WIRELESS AUDIO
Many professional audio products today use Dante to send and receive audio. A frequently asked question is ‘Can we go wireless?’ For the answer we have to go back to the early years of Ethernet in the 1970s, when Robert Metcalfe of the Palo Alto Research Centre in California, USA, proposed a method of sending and receiving data packets between multiple devices connected to a shared conductor: a coax cable. The more devices that were connected to the coax cable, the higher the probability was that two devices would, by accident, start to send packets at the same time, distorting each other’s messages.
The solution Metcalfe proposed was to introduce a ‘packet collision’ protocol. Devices would constantly ‘listen’ to the coax cable activity and only start to transmit data to the cable after confirming that there were no other devices transmitting at the same time. During transmission the device would continue to listen back to examine the signal. If the device detected that its signal was distorted - caused by one or more other devices transmitting to the cable at the same time, then the device would stop transmitting; wait for a random time; and then try again.
Because the random time would differ from device to device, the result was that one of them became the first to start re-transmitting, so the other devices could detect the transmission and delay their own transmission until the coax cable was silent again. This way the system worked, even with many devices connected. The downside was that the more devices that were connected, the more collisions would occur, and the slower the system became.
A few decades later, the coax cable has been replaced by CAT5 twisted pair cable, with separate send and receive connections to an intelligent ‘switch’, so that collisions can never occur. This is one of the reasons why today’s audio networks work so well; in addition to the increased speed – from a few Megabits in the 1970s to a Gigabit or even more today - they are capable of supporting thousands of audio channels with very low latency.
How about wireless radio networks, then? A few small radio frequency ranges have been released by governments worldwide for what are referred to as “Industry, Scientific and Medical” (or ISM) applications. These are ‘Wireless Ethernet’ - nicknamed ‘WiFi’ (2.4GHz and 5GHz), Bluetooth (2.4GHz) and DECT (1.8GHz).
Unfortunately, these networks still use the packet collision protocol, because all devices share the same conductor: the electro-magnetic field in a space. For the most commonly-used wireless network, WiFi, the theoretical maximum speed is limited in comparison to a Gigabit Ethernet cable: just 100MHz overall bandwidth, arranged in up to 14 overlapping frequency bands - ‘WiFi Channels’ - each 22MHz wide. The actual speed, however, depends very much on the WiFi usage in a certain area. The more computers, laptops, tablets and smartphones are connected to a Wireless Access Point, the less consistent the speed and latency.
For this reason, WiFi audio transmission is limited to a few channels and it cannot be used for live audio. For playback of pre-recorded audio, latency is not an issue - this is the reason why the home audio industry embraced WiFi and BlueTooth as a main method of transmitting limited channel count (stereo) audio, for example through Apple’s AirPlay or Yamaha’s MusicCast systems.
For higher channel counts with lower latency, manufacturers can use proprietary protocols using the same ISM band as WiFi - 2.4GHz or 5GHz - but they have to use all the tricks in the book to support multiple channels. An example is the Line 6 wireless microphone system, which supports up to 14 audio channels using antenna directivity and diversity to cope with WiFi traffic in the same area, as well as frequency diversity using two or even four carrier frequencies simultaneously per audio channel to achieve a consistent connection.
The answer to the question ‘Can we go wireless?’ for networked audio is absolutely yes but, unless you use one of the few dedicated (and very expensive) laser-based systems on the market, you will have to use one of the ISM range radio frequency systems such as WiFi, with a much more limited scope compared to cabled Gigabit audio networks like Dante.
