This week we spoke with Live Sound Applications Engineer Don Boomer of California based manufacturer Line 6, which makes a well regarded line of 2.4 GHz wireless microphones. We explored the benefits of digital wireless, the challenges of working in the 2.4 GHz band, and why Wi-Fi and other data protocols are such a threat to wireless microphones operating in unlicensed spectrum.
What is Wi-Fi?
It’s a communications system for computers. It sends digital information back and forth, not necessarily in real time. In fact, there’s no expectation that the data is in real time.
What is 2.4 GHz?
2.4 GHz is the frequency of a radio band on which you can make different kinds of radio communications, including Wi-Fi. But Wi-Fi is not really suitable for live music because it’s not in real time.
That’s always been the hurdle to get over with digital wireless audio. It’s easy enough to transmit data from place to place, we do that every day, but to do it with high audio quality and in as close to real time as possible is a trick.
Explain how the Wi-Fi protocol organizes and portions out the 2.4 GHz band.
In the United States, we’re allowed 11 bands for eleven devices in a given place. In Europe, they go up to 13, in Japan they go up to 14. In the United States, where 13 and 14 would be they use for satellite phones. Technically the 2.4 GHz band is open at the top unless you run into someone with a sat phone. I’ve never encountered one, personally.
What most people don’t realize in Wi-Fi is that it’s mostly full of air. You’ll see a big Wi-Fi channel that looks like it’s full of energy. But it’s really like chicken wire, because these devices do not transmit continually.
Wi-Fi protocol channels. Courtesy Kozuch.
Do the FCC rules require all digital devices operating in 2.4 GHz to follow the 11 channels somehow, or just wireless data devices using 802.11?
There are no imposed channels in the band, although there are some power considerations. Different technologies “channelize” by different methods. Wi-Fi, Bluetooth, and Zigby all have different channelizations. It is completely arbitrary and up to the manager of each system.
What digital protocol is Line 6 using to send wireless audio over 2.4 GHz if not 802.11 (Wi-Fi)?
It’s a completely proprietary protocol. What Line 6 does that’s unique is we take 24 bit digital audio and we chop it up into little six bit subpackets, then broadcast the sub packets on four different frequencies within the 2.4 GHz band in a random fashion. We jump all over the band to find unused frequencies because that band very hostile. It’s full of RF.
Since we have four redundancies, it’s not necessary with our system to recover all the data. In fact, to knock a Line 6 mic off the air you have to consecutively knock off six packets. If you knock off five, the mic will still create perfect audio.
Are these duplicate packets being sent across four carriers?
They are not duplicate.
Tell us more about how digital wireless and the Line 6 protocol works.
The simple explanation that’s easy to understand is that over RF I’m sending the digital code for “5,” the digital code for “3”, and a checksum that says “8.” So if the 5 gets there, and the 8 gets there, and the 3 doesn’t, the computer simply writes the 3 back in. And so if all the numbers don’t get there, we can error correct for the numbers that are missing to a point. Plus, Line 6 mics are sending four of these streams in tandem, each on different frequencies, so the probability that the receiver will have enough information to reassemble the packets is higher than with a single stream.
Is this called frequency agility?
No, not specifically. Although we are a frequency agile system, this particular technique is more like what is called channel hopping. Actually, strictly speaking we don’t hop enough channels to call it channel hopping, so we’re using a digital hybrid protocol. We don’t use conventional channel hopping because the more hops that you make, the more time it takes to assemble all those little bits. We want to do it in near real time. Nobody else has been able to do what we’re doing in less than 20 or so milliseconds.
There are other companies that do digital wireless, but they’re sending a single stream, and when you interrupt that stream you knock them off the air. That’s the difference between our radio and everybody else’s. Is that we’re not on a single channel. We’re redundant in moving around over four receiver channels for a single user channel.
So, why not put this proprietary protocol that Line 6 has developed over into other unlicensed bands, or down into UHF spectrum. Why do you use 2.4 GHz specifically?
You have to understand that there are bands called ISM bands. That stands for industrial scientific medical. Now, there are a lot of small ISM bands, but 2.4 happens to be one of the ones that can be used anywhere in the world. So for us, that means we have a single SKU that can be used anywhere in the world. In 194 countries.
A man installs a 2.4 GHz antenna in rural Nepal. 2.4 GHz equipment is legal anywhere in the world. Courtesy Drpaudel.
The penalty for that is that it’s not a very big band, so, it’s crowded and our limitations is that we can’t provide a lot of channels. Because, in that band, for me to provide twelve user channels, I’m using up what other companies would call 48 channels, cause we’re using them four at a time. I need those extra three carriers because it can be very crowded in that band. If I only had one stream I would be off the air all the time.
Because of the unlicensed nature of the spectrum?
Because there’s that much competition for frequencies.
So, what’s innovative about Line 6 microphones is not that they’re able to transmit high quality digital wireless audio, per se, but that they’re able to transmit high quality wireless audio in the presence of hostile interference.
Yes. The FCC does not consider us a wireless microphone, they consider us a digital data device, as well as everybody else that’s doing digital wireless audio.
Digital solves a bunch of problems that analog radio has. Some people who want to talk about the difference between digital and analog sound. There’s a very small difference between the two. It’s not part of the discussion, here. What we’re talking about is the transmission of analog audio through the air as a large, cumbersome, frequency modulated radio wave vs. digital data. Because analog is trying to send a whole lot of information, it needs a lot of dynamic range. Digital is just sending a razor thin line of ones and zeros. I’m either sending a voltage, or not a voltage. It really doesn’t matter what the waveform looks like, as it does with analog.
When a digital system does encounter interference that it cannot overcome, what does it sound like?
It mutes. It makes zero sound. Our radios produce no noise or they produce audio. There’s no turn-on, turn-off noise, fuzzing, or fading. Whereas the quality of the analog signal changes every foot you move away from the receiver as the signal degrades over distance in the air that everybody has been using sounds different at ten feet than it does at 100 feet. In a digital system, signal attenuation doesn’t impact the audible signal at all until it’s so severe that it can’t tell the difference between a voltage and not a voltage.
Our system can only make audio when you have a very specific pattern. So you’re not going to get audio from a taxi cab driving down the street, you’re never going to get noise from spark plugs or fluorescent lights or motors, because they don’t make radio energy that can be converted into audio in our digital system.
Line 6 offers users the option to select between two frequency schemes. Can you explain that?
We have two methods of transmission. In our basic scheme, which we call RF1, we use four radio individual frequencies. So, by the time you use many Line 6 devices, you are pretty much filling that band, since each Line 6 device is using four frequencies. If you completely fill the band up with Line 6 wireless you have little room left for Wi-Fi to operate in.
RFI. Courtesy Don Boomer.
For people that need to use both a Wi-Fi device, like a router or wireless access point (WAP), and 2.4 GHz wireless Line 6 units in the same facility, we created a new kind of transmission where we only use two frequencies for each device instead of four. We call this RF2. Now, it’s not as robust when I only use two frequencies, but, the frequencies are now very specific to Wi-Fi channels. So, if Wi-Fi is the source of interference, and we know what channel the Wi-Fi is operating on, there’s a method that you can use to steer clear of that channel.
RF2. Courtesy Don Boomer.
With RF2, you can run a single Wi-Fi channel, and probably run eight of our channels at the same time, and neither will affect the other at all. Without a Wi-Fi device in RF2 mode, you could theoretically run 14. In RF1 mode you can fit 12.
Other wireless audio manufacturers are currently using 8 channels in 2.4 GHz. The number of channels that Line 6 uses is related to sound quality and reliability. If we chose to lower our self imposed requirements, then we could expand the number of channels.
Because if we were in UHF, 86 MHz would be enough to fit, what? 80 analog mics under good conditions? Why are you only able to fit 12 channels into 86 MHz of spectrum which is actually, comparatively, a lot of spectrum.
Because Line 6 microphones are using four frequencies each, and are therefore four times redundant, and using four times as much spectrum. Nobody does that in UHF.
But, that’s a benefit because we can create much higher quality audio. Because analog RF, theoretically you could probably get 70 dB, but in practice, 50 dB of dynamic range is what you can get with an analog radio. That’s about cassette quality. What everybody does when they’re doing analog is that they have a noise reduction system that is very much like what we use on cassettes, kind of like a dolby dBX kind of thing, to get the noise down. Just like it did on tape, RF noise control gets the hiss down, but there are sonic compromises. It doesn’t do transients very well, it won’t do fast percussive sounds well, they get kind of rounded off. That just all goes away when you do digital.
What do you think the future of the unlicensed bands is going to be? Will other manufacturers start to move up into this specrum?
Absolutely. Let me make a distinction, the quality of radio has almost zero to do with what frequency you transmit at. You’re just looking for reliability. There are some tradeoffs—high frequencies don’t go as far, but you’re not looking for distance anyways. If you want distance, use a telephone.
First, there’s tons of opportunity in the underutilized TV band and unused whitespaces, and big companies want a piece of that opportunity.
Second, from 1962 until 2010, anyone using a wireless microphone was required to have a license. But in that whole history, the FCC had sold less than 1,000 licenses, even though we figure there was probably two million microphones operating illegally. Nobody ever cared. They [wireless mics] did not interfere with television stations.
So, what happened was the FCC eliminated the need for licenses. But, since they eliminated the need for licenses, that opened up the floor for other unlicensed devices, too. So UHF for single channel wireless mics is basically going to go away because of these changes. And manufacturers are going to be abandoning UHF, and will be putting mics wherever else they can find spectrum.
So these higher frequencies are going to be attractive to manufacturers who are looking to come on up?
Yeah, and honestly, 2.4 GHz, is going to become less crowded, because a lot of commercial stuff is going up the 5.8 GHz. Typically the bands of interest for wireless microphones are the 900s, the 2.4s, and the space in the 5 GHz range. We’re going to start to see radios in all of these places.
Do you have any hints for us about what might be around the corner for Line 6?
New stuff. We’re planning on working everywhere. Because, as new devices gobble up more spectrum, we need to keep adapting. We are improving latency, too. We started at right about four milliseconds. Our newest radios run at 2.9 milliseconds, and stuff I have on the bench runs at sub two milliseconds, faster than that even. The target is 1.5. Nobody doing any digital audio is doing anything much better than that. And certainly no digital wireless is.
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