Monday, June 16, 2008

Unused "white space" spectrum and wireless microphones

In previous postings, we've made the case that the TV "white spaces" could be used to provide broadband internet for all Americans without harming current users of this spectrum (TV broadcasters and wireless microphone users). In a new white paper, we make the case that better use of the white spaces could actually lead to new, vastly superior wireless microphones, rather than pose a threat to live events.

The main problem that users of wireless microphones face today is coordination. A single venue may involve as many as 300 microphones. Coordinating the frequency assignments of all these microphones is overwhelmingly difficult, taking hours of careful preparation.

With more permissive rules from the Federal Communications Commission, new wireless microphones could be designed to work in a very different way. Modern signal processing technologies taking advantage of these new rules could help eliminate the coordination problem. These same new technologies would also help guard more effectively against interference of all forms, attacks by malicious hackers trying to disrupt events, or eavesdropping by individuals illegally recording events.

The proposed new rules for the TV white spaces, in addition to improving the lives of millions of under-served Americans by providing low-cost mobile broadband internet access, could also improve the ability of sound engineers to provide audio for live and recorded events without hours of difficult coordination, and without the constant threat of accidental or intentional disruption. And as we've pointed out in a recent filing with the FCC, this can all be done without disruption of existing devices during the transition period.

1 comment:

WirelessTech said...

You left out one big thing.

I keep seeing this same argument everywhere: "if wireless microphones were digital, the interference problem would go away." The problem is that most digital transmission schemes result in a slight time delay while the signal is processed. This causes the audio to be delayed by 50 to 100 milliseconds and sometimes more.

When making a cell phone call, this 'latency' is unimportant for two reasons. First, the listener cannot see the talker -- so they don't care that the sound doesn't exactly match the movement of the talker's lips. Second, the talker's voice is not transmitted back to them after being received by the cell phone network. (Sometimes the phone network allows a little of this signal to leak back to you by mistake, and you hear an echo of your own words. If it weren't so low in level it would be really annoying.)

Here's the problem: sound systems in concert halls, theaters, schools, churches, convention centers, hotels, etc. etc. all operate in real time, with no delay. When you go to a concert and see a closeup of the singer on a giant screen, the sound of the talker's voice matches the movement of their lips, right? Also, performers 'monitor' their own voice or instrument through onstage speakers or earphones so that they can stay in tune and in sync with the other performers. Now imagine that the performer hears themselves as a delayed echo, as you sometimes do on a phone call. Kind of a problem, huh?

As you point out, most digital wireless protocols aren't even allowed to operate in the White Spaces because they take up more than 200 kHz of spectrum. There are a couple of digital wireless microphones that operate in other bands, but they have some significant shortcomings. A company called X2 makes a system for guitar players that operates in the 902-928 MHz band. It sounds good, but it uses up so much spectrum that you can only operate 5 of them at a time. That obviously doesn't work for a rock concert or a Broadway theater, both of which might need 25-50 wireless mics.

Audio-Technica makes a system for conference rooms that uses Ultra-Wideband (UWB) in the 6 GHz range. This product is still very new, but it supposedly sounds good and has very low latency. Unfortunately, it is very expensive, and FCC rules restrict it to indoor use only to prevent interference with the UWB equipment used by rescue teams looking for earthquake victims. And the microphones are designed to sit on a conference table; you can't hold one or clip it to your belt, so it clearly won't work for a U2 concert or a performance of Phantom of the Opera.

The audio industry has been trying for many years to find a digital wireless format that can provide professional-grade sound quality (i.e. much better than a phone call), little or no latency, and that is spectrally efficient enough that it can allow dozens or even hundreds of simultaneous users in one location. But a solution is still a long way off.

Your point is well taken: new wireless technology coupled with revised regulations from the FCC could allow better use of the White Space spectrum. But don't make it sound like this will be easy, quick, or inexpensive.