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making a sound file out of phase

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Nelson May
making a sound file out of phase
on May 9, 2009 at 6:35:03 pm

I have an audio file with some echo in it..not much, and someone told me I can throw the file out of phase, recombine them and it may reduce the echo/reverb.

I have pro tools 7.1 and am not sure if there is a plug in to do this. I looked and didn't see anything. Is this some thing that has to be done manually

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Jordan Wolf
Re: making a sound file out of phase
on May 9, 2009 at 7:37:37 pm

Try it and see what happens.

Step 1: Copy desired audio to another track
Step 2: Select the copied audio
Step 3: Go to Audiosuite-->Other-->Invert

Play them back together and see what happens... :-) The only thing audible will be any difference between the audio in the two tracks. But since you copied the original audio, everything is the same (albeit phase-reversed), and will cancel out completely.

If the original audio had no echo, you could experiment for fun, but if there's environmental echo in there, it's there for good. Sorry :-/

One thing you COULD try, is gating the audio - though it might make it choppy and sound unnatural.


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Bill Davis
Re: making a sound file out of phase
on May 9, 2009 at 7:37:47 pm


Echo is the primary sound followed by a reflection of that same sound - delayed in time.

Phase cancellation, by definition, is adding two copies of the SAME sound together with one of them delayed EXACTLY 180 degrees out of phase with the other.

That process has NO effect on sounds that are already naturally delayed like an echo. The echo is, by definition, a separate sound from the initial sound the echo resulted from.

As a technique, it's valuable in something like balanced audio lines, where you have three wires, one carrying the signal, the other carrying nothing, the final wire the ground. Then when the CABLE picks up RF noise, you can take the whole signal - invert the phase - and mix the result back in - so that the NOISE picked up equally by BOTH conductors is cancelled out. But it only works because the noise you're canceling is DIFFERENT from the signal you're interested in passing.

The only way an echo is "different" is that it's shifted in time. So the traditional way to TRY and compensate is to set up a gate so that you allow the primary attack of the word to pass through and then gate the circuit shut before the echo passes.

Often, that's an "iffy" solution since you often get a mixture of the echos and the next wanted sounds in actual content.

There are also more sophisticated forensic audio tools that mathematically kill sounds below the threshold of amplitude of the initial attack sound.

But in reality, audio usually a lot like paint. Once you mix two of them together it's pretty hard to "unmix" them. At least unless there's something very distinct about the sounds you want to remove compared to the ones you want to keep.

Good luck.

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Sam Mallery
Re: making a sound file out of phase
on May 11, 2009 at 1:49:23 pm

Nelson, do you know how your audio ended up with echo? Is it natural echo from recording in a large space with reflective surfaces? Or did the echo occur another way?

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