In theory, reversing phase on a noisy signal should get rid of unwanted noise, right?
I have a guitar signal I've recorded using a small headphone AC powered amplifier. The sound is great except that it contains a "buzz" that is very noticeable during the silent parts.
I copied the buzz to another track and reversed the phase thinking that should get rid/cancel out the buzz.
However, it did not work.
Theoretically, shouldn't this have worked? But it didn't - any thoughts as to why?
That would only work if the buzz was a steady-state, constant waveform, that didn't vary at all in amplitude, pitch, or harmonic content. And, even if that was the case, you'd need to have a perfect sample of the full cycle of the noise, line it up exactly with the original, and then the phase invert would cancel.
However, in the real world, it doesn't happen that way, and induced noise like that has some variations to it.
If it's a simple ground loop type hum, caused by a cheap power supply or bad ground connection somewhere, then you could get rid of it with very narrow notch filters at 60Hz and the next few harmonics of 60 Hz (120, 180, 240, etc.).
However, if the noise is more complex than that, then you need one of those noise reduction plug-ins where you feed the plug-in a small sample of just the noise by itself to generate a fingerprint of the noise, and then adjust the controls to remove as much of that noise as possible before it affects the audio too much. Sony's Sound Forge (professional version) includes their own Noise Reduction 2 plug-in, which works fairly well. Adobe Audition also has a decent noise reduction plug-in with LOTS of adjustable parameters (almost too complex) to allow you to really fine tune things. Waves has a couple of noise reduction plug-ins as well, Z-Noise and X-Noise. On the really high-end of things are the CEDAR processors, but those are very expensive and mostly designed for professional mastering and restoration work. All of these plug-ins work pretty well if you don't get too aggressive with them.
The best thing is to try to kill the noise at the source BEFORE you record! Ebtech makes some decent, low cost, products that might help, depending on where the noise is coming from. The have a Hum-X power adapter plug that has a transformer built-in to isolate the ground if the noise is from a ground loop. They also have some hum-eliminator boxes, that are basically audio transformers, which will basically isolate the ground from the audio connections and reduce the most common source of hum from electronic instruments.
However, guitars can be much more tricky since it's often the guitar pickups themselves that are picking up noise from electro magnetic fields (emf) from various electronics, or even lights, in the area you are recording in. Some types of light fixtures, or computer monitors, and other electronic gear can create EMF that is easily picked up by certain types of guitar pickups. If you hear the hum when you plug your guitar in, and it changes in volume as you move your guitar around, then you know it's coming from your guitar, and you just have to keep moving around, or turning off unnecessary equipment, until you get it as quiet as possible.
Hope that helps!