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Phase Not cancelling in Daw

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Phase Not cancelling in Daw

Postby dbfs » Fri Feb 21, 2020 5:54 pm

Hi Folks,

Quick question.
If I Use a signal Generator to create a Sine Wave on two tracks in a DAW and invert the phase of one channel, I expected that the signal would cancel.

It does not.

However, If I render one of the signal generator channels to a file and reimport that onto two tracks and then manually shift one of them on the timeline to make then 180 out of phase then I get silence.

I know that there is a difference between polarity and phase, but if anyone could answer this in simple terms it would be great.
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Re: Phase Not cancelling in Daw

Postby CS70 » Fri Feb 21, 2020 6:03 pm

I guess you invert the polarity of the channel?

If the signal generator is set up identically and the mixing engine mixes the samples in correct time order, the only explanation is that the signal generator does not produce exactly the same samples (i.e. waveform).

For example, one implementation may decide to start at 0 radiants but another use a randomized seed to decide at which angle to begin. Both would be sine wave generators but shifting polarity between the samples produced by two distinct instances of the latter type, would not produce zero in almost all cases.
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Re: Phase Not cancelling in Daw

Postby Hugh Robjohns » Fri Feb 21, 2020 6:12 pm

dbfs wrote:If I Use a signal Generator to create a Sine Wave on two tracks in a DAW and invert the phase of one channel, I expected that the signal would cancel.

It does not.

Then either:

1. The sine wave does not have the same frequency and/or relative phase in the two channels. This can happen if you generate the two sine waves independently, rather than generate one tone and copy it to the two tracks with the same time alignment.

2. You are not summing those two channels together at precisely equal levels.

3. There is something else going on to modify one or both tones in some way...

I know that there is a difference between polarity and phase, but if anyone could answer this in simple terms it would be great.

Polarity is a simple inversion of the signal voltage with no movement along the time line -- ie. no physical change of the time alignment.

A phase shift does involve a change in the time alignment of the original signal.
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Re: Phase Not cancelling in Daw

Postby The Elf » Fri Feb 21, 2020 6:16 pm

4. You're listening back to two mono channels, with each channel panned towards a different speaker. This will sound kinda weird, but it will not cancel unless both channels are panned to the same stereo position.
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Re: Phase Not cancelling in Daw

Postby Hugh Robjohns » Fri Feb 21, 2020 6:32 pm

Your 4 would be exactly the same as my 2... then... :lol:

It seems to me that if the thing does cancel when he does the manual phase shift, there must be a proper summation in place.

And as that is achieved when he imports the tone to two tracks (and therefore has the same source), it would make sense that his two original source tracks are simply not time aligned correctly.

If he zooms into the waveform to see a few cycles of the tone it will be very obvious whether or not they are time-aligned correctly on the tracks.

If they are... then there is a plugin in one side or something like that which is messing up the time alignment before the summation.

And if they're not, that would explain it...
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Re: Phase Not cancelling in Daw

Postby The Elf » Fri Feb 21, 2020 6:33 pm

Hugh Robjohns wrote:Your 4 would be exactly the same as my 2... then... :lol:
Hmmmm....
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Re: Phase Not cancelling in Daw

Postby dbfs » Fri Feb 21, 2020 9:25 pm

Hi guys,
thanks for the responses.

I tried it again in a new session and with two new channels in Pro Tools.

Running Signal generator and then using the EQ 3 1 Band which has a polarity button on it to invert the signal on one channel.

It worked and I got silence.
BUT
Then I created a news session and tried to repeat, and it did not cancel.

SO
I then go back to the first session and is still worked.

Now I am genuinely confused. Maybe

However, bouncing a few bars of the test signal, importing that back into the two tracks and then using the avid audiosuite Invert Command does consistently create silence.

The interesting thing is that when Bounce from the two channels in the wonky session, the two files line up perfectly in phase without the polarity invert switched on.

I also checked the mixer in PT to see if there was any delay reported on the channels and it says 0.
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Re: Phase Not cancelling in Daw

Postby dbfs » Fri Feb 21, 2020 9:33 pm

I also just checked this in cubase and it also does not cancel to silence when running the generator in real time.

Do one of you guys want to try on your end to see if I am gone mad.
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Re: Phase Not cancelling in Daw

Postby CS70 » Fri Feb 21, 2020 10:46 pm

Signal Generator TDM, RTAS or ..?

This would be consistent with signal generator code seeding the sine wave differntly depending on where and how you insert it.

The instructions also state pretty clearly that The Signal Generator plug-in is not intended for rigorous test purposes; it is a simple level calibration tool. - which hints that its design may not guarantee phase alignment between instances.
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Re: Phase Not cancelling in Daw

Postby Hugh Robjohns » Sat Feb 22, 2020 12:09 am

dbfs wrote:?..it also does not cancel to silence when running the generator in real time.

If you're running two separate instances of the signal generator, one for each channel, I'm not surprised. They won't be phase-locked to each other, so the two channels will inherently have different signals and inherently won't cancel when summed with a polarity inversion.
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Re: Phase Not cancelling in Daw

Postby Sam Inglis » Sat Feb 22, 2020 9:38 am

The Pro Tools Signal Generator plug-in does have a few notorious quirks relating to mono/stereo use.
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