In my experience, when I use my camcorders in auto mode - specifically, automatic audio gain mode - I hear recorded what you hear. It is especially pronounced when there is no noise when the video is being recorded. The camcorders' auto mic gain is doing its job by increasing the audio gain to grab whatever low volume is available to record.
If the camcorder did not do this, low volumes would be very low and the camcorder manufacturers would get complaints that the mics in the camcorders can't record audio very well.
When I use the manual audio gain control in my camcorders, this disables the automatic audio gain capability - so it is up to me to pay attention to the audio levels going in to the camcorder's mics and getting recorded. When there is no audio I want recorded, I turn the mic gain down. When there is audio I want recorded, I turn up the mic gain to a level that allows for good audio recording levels. It also means that the gain control does not set itself so under low audio level situations, there is no "background noise" recorded...
In my opinion, there is no "magic bullet". Many people do not want to deal with manual audio gain adjustment process. As a consequence, the manufacturers have removed manual audio control from most low-end and mid-range (up to about $800) consumer camcorders. This is a money saver (single circuit rather than two and a switch) for the manufacturers and reduces the cost of the camcorder to the consumer (fewer parts, simpler design).
If the auto-mic gain circuit could be designed to react only to certain audio characteristics, there might be resolution for some people, not all. I presume most folks are not willing to pay extra $ for design and implementation of an even more sophisticated audio algorithm that meets everyone's requirements.
When video recording is made under "regular" audio conditions (i.e., music or TV in the background, someone speaking at a normal voice audio level, etc...) I presume the undesirable background audio to which you refer does not get recorded. That's how it works on my camcorders when auto mic gain is used - basically, the recorded audio sounds acceptable when there are good levels of audio being recorded.
There is an alternative to camcorder-based manual audio gain control use when recording, but it still requires your intervention. Assuming the background noise issue is the same as described above, some sort of "manual intervention" can still happen after recording... Import the video to a computer for editing and adjust the audio level using the video editor of your choice. Export the edited video project to your choice of video file with the "fixed audio".
I hope this helps...
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