FIG. 1: Conventional FM transmission modulates the frequency of a high-power, narrow-bandwidth carrier well below 1 GHz, whereas UWB sends low-power, wide-bandwidth pulses centered anywhere between 3.1 and 10.6 GHz. SpectraPulse is centered at 6.35 GHz with a bandwidth of 500 MHz.
Illustration by Chuck Dahmer
Wireless microphone systems have been a mainstay of performing musicians, offering much more freedom to prance around the stage than their wired counterparts. However, these systems are not without their drawbacks — for example, other signals on nearby frequencies and reflections from structures in the vicinity can interfere with the intended signal. To make matters worse, virtually all conventional wireless audio systems will be completely disrupted when the radio-frequency (RF) spectrum is reallocated to accommodate new services, such as digital TV and wireless Internet access, when analog TV broadcasts are shut down in February 2009.
The problem of interference is inherent in the way most wireless mic systems operate. Essentially, they are little FM radio stations that transmit the audio signal by modulating the frequency of a sinusoidal carrier wave. When that signal is reflected within the environment (called multipath interference) or other signals near the carrier frequency are present (which will be more and more likely in the future), the intended signal is often garbled or lost.
Audio-Technica (www.audio-technica.com) has adapted an all-digital approach that overcomes many of the problems associated with conventional wireless mic systems. Called ultra-wideband (UWB), this approach is different from FM transmissions. Instead of modulating a high-amplitude, single-frequency carrier, UWB sends short-duration, low-amplitude pulses that span a wide range of frequencies encompassing at least 500 MHz around the center frequency (see Fig. 1).
The idea dates back to the late 1800s, when Guglielmo Marconi developed a “spark-gap” system that transmitted Morse code pulses wirelessly across the Atlantic Ocean. After that system was supplanted by more efficient narrow-band techniques, the concept languished for 100 years until it was revived in the form of UWB, which was made possible by the ensuing advance in digital electronics.
The Audio-Technica system, dubbed SpectraPulse, was developed with Multispectral Solutions (www.multispectral.com). It operates in the 6 GHz range with pulses that span a bandwidth of about 500 MHz and last less than 3 nanoseconds (ns). Whereas traditional wireless mic systems transmit about 50 milliwatts of power, SpectraPulse transmits about 40 nanowatts, roughly a million times less. Interestingly, this puts the signal within the noise floor of a typical operating environment, providing unparalleled security against unintended detection, yet the receiver can be programmed to decode the precisely defined pattern of pulses with ease.
A-T's first SpectraPulse product is a wireless mic system designed for conference rooms. Using a technique called time division multiple access (TDMA), up to 14 boundary mics can be used together with no concern about interference, intermodulation distortion, or crosstalk between them — each mic's pulses are precisely timed to interleave with the pulses from other mics in the system. A 16-bit, 24 kHz A/D converter in each mic digitizes audio from 100 Hz to 12 kHz, which is fine for speech applications. The ADC's output is fed to a transmitter, also located in the mic housing. The system achieves a data rate of 8 Mbps over a distance of 75 feet.
Among the many benefits of UWB is low latency compared with other digital wireless audio systems and fast recovery if synchronization is lost. SpectraPulse exhibits a latency of less than 1.2 ms, and it can recover from lost sync in only 3 ms — by comparison, it takes 1 to 2 seconds for a typical FM system to recover from lost sync.
Audio-Technica plans to apply its SpectraPulse technology to music-oriented wireless mic systems. With no need for frequency selection and coordination, as well as immunity from dropouts and interference from other RF sources and multipath reflections, the promise of UWB is definitely worth pursuing.