Noise Cancelling in Software?

doc_verma wonders: "There are directions to build noise-cancelling devices in hardware, but what would it take to create noise cancelling in software? Since computers have a speaker-of-sorts and can possibly have a microphone, why not take the input from the mic, reverse-phase it via software, and output it through the speaker? A noise-cancelling feature would be great to run on servers in a rack. It would also be a great app to run on your laptop when you are on a plane."

Comments

Uh, no.

[QuantumET]

You'd only get noise cancellation at the rough area around the microphone is, assuming you'd account for the speaker-microphone distance. You have to cancel phase _everywhere_ for it to work for a room, and you can't do that, really, without a huge array of speakers, or speakers exactly co-located with noise sources.

So you could do it in software for headphones, since you just need to cancel noise right at the headphones, which is fine. But it's no good for speakers, unless you have very specific configurations of noise sources that lend themselves to simple cancellation. In general, no way.

Re:Uh, no.

[Anonymous Coward]

I am an electrical engineer.

Parent is exactly right. There are two ways to handle noise cancellation. #1 is source cancellation (very hard unless the source is confined). #2 is receiver cancellation (very easy). However, don't expect either method to work at high frequencies.

Sound is a complex beast. Literally. Imagine you could have enough samples to make a 3D spectrum analysis of the sound. What would it look like? You'd see high frequencies "beaming" off of reflective surfaces. You'd see low frequencies "wrapping around" small objects.

To do source cancellation, you have to find a way to negate all of the sound energy before it reaches *any* receiver. Put your computer in a box with a long, narrow air take. Carefully design the intake/exhaust to minimize "port noise". Add noise cancellation *inside* the box. Walk around. You shouldn't hear anything coming out of the box.

If you can't control the source, you have to resort to receiver cancellation. "Why?," you ask. Because if you mess up on the phase, you can/will end up contributing 3dB to the noise source at various frequencies/locations. That's not acceptable.

Receiver cancellation works just like source cancellation, except you put the box around the receiver. In our case, the "box" is headphones. Put a high quality microphone on one side. Feed a phase-inverted signal into the output speakers.

Earlier I mentioned that noise cancellation doesn't work well at high frequencies. Why is that? Delay. At low frequency, the signal changes very slowly. If I'm "off" by 1 microsecond, it won't make any difference to a signal that oscillates at 60Hz. Here's a little exercise for the reader:

F(t) = cos (2 * pi * f * t) - cos (2 * pi * f * (t + d))

In this example, we're considering only one frequency and one point in space. Let f = 60Hz. Let d vary from 0 to 16ms. Observe the results. What happened at 2.79 ms? At 8.33 ms? Nasty, eh? But we're pretty confident our system can handle those latencies without any trouble, so we'll crank this thought experiment up to 4kHz.

Let's do some envelope calculations: our latency has to be under 42 microseconds to do *any* good (i.e. avoid adding to the noise), under 21 microseconds to get a 3dB attenuation, under 4.2 microseconds to get -10dB, and under 0.4 microseconds to get -20dB. No sweat, right? :-)

I'm going to be a bit facetious here and start by examing the CPU aspects. Ok you've got a 3GHz cpu. Let's be "generous" and assume you get about 1.67 instructions/clock. That means you can execute about 5e9 instructions/second (0.2 microseconds/instruction). There's no way you could hope to get any meaningful work done in 2 instructions, so that rules out the possibility of getting -20dB. In fact, there's not much you could hope to do in ~20 instructions, so there goes -10dB. You *might* be able to write a dedicated driver that could handle -3dB. Whooptee freaking doo *twirls finger*. The padding on your headphones probably does a better job. Or just stick some cotton in your ears.

Even if you manage to get your digital hardware up to speed, you've got another problem: the analog hardware. That's right: between your $0.50 microphone and your $0.50 speaker, there's a pretty decent chance that they've got more than a few microseconds of slew error. Even if you bumped those up to $100.00 each, you're still not likely to get it right. In short: don't expect to do -20dB at 4kHz for under a few grand (*).

Last I heard, the $100 sony headphones do about -10dB at 100-400Hz. As far as I can tell, the hardware is only worth about $2, and you're paying $98 in salesman, middleman, marketing and engineer salaries.

(* No, this is not an offer. I wouldn't try building you a pair for less than a million in R&D funds, and even then I couldn't guarantee results.)

Real-time sound

[mbrubeck]

Your computer's sound input and output are buffered in both software and hardware. This means that there will be a minimum latency measured in tens or hundreds of milliseconds between reading input from the microphone and being able to send corresponding output to the speakers. You can make this latency smaller and more predictable with well-designed hardware, drivers, and applications software, but you'll never be able to do the same "real-time" processing that you can do with dedicated hardware.

why not...

[croddy]

why not take the input from the mic, reverse-phase it via software, and output it through the speaker? well, i suppose the howling, squealing feedback would make you forget about the server fans for a few seconds, but i suspect it would just end up giving you a bigger headache in the long run.

Loud Noise

[DavidLeeRoth]

I dont know how to noise cancel, but I know how to create a ton! cat /dev/urandom > /dev/dsp :)

Re:My child once talked for eight hours straight .

[OneDeeTenTee]

the only stores for thousands of miles in any direction sold only alcohol, Mormon cookbooks, and knives.

Sounds like a bad day for the Mormons if you're hungry.