The Gopro Hero 3 USB to 3.5mm audio cable showed up last night and I had a chance to do a quick audio test using the adapter. In these test I used the Rode VideoMic pro plugged directly into the usb adapter and recorded audio at 0db of gain as well as +20db of gain to see how well it performs.
I haven’t had a chance to import this audio into Sound Forge and take a look at it but to me it sounds like the AGC is much better controlled then other devices and doesn’t seem to ramp up the volume in quiet areas of the audio nearly as much. At 0db of gain I can hear the noise floor creeping in, but the audio sounds usable (with a little bit of work). However at +20db of gain, the audio sounds like it starts to have strong compression applied and it sounds like there is some sort of limiter effect being used.
Take a listen to the audio and see what you think. The USB to 3.5mm adapter doesn’t seem to be a complete waste of money, but if you want good quality sound with your Hero 3 videos you’ll probably still want to use dual system sound.