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Comment by jjtheblunt | Hacker News Reader

> In physics, different frequencies don’t interfere with each other—they're always independent.

Are you overlooking harmonics? Are you overlooking the same frequency case of noise cancellation, via phase shift?

You're correct - I oversimplified assuming no doppler effect and a linear time invariant system.

The key is that we can still analyze and manipulate frequency components independently in REAL PHYSICAL WORLD, which enables techniques like OFDM.

Phase shift isn’t different frequencies?

Please check out the 3rd paragraph of this article.

https://en.wikipedia.org/wiki/Active_noise_control

I’m aware of how ANC works. It uses the same frequencies with reverse amplitude. The comment you objected to however was talking about different frequencies.

> reverse amplitude

Right? One of my dumb noobie questions has always been why textbooks explain ANC as "180 phase shift" instead of "reverse amplitude". Do they really do an entire DSP pass to break apart a signal and phase-shift it just to get the output that a minus sign could get? There must be something missing from the plum-pudding model.

It's only a sign flip when the detector and emitter are perfectly in phase. Adding delay causes the required phase shift will drift slightly. Also note that the phase offset of a delay is frequency dependent, so this drift will be as well.

Another reason is that devices and materials have different frequency responses, so an external signal needs to be filtered to match the levels in the device.

I wasn't clear : phase shift has a clear example: think of two sine waves, one whose value at time t is sin(t) and the other sin(t+pi/2). they'd have the same period, and frequency, but would be phase shifted by that addition in the time parameter.

Here's a graphed example.

https://www.wolframalpha.com/input?i=Plot%5BSin%5Bt%5D%2CSin...

For another fun example, here's the residential power in current American homes: two 120v A/C feeds, out of phase so that you can combine them in the breaker box to get 240v A/C.

https://www.wolframalpha.com/input?i=Plot%5B120*Sin%5Bt%5D%2...

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