Author: Lance Glasser

Nine hints for better, cleaner WiFi audio streaming

If at all possible, use the 5 GHz rather than the 2.4 GHz band. The 2.4 GHz band has the advantages that inexpensive or old Android phones cannot receive 5 GHz and that the 2.4 GHz band penetrates walls better and has slightly more range. But 2.4 GHz has numerous deficiencies. First, there are only three non-overlapping 2.4 GHz bands, which contribute to the band being crowded. On top of that, there are numerous other devices using the same band, including wireless phones and microphones, garage door openers, and microwave ovens. Only other Wi-Fi signals will show up if you use a basic Wi-Fi scanner, but all of these devices interfere. Bottom line, 2.4 GHz has way more “pops and clicks” than you will hear on the 5 GHz band. On 5 GHz use a 20 MHz wide channel. Going wider to 40 MHz or even 80 MHz will not make the system faster or increase capacity. What it will do is give more opportunity for other transmitters to interfere. Essentially, a wider bandwidth is just a bigger target. Advanced Wi-Fi access points have the ability to jump from channel frequency to channel frequency so as to try to find a quiet channel. This is perhaps OK for the first hour or so of operation, but if it doesn’t settle down, it will be a problem. Every time the...

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Balancing bandwidth considerations

When looking at a streaming WiFi audio system, there are several potential bandwidth bottlenecks. The Ethernet Switch The Venue Server The Wi-Fi Access Points IP Address space In our last technical blog, here, we looked at the Wi-Fi access points. Access points must be added to reach the total number of clients (smart phones and tablets) that will be listening at one time. Doing some quick calculations on data rates and bandwidth, assume 150 kbps for the communication to each phone and a 50% load capacity on the Ethernet, then 100 baseT (100 MHz) Ethernet can support about 300 phones and Gigabit Ethernet can support about 3000 phones, assuming no other significant traffic. It would presumably be best to use a cut-through switch for minimum latency, but as a practical matter, no one does this because of the cost. ExXothermic has designed a range of systems that can handle different number of clients depending on the processor and network interface, starting with 150 client phones for the most basic offering. Finally, one does need to get enough IP address space in the network so that each phone can grab one from DHCP. The usual netmask of 255.255.255.0, for instance, only provides for about 250 addresses. And don’t forget about the impact of DHCP lease times (we recommend 120 minutes). An IP address for a phone that has left the building...

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Load testing WiFi access points

We have recently been load testing WiFi access points for use with WiFi audio streaming. We had a lot of questions on the results because the specifications of enterprise grade WiFi do not really address our special case. One one hand, they talk about bandwidth, but we have very small packets and are not really bandwidth limited. For instance, an 802.11ac access point with an 80 MHz wide channel could theoretically support 3000 phone. That is way too high. And to maximize the quality of the audio, we set the bandwidth to 20 MHz. This minimizes interference with other WiFi devices as well as with other radios in the band. We do this because if one has high levels of interference, one hears the lost packets. Unlike a system like Sonos, we tune our system for minimum latency and best lip sync, which means we have no time to retransmit lost packets. In our system, one does not hear a single lost packet, but if there are several in a row, one does hear that. On the other hand, if one looks at the specifications for the number of VoIP (Voice over IP) calls an access point can support, one gets a very small number, e.g., 30 or 40. That is too low. VoIP traffic is not synchronized and people can talk at any time. In our system, there...

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Soft gain adjustments

Version 2.40 of our venue server software introduced soft gain limiting for our gain adjustment function. This is for use in cases where the audio volume of a particular source is too low and must be boosted. The gain is adjustable in 6 dB increments. That is, every increment is roughly equivalent to moving twice as close to an audio source. The venue servers take in line-level inputs, but not all sources conform. Here are the gain curves that we have implemented. The soft limiting make the audio output sound more pleasant when over-driven. The control is done most easily by our Cloud Server. Here are what the controls look like.  “0” corresponds to 0 dB, “1” to +6 dB, etc., up to +24...

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