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Resolving Interference Problems

Interference is annoying and often frustrating, especially when the problems are intermittent or when it seems that a problem has been solved, only to have it return later. Efforts to correct interference problems are also often complicated by the fact that there are several kinds of interference and each requires a different approach and a different solution. Sometimes a "quick fix" will work, more often it does not. Unless the problem is very simple, the only really productive approach is an orderly and systematic one that first finds the cause of the specific problem, then applies appropriate corrective action. This section discusses Avoiding Basic Problems, Is It Really The Wireless?, and Identifying Interference.

Avoiding Basic Problems
Sometimes interference problems have very basic causes. To avoid wasting time on an easily correctable problem, check the following items before proceeding:

  • Make certain than no radio transmitters, including the system transmitter or those for other wireless systems, are allowed to come closer than approximately 10 to 15 feet (3 to 4.5 m) to the wireless receiving antennas. This can overload the receivers and increase the chances of interference.

  • Make certain not to allow receiver antennas to touch each other when arranging receivers. Make especially certain that the antennas from one receiver do not touch, or come too close to, those of another receiver. Try to provide at least 10 inches (25 cm) of separation between the antennas of any two receivers.

  • Make certain that all transmitters have good batteries. The low output voltage of weak batteries can cause some transmitters to generate harmful interference. If there is any doubt, install a new, fresh alkaline battery in all wireless transmitters.

  • If you have a "combination" system (handheld + body-pack) with two transmitters on the same frequency, or two wireless systems on the same frequency, make absolutely certain that both transmitters cannot be turned on at the same time.

  • Check the squelch control setting on the receiver. 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 the interference.

Is It Really The Wireless?
Not all noise in audio systems using wireless is due to interference, or even to the wireless equipment itself. Because interference is a known problem with wireless, it is common to blame it for unwanted audio noise. Sometimes however, the real problem is elsewhere. For example, AM radio stations can introduce noise into an audio system through the input circuits of the mixer or amplifiers. Because the interference is plainly a radio station, it is easy to jump to the conclusion that it comes from the wireless. Recognizing the actual source of the problem can help avoid fruitless efforts to correct a nonexistent wireless system problem.

The first step is to determine where the interference is entering the audio system. Try the following:

  • Make certain that it really is a wireless system at fault. Try momentarily turning off the wireless receivers and disconnecting the audio cables. If the problem is still present with the receivers off or with the cables disconnected, the trouble is almost certainly elsewhere in the audio system, not in the wireless.

  • Turn off all wireless transmitters and make certain that all receiver "signal" indicators go out. Listen to the audio system to see if the problem is still present. If it is, the trouble is probably not radio interference, but some type of non wireless interference [1]. If the "signal" indicators do not go out, there may be squelch problems [2].

If the interference is present only when the receiver's RF indicator is on, and the possibility that noise is getting into the audio system elsewhere has been eliminated, it is likely that the wireless is experiencing some type of external interference.

Identifying Interference
The next step is to determine what kind of interference is occurring. There are several kinds of interference, each with a different cause and each requiring a different solution. Unfortunately, actions intended to correct a problem can have some effect even when they do not address the real source of the trouble. This encourages wasting time and effort in the wrong area and can greatly increase the level of frustration. However, if the kind of interference actually occurring is known, efforts to correct the problem are much more likely to succeed.

The keys to identifying the kind of interference present are observing the receiver indications and carefully listening to the audio. A good set of headphones can be a very useful tool in isolating and analyzing interference problems. With headphones it is easy to listen to one wireless channel at a time, even while the system is in use. Unfortunately, a few interference problems only show up under live conditions, when all equipment is in use.

Read through the problem descriptions below to find the one that matches the symptoms that you are experiencing. Then click on the links to go to suggestions for correcting that particular problem.

  • If the main interference problem occurs only when the wireless system transmitter is turned off, first check to see if the receiver squelch control is set to the middle of its range or higher (see squelch problems [2]). If so, there may be a problem with low-level interference [3].

  • 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 [4] is a serious problem that must be corrected in order for the wireless system to be fully usable.

  • 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 [5].

  • 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 [5], non-wireless interference [1] 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 [6] might be the problem. Likely sources include computers, digital delays, effects processors, lighting controllers, and other equipment using microprocessors and digital signal processors (DSPs).

  • 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 [7] will generally affect all wireless systems at a location, not just one system.

  • One type of buzzing sound is actually due to interference from TV stations. If the buzzing sound changes substantially at more or less random intervals and does not seem to be caused by electrical equipment, the problem may be TV interference [8].

  • 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 [9] is quite rare but can be among the most difficult to resolve.

If none of the descriptions seems to quite match your particular problem, or the information is unclear, see Types of Interference. Your dealer may also be able to offer assistance and might be familiar with any unusual local conditions that could be affecting the problem. If your dealer is not able to assist, owners of Audio-Technica wireless equipment can obtain telephone support from the factory.

Non-Wireless Interference [1]
Audio systems can be affected by interference from AM radio stations, TV transmitters, 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. High-powered TV stations can cause problems at distances of two miles (3 km) or more. UHF TV stations are sometimes very powerful (500,000 watts or even more) and can cause interference as far away as four or five miles (6 to 8 km).

AM radio interference typically results in the presence of distorted speech or music in the audio. TV station interference usually takes the form of a buzzing sound that changes in intensity from time to time. AM interference is easy to track down to a particular station by just tuning an AM radio across the local stations. TV interference can be identified by watching local channels and listening for audible changes that coincide with TV scene changes. It is important to remember that the problem is with the audio equipment; it is highly unlikely that any defect at the radio or TV station is at fault.

Radar systems, including airport radar systems, military search and weather radars, sometimes affect audio systems. Radar interference usually takes the form of high-pitched 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 and TV interference, radar interference is likely only when the radar is fairly close, usually when no more than one to three miles (1.5 to 5 km) 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. Some equipment may lack adequate filtering on their input circuits. Usually the best information resource is the equipment manufacturer; most can provide at least some support.

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.

Audio equipment from different manufacturers, and even different models from the same manufacturer, can vary widely in their ability to reject interference. Sometimes the simplest and quickest solution to an interference problem is just to change out the troublesome item of equipment for one that is more robust. When the audio system will be located near high-powered transmitters or heavy electrical equipment, it is wise to consider the possibility of interference in planning equipment purchases. An arrangement that allows for exchange of equipment if trouble is encountered can help avoid resorting to lengthy and more expensive solutions.

Squelch Problems [2]
If the RF indicator on the receiver does not go out when the system transmitter is turned off, there might be low-level interference present. For the purposes of testing for the source of the interference, first mark the position of the squelch control on the receiver, then set the control to maximum (its fully clockwise position). If the signal indicator does not go out with the squelch control set to maximum, it is likely that serious interference is present.

If the signal indicator does go out at the maximum squelch setting, and there is still interference audible in the system, it is likely that the major problem is not in the wireless system. However, there might also be a less-serious low-level interference problem in the wireless. Do not forget to return the squelch control to its original position when this test is complete, otherwise the system range is likely to be considerably reduced.

Non-wireless interference [1] will usually still be audible when the receiver is squelched. Low-level interference [3] is relatively common when using wireless microphone systems, but is not necessarily a serious problem.

Low-level Interference [3]
Most radio frequencies are shared among many different users. Also, many types of electronic equipment, including TV sets, VCRs, 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 operate 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.

It is important to consider that the receiver is doing more or less what it was designed to do - receive a radio signal on a particular frequency. Because the wireless transmitter is turned off, the signal happens not to be the one desired, so it is considered "interference." There are at least three possible solutions to this problem: adjust the receiver squelch control until the weak signal no longer is received, use a wireless system that has special circuitry to reduce the possibility of unwanted audio, or operate in a way that avoids the problem.

Most Audio-Technica wireless receivers have a squelch adjustment control. As this control is turned up (clockwise) an increasingly stronger radio signal is required to open up the squelch circuits and let audio through. Usually, a point can be found where the weak interfering signal can no longer open the squelch circuits and the interference stops. Unfortunately, turning up the squelch control also will reduce the maximum range of the wireless system. This is because a higher or "tighter" squelch setting requires that a stronger radio signal be present for audio to be passed, including the signal from the system transmitter. To avoid reducing range, do not turn the squelch control any higher than absolutely necessary to eliminate interference.

Some wireless systems include special circuitry to reduce the chances that the receiver squelch circuits will let through unwanted audio. Although not foolproof, this special circuitry does greatly reduce the chances that unwanted noise will come through when the transmitter is turned off. However, the special circuitry can do nothing to reduce the effects of interference when the transmitter is on, and the audio might still be noisy or distorted.

It is possible to avoid the effects of low-level interference by simply muting the audio from the wireless receiver when the transmitter is not turned on. By first turning on the transmitter, then bringing up the audio, using the system, then muting the audio, and finally turning off the transmitter, low-level interference is unlikely to ever become a problem. In any case, it is always a good idea to mute the audio from a wireless system when it is not being used because there is always at least a slight chance of some type of interference.

Direct Interference [4]
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, the wireless system will have problems.

TV transmitters are very powerful and can affect wireless systems at distances of 100 miles (160 km) 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.

Surprisingly often, interference problems are due to another wireless transmitter at the location. This can be the other transmitter in a "combination" system that has both handheld and body-pack transmitters, or the transmitter from another wireless system. Often, the existence of the second wireless system is unknown, perhaps because it is in another room or 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 3,000 feet (900 m), 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 the location. It is also advisable to check the nearby area for other operating wireless systems, if this is feasible. The interfering wireless system could be in another part of the building or in a nearby building, perhaps even two or three blocks away.

It is also necessary to check for other wireless transmitters that are on frequencies near that of the system receiving interference. To be safe, it is best that any two wireless frequencies be at least 1 MHz apart. Depending upon the equipment type, it might be safe to use frequencies that are closer together. The best test is to simply turn off the other transmitter and see if the interference stops. If so, the frequencies are too close together. Keep in mind that the required frequency separation increases as the interfering transmitter comes closer to the receiver antennas.

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 more or less 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:

  • Reconfirm that it really is direct interference. If so, interference will almost always be present (in different forms) with the system transmitter either turned on or turned off.

  • Make certain that two wireless transmitters on the same frequency are not both turned on at the same time.

  • Check that the wireless frequency is not on a TV channel in local use.

  • Check for wireless systems that are within approximately 1 MHz of the frequency of the system experiencing interference.

  • Check for other obvious external sources, particularly a harmonic of an FM radio transmitter, cable TV systems and any type of radio transmitter.

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 [5]
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.

Generally, resolving intermodulation problems requires that one of the frequencies involved be changed. However, it is sometimes possible to change the operation of an audio system to avoid combinations of wireless transmitters that cause intermodulation, at least if all transmitters do not have to be on at the same time. Most types of intermodulation involve three frequencies: two transmit frequencies and one receive frequency. Changing any one of the three will correct the problem.

Digital Interference [6]
There are really 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 (2 m). 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 ( 20 cm), 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 (a few meters) and rerouting cables will generally solve even stubborn problems.

Electrical Interference [7]
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. Moving the receiver closer to the transmitter, or using remote antennas that can be positioned nearer the transmitter location, are often very helpful. Do not bring the transmitter closer than approximately 10 feet (3 m) 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.

TV Channel Interference [8]
Interference due to TV signals can sometimes seem similar to that caused by electrical systems. A TV picture transmission carries a strong 30 Hz signal for the vertical component of the picture. The 30 Hz signal is rich in harmonics and can sound much like many types of electrical interference [7], which occur at 60 Hz and harmonics (in the U.S. and some other countries). This can be confusing since TV channel interference is normally the result of intermodulation [5] or direct interference [4].

TV channel interference can usually be distinguished from electrical interference by the fact that the sound quality will change as the scene being transmitted changes. Switching from a dark scene to a bright scene will be particularly noticeable. It is often possible to determine which TV channel is causing the interference by listening to the interference while clicking through the local channels on a TV set.

Random Interference [9]
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. Moving the receiver closer to the transmitter, or using remote antennas that can be positioned nearer the transmitter location, are often very helpful. Do not bring the transmitter closer than approximately 10 feet (3 m) 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).

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