Does it sound anything like HMI lighting? I hear that now and again. But I suppose that's more of a whine.
Recently I just watched a DVD with EXTREME hiss in some scenes coming from the center channel whenever dialogue was present. It was clearly being expanded downward between lines. Let's just say this film was about clocks and orphans. Anybody else notice this?
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It's highly possible the tone wasn't detectable on the stage if it was large enough to be running an X-Curve. And thus if the film wasn't re-mixed for DVD/BluRay as they normally are, it's possible that tone which was always present is now audible in a near-field/mid-field home theater environment. Don't know this for a fact, could be a possibility though.
It was just for reference :), I don't think that is the explanation either. However, it's useful to know it is a feature film.... Films are usually mixed in environments similar to those of a movie theatre; if I am not wrong, speakers in theaters usually don't go up very high in the frequency spectrum, they are quite dull, so it very probable that the hiss that you can hear in those frequencies wasn't very, or maybe at all audible in the mixing stage, so maybe they didn't re-mixed it for the DVD release and that's why you get some hiss, specially if you are listening in some -pretty good- quality monitors.
I hope that my knowledge is pretty accurate, and that this helps :).
Was the hiss noticeable during the louder parts, too? Or just soft parts. Could be a result of improper dither. Compressing audio for DVD, you have to dither down to 12 or 16 bit, which can cause an audible hiss in the lower levels. Just a thought. My guess is residual tape hiss if it's an older film that's been put to DVD without re-mastering (as was mentioned) if not that.
Or maybe it's all about the original location sound recording :)
There is so much needed to be done while shooting. Muting HMIs, Camera noise, HD noise of RED and such, killing reverbs etc... And sometimes location recording team don't get the chance to do their job properly, unfortunately.
And maybe those productions don't appreciate clean dialog so much that they'd ADR such problematic scenes...
I've noticed this as well in certain films, big and small budget. Generally seems to only happen when dialogue is played, which leads me to believe it is from the location sound. I've even once noticed it only for one actors dialogue. Seems like it would be easy to filter out with a narrow band eq or a noise remover plugin. Then again if your hearing doesn't exceed 12K, you're not getting that information.