Camera exposures and audio recording levels are similar. If you shoot under exposed, adding gain in post results in visual noise. Under recording in audio and boosting in post results in bringing up the system noise level.
Overexposing and over-recording have a similar parallel result.
The best thing to happen in pro audio has been the move from 16-bit to 24-bit recording. Those extra 8 bits (when properly engineered and designed) allow sound to be lower in level and still not be as far into the noise floor as 16-bit audio.
This is another of the problems with HDV audio. It is not even 16-bit 48 khz. HDV audio has one fifth the data rate of 16-bit, 48 kHz audio, so noise low record/floor problems tend to be more obvious.