Looking for better audio. Currently using H4N

[Rick Burnett]

The device sends a tone, or noise (some are selectable between tone and noise), into one channel of the camera's audio input. That basically forces the ACG to clamp down and stay there, so it doesn't raise the gain at all when audio in the other channel gets quiet. Using one of these devices also means that you're stuck with only one record channel. There have also been issues with cross-talk between the noise/tone and the primary audio.

Some of them use a very high frequency that you can notch filter it out of the recording as well, and are selectable. I had the beachtek for the 5D and it worked okay. I could pull the tone out no problem without degrading the audio (as nothing of interest to me was in that range).