Music Without Cables? – A Look at Wireless Audio Transmitters

Eliminating the big clutter of cables has lead to a variety of wireless audio transmitter products over the years which promise to untie your home audio equipment. We will take a look at recent products to see whether they can deliver on their promise to eliminate the cable clutter and we’ll examine how changing technology has impacted performance of these products.

Wireless transmission of music has started with the launch of commercial radio broadcasts several decades ago. 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 begun to replace traditional FM radio. Today a multitude of consumer devices exist such as wireless microphones, wireless surround sound kits, Bluetooth audio transmitters, baby monitors etc which eliminate the cord by transmitting audio wireless.

One of the main applications for wireless audio is to distribute music throughout the home and setting up speakers without running speaker cables which is a big problem in homes that are not wired for audio. 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 using a high-frequency signal which is constantly changing in frequency according to the audio signal. This method is called frequency modulation or FM for short. The biggest advantage of FM transmitters is their simplicity and thus cost and range. As a result, most of today’s 900 MHz wireless headphones use FM transmission. However, FM has some major drawbacks.

The first 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 location of the radio or receiver. This is due to the fact that the radio signal is bouncing off walls and will cancel itself out at different locations. This phenomenon is called multi-path fading. Some wireless FM transmitters use two antennae in an attempt to cope with this phenomenon (diversity receivers). In addition, FM transmissions will easily pick up interference from other wireless devices and products.

Bluetooth based audio transmitters have gained popularity recently. Bluetooth is a popular wireless protocol which was designed primarily as an interface between computer peripheral products. Bluetooth audio transmitters will convert the audio signal to digital information and then transmit via the Bluetooth protocol. Bluetooth is fairly robust in terms of interference in is well standardized. Bluetooth, however, has some problems in regard to wireless audio since it was not designed for wireless audio originally. The first drawback is the limited range which typically would be 30 ft or less. The second problem is the fact that Bluetooth does not have enough space to transmit CD-quality audio and therefore the audio is compressed first. This compression will result in a loss of audio quality depending on the degree of compression. The final problem is the fact that Bluetooth will introduce a delay in the signal (also called latency) which is a problem when used together with video since the audio will be out of sync or if used in a surround sound setup where some of the speakers would be out of sync with the hard-wired speakers.

Satellite radio and digital terrestrial radio technologies offer high range but use extensive audio compression leading to degradation of the audio signal and have an audio delay of several seconds.

Newer technologies transmit the audio signal without compression digitally. By avoiding audio compression, the signal will retain the original quality. The audio latency of some of these products is less than 1 ms and therefore these technologies are also being used in wireless speaker kits for home theater setups.

By using error correction, these technologies bluetooth audio module can cope with interference from competing wireless devices which is crucial in today’s noisy environment. Some transmitters operate at 5.8 GHz. This frequency space is less crowded than the 900 MHz or 2.4 GHz bands thus leading to a further improvement of 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 home.

 

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