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1. Every wireless microphone system must operate on a different, specific frequency.
2. All frequencies are shared by a large number of users across the country.
3. Wireless frequencies are shared with TV stations, communications equipment and other wireless microphone systems.
4. Because of frequency sharing, there is always a chance that someone else in the area might be using the same frequency as your wireless system.
5. The U. S. Government sets strict technical requirements for wireless, including limits on maximum transmitter power and available frequencies.
6. There must be one transmitter and one receiver to make a complete wireless system, and they both must be on the same frequency.
7. If any two transmitters are operating on the same frequency, severe interference will result and the wireless system will be unusable. Two transmitters cannot be used with one receiver at the same time.
8. If the frequencies of any two wireless systems are too close together, interference is possible, and one or both systems may be unusable.
9. The actual operating range of a wireless system will vary from as little as 50 feet in heavily crowded indoor situations with a non-diversity system and upwards of 1,000 feet under open outdoor conditions with true diversity. Mipro quotes a 330 foot range with line of sight usage.
10. Diversity wireless systems will have better operating range than similar non- diversity systems.
11. Wireless receivers should have a clear line of sight path between the antennas and the transmitter.
12. Weak or worn-out transmitter batteries are a common cause of wireless problems, including complete failure, poor range, distorted audio and even interference.
13. High-quality alkaline batteries (Duracell, Energizer) will provide the longest transmitter life. Many other types of batteries will have much shorter life, and some may cause performance problems.
14. Wireless microphones are no more prone to feedback than a wired mic. However, since the freedom of movement makes it is easier to accidentally walk in front of speakers, feedback problems are more common with wireless microphones than with wired microphones.
15. The power output of wireless microphone transmitters is very low, only about 10% of that of a typical cellular phone. However, any source of RF energy may interfere with the normal functioning of implanted cardiac pacemakers or AICD devices. A body-pack transmitter should not be worn where it is immediately adjacent to such a medical device. Note also that any medical device disruption will cease when the RF transmitting source is turned off.
1. Make certain that there is a clear, line of sight path between the receiver antennas and the transmitter. Metal objects between the transmitter and receiver, even folding chairs, screens or scaffolding, can reduce range.
2. Remember that human bodies are excellent absorbers of RF energy. Your wireless transmitter may not have enough power to transmit through a large group of people. Generally, the best remedy is to mount the antennas as high as possible to allow for unobstructed line of sight with the transmitters.
3. When a beltpack is worn at the small of the back, the antenna may be trying to transmit through the wearer. This can block some, or even all, of the signal. Try to position the bodypack so it has a direct line of sight to at least one of the antennas. For best performance, the antenna on a transmitter should never be covered. With handheld mics, take care not to cover the antenna with your hand. If you don’t see an antenna on your microphone, it’s probably hidden inside the lower few inches of the mic’s body. Hold the mic closer to its head/capsule to avoid covering it with your hand as you grab the mic. Do not cup the capsule with your hand. This can affect the microphone’s polar pattern and reduce performance as well as increasing the possibility of feedback.
4. If interference problems require a high squelch setting, try to eliminate the interference or change frequency to avoid a loss of range. A higher squelch setting provides better protection against interference. However, since a high setting also can cause a reduction in operating range, set the control to the lowest position that reliably mutes any interference. Mipro uses a sophisticated dual Pilotone squelch system which prevents interfering signals from opening the squelch gate when the receiver is in standby mode. This allows for a lower squelch setting and better range. Mipro’s default setting is 10% which is generally adequate.
5. Do not allow receiver antennas to touch each other. For best results, keep antennas from different receivers at least 10 inches apart or use antenna distribution. If three or more receivers are being used together, antenna distribution is required. This allows all of the receivers to use one set of antennas, reducing antenna clutter and maintaining maximum range and performance.
6. Remote mounted antennas are usually the best remedy for range or drop out issues. Most Mipro receivers come supplied with a ½ wave coaxial antenna that can be remote mounted without requiring a separate ground plane. For even better results, use either an omni directional or bi directional log antenna. These antennas provide increased gain and have the option of using a built in line amplifier to compensate for signal loss from long cable runs. They can be placed on a mic stand or mounted to a wall.
1. If the main interference problem occurs only when the wireless system transmitter is turned off, first check to see if the receiver’s squelch control is set to the middle of its range or higher. If so, there may be a problem with low-level interference.
2. If there is interference when the transmitter is off, and noise or distortion on the receiver audio when the transmitter is on, try turning off all other wireless transmitters. If the interference problem is still present, there might be a high level or direct radio interference problem. Direct interference is a serious problem that must be corrected in order for the wireless system to be fully usable.
3. If there is interference when the system transmitter is on, but it disappears when the other wireless transmitters are turned off, there may be a problem with intermodulation interference. Interference which takes the form of low level-audio tones, whines, whining sounds whose pitch changes rapidly, or audible voices or music (distorted or not) is likely to be caused by intermodulation interference, non-wireless interference or direct interference.
4. If there is interference when the system transmitter is turned on, and it is in the form of buzzy or raspy whines with a distinct cadence or rhythm that changes from time to time, digital interference might be the problem. Likely sources include computers, digital delays, effects processors, lighting controllers, and other equipment using microprocessors and digital signal processors (DSPs).
5. If there is no interference when the system transmitter is turned off and irregular popping, cracking or buzzing noises on the audio with the transmitter on, there may be electrical interference problems. This kind of interference is often caused by electric motors, neon lights, lighting equipment, appliances and other types of electrical equipment. Electrical interference will generally affect all wireless systems at a location, not just one system.
6. Interference that takes the form of crashes of static or short bursts of noise is likely to be caused by lightning, intermittent arcing or defective electrical machinery. This type of random interference is quite rare but can be among the most difficult to resolve.
Audio systems can be affected by interference from AM radio stations, radar systems and many kinds of electrical equipment. The interference usually enters the audio system via the audio cables at the mixer (console) or power amplifiers, or via the AC power lines.
AM radio stations are a common source of problems, but usually only when the station is very high power and nearby. Even with 50,000 watt stations, problems are rare when the station is more than a mile or two away. AM radio interference typically results in the presence of distorted speech or music in the audio. AM interference is easy to track down to a particular station by just tuning an AM radio across the local stations. Mipro’s squelch system eliminates most AM interference.
Radar systems, including airport radar systems, military search and weather radars, sometimes affect audio systems. Radar interference usually takes the form of highpitched buzzes or noise bursts that occur every few seconds. Many high-power radars turn a few times per minute, and cause interference only when they are pointed in the direction of the audio system. As with AM radio, radar interference is likely only when the radar is fairly close, usually when no more than one to three miles away.
These types of interference can sometimes be difficult to eliminate. Improving equipment grounding sometimes helps, especially when the only earth ground is at the electrical panel some distance away. Shielding or filtering audio cables where they enter equipment, especially mixers and power amplifiers, is often of help. Coiling audio cables and the addition of ferrite filters at cable connectors can be very helpful.
Interference can also enter audio systems through the AC power lines. Sometimes this is the path taken by interference from broadcast and radar transmitters. Electrical interference from heavy machinery, lighting equipment and other electrical equipment can travel along the power lines. This is especially likely near industrial areas where arc welders, large motors, high-power electrical controllers and similar equipment is in operation. This type of interference can take many forms, but buzzes at the power line frequency and harmonics, random noise bursts, and static-like noises are most common.
Most radio frequencies are shared among many different users. Also, many types of electronic equipment, including TV sets, DVDs, radio receivers, computers and peripherals, signal processing devices and all sorts of communications equipment may unintentionally generate small amounts of radio frequency (RF) energy. As a result, there are almost always weak radio signals present on any frequency that can be used for a wireless microphone system.
Most of the time, these signals are too weak to affect a wireless system, especially when the transmitter is turned on and the system is operating. However, if the transmitter is turned off, there is sometimes enough signal to open the squelch circuits in the receiver. When this happens, the result will be an unwanted audio signal from the receiver, often in the form of loud noise and static. Unless the interfering signal is strong, it will not significantly affect the audio quality when the transmitter is turned on. Mipro uses a dual pilot tone squelch that prevents most low level interference from getting into your system.
Direct interference occurs when there is another strong radio signal on the frequency used by the wireless system. Depending upon the strength of the interfering signal, the effect can range from weak tones, whistles and whines in the audio to very loud tones or harsh noise that make the wireless system completely unusable. Direct interference can also cause other audio problems such as excessive noise, distortion, unexpected level variations and changes in sound quality and character.
There are a number of possible sources of direct interference to wireless systems, including other wireless systems, TV transmitters, communications transmitters and spurious outputs from communications equipment, computers, digital devices and industrial equipment. The interference source may be intentionally transmitting a signal for use by another type of receiver, or the transmission may be unintentional or accidental. Either way, as long as the other radio signal is present, your wireless system may be affected.
TV transmitters are very powerful and can affect wireless systems at distances of 100 miles or more. When interference is experienced, one of the first steps should be to determine if the wireless frequency is on a TV channel that is in use in the area. Any wireless frequency within a 6 MHz-wide local TV channel is subject to serious interference.
One of the more frustrating problems with wireless is that the RF spectrum is dynamic. TV channel assignments have been changing regularly since the DTV transition began years ago. The FCC is also allowing a new strain of products known as TVBD’s (TV Band Devices) to use the unoccupied TV channels for wireless broadband internet access. Those who set up and use wireless microphones as well as in-ear monitors need to regularly check local spectrum conditions, even when working at a venue they know well.
Many times, interference problems are due to another wireless transmitter within range of your system. Often, the existence of the second wireless system is unknown, perhaps because it is in another room, on a different floor or even in another building. Keeping track of wireless systems can be particularly difficult in situations where visitors bring their own wireless equipment.
The other wireless system might also be a considerable distance away. Interference is possible at distances of up to 2,000 feet (600 m), and even more under certain conditions. When interference is encountered, there should be a careful check to see if there are any wireless systems on the same frequency at your location. It is also advisable to check the nearby area for other operating wireless systems. Mipro has software that will allow you to use your receiver to scan the RF spectrum in your frequency band to find sources of interference. This must be monitored regularly as the other system will not always be transmitting.
Other types of transmitters can cause interference. One quick check is to make certain that the wireless frequency is not twice that of a local FM radio station. FM transmitters generate a small amount of harmonic energy that can affect wireless receivers. Nearby two-way radio transmitters can affect wireless systems. Other types of transmitters and communications systems can have spurious outputs that fall near wireless microphone frequencies.
Cable TV systems often use nearly all the TV channels. If such a system is nearby, it is possible that leakage of radio energy is occurring on a TV channel that is thought to be unused in the area. Similarly, TV sets, VCRs, satellite dishes and other TV and video equipment can cause wireless interference. Cordless phones and other similar devices can also be interference sources. Almost any electronic device that makes or uses radio energy can potentially be an interference source, especially if it is close to the receiver antennas.
Computers and digital devices can cause direct interference. Most computers and digital devices leak at least a small amount of radio energy, often enough to affect a nearby wireless receiver.
Clearly, it is virtually impossible to be aware of every frequency present in a particular location. Given this, the best approach is to first eliminate the most obvious causes of interference, then attempt to eliminate as many of the remaining possibilities as you can, one by one. The recommended approach is as follows:
Once this has been done, attempt to turn off other possible interference sources one by one. If this is impossible, try to test for interference when the equipment is normally unpowered. Pay particular attention to communications and TV equipment, including cable TV gear, cordless phones and two-way radio equipment. Also carefully check out computers, printers, effects processors, lighting controllers and other digital devices. Observe activity in the area and try to determine if changes in the interference correspond to particular events or certain activities.
Sometimes the cause of the interference proves extremely difficult to identify. In other cases, the cause becomes known, but it proves impractical to correct the problem at the source. In these situations, it will usually be simpler and quicker to change the wireless frequency.
If the wireless frequency is changed, the information gathered during the interference search will be very helpful and will greatly reduce the chances of ending up with another problem frequency. In particular, it will allow selecting a new frequency that avoids the local TV channels, other wireless frequencies at the location and other identified problem frequencies. These are the most likely sources of interference, so their elimination will greatly reduce the chances of further problems.
Intermodulation interference or “intermod” is generated in the wireless receiver by the combination of two or more strong interfering signals. Unlike direct interference, it is not necessary that the interfering signals be on frequencies that are close to the wireless frequency. With intermodulation, the interfering signals mix together in the wireless receiver to create an internal signal that is close to the frequency of the receiver. This internally-generated signal then interferes with normal operation of the wireless system.
Intermodulation is usually caused by the signals either from other wireless systems at the location or TV signals. Often, the signal from a wireless transmitter combines with a strong TV signal to create interference on another wireless system. Because signals can combine in complicated ways, it is rarely possible to look at a list of frequencies and pick out those that will cause intermodulation problems. In addition, whether or not a particular set of signals will actually cause serious interference depends greatly upon their strength.
The amount of intermodulation produced in a receiver by external signals increases rapidly as they become stronger. Intermodulation also requires the presence of two interfering signals; removing either one or significantly reducing its level will usually eliminate the problem. These are two of the reasons why it is advisable to keep all transmitters at least 10 to 15 feet (3 to 4.5 m) away from receiver antennas. If intermodulation interference is suspected, one of the first things that should be tried is moving all transmitters further away from the receivers.
A quick test for intermodulation interference is to simply turn off other transmitters in the area one by one. Very often, it will be found that turning off a particular transmitter will completely eliminate the problem. In this case, it is very likely that the problem is due to intermodulation. It should also be noted that intermodulation works both ways; if a particular wireless transmitter interferes with a wireless receiver, it is likely that the second transmitter will also interfere with the receiver of the first system. If so, it is virtually certain that the problem is intermodulation.
When intermodulation is being caused by TV signals and other external sources, it might not be possible to turn off or relocate the sources. Sometimes the wireless receiver can be relocated to another position where one or both of the interfering signals are weaker, reducing the effects of the interference. This rarely cures the problem, but it is a good indication that the problem is due to intermodulation.
There are two general kinds of digital interference: that due to the “clock” oscillators in computers, peripherals and other digital devices, and that which is caused by the processing in a digital device. The first kind is just another form of direct interference that is caused by leakage of radio signals at the digital clock frequency or some multiple. The interference will generally take the form of a steady tone, usually fairly low in level, in the receiver audio.
The second kind of digital interference typically takes the form of a raspy or buzzing whine or tone, often with occasional changes in sound character. Frequently there is a distinct cadence or rhythm present. In some cases, digital interference can result in a high noise level on the audio, including hissing sounds and frying noises. There might not be any detectable tone involved. This kind of interference is caused by the operation of the internal processor (or digital signal processor) in the device. The interference is not necessarily at a multiple of the clock frequency; instead it can be related to the rate at which instructions are being processed. The level and character of the interference can vary greatly with the type and amount of processing occurring, and the interference can disappear entirely if the digital device is not actively processing.
Digital interference is usually relatively easy to identify. The signals involved are not strong and rarely affect wireless systems at distances greater than about 6 feet. The sound of digital interference is also relatively unique, so it is fairly easy to recognize. In addition, it is usually possible to test for interference from a specific device by entering new instructions, starting a new program or simply turning it off momentarily.
Digital interference is usually also relatively easy to resolve. Separating the digital device from the wireless receiver and its antennas, sometimes by as little as 8 inches will often cure the problem. It can also be helpful to separate the power and signal cables of the digital device from those of the wireless receiver. In a few cases, the interference is traveling up the power cable of the receiver and one or the other of the units will need to be powered from another source, or a power line filter installed.
If these simple measures are not effective, it is likely that the digital unit is defective in some way. Government regulations strictly limit the amount of stray radio energy that such equipment can produce; they cannot cause serious interference unless there is a significant defect. In such cases, it will almost always be necessary to correct the problem at the source, as changing the frequency of the wireless is often not effective. Of course, separating the wireless receiver and the digital unit by several feet and rerouting cables will generally solve even stubborn problems.
All phones that use the GSM standard, transmit on frequencies in either the 800 to 900 MHz or 1,800 to 1,900 MHz range, depending on the country and the carrier. They transmit data in RF energy bursts that are short but powerful. These bursts occur 217 times per second at power levels as high as 2 watts, depending on how far the phone is from the nearest cell tower. This 217 Hz “lightning bolt” can easily induce a ragged sounding noise (the now-familiar “dit di-dit di-dit di-dit”) into most audio equipment. The noise can invade at almost any point — at inputs or outputs, through a cable, or directly into a component on the circuit board.
Most of the time, GSM interference occurs when the phone is within just a few feet of an audio device. Audio equipment manufacturers are quickly finding that protecting their products from GSM noise requires extensive design changes — not just the addition of a component or two at the connector. Until such protection is universal, AV technicians need to keep GSM phones away from unbalanced audio lines, including lavaliere and headworn mics, hanging choir/ audience mics, and interconnect cables between equipment. The only instant sure-fire solution: Make presenters turn off their phones.
In an ideal world there would not be any reflections of the radio signals and the signal transmitted would just go to the receiver antenna and not also bounce off nearby surfaces and objects. In the real world the buildings we use create many radio frequency (RF) reflections and when those reflections combine with the direct signal from the transmitter they cause “multi-path” interference. At any given receive antenna location there will be some transmit locations where the reflections either totally or in part cancel out the direct signal. The good news is that diversity receivers with two antennas can almost eliminate the problem since it is highly unlikely that the signal cancels out at two different receive antenna locations. Again, proper spacing and placement of the antennas is critical.
If possible the receiver antennas should be located where there are no obstructions, including people, between the transmitter locations and the receive antennas. One way to achieve this is to locate the receive antennas higher than the transmitters so the antennas have a good line of sight to everywhere the transmitters may be used. The disadvantage of a higher receive antenna location is that the antennas may also pick up distant sources of interference better. Another good location for receive antennas is close to the transmitters. Even if antennas are mounted high and therefore more likely to pick up interference, if they are also closer to the transmitter locations the desired signal from the transmitters will be stronger and better able to override any interference.
This kind of interference is generated by electrical equipment, particularly neon signs, lighting controllers and dimmers, electric motors with brushes, motor speed controllers and various types of high-voltage equipment. In most instances, the interference is caused by equipment that is defective, worn-out, poorly maintained or incorrectly installed. Electrical equipment in good condition rarely creates interference problems. In a few cases, however, older equipment which might have been designed and manufactured with no thought to the interference it could cause can be troublesome.
Neon signs are a well-known cause of electrical interference. Properly constructed and installed signs generally do not generate significant interference unless they are near the wireless antennas. However, neon signs that are old, poorly maintained and whose insulating material is dirty or broken can produce heavy interference. Neon signs that are dimmed can be a particular problem. Defective fluorescent lights, including ones with worn-out tubes, can generate substantial interference. Fluorescent bulbs connected to dimmers, the dimmers themselves, and fixtures with electronic ballasts also are frequent sources of trouble.
Electrical motors with brushes are another common source of interference. This includes many appliances and tools, such as vacuum cleaners, blenders and food processors, electric drills, and tools with variable-speed motors. Motor and lighting controllers, especially older theatrical lighting control units, are frequently the cause of electrical interference. Worn-out or defective relays, particularly motor start relays and high-power contactors can cause interference during switching.
In addition to neon signs, other types of high-voltage equipment can be interference sources. These include worn-out or defective TV sets, X-ray equipment, certain kinds of lighting equipment and several kinds of industrial equipment. In most cases, the interference is generated by arcing due to leaky or defective insulation. Arcing can also be a problem with lower-voltage equipment. Loose contacts, broken wires, cracked insulation and other defects can not only cause serious interference, they can pose a significant safety hazard.
Electrical interference is usually fairly easy to recognize. Most forms cause distinctive buzzing noises at frequencies related to the 60 Hz power line frequency. Arcing has a characteristic frying or crackling sound that is usually readily identifiable. Variable-speed motors can be distinguished by their speed changes and their characteristic whining sound. Interference can often be related to some event such as a lighting change, operation of a specific piece of equipment or the flickering of a lamp. In fact, this is frequently one of the quickest ways to determine the cause of a problem.
When electrical interference occurs, the first step should be to look for the most likely sources. This includes neon signs, fluorescent lights that are flickering or unusually dim, lighting controllers, devices with variable-speed motors and high-voltage equipment. If this does not reveal the problem, the next step is to attempt to relate the interference to a specific event such as the operation of a certain machine (such as air conditioning equipment), use of a certain light or group of lights, or a particular activity. If possible, it will often help to turn off suspected equipment momentarily to see if the interference disappears.
It is possible that the source of the interference is external. In this case it might be possible to isolate the cause by observing activity in the vicinity, determining the time of day the interference occurs and looking for related events such as flashes of light or noises. If it appears relatively certain that the noise source is internal but it cannot be located, it might be wise to have an electrician do a thorough check of the building electrical system.
If the source of the interference is a neon sign, fluorescent lights, a noisy motor, high voltage arcing or some similar cause, it will almost certainly be necessary to correct the problem at the source. In other cases, it might be possible to simply repair or replace the defective unit causing the interference, relocate it further away from the wireless antennas, or arrange that it be turned off when the wireless is in use.
If the source of the interference cannot be identified, but it is not severe, there are a few steps that can be taken to reduce its effects. Increasing the amount of signal available to the wireless receiver will usually help it reject the outside interference. Move the receiver closer to the transmitter, or use remote antennas that can be positioned nearer the transmitter location. Do not bring the transmitter closer than approximately 15 feet to the receiver antennas, however, as this can increase the risk of intermodulation interference.
In some instances, interference enters the wireless receiver through the AC power lines. Quality surge arrestors with effective RF filters can sometimes stop interference in this situation. Note that most “surge suppressors” have little or no filtering; only deluxe models that are specified to include noise and RFI filters are likely to be really effective. Improving grounding, especially when the main power panel is a considerable distance away, is often of considerable help. More sophisticated solutions to interference on AC power lines include isolation transformers, power conditioners and certain types of uninterruptable power systems (UPS units).
Better equipment design and heightened concern about interference has made electrical interference much less common than in the past. When it does occur, defective equipment is now the cause in a majority of cases. Because of the potential fire and electrical safety hazards involved, electrical interference is worth investigating even if it does not present a serious problem for wireless systems.
Random interference, as the name implies, is a result of random high energy electrical events. Examples include lightning, arcing of power lines in the wind, sparking of the feeder lines for electric locomotives, arc welding and defective heavy electrical machinery. Auto sparkplug noise was once a major source of random interference, but modern ignition systems rarely cause any problems. This kind of interference appears as brief bursts of noise of various types, including lightning crashes, pops, clicks and buzzes. The electrical disturbance almost always occurs over a very broad range of radio frequencies.
It is rarely possible to correct this type of interference at the source. Frequency changes are normally ineffective because of the wide frequency range of the interference. However, this type of interference does fall off in intensity as frequency increases, so UHF wireless systems are generally less likely to be seriously affected. The quality of the wireless receiver is also a factor; more sophisticated receiver designs usually provide better rejection of most types of electrical interference.
There are other steps that can be taken to reduce the effects of random interference. Increasing the amount of signal available to the wireless receiver will help it reject the outside interference. Move the receiver closer to the transmitter, or use remote antennas that can be positioned nearer the transmitter location. Do not bring the transmitter closer than approximately 15 feet to the receiver antennas, however, as this can increase the risk of intermodulation interference.
In some cases, random interference enters the wireless receiver (or some other unit in the audio system) through the AC power lines. This can be in the form of radio frequency (RF) noise, or as electrical surges, voltage spikes or noise bursts. Quality surge arrestors with effective RF filters can greatly reduce the effects of random interference in many situations. In addition, they can protect equipment from damage caused by lightning and voltage surges. Note that most “surge suppressors” have little or no filtering; only deluxe models that are specified to include noise and RFI filters are likely to be really effective. More sophisticated solutions to interference on AC power lines include isolation transformers, power conditioners and certain types of uninterruptable power systems (UPS units).
-Kevin Lake