[Home Theater Network HDAV.com.cn] The performance of wireless microphones is often affected by various factors such as frequency interference and user error operation. This article will teach you how to avoid and prevent the most common wireless. Microphone problem.
1: The system itself is not compatible
When using a wireless microphone, there is always a problem of interference between the systems themselves. Although each system has a frequency or interval of several megahertz, intermodulation distortion (IMD) still causes mutual interference between the microphones. If there is not enough megahertz space between the intermodulation signal and the operating frequency of the device, it is difficult for the receiver to pick up the signal from the transmitter. Typical phenomena are crosstalk between systems, frequent signal loss or excessive noise and distortion.
The minimum spacing between frequencies depends on the design of the system receiver, and the entry-level receiver may need to be 1 MHz apart from the nearest nearest system. The more expensive the receiver typically has a narrower adjustment "window", with a smaller intermodulation frequency spacing between each system.
Solution: To avoid intermodulation distortion, select the calculated compatible frequencies. This requires a wealth of transmitter and receiver design knowledge that wireless system manufacturers often have calculated for these frequencies. For example, when only 8 wireless microphones are used together, thousands of calculations are performed to ensure compatibility between the microphones. As a result, most manufacturers have published their system-compatible frequency list. In addition, software can be used to help users identify compatible frequencies in certain situations.
2: The compatibility of the entire system is not enough
There is a different degree of compatibility between the frequencies. If you know the system well, you can use more systems more boldly, but the key is how to balance the compatibility of the whole system.
Most frequency-compatible software has an important assumption in design that all receivers are always on or off (even if some transmitters are occasionally turned off), ensuring that all receivers are not picked up. To the intermodulation signal that may produce noise.
Therefore, the software is designed to leave enough room for intermodulation signals and wireless microphones. If you assume that the sound system operator is playing a more active role in the event, then the system needs to have a wider range of compatibility. In this case, assume that the operator will silence all receivers and all transmitters will remain in the show. The distance between the transmitter and the receiving antenna is similar. These assumptions are completely feasible in the Broadway theater performance, but in the school hall, the system is operated by untrained personnel, and it is not great to achieve the same expected performance. It may be.
Interference is more severe when the transmitter is located very close to the receiving antenna, or when a high-power transmitter is running. That's why in a movie theater, 40 wireless systems work at the same time far more difficult than in school (many transmitters and receivers are very close), there is a system in every classroom in the school, and the transmitters are completely Independent, but each close to their own receiver.
Workaround : To balance the maximum number of system devices with high performance, make sure that the level of compatibility between the frequencies is appropriate for the system you are using. Keep the transmitter at least 10 feet away from the receiving antenna. If the transmitter's RF output power is adjustable, use the lowest transmit power to cover the expected distance between the transmitter and receiver.
3: Interference from other sources such as TV stations
Wireless microphones are also subject to interference from other sources transmitting from the same spectrum. The most common is usually a television station, and the FCC rules require users of wireless microphones to avoid using the frequencies occupied by broadcast stations in the same geographical area.
Solution: Indoors, avoid interference on 40-50 mile TV channels. When working outdoors, keep it within 50-60 mile radius. Since the frequency of each city is different, the suitable frequency of the wireless microphone is determined by the location. Equipment manufacturers often provide guidance to inform users of the different frequencies of use in different cities.
The FCC stipulated that all analog TV stations ceased operation in February 2009. At the same time, the spectrum above channel 51 will be used for other purposes. The wireless microphone frequency above 698 MHz should be adjusted to a lower frequency to avoid interference with new services. As the conversion continues, the TV channels in a particular location may change, so it's best for users to periodically view official profile information.
4: Interference problems with other digital devices
Other wireless audio devices such as ear monitors, intercom systems, and non-wireless devices can also cause interference problems. Digital devices (CD players, computers, and digital audio processors) often emit strong RF noise and can cause interference if installed close to the wireless microphone receiver. For transmitters, the most common source of interference is the GSM mobile phone and the PDA worn by the host.
Solution: When choosing a wireless microphone frequency, be aware of other wireless audio devices. Keep the distance between the digital device and the wireless microphone receiver at least a few feet away.
5: Receive antenna selection and layout
The receiving antenna of a wireless microphone is one of the most misunderstood areas. Errors in antenna selection, layout, and routing can result in short distances in the performance coverage area and low signal strength, resulting in frequent dropped calls. The performance of modern diversity receivers is far superior to the performance of individual antenna types, but to optimize system performance and reliability, antenna selection and layout must be correct.
Solution : To ensure good diversity of the system, the antenna space is guaranteed at least one and a half wavelength (about 9 inches 700MHz). The angle of the receiving antenna should be a "V" configuration that provides better signal pickup performance when the transmitter is moved or placed at different angles.
If the receiver is installed away from the performance area (such as in a cabinet closet or enclosed rack), install a half-wave antenna or directional antenna (preferably above the viewer) remotely so that there is clarity between the transmitters The line of sight. Do not install 1â„4 shortwave antennas remotely as they use the receiver chassis as a ground antenna. The extra distance between the antennas does not significantly improve the diversity of the system, but may better cover a larger stage, church or conference room area. If the antenna is mounted away from the stage, a directional antenna can be used to improve signal reception by picking up more signals in that direction and reducing signal pickup at other angles. If a coaxial cable is used to connect the antenna to the receiver, an antenna amplifier may be needed to solve the signal loss problem in cable transmission.
The loss of the number of signals depends on the specific length of the cable and the type of cable. Therefore, please follow the manufacturer's recommendations and the total net loss of the signal should be controlled within 5 dB.
6: Artificial wireless signal is blocked
The human body may also interfere with wireless signals. The human body is mainly composed of a large amount of water, which can absorb radio frequency energy. In addition, if the user surrounds the hand-held transmitter external antenna, its effective output can be reduced by more than 50%. Similarly, if the flexible antenna on the transmitter is curled or folded, the signal will also be affected.
Solution: Keep the transmitter antenna fully deployed and unobstructed for maximum signal transmission for optimal performance.
7: The transmitter battery voltage is not enough
The battery life of the transmitter is a primary concern for wireless microphones, and users are always trying to reduce the cost of the equipment through inexpensive batteries. Most wireless manufacturers specify alkaline or disposable lithium batteries because their output voltage is stable throughout the life of the battery. This is very important because most transmitters experience distortion or loss of signal at low voltages. Rechargeable batteries often seem to be the ideal solution, but most rechargeable batteries provide a voltage that is 20% lower than the disposable battery voltage, even when fully charged.
Solution: In order to solve the problem of the battery, always carefully compare the voltage output requirements of the transmitter battery to ensure the sustainability of the battery throughout the work. Lithium-ion batteries and rechargeable alkaline batteries usually work continuously, while nickel-metal hydride and nickel-cadmium batteries may last only a few hours. This is especially true for 9 volt batteries, which have similar performance to disposable AA batteries.
8: Unadjustable transmitter
The inherent noise and limited dynamic range of FM transmission make analog wireless audio transmissions have their limitations. To overcome this, most wireless microphone systems typically use two audio processing methods to improve sound quality. A pre-emphasis device is added to the transmitter, and a de-emphasis device is added to the receiver to improve the signal-to-noise ratio of the signal. The compressor and receiver expanders in the transmitter increase the dynamic range by more than 100dB. This makes the volume setting very important. If the audio level is too low, a click will be produced; if it is too high, it may cause distortion.
Solution : For the best sound quality, the transmitter's input gain should be adjusted so that the full modulation occurs at the highest volume, but without distortion.
9: Receiver output level setting error
With so much discussion of frequency, wavelength and antenna, it is easy to overlook the most basic requirements of wireless microphone systems: in order to replace the connection cable between the signal source and the audio system, the receiver is usually equipped with output level control, and large Most wired microphones don't. This provides a better chance of a more accurate match between the receiver output and the input.
Solution : Regardless of the microphone level or the line level, the output level should be set to the highest level that is practical and does not exceed the limit of the input of the audio system. This may be indicated on the input channel of the mixer. It can also be judged by listening to the distortion of the sound.
10: Wireless system settings
The most troublesome problem with wireless systems is that the waves themselves are constantly changing. Since the beginning of digital TV conversion, the analog and digital TV channel radio waves have been constantly changing. The FCC is trying to find a way for consumer products (personal PDAs, smart phones or home devices) to use vacant TV channels for wireless Internet access.
Solution: It used to be easy to know if the VHF band TV channel in the city where the user is located is odd or even. However, when people install and use wireless microphones (as well as in-ear monitors and intercom systems, etc.), they must periodically check the local spectrum status even when working on a site they are familiar with.
Of course, this is far less complicated than we think. First, most wireless device manufacturers now offer online frequency selection tools that are updated with the latest TV channels. Second, both external RF scanners and spectrum analyzers can quickly scan the entire spectrum (including the TV band), and they are more powerful and cheaper, providing a more practical alternative for those who rely heavily on wireless systems. . Finally, the wireless system itself is becoming more complex, and even some entry-level systems can scan the spectrum or find an open spectrum. Some excellent systems can even be connected to your PC or Mac, scan the spectrum, give you an intuitive RF status depiction, calculate a set of optimal frequencies (taking into account other RF devices), and then automatically set up the receiver. . More fresh and fun home theater information, please pay attention to home theater network http:// (WeChat: cnhifi), the country's most influential home theater audio player interactive media website.
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