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Infrared - These devices use an infrared (IR) beam to transmit the sound from the base unit to the headphone, working in much the same way as the remote control for your TV. The range is limited to about 7m, less if you’re away from the center line. Infrared is optical, so you need to have a clear line of sight between the transmitter and headphones. Headphones of this type are not really any good for wandering around the house, but they're great when sitting in front of the TV or hifi.
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Bluetooth - In bluetooth headphones, a low-power radio signal is used to digitally transmit the sound. At the moment, Bluetooth headphones are relatively thin on the ground. MP3 player manufacturers are slowly beginning to include the technology, but Apple seems to be stubbornly refusing (third party accessories are available). The limited range puts them in the same league as infrared, but the portability of Bluetooth devices makes it much more suitable for walk-around listening. As it is a radio technology, line of sight is not required so you can listen to music on your phone without having to take it out of your pocket.
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Radio - If you want to walk around the house and garden while listening to your hifi, this is the one to have. Like Bluetooth it uses a radio signal, but it is more powerful and is FM rather than digital, so it works just like your radio, but on a different frequency. There is usually a choice of two or more channels to avoid interference from other devices or neighbours who bought the same headphones as you.
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So how do I connect it all together?
The general principles are the same for all three types, but may be implemented differently.
At the music source end, i.e. your CD player or TV, there needs to be a transmitter. In the cases of IR and radio, this is usually a small basestation that needs a mains power supply (IR and radio use more power than Bluetooth) and needs to be connected to the headphone socket of the source. Bluetooth, on the other hand, tends to be built into devices such as mobile telephones, or you can get a Bluetooth usb dongle - a small (mine is smaller than my thumbnail) transmitter to plug into your computer. Battery-powered Bluetooth transmitters are also available to use with non-Bluetooth devices such as iPods.
The headphones contain the receiver circuitry and a small amplifier as well as a battery pack to power them. The mains-powered transmitters usually serve as chargers for the headphones, connecting to them via a short cable or a charging cradle.
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Which one should I choose?
There are three factors to consider: the base station, the range, and the size of the headphones.
Taking the last of these first, IR and radio tend to be larger, over-ear headphones, as they need to enclose larger batteries, while bluetooth headphones are available in a wider variety of designs from traditional over-ear to funky lightweight headsets incorporating microphones (for use with your mobile phone).
The base station for IR and radio will need a power socket. Wireless refers to the absence of a wire tethering the headphones to the source, not the entire contraption. Mine (Sony MDR-RF800R) have three wires coming out of the base station: power supply, audio connection (plugs into the headphone socket), and a wire to plug into the headphones themselves to charge them. The presence of these wires makes it a chore to move them from the TV to the hifi or computer and in practice I don’t do it.
The range may seem like a limiting factor, but it really depends how you want to use the headphones. IR headphones seem to me to be ideally suited for TV use - you need to be able to see the TV, so if the base station is on top of the TV, the headphones will be able to see it. Also, you tend not to walk around the house watching TV but will more likely sit directly in front of it at a distance of about 2 or 3 meters.
Bluetooth, on the other hand, has a similar range, but you are probably carrying the source (MP3 player or mobile phone) about your person. You are unlikely, in this situation, to have a power socket following you around, so Bluetooth, if you must have wireless, is the only answer.
The greater range of radio headphones makes them well-suited to general listening where you might be in one place for a while, then want to move somewhere else. I use them at my computer, sitting on the sofa and cooking dinner.
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What is my personal recommendation?
As the IR and radio headphones are about the same size and price, it makes sense to choose the radio ones. They’ll give you much more flexibility to change how and where you listen.
For MP3 and phone use and possibly at-the-desk computer use, choose the Bluetooth ‘phones. For this application, however, they seem to me like overkill. I’d rather spend my money on a pair of wired Sennheisers.