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Phase accuracy of Pro Tools vs Logic

Discussion in 'Logic' started by stefhambrook, Feb 23, 2004.

  1. stefhambrook

    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 ;)
     
  2. David French

    David French Well-Known Member

    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)?
     
  3. deanp920

    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
     
  4. stefhambrook

    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.
     
  5. David French

    David French Well-Known Member

    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.
     
  6. David French

    David French Well-Known Member

    Double post.
     
  7. stefhambrook

    stefhambrook Guest

    Yes, I'd appreciate it, thanks.

    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?

    ;)
     
  8. David French

    David French Well-Known Member

    I may be able to do this today. Are you ever going to answer my questions from the first post?
     
  9. stefhambrook

    stefhambrook Guest

    Yep they were in exactly the same place.

    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....
     
  10. David French

    David French Well-Known Member

    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.
     
  11. stefhambrook

    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?
     
  12. David French

    David French Well-Known Member

    Sorry, I meant to specify. It was LE v6.2.2.
     
  13. stefhambrook

    stefhambrook Guest

    No worries.

    Can anyone get null phase on TDM or HD with the test above?
     
  14. JensenBohren

    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
     
  15. stefhambrook

    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?
     
  16. JensenBohren

    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.

    (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.
    *JensenBohren stands up strait in a near-military fashion and holds out his hand like a begging statue.
     
  17. David French

    David French Well-Known Member

    God, I'm glad I use software with automatic delay compensation!
     
  18. stefhambrook

    stefhambrook Guest

    Thanks for your time Jenson! Thats really helpful.
     

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