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Is this how noise-cancelling headphones work?
by JosephRedfern 12y ago
Is this how noise-cancelling headphones work?
- citizens 12y agoNot sure, but it's how a lot of karaoke tracks are made.
- clay_to_n 12y agoAnd if an artist releases instrumental or a capella versions of their track, you can usually use this to get the other. Only works if they're exactly the same as the original of course.
- psuter 12y agoAnother trick is to invert one of the stereo channels and add it to the other one: this eliminates everything that was mixed in with neutral pan. That's typically where the voice is. It will also remove e.g the drum kick, but this can be salvaged with a low-pass filter. Note that you end up with a mono track.
- deleted 12y ago[deleted]
- derekp7 12y agoWhile on the subject of these tricks -- disconnect the ground from the audio out, and tie the speaker grounds together. This causes the speakers to use each other as ground, which in effect causes the middle voice to cancel out.
- nitrogen 12y agoI used that trick to get a cheap rear channel from my thrift store stereo back in my high school days.
- robszumski 12y agoThat's really interesting. I never knew how they did that. Canceling out the frequencies of the voice makes complete sense.
- robin_reala 12y agoYes. They have a microphone and a circuit that inverts that input and mixes it into the recording.
- mbell 12y agoThat's not really what happens in noise-cancelling headphones. The transforms required are fairly complex, simple mixing is a long way from what is required to achieve good results.
- ethanbond 12y agoThe transforms aren't complex... They're inverted. What's complex is getting the timing right (can only occur at a fixed distance from the ear so they can delay the generated canceling waves appropriately). It's just wave addition. -1 + 1, occurring at the same time, is 0.
- mbell 12y ago> The transforms aren't complex... They're inverted. Again, It's not that simple. Most ANC headphones use a digital feed-forward control system using a variant of the LMS algorithm to provide coefficients to an adaptive FIR filter(s). Generally you also have to apply additional filters to model the loudspeaker / system response. Better systems will use a hybrid analog/digital approach, either way, there is a lot more to it than 'invert the mic signal and add'. The reason that ANC headphones didn't really exist until recently is that sufficiently high performance yet low power DSPs were needed to perform the necessary computations.
- mturmon 12y agoThanks for the correction. I assume that the simple v(t) + -1*v(t) = 0 approach is unworkable because you can't generate the canceling signal (-1 * v(t)) fast enough? By the time you have received it, amplified it, and played it back, it's too late? So you need a predictive approach.
- deleted 12y ago[deleted]
- jayd16 12y agoEveryone is getting really excited because they think this is how it works, but no, not exactly. The shutter sound comes from the same speaker as the anti-shutter sound. The phone is mixing the tracks together and playing nothing. It's not like both sounds are played and they neutralize each other as sound waves.
- lovemenot 12y agoSince it would have more general applicability, I am curious how effective could the noise-cancelling-like method could be? What would be the constraints on its effectiveness?
- wyager 12y agoIt's the same process that would happen if you actually played both sounds. The superposition is just happening in software instead of air.
- ANTSANTS 12y agoInverted sound waves can cancel each other in air, I've heard (or not heard) the effect with my own ears.
- TTPrograms 12y agoRight, but for any point source this is effect is going to vary spatially. That's why you can't make one speaker totally cancel out another speaker over a whole room.
- jayd16 12y agoSure, but my point is that there is only one speaker involved so the cancellation happens in the electronics, before any sound is made.
- kordless 12y agoIt's interesting to think of 'cancellation' in different mediums. Waves in physical space. Radio in electromagnetism. And now 'computed' waves cancel each other too.