching is improper, it may limit the amplitude when the signal is large and cause a popping sound. In this way, the second set of tests changed the output of the two sets of receivers to line output. Judging from the waveform displayed by the audio workstation, even in the case of individual loud-speaking signals, there is no clipping phenomenon.

The receiver is a circuit system with the following components: antenna, filter, amplifier, A/D converter. The navigation and positioning signal sent by GPS satellites is an information resource that can be shared by countless users. For the vast number of users on land, sea and space, users are required to have receiving equipment capable of receiving, tracking, transforming and measuring GPS signals, that is, GPS signal receivers, so that they can use GPS signals for navigation and positioning measurements at any time. Depending on the purpose of use, GPS signal receivers required by users are also different. At present, there are dozens of factories in the world producing GPS receivers, and there are hundreds of products. These products can be classified according to principle, purpose, function, etc.

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.

The third set of tests

The route taken is shown in Figure 1. The starting point is where the receiver is placed, and the ending point is the North Gate Cave. The tested products are: (1) L brand UH2xx (RF power 100mW, 200 series analog radio frequency 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 fourth set of tests

The products tested for interference of rice transplanter signal to wireless microphone work 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, and observes the interference of the mobile phone signal to the tested wireless microphone. 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 high-power search for signals interferes with the reception of the wireless microphone within a few seconds after it is turned on.

The full name of GSM is: Global System for Mobile Communications, Chinese for Global System for Mobile Communications, commonly known as "Global Communication", a digital mobile phone network standard developed by Europe. Its development purpose is to allow all parts of the world to use a mobile phone network standard. Users can travel all over the world with a mobile phone. The GSM system includes several frequency bands such as GSM 900: 900MHz, GSM1800: 1800MHz, GSM-1900, 1900MHz, etc.

The two groups of tested wireless microphone systems did not respond to the interference of the mobile phone signal, and the sound signal output from the receiver was normal. See the attached table for the test results of the critical running frequency distance.

(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 modulated 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, the L brand UHxxx external plug-in transmitter (the so-called "grenade transmitter") has a nominal RF power of 100mW, but 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 its body shell is held by a human being, 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 the 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 sound reinforcement situation, 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, users 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) cannot be generalized.

(7) This report mainly verifies that users are most concerned about working distance and mobile phone interference issues. Other indicators, such as battery life, need to be further imitated 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.

Through this kind of actual measurement, the 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.

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