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Phase Canceling Music&FX: or Audio Difference Matte
I’m pretty sure there is a way to do this, but I really can’t figure it out.
We often get Digi tapes of shows that have a full stereo mix on CH1 & Ch2 and just music and sfx on CH3 & CH4 (i’m assuming for international distribution). Is there a way to phase out the music and sound effects leaving just the dialog and VO? Basically, creating a difference matte, or difference blending mode for audio. I assuming that it probably won’t cancel out completely, but something is better than nothing.
Thanks guys.
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8 Replies
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of course, just phase reverse 3/4 and mix (levels should be the same, probably isn’t on tape)
Dunno if the filter is already in FCP, (it probably is), but any decent mixer can do so on the fly, or just go in balanced analogue and reverse pin 2/3 and you’re done.
(assuming 3/4 is also stereo…)
Bouke
http://www.videoToolShed.com
smart tools for video pro’s -
could you please be a little more descriptive. i’m not sure what you mean by phase reverse.
also, we normally digitize our audio as embedded SDI. how would you do this in FCP or Soundtrack?
thanks for the fast response.
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In Soundtrack create a mutltrack project.
Bring in the full mix to tracks one and two, the music and FX to three and four.
If these are stereo files they will apear with both channels on one track for each of the mixes.
If they are in perfect time alignment, if you toggle off the FX and music tracks, the level of both will decrease.
(To assist, find an FX with a sharp transient, zoom in and adjust the position of the track so the waveform is perfectly aligned visually with the same FX in the full mix tracks.)I know most people use the term “reverse phase” but this technically incorrect for what we want to do.
Phase implies a timing issue. What we want to do is “invert” the polarity of one waveform on a channel relative to its sibling on the other channel.
If they are perfecly aligned, as the waveform is going in a positive direction realtive to the “0” baseline of one chanel, the waveform is going in a negative direction on the other.
If these match perfectly in level, when added together…
They cancel each other out.
+1 added to -1 equals 0
If the levels can’t be matched, at least the level of the music and FX will decrease.In Soundtrack, select the music and FX track (or if on seperate tracks shift click to select both) from the “Process” menu choose “invert”.
Play with the level of this inverted track to get the best results.
Then export this mixed version making sure you “flatten all actions” from the Process menu. -
Phase cancelling the M&E mix to the final mix will not give a perfect cancellation. Firstly the assumption that the two audio tracks are already in phase is false. M&E mixes can be stripped onto a dig master at a later date and are subject to 9 pin sync accuracy. Even if they are in perfect phase they are not the same mix.
The M&E will have been altered to take out dips for narration so it is a slightly different mix.
Ask the audio post people for separate dialog tracks. This should be a basic request for them
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hey don’t get me wrong, we’d all love splits when we can get them. but you know, sometimes you got what you got and you have to make it the best you can. often this isn’t perfect, but it’s better than nothing.
thanks everyone for the replys. booked with clients again today, so haven’t had a chance to try it yet. hopefully friday.
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So I got a chance to try it out and it works awesome. My work flow is a little different, but all the same principles.
-Capture Video, A1+A2 Stereo, A3+A4 Stereo
-Cut into Sequence
-Select A3+A4 and choose Link from the Modify menu (this will unlink the audio from the video so when you send to Soundtrack things don’t go screwy on you)
-Select A3+A4 and send to Soundtrack Pro Audio File Project
-In Soundtrack choose Process>Invert and Save
-Back in FCP everything should play just fine. You can even select the Video and 2 stereo pairs and relink them.
Thanks again for the help.
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Glad the “invert” polarity worked for you.
And just repeat myself, (because I’ve been in the audio business for 30 years), this has nothing to do with “phase”.
“Polarity:
In electronics, two points that have opposite electric potentials (one is positive, the other negative). This is not the same as being 180 degrees out of phase (although the results can be similar).
Phase implies a relationship with time, polarity does not.
What most engineers, consoles and preamps refer to as a “phase” switch is actually a switch reversing signal polarity.” -
Phase
Audio waveforms are cyclical; that is, they proceed through regular cycles or repetitions. Phase is defined as how far along in its cycle a given waveform is. The measurement of phase is given in degrees, with 360 degrees being one complete cycle. One concern with phase becomes apparent when mixing together two waveforms. If these waveform are “out of phase”, or delayed with respect to one another, there will be some cancellation in the resulting audio. This often produces what is described as a “hollow” sound. How much cancellation, and which frequencies it occurs at depends on the waveforms involved, and how far out of phase they are (two identical waveforms, 180 degrees out of phase, will cancel completely).
Phase Invert
A switch found in the input sections of mixing consoles and mic preamps.
The term “phase invert” is actually a misnomer, since what the switch really does is invert the polarity of the signal in that input (correct usage would be “polarity invert”).
Its intended use is to correct for balanced lines and mics that are wired backwards. In some cases toggling the phase invert switch may make a sonic difference if signals are out of phase, but doing so will also put that signal out of polarity with the others.
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