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Phase & Waveforms

Discussion in 'Recording' started by Jeemy, Mar 23, 2005.

  1. Jeemy

    Jeemy Well-Known Member

    I think I know the answer to these questions but wouldn't mind verification or correction if somebody would be so good.

    I just finished tracking a bass guitar part using an AKGD112 on the 15", a Rode K2 about 50cm back from the 4x10" and a Sansamp DI.

    When I zoom right in to the waveforms of the tracks, the AKG gives a very sinusoidal wave.

    However the DI wave is a good bell curve below the line, but has a plateau on the ascendance of the wave upwards.

    The Rode has this same plateau but below the line. Obviously they are out of phase. So I flip one and then shift it back a millisecond to get it right.

    What I want to know is why one of the waveforms varies so much? Surely they should have minor deviance only that is hard to see, not obvious differences like this?

    And if they do have differences this big, can I ever get them truly in phase? Even if I line up the points at which, and directions in which, the waveform passes through zero, surely some comb filtering or other cancellation will be caused by this varying wave shape.

    Rest assured I don't want to use all 3 tracks at mixdown but I would appreciate some advice on the theory of this,

    Thanks in advance,

    Jamie
     
  2. Kev

    Kev Well-Known Member

    We shouldn't use the word phase for this.

    Polarity is closer to the true.

    Real phase through different signal paths can vary as much as you have seen and NO they can never be IN PHASE through out the whole frequency range.

    Phase and Frequency are inherently tied together and change through out the frequency range of things like speakers and distortion units and EQ's ...
     
  3. dpd

    dpd Active Member

    Actually, since you have 3 versions of the same signal, you aren't talking polarity (unless one channel is inverted from the others). What you are trying to do is align the delays among the three signals. You can do that with time-aligning all three waveforms on a zero crossing of a particular transient.

    Once you do that, the phase between any two of those signals will be a measure of the difference in frequency responses of the signal chain (e.g the AKG and the DI, the AKG and the Rode, the DI and the Rode). You can't phase-align at all frequencies, as Kev says, because there is a phase difference amongst these channels.
     
  4. Kev

    Kev Well-Known Member

    and sometimes those delays may actually be technically correct ...
    the time it takes to get to the distant OH mics for example.

    multitrack recording is all a bit of a fake anyway and often there are no absolutes ... just choices.

    Grammys have be won using a variety of techniques ... and rules etc ...
     
  5. dpd

    dpd Active Member

    ^^^ yep. If you start time-aligned, then at least you can correlate the delay you use with the sound you hear (if you are curious). Gotta apologize, in my real job I have to worry about time-aligning up to 40 underwater microphones. If you don't do that first, and try to combine (i.e. 'mix') them, you get junk and that's a bad thing.
     
  6. David French

    David French Well-Known Member

    Oh I've gotta hear more about this!
     
  7. Kev

    Kev Well-Known Member

    yeah
    ... me too

    40 mics underwater !!
    That's a hell'of'a water polo match ?
     
  8. dpd

    dpd Active Member

    I've worked in the Anti-Submarine Warfare industry for 30 years. If I told you any more, I'd have to kill you! :D

    Suffice to say, we have the same problems as in recording: time-alignment of signals, preamplification of nanovolt audio signals, high dynamic range issues, A/D conversion quality (incl. Sigma-Delta), digital signal processing designs, mixed-signal processing (how to keep analog clean with DSP devices around), phase and amplitude equalization, driving cables w/o loss of signal fidelity, etc.

    Pretty much all of the concepts translate to the audio recording and processing biz. But, you guys need to (and do) keep me on my toes - sometimes what we do in my biz doesn't make sense here.
     
  9. Kev

    Kev Well-Known Member

    cool ... 8)

    With the recent earthquake stuff happening around the world I had figured the answer was going to be about seismic and tsunami type monitoring ...

    Never saw the sonar thing coming and yet ... It probably was the obvious pick.

    tell us more ... I won't tell anyone ... :D
     
  10. dpd

    dpd Active Member

    sorry, Kev - I can't... Back in the 70's we used to make big (and I mean BIG) buoys for NOAA that gathered oceanographic data. I don't know who makes those now, but that's where (I think...) some of the tsunami data was gathered.

    Sorry to hijack this thread.
     
  11. David French

    David French Well-Known Member

    Cooooooool! Thanks for sharing my Indiana brother!
     
  12. Kev

    Kev Well-Known Member

    yep
    double coool
     

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