IR Listening Systems
Oct 1, 2006 12:00 PM, By Bruce Borgerson
IR units provide a secure and flexible hearing experience.
For infrared (IR) listening systems, all market indicators point skyward. Whether for assistive listening or language translation applications, sales prospects simply couldn't be rosier.
The reasons? First, with populations aging, more people are developing hearing impairments. In addition, with new legislation in some jurisdictions requiring audio description for the visually impaired, a single-channel system will no longer suffice. A third factor: Immigration patterns are creating more multilingual communities, which, in turn, boosts the need for second- (or even third-) language translations. Finally, post-9/11 security consciousness has prompted a move to keep all government proceedings strictly contained within four opaque walls.
Though the aging trend has benefited radio frequency (RF) ALS systems as well, the other three trends clearly favor IR systems. This helps the light-based technology overcome a relative cost disadvantage. Across the board, per square meter of coverage, an IR system usually demands a bigger budget outlay. Where a single FM whip antenna can take care of a large auditorium (and most adjacent rooms as well), an IR system may require two, four, or more IR emitters, depending on architectural impediments. Installations costs therefore run higher, and more system components are needed.
Nevertheless, in many installed applications, IR is the clear winner, regardless of cost. That's primarily because, with rare exceptions, an RF system will provide only one audio channel at a time. IR systems routinely offer two and four. Also, an RF transmitter broadcasts its signal throughout the immediate neighborhood — whether or not such “public access” is desired. IR signals are fully contained by opaque walls.
The attendant security concerns of RF broadcast have made IR systems mandatory in courtrooms and, increasingly, in nearly all other government applications. Signal containment also makes IR the dominant technology in schools and universities where adjacent classrooms use systems simultaneously: Security is less of a concern, but keeping track of RF channel frequency allotments can be an operational headache. Cinemas prefer having one standard frequency (usually 95kHz with 250kHz added for stereo or second language), as headsets can go anywhere in the multiplex without switching channels. Finally, today's wireless-saturated Broadway shows certainly don't need yet another RF system pumping into the pervasive electromagnetic soup.
These trends have combined to reinforce a thriving business for companies currently providing IR technology for the broad installation market.
The following survey focuses on broadband systems that offer high-quality reproduction (potentially full audio bandwidth) on one, two, or four channels. In varying configurations, such systems find applications in houses of worship, conference centers, legitimate theaters, cinemas, educational facilities, courtrooms, and government buildings. Not included here are multichannel (more than four), narrow-band systems targeted specifically at multi-language translation for large international conferences.
JUST A FEW FREQUENCIES
For many years, all IR systems operated on a single carrier frequency: 95kHz. This single standard was handy because it meant that any IR headset could be used with any IR system.
The number of frequencies has expanded in recent years, both to accommodate more channels and to skirt around potential interference from some high-intensity lighting systems.
Manufacturers who still offer the 95kHz claim that new refinements in their circuitry have overcome the interference problems. However, the shift up into the 2.3MHz to 3.8MHz range remains a clear trend — primarily because it more easily accommodates 2- and 4-channel systems.
Nevertheless, that means the IR systems of the world still use a relatively small number of carrier frequencies — six in all — with 250kHz commonly paired with 95kHz for 2-channel operation. Compared to the channels allotted in RF spectrum, this limited frequency choice does foster a greater degree of compatibility among transmitters and receivers from various makers.
Whereas the carrier frequencies are cleanly defined, definitions of coverage areas are not. Some latitude must be allowed here, as there is no universal agreement as to what constitutes acceptable background noise levels. As a general rule, the more diodes in the emitter and the higher the radiated power (the two usually track together), the larger the potential coverage area. The only absolute rule is that doubling the number of channels carried by each emitter will halve the effective coverage area.
Other factors affecting coverage include amount of direct sunlight (which can introduce noise) and the reflectivity of walls. Most manufacturers will not assume wall or ceiling reflections when making their coverage calculations, so the resultant coverage areas usually form oval or toy-top shapes. But, in most cases, a large area of light-colored wall surface will greatly expand the effective coverage area, often extending a signal into corners of a rectangular room. The dark, fuzzy walls inside a cinema, however, are a completely different story.
MODULAR AND INTEGRATED SYSTEMS
IR systems come in two broadly defined categories: modular and integrated. The modular systems comprise separate emitters (also called radiators) and transmitters (also known as modulators). Many will have separate power supplies as well. Modular systems are the best choice for larger rooms, or rooms with architectural impediments that preclude line-of-sight from only one or two points. With this approach, only one or two transmitters (depending on channel requirements) can be linked to any number of emitters, sometimes of various sizes, as needed to cover the space.
The integrated systems come as a single package, with the modulator/transmitter function built into the emitter unit. This can be more cost-effective in smaller installations, but it limits flexibility. Some systems take a middle course, offering an integrated master unit that can be paired with an emitter-only slave unit.
AUDEX: 95KHZ SPECIALISTS
Audex has opted to stick with the tried-and-true 95kHz standard, and the company offers an impressive array of products for medium- to large-scale mono systems using this carrier. At the core of Audex modular systems is the EX-B-3 switchable base ($588). The 1RU unit offers two XLR inputs with three sets of outputs providing signal and power for up to three emitters. Also included are expansion inputs and outputs to allow master or slave function with other units, and a single emitter diode on the front panel for control room monitoring. The EX-BEX-6 infrared base expansion unit ($510) has no audio inputs, but provides power and modulated output for up to six emitter units. Audex offers three emitter models: EX-PS1, EX-PS3, and EX-PS9.
The Listen LT-82 ($680) operates on any one of four selectable carriers in the 2.3MHz to 3.8MHz range; up to four may be connected to provide up to four mono or stereo signals through connected IR emitters. Features include mic input (with phantom power), L and R audio line inputs and outputs (on RCA), modulated signal inputs and outputs, defeatable AGC, input select, and headphone jack. Up to two emitters may be directly powered via — interestingly — standard Cat-5 cables. The LT-82 comes in a .5RU format and the power supply is included.
Sennheiser's modular systems begin with the SI 1015/NT IR wideband modulator ($715). Also in a .5RU format, this unit provides discrete 2-channel or stereo signals at 2.3MHz and 2.8MHz. Modulated carrier signals for two emitters are available on either F-type connectors or a barrier strip, with emitter DC power also on the barrier strip. Audio inputs are on balanced XLR connectors. Monitor diodes and level controls for each channel are on the front panel.
Sound Associates started out making bulletproof systems for the Broadway theaters, and has since expanded into wider markets. The company's keystone unit is the SA601F single-channel transmitter ($735), available in both 95kHz and 250kHz versions. The premium audio preamp features adjustable gain and accepts line or mic inputs, with phantom powering available. On the front panel of the 2RU half-width module is a 10-segment true modulation output meter and five diodes for IR output monitoring.
The Williams Sound MOD 232 ($1,120) operates on any two of four selectable channels in the 2.3MHz to 3.8MHz range. Consequently, a single unit can offer discrete 2-channel or stereo operation, and a pair of units can be linked for a 4-channel system. Front panel features include level, 10-segment LED meters, compression ratio adjustment trimpot (1:1 to 4:1), and headphone jack with channel select switch. Rear audio inputs are on combo (XLR and 1/4in.) jacks. A DIP-switch array selects a number of functions, including mic or line input level, compressor gain, input mixing, and carrier frequency. F-type connectors are provided for two outputs and one linking input. Williams' companion emitters are powered locally.
INTEGRATED, YET INDIVIDUAL
Key features and specifications for the integrated units are given online in the charts I created. Although all are self-contained packages, some individual characteristics are worthy of note.
Audex has three entries in this category, again all using the 95kHz carrier. The 165-diode EX-PS3-C accepts line level and up to four microphones on 1/4in. jacks. Levels are adjusted by an AGC circuit. For added flexibility, expansion DC power and signal are carried on input and output XLR connectors. A variant version, the EX-PS3-T6, accepts line and speaker level audio inputs, and is targeted at the cinema market. The diminutive (1.2lbs) SA-PS1-U2 is a 55-diode unit with a wide, onion-shaped coverage pattern suited to classrooms and courtrooms up to 2,500 square feet. One option is an electret microphone that plugs directly into the back of the unit, allowing use without a separate microphone.
Phonic Ear maintains forward and backward compatibility with systems operating at both the 95kHz/250kHz and 2.3MHz/2.8MHz standards. The Starsound PE420E system is single channel, although it can be linked to a slave PE421E emitter using the alternate frequency for a 2-channel system. The PE600E system is 2-channel (with consequent reduced coverage area) and can be used for either discrete channel or stereo, depending on headset choice. Both systems offer a variety of accessories and headset options, including IL neckloops for compatible hearing aids.
Koss is a big name in headphones, and has leveraged that reputation into an entry-level system that offers cost-effective performance for smaller to mid-size rooms. Nady Systems is another well-known name from other markets, and is making a foray into IR ALS with two pre-packed systems (headphones included). One, the IR-200 is a fairly straightforward application of 2.3MHz/2.8MHz technology. The IR-400 4-channel system is a bit peculiar, claiming to use “advanced PPM (Pulse Position Modulation) IR technology” in the 300kHz-to-900kHz range. To what extent this would be compatible with other systems is open to question, because details from the company are sketchy.
FOR MOVIES AND TRIALS
USL targets the cinema market, and that focus is easily apparent in the way the company's products are constructed and packaged. The appearance reflects no-nonsense, industrial-strength approach that eschews microphone inputs and offers instead only line and speaker level inputs. Despite the cinema marketing emphasis, USL units are worthy of consideration for other applications. For example, I installed a hand-me-down USL system (from a local theater chain) in a church 10 years ago, and the congregation has found the gift performing to full satisfaction.
Sound Choice systems takes us out of the movie house and straight into the courthouse. Unique in both design and appearance, this company's SC-186K targeted squarely at the courtroom market. The basic unit comes as a turnkey system with a built-in pressure-zone microphone, appropriate equalization, and an AGC/limiter circuit. Basically, you place it somewhere in the courtroom (where the polished wood exterior will blend nicely with the décor), turn it on, and forget about it. Sound Choice is developing a variant 2-channel mono product: One channel is for all to hear, except during sidebars and other confidential discussions, when the “general” channel will be muted and only the second “privileged” channel will remain on for those authorized to hear.
CHOICE OF RECEIVERS — MAYBE
When specifying and configuring modular transmitter/emitter systems, you do not want to mix and match components from various manufacturers. You could find yourself trying to plug an RJ-45 connector into an XLR receptacle. There simply is no standardization at that level.
However, at least in theory, any receiver should work just fine with any transmitter/emitter using the same carrier frequency. This does allow some latitude for shopping around to get the best combination of features, price, and manufacturer support. Still, the caveat emptor warning applies. When something goes wrong, it's always best to avoid situations where two manufacturers' fingers are pointing in opposite directions.
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