Technology Showcase: Wireless Systems
May 1, 2004 12:00 PM, By Bennett Liles
Wireless microphone systems have come a long way since the receivers looked like a pair of saddlebags, used oddball batteries only available from the manufacturer, and worked pretty well as long as nothing moved. A number of specific advances have opened the world of wireless mic systems to a vast professional and consumer market.
Probably the most widely implemented and obvious modern feature is diversity reception. In its various versions, diversity reception has given wireless microphone systems the reliability they needed to break into widespread use. New squelching techniques have also boosted reliability and performance, extending range and cutting the incidence of obnoxious noise bursts. Pilot tone muting has eliminated the problem of having a local broadcast station suddenly blasting out of the receiver when the microphone transmitter is turned off. Most of the current systems transmit at a maximum RF power of 50 mW. Transmitters have sprouted into many useful tools from electric guitar pickups to snap-on conversion units for handheld, hard line microphones. With the modern variety has also come a need to match the specific features and specs for each system to its planned application. Here is a look at some features and how they work.
Certainly the most marketed term in this field, diversity reception, has several forms, and the manufacturers have given those forms several names. The main issues are cost and size versus performance. The simplest form of diversity reception uses just two antennas and a passive combiner, essentially a splitter turned around backward to feed the front end of a single receiver. The purpose of diversity reception is to prevent dropouts, and in this the simple use of two receiving antennas provides no real advantage. It can even make things worse than with a single antenna because if the signal is strong but out of phase between them, each antenna can actually cancel the other one out.
Another type of diversity reception uses phase detection and RF switching to select the best antenna signal at any given moment, and that is where meaningful results begin. Once the signal begins to deteriorate, a phase-switching circuit samples the RF output level and reverses phase on one antenna if that produces a stronger combined signal. The simplicity of this method has given it a wide presence in field systems in which compact size and low weight are important considerations. The trade-off is that the RF signal has to deteriorate before any corrective action is taken. Also, the RF switching, when not coordinated with a squelch circuit, can produce pops and clicks in the audio output, and the RF phase switching can momentarily make things even worse until the circuitry senses this and reverses one antenna's phase again. There is a variation on this scheme that coordinates the phase switching with the squelch operation so that the switching is not heard.
One of the most widely encountered terms in this area is true diversity. One hallmark of so-called true diversity systems is that the switching between signals is done at the audio level while the sensing is still made on the RF signal. That is more potent in combating dropouts but adds little to the maximum operating range. Requiring more physical space than simpler, single receiver methods, audio switching is usually found in studio rackmount installations where space and weight considerations are not the paramount concern. For quiet switching that momentarily has the stronger signal, the switch occurs at the zero crossing point of the audio signal while the voltage level is at minimum. The main enemy of this arrangement is RF interference that can outsmart the RF sensing into selecting the wrong audio signal. To maximize success with audio-switching wireless systems, high selectivity is required on each receiver. The spec is rated in decibels, and higher numbers are better. It is also vital to have both audio outputs carefully matched in level so that the audio switching is not heard. If the audio levels vary by 3 dB or more, the switch will be obvious.
The ratio diversity or panning diversity system combats the problem of detectible audio level switching by incorporating a panning circuit into the audio-switching process. The audio output of both receivers is used simultaneously but proportionally. A ratio diversity system is designed to smoothly hang on until the last gasp before squelching out on a low RF signal. This characteristic makes them a strong choice for permanent installations in challenging environments exhibiting multipath interference.
All these diversity schemes benefit from high receiver sensitivity and proper squelch levels for the situation. Some squelch systems operate on the received RF level alone and mute at a preset threshold. This point is optimally just above the level of background RF noise in the local area. However, if strong interference is present, the system can be fooled into allowing the unwanted signal into the receiver. This problem spawned a feature known as pilot tone muting, in which a higher-than-audible tone is broadcast along with the microphone signal. The tone activates the front end of the receiver, and it passes the audio signal. If the pilot tone disappears rather than receiving noise, the tone-coded receiver is muted. Some systems combine the pilot tone with high-frequency noise level measurement to sort out interference. The best that any of these squelch techniques can accomplish is muting the sound output during poor reception. Better reception relies on proper antenna placement, impedance matching, and elimination of interfering signals.
If problems persist with a receiver having two antennas, better results can often be attained by separating one antenna and mounting it externally through a coaxial cable connection. Often, particularly at VHF frequencies, the antennas as mounted directly on the receiver are too close together to get a diversified signal. Ideally, they should be mounted more than half a wavelength apart. In VHF this distance would be from five to six and a half feet. For UHF the optimum spread is over 20 inches. Impedance matching is critical with the miniscule RF levels from the antenna. This element, whether passive or with internal RF amplification, typically has a 50ž output and should be connected with 50ž coaxial cable such as RG-58 for maximum energy transfer.
Proper polarization certainly helps reception, and that means having both transmitter and receiver antennas oriented vertically. Placing a body-pack transmitter horizontally on a podium or table can attenuate the received RF signal by as much as 50 percent. Placing the transmitter against a human body, as is usually the case, also dampens the transmitter's output. Most handheld microphones and clip-on transmitters incorporate internal antennas that are electrically loaded to behave as a longer wire. In field applications in which durability and compact size are a plus, helical wire encased in a rubber-ducky antenna is used, though it offers a more limited bandwidth. The most efficient and widely used design is the quarter-wave whip. A single pair of these antennas can serve as many as three or four receivers, but if more than that is needed, an active antenna divider is used. That will maintain proper levels and impedance over a larger number of RF outputs. Ground plane antennas will include four elements angled 45 degrees down at the base of the vertical element and offer more gain than a simple quarter-wave whip. Log periodic dipole arrays may take the shape of a vertical TV antenna with director and reflector elements, or it can be enclosed in a plastic shark fin unit that mounts to a wall or mic stand. These also have a somewhat more directional pickup pattern and more gain than the quarter-wave whip.
Companding operation and high-frequency audio response are two audio aspects to judge in a wireless microphone system. Many transmitter designs have companding characteristics that are not user-adjustable. A good evaluation of this feature involves turning up the monitor volume on the system until amplification noise can be heard. Then a loud sound such as a hand clap is made near the microphone. How fast does the noise level go down, and how long does it take to return to the previous level? The faster the attack and release, the less noticeable the companding will be. If car keys are jangled near the microphone, does the audio output sound like keys or does it more resemble footsteps in a bed of gravel or crackling flames? If the highs sound distorted, they will come through on the sibilant sounds of a human voice.
An understanding of the basics in wireless microphone operation and a few handy tests will help a great deal in sorting out the best system for any application.
Among its vast array of wireless products, AKG Acoustics offers the WMS 4000 wireless microphone system. It consists of the HT-4000 handheld transmitter with a versatile provision for three dynamic and three condenser AKG mic heads using a quarter-wave antenna. The backlit display includes battery condition, channel view, and mode selection. The PT-4000 body-pack transmitter allows input of both mic and line-level sources, a very smart feature. Users can expect to have about 50 UHF channels available. The PT-4000 and HT-4000 transmitters will operate for about 15 hours on two AA batteries. The SR-4000 receiver sports pilot tone muting and true diversity with separate antenna, receiver, and demodulator sections. A data port allows for PC monitoring of receiver status.
The Audix RAD-360 UHF wireless system features 193 selectable frequencies and a dual tuner, true diversity receiver operating in the UHF band between 638 and 806 MHz. The handheld transmitters feature OM-series dynamic microphones and have a gain setting control that will enable a -10, -20, or -30 dB pad. The body-pack transmitter, constructed of ABS composite, is housed in a metal cradle. Mic/line input adjustments are provided, as well as an additional 20 dB sensitivity/gain control. An optional amplified antenna distribution system (ADS-4) is available, allowing as many as four systems to be run off a single pair of antennas and four systems to run off of one DC power supply.
Audio-Technica U.S. markets its 7000 Series professional wireless system, a versatile product that includes the ATW T-75 body-pack transmitter, the ATW T-76 hand-held dynamic mic transmitter, and the ATW T-73 condenser microphone transmitter. All operate on 9V batteries in the UHF band on 100 available channels. Each transmitter features a user-adjustable modulation level that can be set with a small tweaker tool that is included. The rackmountable receiver for these units features XLR and quarter-inch output connectors at different audio levels, and these can be used simultaneously to drive mixers and guitar amplifiers. A wide variety of accessories includes different color and style microphones, receiver antenna models, clamps, adapters, and windscreens.
Azden Corp.'s Performance Series UHF wireless microphone system includes the 41HT and 41HT3 handheld transmitters, the 41BT body-pack transmitter, the 41XT plug-on transmitter, and the 41UDR and 41DRH receivers. The 41HT and 41HT3 operate on any of 63 UHF frequencies and are powered with alkaline AA batteries that should last for six to eight hours. The 41XT plug-on transmitter uses a single 9V battery. The transmitters offer a selection of nine channels within each of seven frequency groups. The power LED will change from green to red when battery power is low. Azden's 511DRH receiver has twin LED indicators that indicate antenna reception and signal strength as well as LED indicators for audio signal strength. The receiver has channel selection and power on/off mounted on the front panel, and the rear panel has the XLR balanced and ¼-inch unbalanced output connectors, a variable squelch control, a variable output level control, and two BNC antenna connectors.
Califone International offers a system that combines low price and simple operation. The R1000/Q1000 system features the single-channel VHF R1000 receiver and the Q-1000 handheld wireless microphone. The R2000/Q2000 system offers a selection of two VHF frequencies and includes the Q-2000 dual-channel handheld mic and the M-1000 belt-pack transmitter for use with the MC-16 lapel mic. The receiver is rackmountable individually or in pairs.
Electro-Voice offers the RE-2 UHF wireless microphone system featuring Auto-ClearScan. This feature scans the available channels and displays them for manual selection or automatically selects the clearest available channel. That could be a useful feature in environments in which many RF microphone systems are in use. The receiver has a switched mic or line-level XLR output and a line-level, ¼-inch output connector. On the CSR-1000 receiver, Posi-Phase diversity reception uses both antennas simultaneously and makes continuous relative phase adjustments between the two. The CSH-1000 handheld transmitter has gain control, channel selection, and a setup menu display and incorporates an internal half-wave antenna. The CSB-1000 body-pack transmitter has a detachable quarter-wave antenna, and full setup display and will continue operation for about eight hours on a single 9V alkaline battery.
Lectrosonics has introduced a digital wireless microphone system with encryption technology. The UM700 encrypted digital wireless transmitter features 256 synthesized UHF frequencies, an input limiter, and manual adjustment of low frequency rolloff from 35 Hz to 180 Hz in a machined aluminum case. The unit also has a 5-pin connector that can accept line- and mic-level sources and transmits the signal with a detachable quarter-wave antenna. The operational frequency is set with two small rotary dials on the case. The UDR700 digital diversity receiver keeps both antenna signals reinforcing rather than canceling each other by performing continuous relative phase adjustments. The audio output section has a variable attenuator and phase reverse switch. The 128-bit encryption key is selected by connecting the transmitter and receiver with a short cable, and the receiver senses and times button pushes during the setup and locks itself and the transmitter to the same key without the operator knowing what the key is. Several transmitters can be set up to use the same key. The key is never transmitted between the two except through the connecting cable at setup. This feature is valuable when used in a training environment in which competitors could be trying to electronically eavesdrop.
Among many other wireless products, Lightspeed Technologies offers its 9000DX Quad RAK dual diversity UHF wireless microphone system. This is a 1 RU rackmount chassis containing from one to four dual tuner receivers and an integral antenna divider. It has 1,000 selectable frequencies in five groups, dual band companding, and a maximum signal-to-noise ratio of 105 dB. The BP-800 body pack and HM-800 handheld transmitters have a maximum RF output level of 50 mW and use a pilot tone muting system described as tone squelch. Each should provide about eight hours of continuous operation on two AA batteries.
Mipro Electronics' ACT Series of wireless microphone systems includes the ACT 707S single-channel UHF true diversity receiver, the ACT 707H handheld wireless transmitter, and the ACT 707T body-pack wireless transmitter. The ACT 707 four-channel receiver is housed in a 1 RU metal chassis and features PLL synthesized true diversity reception with pilotone muting and NoiseLock dual squelch for maximum range. Color LEDs indicate RF and audio signals, diversity metering, and battery life. Displays include group, channel, frequency, performer's name, squelch level, mute off/on and address. These displays allow manual channel and frequency settings, but there is also an automatic channel-scan feature that will seek out and lock to interference-free channels. The receiver's Automatic Channel Targeting button quickly locks the transmitter to the same frequency. The system operates in the range of 629 through 950 MHz. The ACT 707T body-pack transmitter features the same LED indicators and the handheld mic and contains an impedance switch for operation with microphone or electric guitar and uses an RF output power of 10 mW. The system will interface with a PC for complete control of as many as 64 channels.
Nady Systems has introduced its new UHF-16 PLL synthesized wireless mic system featuring 16 user-selectable frequencies within several 25 MHz bands in the range of 720 through 865 MHz. The matching microphone products include the UB16 body-pack transmitter and the UH-16 handheld transmitter. The UB-16 is adaptable as a rackmount installation or as a standalone unit for mobile setups. The backlit front-panel display indicates RF and audio parameters and battery level. The UB-16 body-pack transmitter has three-way input switching for use with a lapel mic, a guitar output, or a head-worn mic and includes phantom powering in the microphone switch positions. The UH-16 and UB-16 operate on two AA batteries and have Off/Standby/On switches, Up/Down channel selector buttons, and low-battery LED indicators.
Peavey has the Pro Comm PCX U-302, 30-channel UHF wireless microphone system, a 900 MHz system featuring frequency tracking and muting circuit, RF signal and audio level indicators, balanced and unbalanced outputs, a battery level indicator, volume control, and frontmount antennas. The product can be purchased as a body pack/receiver or handheld/receiver.
Sabine offers its True Mobility UHF SWM 3000 wireless microphone system with 30 PLL synthesized channels, true diversity reception, and dual squelch operation. The receiver includes a dual 9V NIMH battery charger, RF and audio signal meters, balanced and unbalanced outputs, adjustable compressor and de-esser, and Sabine's FBX Feedback Exterminator, which is also available as a separate product. The lapel mic/receiver combination is available as the SWM 3000LX, and the handheld combination is marketed as the SWM 3000H. The receiver is designated as the SW30-R. Sabine also offers the SWM 7000 system (see the April 2004 issue for a review).
Samson Technologies Corp. has achieved a breakthrough in microsizing the UHF wireless transmitter/receiver with the introduction of the Airline wireless system. The AR1 microreceiver operates in the 801 through 805 MHz range and can function for as long as 14 hours on one AAA battery. The receiver has RF/power-on and audio clipping LED indicators, volume control, a balanced XLR output, and an unbalanced ¼-inch output. The AX1 handheld transmitter plugs in to the XLR connector on any dynamic handheld microphone to convert it to wireless operation. It also features a power-on/battery level LED and single AAA battery operation, though for shorter-duration use, rechargeable AAA NiCd batteries can be used. The UR1 uses diversity reception with two independent receivers as part of the UHF Series One wireless system. Each transmitter/receiver combination retails for about $500.
Sennheiser Electronic Corp. has found a wide market for its 5000 Series UHF wireless system with the EM 1046 wireless receiver mainframe. This unit offers a new dimension in expandability through the addition of receiver modules into the mainframe chassis. Each receiver module can be tuned to any of 4,800 separate frequencies. The display on each module can be programmed to show a user-defined label or to exhibit its operating frequency. The EM 1046 can be used with an active, multioutput antenna system for large shows and with the SKM-5000 handheld, interchangeable-head mic operating at an RF output of 50 mW. The microphone features an audio response of 70 Hz through 20 kHz and 16 user-selectable frequencies. The SK-50 and SK-250 body-pack transmitters operate on RF outputs of 50 mW and 250 mW respectively and feature tunable operation and Lemo connectors to interface a wide variety of Sennheiser lapel mics. The SKM 5000-UHF lists for about $2,500, the SK-250 UHF body-pack transmitter lists for about $3,750, and the SK-50 body pack costs about $3,000.
Shure's ULX UHF wireless microphone system comes in standard and professional models. The ULXS4 receiver has more than 1,400 selectable frequencies, a five-segment audio meter, an RF presence LED, detachable quarter-wave antennas, and a handy output that is mic/line switched. The diversity scheme used is called predictive diversity, and it uses a CPU that reads both antenna RF levels and only switches if the other has a better signal, not after the signal degrades. Automatic frequency selection speeds the setup and finds an interference-free channel. The ULXP4 professional model adds group scanning and lockable settings. The ULX-1 body-pack transmitter and ULX-2 handheld transmitter offer a three-segment battery life gauge, a backlit group/channel display, frequency and power settings, control lockout, and eight-hour battery life. Each transmitter/receiver pair costs about $800 to $950.
Sony Electronics of America markets the versatile UWP Series UHF wireless microphone system among its many professional wireless products. The URX-R1/6668 tunable receiver comes in a half-rack chassis with space diversity reception on TV channels 66, 67, 68, and 69. This diversity method is one that senses and selects the signal-to-noise ratio between its two antennas. Features include balanced and unbalanced audio outputs, pilotone muting, a switched mic/line XLR audio output, and a front-panel headphone jack with monitor level control. The UTX-H1/6668 handheld wireless microphone offers TV channels 66 through 69 operation, user-adjustable input attenuation, six-hour use on two AA batteries, and an LCD. The UTX-R1/6668 receiver lists at less than $500, and the UTX-H1/6668 costs about $400.
The Telex Communications FMR-1000 UHF wireless microphone system has Advanced ClearScan for automatic group and channel selection among more than 950 UHF channels. A nice addition to the receiver is a front-panel parametric EQ control with level, Q, and frequency adjustments. Normally, a mixer would be required for these features. Both balanced and adjustable unbalanced outputs, battery level, setup screen, and RF signal strength metering are included. The HT-1000 handheld transmitter uses detachable mic elements and has selectable RF output at 5 mW or 50 mW. The WT-1000 body-pack transmitter has a 40 dB input level adjustment and a level switch for microphone or instrument usage. Both mics and the receiver cost less than $1,500.
TOA Electronics' Password Series wireless microphone system includes the WT-4800 receiver, the WM-4200 hand-held transmitter, and the WM-4300 belt-pack transmitter. The system operates among 16 frequencies in four groups for a total of 64 available UHF channels from 692 to 722 MHz. The receiver front panel includes a diversity activity display, channel/bank selector, and power switch. Available on the back panel are balanced, unbalanced, mic/line switched output, and a second unbalanced output. The system also features tone-key, carrier, and noise squelch to avoid interference. The system (Password-HL) consists of the WM-4300, the WM-4200, and the WT-4800 receiver for less than $1,000.
Vega, owned by Clear-Com Communication Systems, offers the U-2020, 100-channel wireless system. The R-2020 receiver features balanced and unbalanced audio outputs that are mic/line switched, five-position RF, and audio level LED meters. The handheld transmitter has detachable elements for an array of polar patterns and 50 mW RF output power. The body-pack transmitter will accommodate several popular lapel microphones and includes a mic-level input adjustment. The system — including body-pack transmitter, handheld transmitter with selectable head element, and receiver — lists at about $1,500.
Tremendous advances in several technologies have brought wireless mic systems from experimental toys to solid, professional performers. Most of these manufacturers offer a wide range of wireless systems, including those using the VHF band. Headsets, intercoms, instrument pickups, and individual ear monitors all make use of wireless technology. Beyond the basic features of these, the potential user has a wide array of parameters to choose from in fine-tuning a final selection.
Bennett Liles is a freelance television production engineer and audiovisual technician in the Atlanta area. He specializes in government video production, distance learning, and videoconferencing.
What Will DTV Do to Wireless?
Digital television will cause changes in some of the frequency bands now used by wireless microphones. TV stations have been allocated a second channel on which to begin broadcasting digital signals, and some of these may cover present wireless areas. Public-safety agencies will begin using TV channels 63 and 64 (764 through 776 MHz) and 68 and 69 (794 through 806 MHz) for two-way radio. That may cause interference to wireless mic systems on those bands. After December 31, 2006, new commercial wireless services will begin using TV channels 60 through 62 (746 through 764 MHz) and 65 through 67 (776 through 794 MHz). The FCC has ruled that wireless microphone systems may continue to operate on all the present TV channels that they currently use. As to how the new digital transmissions will affect your wireless mic systems, the most likely effect of DTV interference will be a substantial reduction in range.
For More Information
Samson Technologies Corp.
Sennheiser Electronic Corp.
Sony Electronics of America
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