Over the last couple of years, users have enthusiastically embraced new communications products using GSM cell phone technology. The acceptance and growth of messaging capability has led to new user habits. With the introduction of multipurpose devices featuring Personal Digital Assistant, web access, instant messaging as well as voice capabilities, it is common for users to place their device on a conference table or podium for easy viewing and access. In fact, it is not uncommon to place or prop the devices close to or on a microphone located on a conference table or podium.

The GSM radio transmission protocol is a digital format, where the voice and media data is transmitted on a series of radio frequency pulses. In the GSM technology, the base pulse rate is 217 Hz. These pulses are very short, but also very intense. The intense radio frequency energy can couple a significant amount of interference into nearby audio electronics devices, such as microphones. Active electronic devices, such as transistors and integrated circuits, can detect this signal. The base repetitive pulse rate, along with harmonics, falls in the audio frequency range. The resulting “buzz” is very unique and recognizable, not to mention undesirable.

Shure engaged their development teams to redesign Microflex microphones to be highly resistant to radio frequency interference such as might be experienced with GSM cell phone technology. Because of the intensity of the interfering signal, advanced electromagnetic interference (EMI) techniques are employed in the new designs to provide robust shielding and RF filtering for the audio circuits. Design techniques include multilayer circuit boards for microphone preamplifiers, improved grounding techniques, tighter enclosures for improved shielding, as well as improved microphone cable and audio connectors. The combination of improved techniques, known as CommShield® Technology, is utilized to deliver Microflex microphones that are highly resistant to intense nearby radio signals. The new designs have been extensively tested in Shure’s RF laboratory.

The newly designed Microflex microphones are poised for the future. New product designs are now released and available. Product package labeling identifies the new designs with the phrases “RF Filtering” or “CommShield Technology for RF Filtering”.  Shure Easyflex microphones, now discontinued, also feature Commshield technology.

Please contact the Shure Applications Engineering Group (1-800-516-2525 prompts 2, then 1) with any questions or for additional assistance, or check out our FAQ database.

Registered trademark update: November 23, 2009

An RF scanner is an electronic device designed to pick up radio activity over a certain range of frequencies. A car radio is a type of RF scanner focused on a very specific band of frequencies, commonly known as AM and FM radio. The practical application of an RF scanner to wireless microphone systems is two-fold: 1) To search for clear frequencies in a new installation, or 2) to troubleshoot interference problems at an existing installation site. If the use of traveling frequencies (169-172 MHz) is planned, it is essential to scan for potential problems ahead of time due to the ever-changing nature of that frequency range. [For more information on the problems with traveling frequencies, contact Shure for the Applications Bulletin, Traveling Frequencies: No Longer a Trouble-Free Solution. ]

Most RF scanners are simple to operate, and offer a variety of methods for frequency scanning. The simplest method to check for interference is to enter the frequency of the wireless microphone system in question and listen for any audible signals. Remember, anything that is picked up by the scanner when the wireless transmitter is turned off is a potential source of interference for the wireless system. Certain types of interference will produce distinctive sounds. Paging systems produce a series of beeps and crackling noises. The video signal carrier of a television station is identified by a steady buzz, and the color signal is usually heard as a high frequency whine. Audio information is, of course, easily recognizable. Be sure to check adjacent frequencies as they can also cause interference even though they are not on the exact frequency of the wireless system.

This method is useful for checking one or two problem frequencies, but can be cumbersome when searching for clear frequencies at a new site. In these situations it is best to use the scanner’s search or scan function. This function will automatically scan through a preset or programmed range of frequencies, stopping wherever there is significant RF. Avoid these frequencies.

Some scanners offer the ability to search repeatedly over a certain range. This feature allows the scanner to catch intermittent transmissions missed on initial scans. To further refine this technique, a scanner can be connected to a voice-activated tape recorder and operated over an extended period of time, repeatedly scanning a particular frequency range. The tape recorder will activate whenever the scanner catches an active frequency. This is useful for catching intermittent interference on-site, instead of spending an entire day waiting for the problem to occur. Most scanners are equipped with a headphone output that can drive the record (or line) input of a recording device. Voice-activated (VOX) recorders are available from Radio Shack. See the below diagram illustrating the connection of a scanner to a tape recorder:

Revision: 2/5/2007

To make effective use of an RF scanner, be familiar with the concept of squelch. Most quality scanners are equipped with a squelch control which allows the user to eliminate unwanted background noise on normally inactive frequencies. Care must be taken when setting the squelch control. If squelch is set too high, significant sources of RF interference may be overlooked. If squelch is set too low, the scanner will frequently stop on background noise when using scan or search modes. To set the squelch control, select a known clear frequency, e.g., an unused local TV channel, and rotate the squelch control until the background noise disappears, then add just a bit more squelch for extra insurance.

When scanning for potential sources of interference, it is only necessary to search the frequency ranges where wireless microphone systems are assigned. The most common (and most crowded) frequency range for wireless microphone use is high-band VHF, which extends from 174 to 216 MHz. The “traveling” frequency range is 169 to 172 MHz. The spread of UHF frequencies is much wider, from 470 to 806 MHz, and can take a long time to scan. If you are considering a UHF system, first determine the operating range of the wireless system in question and scan only those frequencies. The UHF band is much less congested than VHF, making it easier to locate unused frequencies. Frequencies outside of those used by wireless microphones do not normally cause problems. Commercial AM and FM radio bands, for example, are well below the VHF band and are unlikely to cause interference. However, occasional interference can occur in cases of extreme proximity (less than a mile) to a high-power radio transmitter.

RF scanners are not difficult to find. Several electronics catalogs carry scanners, as well as Radio Shack. For information on where to purchase scanners, contact the manufacturers directly. In addition to Radio Shack, contact AOR (www.aorja.com) through their US distributor, Electronics Distributors Corp. at 703-938-8105, or for Bearcat scanners (www.uniden.com) call the Uniden Answer Line at 900-225-4822. Expect to spend between $300 to $400 for a quality scanner. The single most important feature in determining which scanner to purchase is what frequency ranges it covers. Many scanners exclude the high-band VHF range, which makes them useless for VHF wireless applications. Be certain that the wireless microphone frequencies mentioned in this bulletin are covered by your choice of RF scanner. Other useful scanner features include a good squelch control, the ability to limit the scan range, and programmability. This last feature allows a variety of frequency ranges to be saved in memory and recalled at any time. Finally, a headphone output or line output is useful for aural recording of RF interference. An investment in a scanner with these basic features will prove invaluable for all wireless microphone installations.

Distance Old

20 X log (   ————    )

Distance New

Example: How much will the gain of the sound system increase if mic is moved from 12 inches from talker to 3 inches from talker?

12/3 = 4

log 4 = 0.6

0.6 x 20 = 12 dB of gain increase

Below is a number of open mics formula from Shure Applications Engineering:

NumberOpenMicsOld

10 X log ( ————————- )

NumberOpenMicsNew

Example: How much will the gain of the sound system decrease if the number of open mics goes from 4 to 9?

4/9 = 0.44

log 0.44 = – 0.35

-0.35 x 10 = -3.5 dB of gain change

For a steady state sine wave tone, the difference between the average level (VU) and the peak level (PPM) is about 3 dB. But for a complex audio signal (speech or music), the difference between the average level (VU) and the peak level (PPM) can be 10 to 12 dB! This difference between the reading of a VU meter and a PPM is known as the crest factor.

A VU meter and PPM also have different ballistics (acceleration/deceleration rates). If a 1kHz steady state tone is fed into a VU meter, it takes 300 milliseconds (0.300 seconds) for the meter to stabilize. However, the PPM stabilizes within 10 milliseconds (0.010 seconds). As the VU meter displays an average volume of the audio signal, it must “sample” the audio signal over a longer time period than the PPM.

Because of the crest factor and the difference in ballistics, a VU meter and a PPM will display the same speech/music audio signal in very different ways. Therefore, using a steady state tone to line up a VU meter with a PPM is not effective unless these differences are taken into consideration. Analogy: A mini-van (VU) and a sport car (PPM) will cruise side by side at a constant 60 MPH (steady state tone). But they will not cruise side by side if each vehicle accelerates to 100 MPH and brakes to 20 MPH many times and as quickly as possible (speech/music signal). Though both vehicles are performing the same function, the location of each vehicle (position of each meter indicator) will be very different.

The VU meter closely corresponds to the level sensing mechanism of the human ear. It provides a useful indication of the subjective loudness of different programs and is very useful when matching levels between programs. But the VU meter does not give an accurate indication of peak signal levels because of its relatively slow ballistics. In practice, a VU meter will under-indicate the peak signal level by 8 to 20 dB.

Here is a rule of thumb when using a steady state tone to align a VU meter with a PPM. When the VU meter indicates “0″ (typically a +4 dBm level), the PPM should be set to read 20 dB below its maximum full scale reading. For example, when the VU meter of a Shure FP mixer reads “0″, the PPM on a Sony Beta Cam with a “+12″ full scale reading should be set to read at “-8″. Like any rule of thumb, this one may vary depending on the actual specifications of the products in use.

Note: Meters marked with the symbols “VU” or “PPM” may not actually meet the international standards for such meters. The best advice is to listen critically while recording and not rely solely on meter readings.

For additional reading:

Ballou, Glen; “VU and Volume-Indicator Meters and Devices”; Handbook for Audio Engineers; Howard Sams & Co., Indianapolis, IN; ISBN 0-672-21983-2

Sound System Equipment – Part 10: “Methods for specifying and measuring the characteristics of peak program level meters”; BS6840: British Standards Inst., 2 Park St., London W1A 2BS

Phantom power is a dc voltage (11 – 48 volts) which powers the preamplifier of a condenser microphone. Phantom power is normally supplied by the microphone mixer, but may also be supplied by a separate phantom power supply. Phantom requires a balanced circuit in which XLR pins 2 and 3 carry the same dc voltage relative to pin 1. So if a mixer supplies 48 volts of phantom, XLR pins 2 and 3 of the microphone cable each carry 48 volts dc relative to pin 1. Of course, the mic cable carries the audio signal as well as the phantom voltage.

Mixers that supply phantom power contain current limiting resistors which act as control valves. If the microphone or cable is improperly wired, these resistors limit the flow of current to the microphone and thereby prevent damage to the phantom supply circuit.

A balanced dynamic microphone is not affected by phantom power. However, an unbalanced dynamic microphone will be affected. Although the microphone will probably not be damaged, it will not work properly.

Bias is a dc voltage (1.5 – 9 volts typically) that is provided on a single conductor.

Unlike phantom power, bias does not require a balanced circuit. Bias supplies power to a Junction Field Effect Transistor (JFET) connected to the output of an electret condenser mic element. The JFET acts as an impedance converter which is a necessity in any microphone design that uses a condenser element. A condenser element has a high output impedance (>1,000,000 ohms). The JFET input loads the output of the condenser element with an even higher impedance (>10,000,000 ohms) to minimize loss of signal level. Also, the JFET output provides a low source impedance <1,000 ohms> to feed the microphone preamplifier.

In some condenser microphones, the bias voltage must be supplied on the same conductor as the audio. Condenser elements with a built in JFET use this configuration and employ a single conductor, shielded cable. Other condenser microphones utilize separate conductors for bias and for audio. Consult the manufacturer’s data sheet to find out the exact wiring configuration.

A dynamic microphone should not be connected to an input that supplies bias voltage (such as a wireless transmitter) because the audio and the bias voltage will travel down the same conductor. If this occurs, the frequency response of the microphone may be altered or the audio signal distorted. If a dynamic microphone must to be connected to an input with bias voltage, a blocking capacitor must be used. The blocking capacitor is placed in series with the hot conductor of the microphone. The capacitor passes the audio that is present on the hot conductor while blocking the dc bias voltage. The capacitor must have enough capacitance to pass the audio signal without degradation. The exact value depends upon the electronic characteristics of the microphone circuit and must be calculated for each situation.

Remember, in a typical electret condenser microphone, it is the JFET that requires unbalanced bias and the preamplifier that requires balanced phantom power. Therefore, a condenser microphone that requires phantom power will not work with an input that only supplies bias, e.g. a wireless transmitter.

Once again: phantom and bias are not interchangeable!