School Video News Camera I/C Systems

Camera Intercom Systems-Part One

For those who know as much or more than we do about ‘wireless’, and there are plenty of those, this discussion may be overly simplified. But, for the rest of us, it can be a confusing and complex topic. This will be an attempt to clarify ‘wireless’ as it applies to headset intercom systems and to our attempts to offer affordable options. Any suggestions on how to more clearly describe a function or correct an error will be appreciated.

First, some key terms necessary to understanding the different types of ‘wireless’ equipment that can be part of a headset communications system:

Transceiver: Any two-way radio that can both transmit and receive radio signals. They can be portable (hand-held), mobile, marine, base stations, repeaters, etc.

Repeater: A repeater is a specialized transmitter, receiver and controller that picks up signals from a portable or mobile transceiver and retransmits that signal at a much higher power on a different frequency.

Simplex: Transceivers that are press-to-talk, release-to-listen. Common portable transceivers (walkie-talkies) are simplex, as are most mobile transceivers. Users may transmit and receive but never do both at the same time. Only one person in a group can successfully transmit at one time. Some portable transceivers are equipped with voice-actuated circuitry (VOX) and begin to transmit when the user begins to speak. They are still simplex, and this feature is rarely usable in a performance environment

Full Duplex: Full duplex wireless transceivers can transmit on one frequency and receive on another frequency simultaneously. There is no push-to-talk requirement. Your cordless or cellular telephones are full duplex, as are the wireless intercom beltpacks sold by Clear-Com®, Telex®, and HM Electronics®, etc. In a wireless intercom system there is a central ‘base’ station (really a ‘repeater’) that receives transmissions from each beltpack on that beltpack’s unique, assigned, transmit frequency, and retransmits back to all of them on the one receive frequency that all of them share. Since the base station is capable of transmitting constantly, everyone on the system can hear everyone else full time, while simultaneously transmitting her/his own voice to the group.

Half-Duplex: Half-duplex describes a system which consists of simplex portable transceivers (walkie-talkies) or simplex mobile or marine transceivers, but, rather than communicating directly with each other, they talk through central transmitter/receiver called a repeater. The transceivers are programmed to transmit on one frequency and receive on another. The repeater receives all the transmissions from the transceivers on first frequency and retransmits them on the other.

The repeater is designed to transmit constantly. (Portable transceivers are not designed to do this.) A half-duplex system is primarily used to increase the range of a group of hand-held, mobile, or marine transceivers, as the repeater can be many times more powerful than the portable or mobile transceivers, and it can be equipped with a raised antenna sophisticated enough to pick up the weaker transmissions from the portable transceivers from a greater distance. Half-duplex systems are very common. It is probable that your local police, fire, and emergency services are using them. The repeater in a half-duplex system is different from the one used in a full-duplex system in that it receives only on a single frequency, so only one portable transceiver can talk to the repeater (and thus be heard by all the other portables) at a time.

Because the half-duplex repeater is transmitting constantly, the portable transceivers can receive constantly and are going to hear all the voice ‘traffic’ on their frequency. But, because the portables are simplex, they will have to press a talk button to transmit.

How does all this apply to headset communications systems used in our industry?

A. Simplex systems may work for you. Walkie-talkies are by far the least expensive of the options here. Purchase as many transceivers as you need, fit them up with earsets, headsets or lapel type speaker/mics and you’re ready to go. There are a bewildering variety of earsets available for portable transceivers. Finding one suitable to you should not be difficult. You can also use full-sized intercom headsets with these transceivers, including ours. Cheap consumer oriented transceivers seldom satisfy the needs of a professional technical crew. FRS and 49mHz or 900mHz wireless headsets might work in a very rural setting but in an urban area you cannot assume that you will have exclusive access to any particular frequency. You should also be aware that these devices are for personal communications and the FCC forbids their use in a commercial application. If there’s a crowd control component in your use of transceivers, you should be particularly careful to play by the FCC’s rules. To play by the rules you should purchase and license transceivers intended for this kind of application. We distribute ICOM radios and have an obvious bias, but there are several good product lines available, including, of course, Motorola®. The licensing process does not have to be daunting or outrageously expensive. There is one set of frequencies set aside by the FCC for limited business applications where you can go up on the FCC’s site and complete the licensing form online. We’ll provide directions. The number of these frequencies is limited and you are not guaranteed exclusive use of any of them in your area. We are told that the fee is currently $100. If you are ready to take the professional approach though, there are services available that will take your application, do a frequency availability analysis for your area(s), complete the rather complicated application for you and submit it to the FCC. They’ll even get you a ‘station temporary authority’ (STA) to be on the air until the FCC license comes through. You’ll be ‘legal’ from day one. You can license up to four frequencies for 10 years for $285 total, and each subsequent 10-year renewal is currently $45. That includes the license fee and the services provided. We can provide you with access to a well-respected service. You are being granted a license to operate on a specific set of frequencies, not a license for your transceivers. You can operate on your assigned frequencies with any transceiver that is FCC type approved.

There are special frequencies that are set aside for itinerant use. If you are traveling, you don’t have to concern yourself with ‘stepping on’ someone’s licensed frequency in another part of the country.

B. Full Duplex wireless beltpack systems are obviously suited to production intercom people. They do, however, have some ‘issues’ associated with them.

The first is cost. These systems are not cheap to manufacture, and providing support is very costly. From one major manufacturer, a single channel base station and four wireless beltpacks will cost the end user over $7500, and that’s before adding any headsets, spare rechargeable battery packs, battery chargers, or special antennas. There’s aggressive competition so it’s only fair to conclude that the manufacturers who are building them are selling at the lowest prices they can offer and still make enough to cover the products themselves, and the support structures necessary with radio frequency (RF) gear - as well as make a fair profit. The FCC has some very serious rules about the qualifications of people working on RF equipment including not only design but programming, quality control, and repair. At least one of these people is required to have their general class FCC license posted in their work area. The cost of the equipment used to work on RF gear is very high and must constantly undergo FCC mandated calibrations. None of this is free. When you look at a $1000 wireless beltpack and wonder where the money went, well, now you know. It’s not gouging. That’s what these things are worth! The case and components are only a small part of the manufacturer’s real cost.

The second is the built-in limitations. Wireless intercom systems base stations usually limit the number of portable stations that can be used to four or five, after which you have to add another base or sub-base, again adding substantially to cost. In the $7000+ system referred to above, an extra base station prices out at $3000+. Every wireless system has to find enough available radio frequencies to allow for one frequency for each portable plus one more for the base. That sounds easy, but in this day of wireless everything, it’s not always simple to accomplish. Manufacturers have responded by building ‘frequency agile’ or ‘spread spectrum’ products that can either find available frequencies themselves or allow the technician to set the frequencies simply and quickly. These more sophisticated products are expensive to build and still don’t guarantee that the needed number of frequencies will be available every night and in every location, or that the same frequencies used in rehearsal or sound check are going to be clear of competition come show time. One well-known wireless manufacturer has even recommended that you run a parallel, cabled system in critical situations.

If wireless beltpacks are used in their full duplex mode, they are both transmitting and receiving constantly. The transmit function eats batteries and that can become an issue. Provision has to be made for replacing batteries quickly should a transceiver go down unexpectedly during a performance. Most of them use conventional, readily available cells, but that doesn’t help if they are not immediately available. The most practical way to preserve battery power is to turn off the transmit function except when you need to speak. But that effectively eliminates the full duplex benefit because now you must press-to-talk.

Wireless beltpacks transmit a very weak signal. You may find situations where the base station will require a special or very carefully located receive antenna. They are not particularly expensive but getting an RF cable from the properly placed antenna to the master can be a challenge.

There’s no question about it! If your application makes it a practical necessity, if you have the budget, and if you can technically support it, a full duplex system can add some considerable convenience.

C. Half-Duplex systems have not been used widely in our application. Always considered a means of getting extra ‘range’ for low powered portable or mobile transceivers, their use in a confined area didn’t seem to be appropriate.

But we think that you might want to reconsider a half-duplex system when you read Part Two next month.