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Music Without Cords? - An Overview Of Wireless Audio Transmitters
Over the years a whole array of wireless audio transmitters have emerged which are supposed to cut the cords at home when connecting audio equipment. We will investigate the impact of new technologies on the performance of these devices. In addition, we'll examine if they keep their promise of removing the cable clutter.
The first form of wireless audio transmission was in the form of AM and FM radio stations. FM radio is still popular until today although some newer technologies such as satellite radio and digital radio formats such as HD radio and DAB radio have started to substitute traditional FM radio. Today's wireless consumer products such as wireless speaker kits, baby monitors, Bluetooth audio transmitters and wireless microphones eliminate the cord.
Many houses which are not wired for audio are ideal for using wireless audio to distribute music all through the home. The technologies used these days all have pros and cons as we will examine.
Let’s look at the most traditional technology first: FM broadcasts. FM audio transmitters send the audio by employing a high-frequency signal which is constantly changing in frequency according to the audio signal. The technology, which is called frequency modulation (FM), is fairly simple and cost-effective to build and offers high range. As a consequence, most of today’s 900 MHz wireless headphones use FM transmission. However, FM has some major drawbacks.
The first problem is the fact that the transmission will pick up noise and thus there will be some hiss or static. The amount of noise will actually vary depending on the position of the radio or receiver. This is due to the fact that the radio signal is reflecting off walls and will cancel itself out at different locations. This phenomenon is known as multi-path fading. One method to cope with this problem is employing two antennae instead of one. Devices using this method are known as diversity receivers. In addition, FM transmissions will easily pick up interference from other wireless devices.
Another technology which has gained popularity recently is called Bluetooth. Bluetooth was originally designed to connect computer peripheral devices wirelessly. Before the signal is broadcast, Bluetooth transmitters will convert the audio into a digital format. Bluetooth is relatively robust in terms of interference and is well standardized. Bluetooth, however, has some drawbacks in regard to wireless audio since it was not designed for wireless audio originally. The range of the transmission is only 30 ft or less. Moreover, Bluetooth does not have enough space to transmit an uncompressed CD-quality audio signal. As a result the audio is compressed prior to the transmission. Due to the compression, the audio quality will be degraded compared to the original. The final setback is the fact that Bluetooth will introduce a delay in the signal (also called latency) which is a problem when used together with video because the audio will be out of sync or when used in a surround sound setup where some of the speakers would be out of sync with the hard-wired speakers.
The technologies used for satellite radio and terrestrial digital radio offer high range. However, they also use extensive audio compression. Further, a delay of up to several seconds is introduced.
Another technology transmits the audio signal without compression digitally. This technology is used in wireless audio transmitters from Amphony. By eliminating audio compression, the signal will retain the original quality. The audio latency of these products is smaller than 1 ms and as a result this technology is also used in wireless speaker kits for home theater setups.
To be robust against interference from other wireless products, this technology utilizes forward error correction. This mechanism can repair errors during the transmission. Rather than utilizing the crowded 900 MHz and 2.4 GHz frequency bands, Amphony products operate at 5.8 GHz and thus offer high robustness. An advantage over Bluetooth is the ability to send audio to any number of wireless receivers from a single transmitter. This is significant for distributing audio to multiple rooms in the house.
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