Technology Information

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

The first type of wireless audio transmission was in the form of AM and FM radio stations. FM radio is still popular today. However, newer technologies have begun to replace it. These include DAB broadcasts, satellite and HD radio. Today’s wireless consumer devices such as wireless speaker kits, baby monitors, Bluetooth audio transmitters and wireless microphones cut the cord.

One of the main application for wireless audio is distributing music throughout the house and setting up speakers without running speaker cords which is a big difficulty in houses that are not wired for audio. Not every technology is ideal 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. This technique is known as frequency modulation or FM for short. The biggest advantage of FM transmitters is their simplicity and thus cost as well as range. While the majority of today’s 900 MHz products utilize FM transmission, FM has some major problems.

The first problem is the fact that the broadcast 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. 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. Some wireless FM transmitters use two antennae in an attempt to cope with this phenomenon (diversity receivers). Another difficulty is that FM radios easily pick up interference from competing wireless transmitters.

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 data and then transmit via the Bluetooth protocol. One of the advantages is the high robustness against wireless interference. Still, Bluetooth was not designed for audio transmissions. Consequently is does have several problems. The first downside is the limited range which normally would be 30 ft or less. The second problem is the fact that Bluetooth does not have sufficient space to transmit CD-quality audio and as a result the audio is compressed first. This compression will result in a loss of audio quality depending on the degree of compression. Bluetooth will introduce a delay or latency to the audio signal which is a problem for surround sound and video applications because the wireless speakers will be out of sync with the remaining loudspeakers and video.

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. The audio signal will retain CD quality because no audio compression is used. 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.

This technology is rather robust against radio interference from other wireless transmitters by employing a method known as forward error correction. 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 transmit audio to any number of wireless receivers from a single transmitter. This is significant for distributing audio to several rooms in the residence.

Recently more and more wireless audio transmitter products have emerged which promise to bring the ultimate freedom of broadcasting music throughout the home. We will look at the most widespread technologies for wireless audio and give some advice for choosing the best wireless audio product.

If your home is not wired for audio then you face quite a problem when you want to get your music from your living room to your bedroom. Frequently the audio source cannot be moved. Running speaker wires between rooms will be expensive and for that reason a lot of people are looking for other options. There are a number of technologies solving this problem. These include infrared wireless, RF wireless, wireless LAN (WLAN) and powerline.

Infrared wireless audio products are limited to line-of-sight applications, i.e. only work within a single room since the signal is broadcast as infrared light which can’t go through walls. This technology is often found in wireless speaker kit products.

RF wireless products broadcast the music as RF waves - either by using FM transmission or digital transmission - and can for that reason easily transmit through walls. FM transmitters are the cheapest alternative. They provide good range but the audio signal is prone to audio distortion and noise and is extremely susceptible to interference from other wireless transmitters.

Products utilizing digital wireless audio transmission, such as Amphony audio transmitter products, employ a digital protocol in which the audio is converted to a digital signal prior to transmission. This method ensures that the audio quality is entirely maintained. Some transmitters utilize some type of audio compression, such as Bluetooth transmitters, which will degrade the audio to some degree. Transmitters which send the audio data uncompressed will achieve the highest fidelity.

Wireless LAN (WLAN) products are useful when streaming from a PC but will add some amount of latency or delay to the signal because wireless LAN was not originally designed for real-time audio streaming. WLAN receivers usually require buying a separate LAN card to be plugged into every receiver.

Powerline products employ the power mains to distribute audio and provide great range but run into problems if there are individual mains circuits in the house in terms of crossing between circuits. Another problem confronting powerline products are strong power surges and spikes. Such surges can cause drpouts in the audio due to errors in the transmission. To safeguard against these errors, powerline products normally build a delay of several seconds into the transmission.

Here are some recommendations for selecting the optimum wireless audio system: Try to find a system that can run several wireless receivers from a single transmitter. Ideally an unlimited number of receivers should be supported. That way you don’t need to purchase additional transmitters when you begin adding receivers in different rooms of your house. Some devices have some type of error correction built in which will help guard against dropouts in case of strong wireless interference. Pick a digital RF audio transmitter to guarantee that the audio quality is maintained. Make sure the audio delay is less than 10 ms if you have a real-time application such as video.

Pick a transmitter that has all of the audio inputs you need, e.g. speaker inputs, RCA inputs etc. Make sure that you can purchase additional receivers later on as you expand your system. Check that you can get receivers with speaker outputs for connecting regular loudspeakers as well as receivers with line-level RCA outputs. Choose a transmitter that can adjust the audio volume of the input stage. This will give you the flexibility to connect the transmitter to any kind of equipment with different signal levels. Otherwise the audio may get clipped inside the transmitter converter stage or the dynamic range is not fully utilized.

For high amplifier power efficiency and best sound quality, check that the amplified receiver has a built-in low-distortion digital amplifier. Verify that the amplified receiver can drive speakers with the preferred Ohm rating and that it is small and easily mountable for easy installation. 5.8 GHz wireless products normally have less problems with interference from other wireless transmitters than products operating at 900 MHz or 2.4 GHz.