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Phase problems? How do you know?

Discussion in 'Recording' started by robchittum, Nov 12, 2003.

  1. robchittum

    robchittum Guest

    I have read a lot about phase issues, but don't really understand what it is, or what things would sound like when there is one. Please help me understand this.

    Rob
     
  2. Ethan Winer

    Ethan Winer Active Member

    Rob,

    > I have read a lot about phase issues, but don't really understand what it is <

    Phase problems generally occur when one or more miked sources are combined. If you have two mikes on a single performer, and they are panned hard left and right, there won't usually be phase problems unless the music is played in mono. The problems are caused by some frequencies being boosted and others cut - comb filtering - due to time delays between the two sources. The sound is similar to that of a flanger or phaser effect.

    I'll let others elaborate.

    --Ethan
     
  3. Kurt Foster

    Kurt Foster Distinguished Member

    Phase cancellation occurs when two or more mics are used and they capture the signal at different points in time or the waveform. What results is one source will be playing the signal at the top of the waveform and another at the bottom of the waveform. This is 180 degrees out of phase and completely cancels it's self out. Moving the mic a bit will reduce the degree of out of phase condition, where you can hear the signal but it is weak and not as full sounding. Using a set of headphones you can move one mic around until it sounds the fullest. This is where it is totally in phase. I have found that even though something may be in phase, it still can be time smeared. You may be capturing both signals at the same point in the waveform but a different places.. this destroys definition. That is the reason I usually don’t go for all the multi miced set ups. I like pin point definition. If I can’t get it to sound good with one mic, I change the mic / pre / instrument / amp / player..
     
  4. AudioGaff

    AudioGaff Well-Known Member

    For an example of what out of phase sounds like, change the polarity on just one speaker. (swap the + and -) and listen to some music for a few min, then swap it back and play the same music.
     
  5. Alécio Costa - Brazil

    Alécio Costa - Brazil Well-Known Member

    there is a very simple rule, called the 3:1. with this approach, you may avoid lots of headaches.

    For example, you are capturing the same source with 2 condensers...
    Condenser 01 and condenser 02 will be Lc=1 foot ( 33 centimeters) from the source.

    The distance between the two microphones must be equal or greater than 3 x Lc = 3 feet ( 99 centimeters).

    I have recorded horn bands and Orchestras with up to 45 musicians with over 16 microphones and no serious phase issues. Soundcheck and rehearsal days are crucial for a nice semilive recording.


    :)
     
  6. Ethan Winer

    Ethan Winer Active Member

    Rob,

    > I'll let others elaborate. <

    Okay, maybe I better elaborate. :D

    Kurt has the right idea. When you have two similar sources mixed together equally, and one has a phase shift at a given frequency of 180 degrees, that frequency will be greatly reduced in level. But smaller amounts of shift also reduce the level, just by a smaller amount. The same lesser effect happens at 180 degrees but with the two levels not quite equal.

    That said, a difference in mike distance is not really phase shift. Rather it's the more simple case of time delay. The main difference is that phase shift reduces [or enhances] one frequency, where time delay creates a comb filter that affects a series of related frequencies. Also, with two mikes, the farther one often has less highs, so the cancellation is less complete at the high end of the series.

    All of this is totally unrelated to polarity reversal, which is what happens when you reverse the wires on one loudspeaker of a pair. A lot of people call that phase reversal, but that's the wrong terminology.

    Finally, Alécio is correct about the 3 to 1 rule. But what matters is not the distance between the two mikes, but rather the difference in distance between each mike and the source. For example, let's say you're miking an acoustic guitar player and a tambourine player who are sitting next to each other. The guitar mike will pick up some of the tambourine, and vice versa. So make sure the guitar mike is at least three times farther from the tambourine than the tambourine's own mike. Same for the tambourine mike, which must be at least three times farther from the guitar than the guitar's mike.

    --Ethan
     

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