A Production Intercom White Paper on Wireless Intercom

Transcription

1 A Production Intercom White Paper on Wireless Intercom 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, 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 voiceactuated 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 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.

2 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 handheld, mobile 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 repeater systems are very common. It is probable that your local police, fire, and emergency services are using them. The repeater in a halfduplex 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.

3 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 $ 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 pressto-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

4 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 further. How Does Production Intercom Address Wireless? First, all of our solutions involve augmenting or creating a cabled system with a few wireless stations. From thousands of telephone conversations about wireless over the years, our customers, when asked how many in their crew actually need to be wireless, have almost universally answered 2 or 3. Though the solutions we offer can go way out beyond 2 or 3, all of our assumptions have been based on 2 or 3. Here are our suggestions, going down the same list of system types. AA. Simplex system with a Production Intercom twist: Old friends and customers can skip this part. We ve been doing this one for almost 20 years. Our AD913 permits a group of simplex transceivers to communicate, back and forth among them and with a cabled intercom system. It is simple to use and truly inexpensive. You ll need as many portable transceivers as you have crew that need to be wireless, plus one. The AD913 connects to the cabled system just like it was another cabled beltpack. The combination of the AD913 and the extra transceiver connected to it becomes the base station for the wireless portion of your system. The extra transceiver is connected to the AD913 with a cable we ll provide. When the roaming portable transceivers transmit, their signal is picked up by the transceiver that is part of your base, and fed into the cabled system by the AD913. The wireless portion of the system is still simplex, so we have to find a way to cause the base transceiver to transmit when cabled stations want to speak to the remote transceivers. This is done using the signal light system in the cabled portion of the system. Pressing and holding any signal light button (on the same circuit as the AD913) in the cabled system will cause the AD913 to switch the base transceiver into its transmit mode, and whatever communications traffic is on the cabled system will be heard at the remote transceivers. You ll only have to adjust the levels in and out of the AD913 so that the transceivers are being heard at the same level as the cabled beltpacks, and the cabled beltpacks are being heard at the same level as the transceivers. It s simple and quick to make these adjustments, which might not be required at all since every AD913 is set and tested at normal levels before it leaves the factory. All you have left to do is choose what headsets you want to use with the remote transceivers and the system is complete. We have a broad selection of headsets and lapel speaker/mics. It is not necessary to carry the transceiver around in your hand. Sling it around behind your back and use the in-line PTT switch on the headset cable. The switch can hang on your belt, clip to a shirt pocket, or just be hanging in front of you. There are a huge number of headset styles and qualities to choose from.

5 BB. Full Duplex Systems: Save your pennies. If we ever figure out how to build a full duplex system that is either better or less expensive (or both) than the products currently available, we ll sure let everyone know about it. CC. Half Duplex Systems: The AD913 Simplex System discussed above has four or five legitimate knocks on it: 1. In the real world of performances, technical crew members are getting the information they need not only from instructions being transmitted specifically to one individual, but also by monitoring the general traffic on their intercom channel(s). They can t do that in a simplex system. 2. The persons calling the show from master stations do want to reliably assume that the crew members on wireless are hearing them, and don t want to have to remember to press a call light button to transmit all the time. 3. Using the call light to trigger transmit to the wireless crew, effectively makes the call function unusable for its original purpose. The light is going to be flashing on and off, all through the performance. If there are any signal devices like our Blazon strobes in the system, they too are going to be triggered unless they are isolated on their own circuit. (We re about to announce a cure for that!!) 4. The noise often heard in the headphones (k-chzz) each time the base transceiver is keyed to transmit could be annoying. 5. Wireless crewmembers equipped with simplex transceivers must press their talk button to communicate back to the stations on the cabled system. A half-duplex system can eliminate the first four problems and we would argue that the fifth is seldom a real issue. In fact, people calling the show, stage managers, lighting directors, etc., generally want crew members to keep their headset microphones turned off except when they need respond to a call, or need to report. (Some manufacturers actually offer an accessory that will permit the person calling the show to shut everyone s microphone off.) But they do need to listen constantly. Here s how it works. After looking at the available products and prices, we chose to use a professional quality, off-the-shelf, ICOM repeater with some special preparatory modifications and connect it to the cabled intercom system through our new HD903 halfduplex adapter using a custom DB25 cable. The HD903 takes care of matching the audio out level from the repeater to the audio in level of the cabled system and the reverse. Plug a standard intercom cable (2-conductor shielded with male and female 3-pin XLRtype plugs) into the back of the HD903 and into any convenient jack in the cabled system. It could be a special run from the cabled system s master station but it does not have to be. An available loop-through jack on the back of any belt pack would do just as well. The remote transceivers can be any high quality UHF portables that have been programmed to operate on the frequencies set up on the repeater. If you choose our ICOM portables, we can do all this for you.

6 When you activate` the repeater, any transceiver on the chosen frequency will hear the uninterrupted audio from the cabled system and the other portables. Press the talk button on the portable transceiver and you will be heard on the cabled system and by the other portables. The receive side of a repeater, to which the cabled system is always connected, is inherently noisier than the audio on our cabled systems, so the HD903 has a circuit to eliminate that noise. If you don t already have a cabled system, you can run up to 5 belt packs directly off 3- pin XLR jack on the back of the HD903. This will require that you connect the small power supply that comes with the HD903. If you already have a cabled system, the HD903 will be powered by your intercom power supply. Because you will be using conventional portable transceivers, all of the accessories and gadgets made for them can be used, from lapel mounted speaker mics to concealed earspeaker/microphone combinations like those used by the secret service. The batteries available for these transceivers have capacities as high as 1800mAh and will operate at 4 watts output for as long as 14 hours. Since you ll probably be operating at 1 watt, battery life becomes a non-issue. The new fast chargers made for the portables will recharge a battery in 40 minutes. Having a second battery for your portable will allow you to make a quick change. On ICOM and other top brand portables this is a snap-out, snap-in, almost instant procedure. Should one of your portables be damaged, lost or stolen, a replacement can be easily found. Any two-way radio shop in any town should be able to solve your problem. When it s not performance time you have a very powerful repeater based system to coordinate setup, teardown, or sending somebody out for a pizza. All of the points regarding FCC licensing, etc. in section AA above, dealing with straight simplex systems, also applies here. It s more than worth it to properly license your transceivers. You will get protection from others in your area using the same frequencies. You will be able to use the full power of your transceivers, including the repeater, which will give you far more coverage and reliability of communications than any other system discussed here. And you ll be cooperating in the campaign to share those precious frequencies in the most efficient way. Oh, and you won t get a nasty letter from the FCC. Huge changes are coming in the availability of frequencies and in the spacing between frequencies required by the FCC. The spacing between frequencies is going to be cut in half, for the third time (from 25 to 12.5 to 6.25), and only the best transceivers are going to be able to operate without slopping over onto the adjacent frequencies. Those that can t will soon be unusable. The bottom portion of VHF television channels (2 through 6) is going to disappear within the next couple of years, and the top half (7 through 13) not long after. This, and the advent of high definition TV, is going to result in massive reallocations of frequencies. The ICOM portables, and the repeater that we are recommending, are already engineered with that in mind. The half-duplex system isn t perfect. We believe that the only serious drawback is the fact that only one portable can be successfully transmitting at a time. If you ve ever joined in a multi-party conference call, you might not consider that such a disadvantage.

7 What does happen? If two, or more, portables try to transmit at the same time and one of them has a more favorable position vis-à-vis the repeater s receive antenna, that portable will win the competition. In the unlikely situation where two portables are in exactly the same relative position, and both start transmitting at the same moment, the repeater may alternate back and forth between the two creating a jumble. In practice it s not much of a problem for crews who are familiar with one another and where a voluntary protocol evolves. If it does become troublesome and you are using ICOM portables we can program them and the repeater to implement a busy channel lockout feature. When any of the portables begin to transmit, the others, when they press the transmit button, will get a beep/beep/ beep tone indicating that the channel is in use. This is simply a programming function. There is no hardware involved. We don t do it to all of them because we have learned from 20 years of selling our AD913 system that it s just not much of a problem for most of our users. Questions, comments, corrections? support@beltpack.com Rev