Home owners long for the ability to place speakers anywhere in the home without having to worry about running audio cables and/or power cables across the floor. Similarly, they would like to be able to walk around the house wearing headphones or earbuds listening to their favourite music without disturbing other family members who may not share their musical interests. But wireless technology to date has always come with significant compromises, such as lower audio quality, poor radio performance, and frequent battery replacement/recharging, that have prevented the application from taking off.
This article discusses the audio quality characteristics that wireless technologies need to satisfy this application, and the trade-offs various wireless technologies present. Audio quality covers a range of topics involving metrics such as dynamic range and total harmonic distortion. In the context of wireless audio streaming, the performance of the radio channel also has an impact that must be considered in the category of audio quality.
Analog vs Digital
Most people are familiar with 'radio static'. When audio is carried over a radio channel using analog technology, there is no opportunity to correct any errors that occur during transmission so any small corruption of the radio channel results in an audible artefact in the audio stream. And radio connectivity in the real world is always imperfect.
Radio interference is pretty much unavoidable, especially when talking about consumer electronics devices using shared radio spectrum. But even when there are no other radios nearby, interference can be caused by multi-path fading, which is essentially the radio interfering with itself by bouncing off walls.
So when considering how to wirelessly stream high quality audio, we immediately turn to digital technology. Digital transmission of audio gives us the ability to detect transmission errors, correct them before they reach the listener, and take steps to avoid future errors.
Digital radios for consumer electronics, including wireless LAN (802.11b/g), Bluetooth, Zigbee, cordless telephones, and baby monitors use unlicensed radio spectrum, known in the U.S. as the Industrial, Scientific and Medical (ISM) bands. The spectrum from 2.40GHz to 2.48GHz is an example of one of these bands. Any device wishing to communicate in this band must be able to deal with the interference generated by these other devices, as well as avoid causing undue interference with the other devices.
A robust overall solution must be able to re-transmit data corrupted by interference at another time and possibly on another radio channel when and where there is no interference. The ability of a wireless digital audio solution to do this depends on several factors:
- The size of the audio buffer determines how long the system can wait for interference to clear up before the audio stream is affected. The buffer allows the speaker/headphone to continue to play buffered data while the radio connection is being restored. Although larger buffers can deal with more interference, they also introduce more latency that can be unacceptable in certain applications.
- The peak bit rate of the radio channel determines how much time the radio must be active to transmit the audio data (the longer the radio is on, the more likely it will get affected by interference), and how quickly the radio can transmit data that has built up in the audio buffer to get ready for more interference.
- Minimum bandwidth requirement refers to the amount of spectrum required by the wireless solution (the more spectrum consumed, the more likely it will be affected by interference).
- Dynamic frequency diversity refers to the ability of the wireless audio solution to quickly move to another frequency, or channel, if the current channel is experiencing poor performance. Speed is critical.
Different solutions make different trade-offs of these elements. For example, several proprietary radio solutions use a Wi-Fi (802.11b/g) RF front end with a proprietary baseband. These solutions benefit from higher bit rates, but suffer from larger spectral footprints and may not be able to switch frequencies quickly.
Bluetooth solutions have a relatively low bit rate (forcing the use of lossy audio compression) and use Frequency Hopping Spread Spectrum (FHSS) to get frequency diversity, but still have a minimum spectral footprint of about 20MHz which is similar to Wi-Fi (see figure below).
SMSC's Kleer technology is an example of a technology that has sufficient throughput to transmit lossless uncompressed audio, and only requires 3MHz of spectrum with 16 channels to select from. It is much easier to find 3MHz of available spectrum than it is to find 20MHz+, thus this type of radio is much less likely to cause interference (or suffer from interference) while coexisting with other radios (including other Kleer radios) in the same band.