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Phase switches-thoughts and concepts

Discussion in 'Recording' started by jmm22, Oct 22, 2010.

  1. jmm22

    jmm22 Guest

    I recently picked up a D-tar Mama Bear, which is an emulation device meant to simplify and improve the sound of various acoustic guitar pickups. I am just now toying with it, and have no opinion yet, although they are liked by many guitar players, and are used to add some air and realistic guitar sounds, and remove quack. The device is for those using only guitar pickups in recording or live settings.

    In any event, it has a phase switch. However, my thinking is that phase issues are a continuum, meaning that two devices (say the guitar and amp in this case) could be perfectly in phase (depending on location of the two devices of course) perfectly out of phase, but more importantly, somewhere in between.

    Now I am wondering about this in between phase. It seems to me that there are theoretical locations where the phase switch will not help phase issues. If this thinking is right, why would the manufacturer's (yes, I have seen other kinds of devices with just a simple phase switch) not make a continuously variable phase control?

    Any other thoughts on the value of the simple phase switch would be welcome.
     
  2. Link555

    Link555 Well-Known Member

    Very useful for applications like a snare drum top mic and bottom mic. Flip the phase on the bottom mic ....and see the body come back
     
  3. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Your thinking is correct. You can't switch phase, you can only shift it. It's a typically mislabeled polarity switch. You can, however, attempt to address phase issues with a polarity inversion (reversal, flip). How successful it will be depends on what frequencies cancel and which ones add, and your desired outcome. It's commonly used when there are multiple mics picking up one source (intentionally or otherwise), like two mics on a guitar or nine mics on a drum kit. In the case of the guitar it's intentional that the source gets into both mics. In the case of drums there is always a certain amount of unavoidable bleed, e.g. from the snare into the tom mics.

    Phase measurement is frequency dependent. That is, when describing phase you have to specify the frequency. A relative delay between to otherwise like signals yields a phase shift in proportion to frequency. So 1ms delay causes phase shift of about 360 degrees @ 1kHz and 180 degrees @ 500Hz and 90 degrees @ 250Hz. You could use a delay on one signal to match it up with the more delayed signal to correct for that kind of phase interaction. But a filter, acoustic or electronic, can cause phase shift that's not necessarily proportional to frequency or even in the same direction, and delay probably won't bring them into alignment across the spectrum.

    [Edit] To answer your question, what is the switch good for, it may be valuable in live situations where low frequency feedback is a problem. If a low string tends to "run away" it's because it's in phase with the sound coming from the monitor or main speaker. Flipping the polarity will change which frequencies are in/out of phase, though it may just move the feedback to a different frequency.
     
  4. jmm22

    jmm22 Guest

    Interesting. So if I read that right, a phase switch will put some frequencies back in phase, but will inevitably put others out, and that whichever orientation is less objectionable, or more pleasing to the ear is the right one to go with.

    Is that a fair extrapolation?
     
  5. TheJackAttack

    TheJackAttack Distinguished Member

    Close enough.
     
  6. IIRs

    IIRs Well-Known Member

    Flipping the polarity (which is what your phase button actually does) will result in a 180 degree phase shift for all frequencies. Whether that (overall) results in more phase cancellation or less will depend on the phase relationship to begin with: frequencies that were cancelling will now be re-inforced, and vice versa.

    Yup. :biggrin:
     

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