New Wireless Headset System- Half Duplex

ricc0luke

Active Member
Joined
Feb 23, 2004
Location
Central Illinois
I was just looking at Production Intercoms website, and I came across this new type of wireless system- Half-Duplex. There are one or two disadvantages, but it is cheaper, and if you are only using one or two wireless there are barly any disadvantages. There are also many perks too.... for instance, it does not eat up as many channels as full-duplex systems do. Below is a section I copyed from their website-

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.

http://www.beltpack.com/howtheywork.htm
There is a diagram as to how they work at- http://www.beltpack.com/HD903-diagram.pdf

Cool stuff-


What do you all think? Has anyone used this?
 

ricc0luke

Active Member
Joined
Feb 23, 2004
Location
Central Illinois
Here is some more info simply because I really want to hear what other people think about this-

CC. Half Duplex Systems (CLICK to open our diagram in a new browser window):

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 half-duplex 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 XLR-type 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.

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 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. 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. 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 indicting 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.
 

cutlunch

Active Member
Joined
Jan 12, 2005
Location
Auckland, New Zealand
An interesting system, but in a school system would it be worth the cost? From some of the prices I have seen just to setup the base station would cost about two grand. Maybe for a production company it would cut down on long cable runs. How much does licencing cost in the States.

It would be nice but if your venue is small enough you could do it with the walkie-talkies that have that caller selection thing that you can set all your walkies to the same call tone.

I might put it in a brand new facility or maybe in a multipurpose area eg. gym where you can't leave in permanent cable runs for the coms.
 

Andy_Leviss

Active Member
Premium Member
Joined
Dec 26, 2004
Location
NYC
This isn't anything particularly new. Interfaces to allow a ClearCom system to broadcast via a two-way radio with the call button as the PTT button (as well as to allow the CC system to listen to the radio) have been around for a long time. It's just a small box that plugs right into the external mic/speaker/headset jack of whatever radios you're already using.
 

ricc0luke

Active Member
Joined
Feb 23, 2004
Location
Central Illinois
Its not the same two way radios into your existing headset system. Two way radios are simplex. When you combine them together you get half the system full duplex and half simplex all talking together. With this new system the wireless part uses half duplex (hence the name) instead of using simplex. It is not the fullduplex that the traditional wireless headsets use, but this system uses less channels- a real plus if you are using lots of wireless mics. There is more info on the differences between the systems at:

http://www.beltpack.com/howtheywork.htm