Just bought cx900 today...
The noise is kind high..I know this is underexposed. This is 0db gain...but still I feel the noise is crazy...
Do I get a bad copy???
As you said: This is underexposed. Just not enough light for the camera to record a proper image. First physics, then math: The codec brakes.
I suppose you're new to filming? Most people are surprised about how much light cameras need.
Slightly bigger sensors like the 900 has help to some extend, but still can't do magic