Shure Notes for Houses of Worship #31

"The fact is, we’re already dealing with the DTV stations – both the analog and DTV stations that are presently on the air."

"Radio is Voodoo to most of us. You can’t see it, you can’t feel it …. it’s mysterious."

"Anyone who has ever compared a UHF system to a VHF system has noticed that UHF antennas are shorter than VHF antennas."

"The ‘white spaces’ are not for sale to anyone."

"The fact is, there are no frequencies that are completely free from interference because there are no frequencies that are reserved for wireless microphones."

"There is no antenna array and no requirement at all that the cables or the antennas be matched in any way."

"Best advice: Use the same brand and the same series."

700MHz range

The ‘700MHz range’ is actually comprised of 698-806 MHz and was formerly occupied by TV channels 52-69. Shure no longer manufactures wireless microphone or personal monitor systems designed to operate in this frequency range, but some older gear such as UC, UHF, PSM®400 and PSM®700 (UA, UB, or HF bands) may become problematic over the course of the next several years. (See Product Spotlight in this issue for rebate/trade in program)

"All manufacturers share the same common frequencies; none own a specific frequency band."

Print this article

MYTHBUSTERS:

THE TRUTH ABOUT WIRELESS

About a year ago, we devoted an entire Shure Notes issue to commonly held myths about pro audio. A reader favorite, we decided that in the flurry of misinformation about wireless systems (especially as it relates to the ‘700 MHz’ band), another Mythbusters feature was in order.

Once again, we turned to Shure Associates Tim Vear, Senior Applications Engineer and Mark Brunner, Senior Director, Public and Industry Relations, for the dynamite we needed to explode several of those most often heard by Shure’s Applications Engineering Group.

Cell phones always interfere with wireless microphones.

FALSE.

There is no direct radio interference from cell phones or wireless microphones or in-ear personal monitors because they operate on completely different radio frequency ranges. There can’t be any direct interference between these things.

However, there is a characteristic noise that can be created by GSM mobile devices which include Blackberry, PDA devices and some phones that can generate audible noises in audio systems in general through microphone lines – and occasionally in wireless microphones specifically lapel or headset microphones where the exposed bit of wire that connects the mic to the bodypack transmitter can pick up this noise from the GSM device. The result is that characteristic chirping noise that you hear in your computer speakers when you’re about to get a phone call.

It sounds like this.

Play

It doesn’t enter the wireless radio circuitry. It actually gets into the audio cable and is demodulated into an audible noise.

And here’s a related myth:

Cell towers cause interference with wireless systems.

Also FALSE.

We know churches that have had their bell towers built by mobile phone companies. These bell towers are located near the sanctuary — on church property. These are disguised cell phone towers and they’ve been around for years. If churches – heavy users of wireless systems – were experiencing interference, these wouldn’t exist.

DTV stations will cause new problems with wireless systems after February 17, 2009.

FALSE.

There won’t be any new problems. The fact is, we’re already dealing with the DTV stations – both the analog and DTV stations that are presently on the air. Wireless systems have to avoid both of these types of stations and we’ve been doing that for years.

The only thing that may be a slight hiccup after February 17th is that a handful of the DTV stations are presently operating above Channel 51 will have to move down to the below 52 range. There are going to be some cases in some cities where a range that is presently unoccupied will have a TV station occupying it after February 17th. But that’s not common.

It’s worth staying up to date Shure’s our online Frequency Finder that will reflect any new assignments. The FCC also has a similar tool.

It’s a bad idea to use wireless systems near airports or military bases.

FALSE.

The question comes up when users are having problems and are concerned that there’s an airport or a military base within several miles. The implication is that they are using some frequencies that are the same as those used by wireless systems.

It’s not true. The frequencies used by aircraft communications, radars and military operations are not in the broadcast band. There’s no direct interference from any of these sources. This is NOT Spinal Tap.

Radio is Voodoo to most of us. You can’t see it, you can’t feel it …. it’s mysterious. So there’s a general sense that any kind of radio device can interfere with any other kind of radio device – a cell phone, a walkie-talkie, a remote controlled toy, a garage door opener, a remote keyless entry. But they all operate on different frequencies and they don’t talk to each other. That’s the missing link – understanding frequency bands.

The larger the antenna, the better the reception.

FALSE.

The size of the antenna is directly related to the wavelength of the radio signal that you are trying to transmit or receive. If you are trying to operate on a specific frequency, then the proper antenna is a specific length.

It may be a quarter-wave antenna or a half-wave antenna. If you could actually see the radio wave in space, a UHF radio wave would be approximately a foot and a half long. Roughly. And so a quarter wave antenna would be roughly 4 or 5 inches long. A half-wave would be 8 or 9 inches. If you made that half-wave antenna 12 inches long, it would actually decrease the performance of the antenna because it’s no longer matched to the wave. What is actually going on is that the antenna becomes a resonant system for a specific frequency and that’s where it has its maximum sensitivity.

Anyone who has ever compared a UHF system to a VHF system has noticed that UHF antennas are shorter than VHF antennas. Here’s why: UHF frequencies are much higher than VHF frequencies, so the wavelengths are much shorter. The higher the frequency, the shorter the wavelength and the lower the frequency the longer the wavelength. That’s why the ‘rabbit ears’ are designed for VHF stations.

Mobile phones have short antennas because they operate on much higher frequencies than our wireless microphone systems. Antennas get progressively shorter as the frequencies get higher; that’s why big antennas are terrible for UHF systems.

The FCC is selling the “white spaces” to leading search engines, service providers and software giants.

FALSE.

The white spaces are the unused television channels in any local area and although the television band is now smaller than it used to be – instead of TV channels 2-69, it is presently TV channels 2-51. There are still plenty of open television channels in the range and that’s what is being called the ‘white spaces’.

The white spaces are not for sale to anyone. Wireless microphones have operated here in the past and will in the future, and they can be tuned to any of the open channels. The FCC is considering allowing broadband Internet access devices to operate in these spaces, but the rules for those devices are still being drafted.

There are some wireless microphone frequencies that are completely free from interference.

FALSE.

This is a myth that is being propagated by some pro audio manufacturers. The fact is, there are no frequencies that are completely free from interference because there are no frequencies that are reserved for wireless microphones. Even if there were, you would still have interference from other wireless microphones occupying that frequency band.

There are no “safe frequencies”. All of the radio spectrum is allocated for different uses by different types of equipment. Every wireless microphone operates in a frequency range that contains other operations. There is no exclusivity in the radio spectrum for wireless microphones.

The best advice – use equipment that is as broadly tunable as possible.

It is essential to use exactly the same length antenna cables for the two antennas of a diversity wireless receiver.

FALSE.

This myth arises from users who have had direct or indirect experience with CB radio equipment — installing antennas on vehicles where in order to get a high-powered transmission capability forward or behind the vehicle, they had to install two antennas on either side of the truck. Those antennas were coupled to the same radio; the cables and the antennas had to be perfectly matched, so that the antenna array had a very well defined pattern.

The antennas in a wireless microphone system are never really mixed together. One antenna goes to the A side of the diversity receiver and the other goes to the B side. There is no antenna array and no requirement at all that the cables or the antennas be matched in any way.

You can use the combination of an omnidirectional antenna and a directional antenna, a quarter-wave antenna with a half-wave antenna. The antenna cables can be different lengths. You can mount one on a receiver and one on a 50-foot cable. There’s no requirement for any kind of symmetry in the arrangement.

I can use any transmitter with any receiver as long as the frequencies match up.

FALSE.

This is the result of confusion with other radio system characteristics that do need to match up.

Yes, you can tune a receiver to a transmitter, but they still may not function properly if the squelch systems on those units are different. Even though the receiver is picking up a radio frequency from that transmitter, you may not hear any audio because they use a different tone key squelch. The receiver just won’t respond to that transmitter.

DEFINITIONS

Companding
“Companding” is an audio signal processing technique typically used to extend the dynamic range of wireless systems. It is a two-part process: the audio signal level is first compressed by a certain ratio in the transmitter, then it is expanded by the same ratio in the receiver. The companding process may be accomplished by analog or digital methods. The word itself is an amalgam of “comp(press)” and “(ex)pand”.

Tone Squelch
“Squelch” is a radio industry term that refers to various methods for muting the audio output of a receiver in the absence of a usable signal. When the transmitted radio signal is weak or lost at the receiver, the result may be a very loud audio noise output. Simple squelch circuits mute the receiver output when the received radio signal strength falls below a certain threshold. More advanced squelch circuits (sometimes called “noise” squelch) measure the audio noise level and can act before the actual noise level becomes objectionable. Finally, tone squelch (sometimes called tonekey squelch) circuits are triggered by the absence of an identifying pilot tone that is normally sent by certain transmitters. This insures that a corresponding receiver will be squelched (muted) even in the presence of a strong interfering radio signal if it does not contain the proper pilot tone. In addition, the pilot tone can be used to send data (such as battery voltage) from a transmitter to a suitably designed receiver.

The other problem is that even if you can get around the squelch problem and get both to respond, the companding systems may not be compatible. And that’s not only a problem between different manufacturers; it’s a problem within the same brand and model lines.

In the case of components with different audio schemes the sound may be seriously compromised. Depending on the type of mismatch, one result may be a muffled sound with no high frequency response or an overly bright sound with too much high end because the pre-emphasis and de-emphasis curves aren’t compatible. In addition, the dynamic response may result in a “squashed” sound or a very “jumpy” sound because the compression and expansion schemes don’t jive.

Best advice: Use the same brand and the same series.

I can’t use a wireless system in the 700MHz range anymore.

FALSE at present.

Likely TRUE in the future.
There is a proposal from the FCC that disallows the use of wireless systems in the 700 MHz range, potentially as early as February 17, 2009. It has not yet been mandated, but it specifies that equipment in this range not be used after the DTV transition date. That could mean effective February 17, 2009 or over a specified transition period.

Because new users will eventually occupy this range - telecommunications operations and public safety operations – equipment in this range may become unusable over time.

I don’t need to worry about interference between Shure wireless systems and other manufacturer’s wireless systems because they each have their own unique frequencies.

FALSE.

This is still a common myth.

Some users believe that if they have equipment from Shure and another manufacturer, they needn’t be concerned with frequency interference or frequency coordination because each manufacturer has its own special frequency.

This is untrue. All manufacturers share the same common frequencies; none own a specific frequency band. When coordinating multiple wireless systems, you need to be aware of the actual frequencies, not the name of the manufacturer.

A good frequency-agile system will tune over the widest possible spectrum of the UHF band. The wider the tuning range, the more likely that the user will be able to find clear frequencies to use. For example, the least expensive systems tune over a narrow band of 12 MHz. The most advanced can tune over a range of 60 MHz or more, so there’s a clear progression of tuning ranges as you move up the food chain – that’s especially important for customers who are using multiple wireless systems or are touring.

RESOURCE ROOM

Here are some helpful places on the Shure site where you can find more comprehensive information on:

White Spaces Overview
Includes a general discussion, a “What You Need to Know” podcast and industry comments to the FCC.

Wireless Frequency Finder
Enter your zip code and wireless series, then choose channels or channels and frequencies to identify available bands.