A large number of wireless audio transmitter products have emerged over the years. They promise to cut the cables to your audio equipment. Can these products make good on their promise to eliminate the cable clutter? What influence has changing technology had on their performance?

Wireless transmission of music has begun with the launch of commercial radio broadcasts several decades ago. While still being popular now, FM radio is now being replaced by recent technologies such as HD radio, satellite radio and DAB broadcasts. Today a variety of consumer devices exist such as wireless microphones, wireless surround sound kits, Bluetooth audio transmitters, baby monitors etc which cut the cord by transmitting audio wirelessly.

Many houses which are not wired for audio are ideal for using wireless audio to distribute music throughout the home. Not every technology is perfect when it comes to wireless audio as we will find out.

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 continuously changing in frequency according to the audio signal. The technology, which is known as frequency modulation (FM), is fairly straightforward and cost-effective to build and offers high range. Most of today's wireless consumer devices utilize FM transmission. FM, however, has serious disadvantages in comparison with new technologies.

The FM signal will pick up static. Therefore there will be noticeable background noise. The amount of static will depend on the position of the transmitter and radio. The reason for the varying quality is a phenomenon known as multipath fading which is a result of reflected radio waves cancelling each other out. Using two antennae is one method some devices utilize to cope with this problem (diversity receivers). In addition, FM transmissions will easily pick up interference from other wireless products.

Bluetooth based audio transmitters have gained popularity recently. Bluetooth is a common wireless protocol which was designed primarily as an interface between computer peripheral products. Before the signal is sent, Bluetooth transmitters will convert the audio into a digital format. One of the advantages is the high robustness against wireless interference. On the other hand, Bluetooth was not designed for audio transmissions. As a result is does have several problems. The first disadvantage is the limited range which typically would be 30 ft or less. The second drawback is the fact that Bluetooth does not have enough space to transmit CD-quality audio and as a result the audio is compressed first. As a result of the audio compression, the audio quality will decrease. Another drawback is that Bluetooth will introduce a delay or latency to the signal during the transmission. This delay is a particular drawback for video and surround sound applications. In these applications the sound from the wireless speakers would be out of sync with the video and remaining speakers.

Terrestrial digital and satellite radio technologies offer high range but use extensive audio compression and have an audio delay of up to several seconds.

Another technology transmits the audio signal without compression digitally. This technology is used in wireless audio transmitters from Amphony. By avoiding audio compression, the signal will preserve the original quality. The audio latency of these products is smaller than 1 ms and therefore this technology is also used in wireless speaker kits for home theater setups.

To be robust against interference from other wireless products, this technology uses forward error correction. This mechanism can repair errors during the transmission. Amphony products use the 5.8 GHz frequency band which is less crowded than the 900 MHz and 2.4 GHz bands. The result is high reliability. In contrast to Bluetooth, any number of wireless receivers can operate from a single wireless audio transmitter which is a concern when distributing audio to several rooms within a residence.


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