Crazy Idea: Noise cancelling RockBox

[keuleJ]

Hey guys, I had just a crazy idea:
Would it be possible to use the internal mic of certain targets (H1x0 etc.) to cancel background noise?
OK, maybe the distance between the mic and the headphones would be a problem.

See http://en.wikipedia.org/wiki/Noise-cancelling_headphones

[Yotto]

Not to mention the fact that it's digital, so there is a (very small but still nonzero) processing time between recording and playing.  So the player would be canceling what happened 0.1 seconds ago, which would sound just odd.

[keuleJ]

Not to mention the fact that it's digital, so there is a (very small but still nonzero) processing time between recording and playing.  So the player would be canceling what happened 0.1 seconds ago, which would sound just odd.

Thats probably the reason why they say that the technique is good at cancelling continuous sounds (such as the humming onboard of a plane) and not so good at cancelling "sudden" sounds.

[Jillorious]

Just remember: most internal mics in todays' DAPs pick up the hard drive whim every time the hard drive spins up, but is only audible in a recorded file, but while listening to music the whim doesn't get to your ear. Now, if noise-cancelation will be implemented, the software will constantly generate a 'noise cancelation sound wave' for the hard drive whim. Miserable.

[Llorean]

Maybe disable noise-cancelling while the disk spins up, or at least attempt to identify what that sound is and dampen it somewhat? It's not like Rockbox doesn't know when the disk is going to spin.

[Jillorious]

BTW (might be a bit off topic) if rockbox CAN identify the whim; wouldn't it be a good idea if it filtered it while recording, so that the resulting file is clean just as with an external mic?

[Llorean]

I'm not so sure it could be done at anything remotely like a quality that would improve recordings. I imagine you'd just distort the sound behind it. Though I could hardly say for certain. In the context of noise cancelling though, it could certainly prevent a spike in the noise cancelling intended to remove a sound the user won't here.

[saratoga]

I think you need to actually record sound hitting the ear itself, not just sound in the room since phase will be different if you're off by even a short distance.  You could probably do it on portables that had an external mic jack, but I don't see how you could make this work in general since i don't know of any headphones with a proper mic built into the ear piece.

[Jillorious]

I think you need to actually record sound hitting the ear itself, not just sound in the room since phase will be different if you're off by even a short distance.

Even for continuous noise?

[JdGordon]

Even for continuous noise?

yes, for any sound...
the way sound moves is horrible physics, the difference 30cm can make is rediculous... it could be that the mic picks up noise that your ears dont hear because of the phase, or vice-cerse the mic could be missing the very annoying noise its supposed to be cancelling.

[amar]

Or even (if the phase is completely out and I remember my physics) amplify the annoying noise

[saratoga]

I think you need to actually record sound hitting the ear itself, not just sound in the room since phase will be different if you're off by even a short distance.

Even for continuous noise?

Imagine a 1KHz continuous tone.  Wavelength is (330m/s)/1000Hz.  To avoid amplifing the noise, you need to be within 1/2 that distance just to break even.  To get any real benefit, you probably want to be within 1/10.  So the worst you'd want is 3.3cm from the inside of your ear.  And realistically, even that isn't going to work very well.

[Jillorious]

TYVM for clarifying matters.