Phase accuracy of Pro Tools vs Logic

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stefhambrook

Guest
In logic if I run duplicate audio on two tracks and phase reverse the signal from one with a plug in I get total phase cancellation as expected at equal volume.

If I do the same in pro tools (or on a DAE track in logic) the result is PARTIAL phase cancellation. Also phase reversing one signal offline in PT, prior to playback does not improve matters so it's not processing causing the phase slip but the summing, it seems.

These are my observations based on two PT systems. (One Mix Plus and one HD.)

Surely a DAW should be completely phase accurate between tracks. Is this an acknowledged weakness in PT or a fault in the systems I tested?

Many thanks ;)
 

David French

Well-Known Member
Joined
Jun 19, 2002
Location
Indiana
Interesting. Have you tried zooming in to the point you can see the samples and checking phase alignment visually? Also, phase cancellation of correlated sources is frequency dependent and can vary between zero and twice the amplitude of the original signal. Are you hearing results that support this (i.e. comb filtering)?
 
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deanp920

Guest
On a similar note, I imported Metallica's 'Of Wolf and Man' into Digital Performer as a stereo, 24 bit file.

I then copied the file to 24 stereo tracks in the mixing board, moved the faders all over the place, and summed everything to a master fader at about -3 db with the master fader set at unity.

I used 'Bounce To Disk' to create a 24 bit stereo file of the mix.

Then I normalized it with DP's algorithm.

I took the original file and normalized it, also.

I put these two stereo tracks on two separate stereo faders in the mixing board at unity gain.

I put a phase reverse plug-in on the original, normalized track, and BAM!

Complete nulling.

I couldn't hear a ghost of anything, even with my HR824's cranked.

A .1db adjustment in either track or any other subtle change I tried wrecked the nulling.

To me, that put a lot of my faith back in DP, at least for the summing accuracy.

Dean
 
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stefhambrook

Guest
Thanks for your responses.

What I'm basically saying is that I haven't been able to get Pro Tools to produce a null signal by phase cancellation either using a plug in or off line processing.

So this leads me to question the phase accurracy of the systems operation when the audio is supposed to be in phase.

If anyone could be so kind as to test this and post the results I'd be grateful.
 

David French

Well-Known Member
Joined
Jun 19, 2002
Location
Indiana
What exatly do you want us to test? If you would like someone to take a track into PTLE, phase reverse it, and report back, i'm sure one of us could do this, but are you suggesting that your PTLE system has phase problems and maybe others do not? Also, could you answer the questions from my previous post?

FWIW, I use Cubase SX 2.0, and inverted tracks cancel perfectly for me.
 
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stefhambrook

Guest
What exatly do you want us to test? If you would like someone to take a track into PTLE, phase reverse it, and report back, i'm sure one of us could do this,
Yes, I'd appreciate it, thanks.

but are you suggesting that your PTLE system has phase problems and maybe others do not?
I'm saying I don't know. That's why I'm interested in other peoples results from this test.

So please, all those who have a spare moment on a pro tools rig, I'm inviting you to:

1. Duplicate a region and put it on an adjacent track in exactly the same point in the timeline.

2. Phase reverse it using the phase switch in the one band EQ plug in. And alternatively ditch the plug in and use audiosuite to phase reverse the audio of one region. Are you getting a null output at equal gain?

;)
 
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stefhambrook

Guest
Interesting. Have you tried zooming in to the point you can see the samples and checking phase alignment visually?
Yep they were in exactly the same place.

Also, phase cancellation of correlated sources is frequency dependent and can vary between zero and twice the amplitude of the original signal. Are you hearing results that support this (i.e. comb filtering)?
I've been talking about cancellation of IDENTICAL sound files so there would be no comb filtering in this case. I think I have been hearing comb filtering effects on drum mixes in PT but this is subjective. Perfect phase cancellation is easier to verify than perfect phase correlation....
 

David French

Well-Known Member
Joined
Jun 19, 2002
Location
Indiana
Just tried it. With both the phase reverse on the one band EQ and the invert function form audiosuite, I got perfect cancellation. By the way, correlated means identical or nearly identical, like multiple copies of the same signal by electrical means or by a single acoustic reflection. When two correlated sounds are mixed together with even a very small phase angle between them, you will get comb filtering.
 
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stefhambrook

Guest
Thanks David.

This is odd. I wonder why both the systems I tried it on don't work as perfectly as yours. Did you try this on TDM, HD or LE?
 
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JensenBohren

Guest
OOh! OOOh!
My Euriditity knows the answer to this!

You've been doin this on TDM and not LE systems, correct?

There is a very slight timing that it takes the signal to go from one place to the card, get processed, and then go back where it came from. Yes, it's not technical, but I havn't read my books in six months...

If you want the phase to be nulled, apple click on... one of the areas of the fader and it'll tell you the latency. There's a plug in that I think is called 'Time Adjuster' that you put on the track without latency and time adjust it forward in time so the latency is covered up.

That, or I'm an idiot that cannot remember lessons from nine months ago. :p
 
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stefhambrook

Guest
Aha! So it appears the LE version of PT has automatic plug in latency compensation but not TDM or HD. Interesting. I'll give that apple click a try. Do you remember exactly which part of the fader?
 
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JensenBohren

Guest
It's been over six months since I've been in front of a non-ProTools Free machince, and all I can tell you now is that it's above the fader and... actually, let me go get my textbook.

Ah. Here it is.

from Pro Tools 201: Pro Tools Essentials, page one hundred and fourty.
DSP Induced Delays
One final topic worthy of mention is DSP Induced Signal Delay. In all digital systems (including Pro Tools), digital signal processing (DSP) causes signal delays of varying amounts. These delays occur as a direct result of using busing, iserts, sends, and audio interface inputs and outputs. The overall effect of these signal delays can vary from as short as one or two samples, to as long as several milliseconds, depending on the t ype of processing or signal routing being performed.

DSP Induced Delays can accumulate as more and more plugins aand other signal processing is applied to an audio signal. In most cases these signal delays are not percpetible, but in certain scenarios, you may want to compensate for he delay.

*When a single track or group of tracks has a considerable amount of delay from a processing-intensive plug-in.

*When you have a grup of tracks where phase coherence is critical, such aas sterio drum overhead tracks, and you are applying different procdesses to some of those tracks.

Pro Tolls TDM systems allow you to compensate for DSP Induced Delay by applying sample-level delay to tracks with the TimeAdjuster Plugin.

Using the TimeAdjuster Plugin (TDM only)

You can use the Time Adjuster TDM Plugin to apply an exact number of samples of delay to the signal path of the tracks you are working with. Up to 2048 samples of delay are avaible.
(Jensen's interruption)TIP FROM FULL SAIL's Brian Smithers, Overlord of Advanced Workstations-- If you need more than that, put another occurance of Time Adjuster on the track.
Digidesign TDM Plugins display their delay values in the track delay indicators in the Mix Window. TimeAdjuster can be used to match these delay values for tracks that need to remain in phase.

In the Mix window, command-click or control-click the Track Level Indicator to toggle between Level (vol), Peak(pk), and Channel Delay(dly) Indicators. Delay values are shown in samples.

To use Time Adjuster to compensate for time delay:
1. Aplly the Time Adjuster Plugin to the track whose delay you want to increase, and Command-click its Track Level Indicator until the Channel Delay value is displayed for that track.

2. Change the delay time in Time Adjuster by moving the Delay slider or by intering a vlaue in the Delay field, until the track delay value matches that of the first track.

Test the delay values by duplicating an audio track and reversing its phase while compensating for the delay.
*JensenBohren stands up strait in a near-military fashion and holds out his hand like a begging statue.
 
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