Audio-Technica's System 9 is a four-channel frequency-agile wireless system designed to provide rock-solid performance along with easy setup and clear, natural sound quality. Featuring sleek, stackable, contemporary styling, System 9 is available in handheld, headworn, guitar, lavalier and body-pack configurations. The system offers four compatible user-switchable VHF channels (169.505, 170.245, 171.045 and 171.905 MHz).The versatile ATW-T901a UniPak body-pack transmitter has both a high-impedance input for instruments and a low-impedance input with bias connection for use with dynamic and electret condenser microphones. The body-pack transmitter use internal AA batteries and have Power/Mute switches and input Trim (level) adjustments.