1. Home
  2. Electronics Design
  3. Active noise reduction, anything new on the marked

Active noise reduction, anything new on the marked

Ah, been smoking enough crack and marijuana that you still think it's the

20th century? Well, explains itself well enough. (c; (c;

Ya, works great (I haven't tried it personally).

Well, I suppose an FFT, phase shift and composition back to time domain could work, but the usual way is to INVERT the signal with a single transistor. ;-O!

Well, okay...20 transistors...(inverting op-amp, mic gain, output, double for stereo.)

Note that, noise cancellation works great for localized noise, but anything slightly too close or too far from the cancelling source isn't going to have a perfect null. If you set up a noise cancelling device in a room (as if), the chances are it'll actually double (or more?) the noise in other areas!

In a room, you also have acoustics against you, since the cancelling amplifier has to detect the noise in the first place, requiring a microphone. It takes a certain time for the sound to pass through the air, to the mic, to the amplifier and back to the speaker, and the chances are, some frequency is going to be amplified repeatedly until some element (mic or amplifier) is at its maximum signal level. This is otherwise known as FEEDBACK...

As a result, noise cancellation is most effective for very local cancellation, like headphones. Tack mics on the outside of a pair of 'phones, add an inverting amplifier, adjust gain and you can have reasonable cancellation inside your ears.

Tim

-- Deep Fryer: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams
Loading thread data ...

In the 70s- 80s I have been reading a lot about noise reduction by simply producing the same spectrum shifted by 180 degrees.

I have not heart anything about this since then. I wonder if there is anything or anybody working on this. Or this idea simply did not work. I guess it would require very fast computer to calculate the same spectrum shifted by 180 degrees, so when the waves meet would, the output would be wave with amplitude equal or close to 0.

Reply to
Tom

If you change "180 degrees" to "inverted", it does work, and is in fairly common use right now.

John Perry

Reply to
John Perry

simply

And change "spectrum" to "waveform".

Reply to
Richard Henry

Noise is random. The concept can't work.

Graham

Reply to
Pooh Bear

For headphones yes.

Actually I had something else in mind when I replied but nm.

Graham

Reply to
Pooh Bear

Not so, it is just computationally intractable. You can separate the various components of a signal by using autocorrolation. You must apply it recursively with auto adaptation until all the signal separations converge. If you are working on a four person band with two singers it takes about 1 pentium GHz hour per minute of audio. For a twenty piece "orchestra" figure on fourty times the time. It is NP hard after all. There should be one dominant noise source and one or more lesser ones. I have a recording that has three nearly equal power noise sources (about 40 dB below the signal). I figure if i can ever get it noise reduced that i would like a copy of the three noise sources without the main signal for a test signal to add to recordings to test noise reduction systems.

--
JosephKK
Reply to
Joseph2k

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.