Anyone mind explaining, "phase cancellation?"

[Mr. Liszt]

I noticed an interesting article in the new "Tips & Techniques" wiki titled, "Noise Guitar Amp:"

http://www.gearslutz.com/board/tips-...uitar-amp.html

Discussed is the removal of unwanted signals through the process of "phase cancellation." I am familiar with the practice of avoiding PC when tracking through the 3 to 1 method and I think I understand a tiny, tiny, tiny bit why PC happens and its effect (removal/cancellation of a signal through a mirrored [symmetrical?] image copy of the wave form?) but I don't understand how to use it after tracking is done in the mix. I'd like to experiment with this technique but I don't understand how to even just simply reverse the phase of a track either in my DAW, Logic 8.

[3rd world order]

pretty simple really... record a full track of amp noise thru the same signal path, flip the phase with INVERT in protools (or whatever) and add it to the guitar track.

i'm kind of surprised it works...

i'll have to try this


you have to track this in the beginning or it wont work.. the idea is to isolate the "noise" with the same signal path.. same mic, position, gain, etc...

[MarkRB]

Phase cancellation happens where one waveform is going up and the other going down, therefore negating each others effects.

If the waveforms are exactly opposite you will have absolute cancellation or a 'null'.

Knowledge of phase is really very usefull, for instance if you have a full mix and an acapella (of the same track obviously) by inverting the phase of the acapella and playing them side by side you can completely remove the vocals from the full mix.

Or even guitar amp noise from a guitar track, although that won't eliminate it completely as the noise will have a certain amount of random variance that you won't be able to recreate perfectly.

Hope that all makes sense.

[donsolo]

it's trickier than that though

most guitar noise is 60hz...

That oscillates so if you don't time-align it to cancel properly, you'll wind up with MORE noise

[Flymax]

Originally Posted by MarkRB

Ph for instance if you have a full mix and an acapella (of the same track obviously) by inverting the phase of the acapella and playing them side by side you can completely remove the vocals from the full mix.

U Ever Try That?

[MarkRB]

Yes I've done it a few times. I've even done vocals down mixes for ppl that way.

[Flymax]

I'd think it wouldnt work because of FX and such..
I'll try it..

[seedee701]

Originally Posted by Mr. Liszt

I don't understand how to even just simply reverse the phase of a track either in my DAW, Logic 8.

on the better desks there is a phase-flip button, for each channel. usually right next to the gain button.

in logic (7) in the 'helper' plugin-folder you find a 'gain' plugin. this has a phase switch

also there are many 3rd-party plugins (EQ COMP...) that include a phase-button (waves mcdsp urs...)

in sample edit -> functions you can render an audio-file with the 'invert'-command

i remember the first time i tested theory years ago:

duplicate a track and invert the phase on one with one of the above methods.
set faders to 0
hit play, the meters move but ... silence ...hmm i don't quit believe...

i set the volume higher ... hmm still nothing happens... realy nothing?? more volume....

i mute one track BANG!!

physics in action

[Mr. Liszt]

Originally Posted by seedee701

on the better desks there is a phase-flip button, for each channel. usually right next to the gain button.

in logic (7) in the 'helper' plugin-folder you find a 'gain' plugin. this has a phase switch

also there are many 3rd-party plugins (EQ COMP...) that include a phase-button (waves mcdsp urs...)

in sample edit -> functions you can render an audio-file with the 'invert'-command

i remember the first time i tested theory years ago:

duplicate a track and invert the phase on one with one of the above methods.
set faders to 0
hit play, the meters move but ... silence ...hmm i don't quit believe...

i set the volume higher ... hmm still nothing happens... realy nothing?? more volume....

i mute one track BANG!!

physics in action

Thanks seedee,

I tried this but ran into a snag that I am trying to figure out. I recorded a simple track of speech. I then created a new track and cloned track 1 data to track 2. I opened track 2 in the Sample Editor and selected "invert" as you said. Track 2 did indeed invert but the only problem is that track 1 also inverted. Must be something simple that I am missing. I don't know why a command for one track would effect another. Some kind of preference setting somewhere?

[seedee701]

you need to duplicate the AUDIOFILE before rendering

check out the difference between audio region and audio files in logic.

regions lay around on your tracks in logic and refer to an audio file (on your hard-disk). many regions can refer to one single audio file. in sample edit you make changes to the actual FILE. in arrange you make changes to regions.

you actually inverted the FILE that both tracks where referring to.

[AllAboutTone]

I use Phase cancellation all the time to get a more controlled track if needed, works great on kick as well, to me mixing one tarck in a little puts a nice bounce airy mix to a track, also can give the track a more open sound at times.
I do not copy though, i use 2 mics on the source at the same place.

[slaves666]

I have a Les Paul Junior that makes a lot of noise but sounds killer, but maybe I can get around it with this tip....thx.

[Mr. Liszt]

Originally Posted by slaves666

I have a Les Paul Junior that makes a lot of noise but sounds killer, but maybe I can get around it with this tip....thx.

I have the same amp! I love it!! I searched amp after amp and tried this one on a whim. No contest once I heard this thing. Pricey one knob and 5 watts but worth every penny. I don't have too much trouble with the noise though. What guitar(s) are you using?

[Sigma]

it is how we did strings..we played speakers in the studio so the string players didn't have to wear headphones ..when we doubled we inverted the phase of the speakers..when yaa played it back together at the same level..they were gone

[Mr. Liszt]

Figured it out by using the mentioned gain plug in with the phase inverter. Amazing! This could be useful.

Sigma, I've never recorded a large of ensemble of strings but if I ever do, in the absence of a headphoned conductor I can see this really working out as a viable alternative to having 50 pairs of headphones and a snake bed of wires.

[Jules]

Originally Posted by FLYMAX

I'd think it wouldn't work because of FX and such..
I'll try it..

Any random action stuff like reverbs and tape recorder wow & flutter - will compromise the efficiency of it working..

So if you had a lot of compression on the guitar amp signal, that will monkey with the level and tone of the background noise on that track , rendering it slightly 'different' (read not 100% phase flipped) from the 'noise only' recorded track..

So to repeat, if there are ANY variables - the trick works to a variable degree (sometimes well, sometimes not at all, sometimes only half working) etc

But its definitely work experimenting with..

Its a fun "pure science" issue that every engineer should have some experience with

[MarkRB]

Yeah I should point out that the stems I use are generally printed at the same time as the mix so are identical.

Although I will experiment with a 2 pass reverb and see how far out they are, just for the hell of it.

EDIT****

I just tried a quick in the box version of this test.

1 vocal file, 1 Digi Medium delay 120ms 40% Feedback and 1 Digi Reverb 50% mix, 4 second decay.
2 passes BTD, no master fader. Import files, invert one.

Result: Absolute Cancellation all the way through!!!

It seems plugins are less random than you would imagine.

I will try some more exotic plugins when I get a few minutes, might also try some hardware at some point. I find this stuff quite interesting (which is kind of sad).

[Blast9]

Originally Posted by Mr. Liszt

Thanks seedee,

I tried this but ran into a snag that I am trying to figure out. I recorded a simple track of speech. I then created a new track and cloned track 1 data to track 2. I opened track 2 in the Sample Editor and selected "invert" as you said. Track 2 did indeed invert but the only problem is that track 1 also inverted. Must be something simple that I am missing. I don't know why a command for one track would effect another. Some kind of preference setting somewhere?

As seedee mentioned, you need to click on the file in the audio window and select file >>> copy or "save as" (can't remeber)

You can then double-click the copy and "invert " from one of the menus up top (sorry to be vague... am not sitting in front of my DAW at the mo)

[sleeps]

Originally Posted by MarkRB

Knowledge of phase is really very usefull, for instance if you have a full mix and an acapella (of the same track obviously) by inverting the phase of the acapella and playing them side by side you can completely remove the vocals from the full mix.

Indeed this method works very well. I do quite a bit of mixing for commercials, informational videos, etc. This allows me to isolate the vocals of a song so I can mute/unmute them in sections where their may be an interview, but only the music should be layered.