Avian’s Blog

It sounds strange, but analog FM wireless microphones (the kind you find in studios, theaters and soon-to-be-restaurants) are one of the obstacles in the efficient re-use of the UHF band that has been freed with the digital switchover. They operate in the same frequency band as the now silenced analogue TV broadcasts, but on the other hand have never been well regulated. While locations and frequencies of TV transmitters are well known, microphones can appear on a lot of different channels and practically in any location.

New devices operating in the TV white-spaces can't be allowed to interfere with them, so they either can't use frequencies where microphones appear (leading to unused spectrum) or they must detect them using spectrum sensing. Reliable and energy-efficient detection of wireless microphones at low signal levels is an open research topic.

To test different ways of detecting microphones, IEEE has come up with simulation waveforms that can be easily reproduced on lab equipment and are reasonably similar to what microphones transmit in real-life usage. They simulate a silent microphone, microphone used with a soft voice and microphone used with a loud voice by frequency modulating a sine wave with different baseband frequencies and deviations (described in this Word document).

For example, here's how a soft voice simulation looks like on a spectrum analyzer when transmitted by a big and expensive Rohde&Schwarz vector signal generator. This is a 3.9 kHz tone modulated using a 15 kHz deviation:

And this is the same signal transmitted with an USRP N210:

As I mentioned before, VESNA also has some signal generation capabilities. This is how it looks like when the signal is simulated using a direct digital synthesis algorithm and a 4FSK modulator on the CC1101 transceiver:

The order-of-magnitude in price difference in hardware shows itself well enough here as transmission from VESNA obviously differs quite a bit from the desired. But still it's good enough for some detection experiments and of course, you can't have a USRP mounted on every light pole.

Although I haven't yet done detailed measurements, VESNA's transmission actually does seem to comply with FCC's requirement regarding spurious levels for wireless microphones (but not with more stringent European ETSI regulations).