The ATW-T901A system 9 frequency-agile VHF body pack transmitter from audio Technica is a single-channel transmitter, although it is a frequency Agile system, which means you can select any one of the four frequencies (169.505, 170.245, 171.045, and 171.905 MHz) and use up to four system 9 wireless systems at the same time, if each system is set to a different frequency.