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Phase Interference

Discussion in 'Recording' started by fiast, Dec 3, 2009.

  1. fiast

    fiast Guest

    hmm ..i just watching internal mixing by steinberg and there topic about phase interference ..actually i don't know what he say about phase interference ..can someone share to me about phase interference that can make me understand that ..thx btw
     
  2. EricIndecisive

    EricIndecisive Active Member

    from what i learned in physics this year, this is what it means. this applies to say, speakers playing in a room, but i think the same concept is also applied to microphones picking them up.

    you have 2 sound waves. you know the shape of them, like this: /\/\/\/\/\/

    well, when the tops and bottoms of these waves match up perfectly, the sound is amplified and louder (constructive interference). however, if the top of each wave coincides with the bottom of the other, the sound actually cancels out (destructive interference).

    like this: phase.gif you can see as they shift one wave the volume decreases and visa versa.

    In the speaker example, due to the way that sound works, there are spaces in the room where sound actually cancels out. This was telling us about a home theater setup, and you do not want to place the seats in one of these dead spots.

    I'm assuming for microphones it is the same thing - that mics that are not positioned properly pick up recordings that are out of phase and have a reduced volume / tonal properties and wouldn't be good for a recording.
     
  3. Kev

    Kev Well-Known Member

    in a multi speaker system there will always be dead spots at different frequencies
    in stereo the centre line is pretty safe but there will be stereo mix effects that show the dead spots
    don't overcomplicate things but be aware of the technicals

    in addition to the display above which is for a two single tones of near even frequency and this will produce a beat frequency
    there is Comb Filtering which will occur when a single sound source is mixed together with a phase shift
    you get peak and valleys in the EQ spectrum and this can sound bad and annoying
    however it is the basis of some Effects where the phase shift is changed with an LFO (low frequency oscillator)

    phase and polarity is a big subject
    take your time and investigate the variations one at a time
    as it can get all very complicated if you try for max knowledge in one hit
     
  4. fiast

    fiast Guest

    hmm ..actually i still don't get it ..
    maybe sarNz and Kev wanna give me example about phase interference so i can imagine that ..thx for sharing btw ..
     
  5. Kev

    Kev Well-Known Member

    try two mics on a guitar amp
    one in close
    one 6+ ft away

    the mic 6ft will have more room/reverb but will also be slightly late compared to the close mic

    set the close mic ito one side of the stereo pan and then fade in the far mic ... also to the same speaker
    the tone will change and may even sound uncomfortable

    better still is to get someone to walk around the room with the second far mic and listen to the tone change

    Multiple mics across a drum kit will have the same issues
    but on a drum kit with short bursts of sound it can be hard to recognize the issues
    until you get to know the sound

    second try

    take a simple recorded track and set it to one speaker
    take the same audio file and put it on a second track
    set you DAW to bump the second track by small increments ... say 1 ms ... perhaps 5 ms

    as the two tracks are played IN phase ... the same time things will sound good ... just 6db louder than a single track

    but as you bump the second
    listen for that tone change ... comb filtering is happening
    and it changes with each bump
     
  6. fiast

    fiast Guest

    it's like take 1 guitar track than copy it but use ADT to copy track pan L full in original guitar track and pan R to the copy one to become stereo guitar sound ..right ?

    if phase interference like that ..i should already know .. :shock:
     
  7. jg49

    jg49 Well-Known Member

    Partly, you have to delay the second track by moving it 5 ms later than the first track, which will make it sound fuller then collapse both tracks to mono and you should hear the difference in tone.
     
  8. Kev

    Kev Well-Known Member

    as jg49 said

    in mono is where you hear the problems

    but in stereo it can be an interesting trick ( a widening thing )
    out of phase content is valid for a stereo mix
    IF you know what you are doing and can control the problem it causes

    a better trick is to record a second guitar as close as possible to the first
    OR
    use a delay that has an LFO and you can slide the amount of delay so the Left Guitar moves a bit
    then the phase error changes and so to the Comb Filtering
    try not to create chorus and the stereo spread of the guitar can move a little ... I think it is more stylish sound compared to the straight 5 or 10 ... 15ms delay to left
     
  9. fiast

    fiast Guest

    wow thx all ..that really help ..hmm about " a widening thing " is like take guitar rhytem 2 times with doing the same thing ?that will become stereo guitar rhytem ?like that u mean or not ?btw sorry if my english bad ..hehehe
     
  10. jg49

    jg49 Well-Known Member

    Kev"use a delay that has an LFO and you can slide the amount of delay so the Left Guitar moves a bit
    then the phase error changes and so to the Comb Filtering
    try not to create chorus and the stereo spread of the guitar can move a little ... "

    Use a delay with LFO (low frequency modulation?) is this a delay with a LFO setting on it, similar to synths, or something you have to hear? I was under the impression that LFO was near or below the range of hearing though I do understand it alters how we percieve sound. Just not sure exactly what you meant here.
     
  11. Kev

    Kev Well-Known Member

    yes it is a synth trick/feature

    the frequency of the LFO is often down in the 1 and 2Hz area .. perhaps lower at 0.5

    you don't hear this directly ... you can't

    what it does is drive the control of the delay amount inside the workings of the delay unit

    soooo

    if you have set the delay for 5ms
    the LFO will change this setting at a rate of 1Hz by the depth you have set

    the depth setting is the sort of control you would use in a chorus or flange

    so depth is like More swirl ... Less swirl

    we don't want to make chorus ... this would be much high delay settings

    the widening effect of 5 to 10ms is changing very slightly and so when in MONO
    that phase sound is not static
    it helps ... slightly ... to cover up the fact you have used a cliche trick of widenning
     
  12. fiast

    fiast Guest

    >,< hmm i think my knowledge not yet reach this topic huhuhu ...but thx ..i understand a little bit about phase interference ..about LFO or something else i will learn it again ..thx Kev and jg49 ..hehehe
     
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