Miniature wireless microphones reveal vocal relationships in a group of small laboratory animals

The spread in transmitting frequency is used to enable the simultaneous recording of up to 12 animals. For each animal one needs a separate receiver. Transmitting frequency is about 300 MHz. All experiments were done indoors. Crossed yagi antennae (Winkler Spezialantennen, Kreuzdipol 300, directional antenna tuned at 300 MHz) were connected to AOR 8600 receivers (AOR, Ltd., Japan) that were modified to an audio bandwidth of 12 KHz. Signals are received as FM, at an intermediate frequency bandwidth of 100 KHz. The antennae were less sensitive to changes in orientation of the transmitters relative to the antennae, for instance when the animal moved. To compensate for frequency drift due to a change in battery voltage or temperature fluctuations, the frequencies of the transmitters were tracked by the receivers using a custom-built software based tracking system.