With the Canon 7D it is very important to choose a model with "AGC Disable" because internally the camera Auto Gain cannot be disabled. This means whatever audio you feed in the 7D will try to lift levels whenever the feed in becomes lower in volume.
The Beachtek DXA-SLR does this by feeding in a constant but inaudible low frequency tone on one of the channels. It doesn't actually disable the the camera AGC it just gives it a constant reference level of audio to stabilise it's normally dramatic fluctuations in level.
There is no after market hack (a la Magic Lantern) available for the 7D to over ride it's AGC and allow you to set in camera audio levels manually.
Cinematographer, Steadicam Operator, Final Cut Pro Post Production.
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