invert phase
by Tom Morrow
on
Oct 14, 2009 at 3:43:11 pm
I have a project that has a ground loop noise on one channel and my good audio and the ground loop noise on the other channel.
I have separated the stereo pair and sent it to STP. I have selected one of the channels and chose process/invert. I am not getting much effect doing this. I took the same separated tracks into Macki's Tracktion software (trial version, so I am cut off before completion) added the 4 way eq to one of the tracks and selected invert phase and was able to almost completely eliminate the noise.
Any tips on getting this result using STP. I would buck up for the other software but my department is holding the reigns tight lately.
I am definitely a novice using STP so I figure I am missing something.
Re: invert phase by Jordan Wolf on Oct 15, 2009 at 5:48:47 am
First off, you are not affecting the signal in time, so the proper phrase is "polarity", not "phase".
It's a common misnomer - phase is time-dependent (you adjust the positive/negative placement of the sound wave in time); polarity is purely mechanical (positive voltage/negative voltage) and does not deal with time.
Secondly, ground loop noise finds its base frequency at 60Hz (in the US) and 50Hz (Europe, etc.). There can be harmonics (overtones), but they are not always a problem. You'll want to use a parametric filter to notch out the offending frequency. Start at 60Hz, with a narrow Q/Bandwidth, and boost the filter. Adjust the frequency until you find the loudest humming happening - that is the fundamental frequency, the rest are harmonics. Apply cut to the filter until it disappears or is out of the way enough to not be troublesome. IF IT AFFECTS THE AUDIO NEAGTIVELY, YOUR Q/BANDWIDTH IS SET TOO WIDE. Narrow it down and try again.
You can also use Spectrum View to ascertain what the fundamental frequency is and zone in on a solution that way.
Re: invert phase by Tom Morrow on Oct 15, 2009 at 1:53:31 pm
I have tried doing what you recommend with some success but I will give it another go. My noise is not exactly a hum but a whistle that pulses. It kinda sounds like an old squeaky wheel or cicadas droning in the trees. The phase inversion does a pretty good job on it in this Tracktion Software.
