AGC (auto gain control) is a feature built into some HDSLRs like the Canon 7D and T2i. This function causes your audio levels to raise or lower depending on how loud the audio being recorded is. This may be a good function for event shooting or news gathering when you don’t know the audio level of the location you are shooting, but it is horrible for narrative work.
While it is recommended that you use a separate recording device for capturing audio, like the Zoom H4n, if you need to record audio on your HDSLR and it has AGC, you can now use a cable system to disable it.
Sescom makes a Y cable that allows you to connect an audio device like an MP3 player and play a tune that disables the AGC while recording clean audio on your HDSLR. Find out more information on Markertek’s website.