High pitched tones in recording from Germany

[daeron80]

The video team from my department just returned from Germany where they recorded a large number of videos in a variety of locations - old churches, castles, outdoor settings, etc. They recorded using Sanken lapel mics, Lectrosonic transmitters and receivers set to frequencies that are safe in many part of the US, and a SoundDevices 702 tracking to SD card. We use this equipment relentlessly here in the States and get excellent results.

But all the files that came back from Germany contain a set of constant tones at 3120 Hz and its harmonics. They are just loud enough to be noticed in the silences between words. iZopope RX handles it fine. But just curious. Any idea what could have caused it?