The Challenges of Digital Audio (Part 3)
By Alan Ruberg, Systems Architect
This is Part 3 of a 4 part series; here is Part 1.
Peter Deutsch, the creator of Ghostscript and widely credited with the list of the 7 fallacies of distributed computing, once said something like “you know you’re networking when someone you don’t even know can ruin your day.” This is especially true with wireless communications because there are so many good uses of radio but very limited resources that we all share – like it or not. Frequencies are licensed for the benefit of everyone: police, air transportation, maritime, radar, television, radio, mobile phone, and satellite.
Error correction and concealment as described in Part 2 of this series can only work to a point. If the band is used too heavily, there will be audio loss. The few frequencies available for individual use are mostly found in the Industrial, Scientific, and Medical (ISM) bands set aside for non-communication applications. This means that some device could come along and indiscriminately ruin our day. The microwave oven is a good example. It heats food efficiently by using the exact absorption frequency of water. 2.4 GHz WiFi (802.11b, g, n), Bluetooth, wireless phones, toys, baby monitors, and video cameras use this frequency. Because of its popularity, the sheer amount of traffic is staggering, as shown below. It’s going to get even worse because more of your neighbors are using it for more things.
This figure plots the 2.4 GHz WiFi band on a trade show floor. Each colored box indicates a single network and its utilization. In the near future, you can expect this much congestion in an apartment building. I couldn’t use 2.4 GHz at this conference so I set up our network in another band…
To relieve pressure at 2.4 GHz, a new band was made available for radio local area networks spanning from about 5.2 GHz to 5.8 GHz. This huge allocation (nearly as much as the television channels combined) is called the Unlicensed National Information Infrastructure (U-NII) band in the US and is shown below with a plot from the same trade show.
There are two things to notice about this plot: (1) the networks are more spread out (over 5x the 2.4 GHz bandwidth is available), and (2) there is very little traffic between channels 52 and 140. A 7.1 wireless speaker system using WiSA-certified technology is on channel 140.
Regardless of the implication, the U-NII band is shared with military, aviation, and weather radar. WiFi channels 36 through 48 are simply operated at low power to avoid interference. Channels 149-165 are available in countries that allocate the 5.8 GHz ISM band. Most networks use these two sub-bands, otherwise, on channels 52 through 140, they must monitor for radar and use Dynamic Frequency Selection (DFS) to avoid conflicts. This causes performance problems for computer networks, but not for WiSA-compliant technology.
In our next installment of Challenges of Digital Audio (Part 4), Alan will discuss the ins-and-outs of U-NII and how WiSA-compliant devices use DFS as a first line of defense to provide great quality and a robust listening experience.