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Are these tracks in phase?

Discussion in 'Recording' started by ClarkJaman, Aug 9, 2012.

  1. ClarkJaman

    ClarkJaman Active Member

    Screen Shot 2012-08-09 at 10.24.24 AM.jpg

    I recorded an amazing cello player the other day, and I wanted to practice stereo micing. The only matched pair I have is a couple of Apex 180s, which obviously aren't the best mics- I need to invest in a better set of pencils. I'm not happy with the sound that I got from them so I am going to use the C414 and V69 in the mix and discard the 180s. But I can't seem to tell if the 180s are in phase or not. I set them up at the same distance from the cello and pointed them at the same place, and kept on moving them around to try and get the waves in phase, but the wave forms don't seem to be in sync like the other two mics (C414 and V69) are. Am I doing something wrong?

    Thanks in advance for your help guys.

  2. Boswell

    Boswell Moderator Distinguished Member

    Although it's a bit hard to tell from your screenshot without any time scale, the upper channel of those two does not look correct. Do the two channels sound similar if you solo each one in turn as centred mono, or is one OK and the other distorted? What happens if you try interchanging the mics firstly at the end of the cables and then at the plugs into the pre-amps? What pre-amps or interface are you using?
  3. ClarkJaman

    ClarkJaman Active Member

    Hey Boswell! Thanks for joining in. I appreciate your expert input.

    Here is another screenshot of a different section of the same instrument with a samples ruler. Is that what you meant?
    Screen Shot 2012-08-09 at 11.50.45 AM.jpg

    Both mics sound fine. No distortion. The sound good for a $100 pair, but its a little roomy, and the room doesn't sound great.

    I am using the built in preamps on the Line6 UX8 interface.
  4. bouldersound

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

    Unless you're using a coincident pair some frequencies will always be out of phase. Sound radiates from an instrument like that in complex ways, including the fact that a particular frequency's phase may vary with direction. It's also radiating from the entire surface, and the mic is going to pick that up somewhat regardless of how it's aimed.

    Using two mics to capture one source doesn't automatically count as a stereo mic technique. There are many specific techniques that each have their strong and weak point, different stereo imaging results and degrees of mono compatibility.

    I'd just use a single mic. The only reason I would bother with more would be if the room sound was especially attractive and/or if it were a solo cello recording that needed something a little more interesting than mono. If there are any other instruments then it's back to one mic.
  5. Boswell

    Boswell Moderator Distinguished Member

    The best I can get out of the screenshots is that the mics could be out of phase. Have you got a phase invert set in your DAW, or could one of the mic cables be mis-wired? I can't read the numbers on the sample ruler by the time it gets through the screenshot resolution mangling.
  6. ClarkJaman

    ClarkJaman Active Member

    Thanks Bouldersound. I wasn't using a coincident pair. I was using a spaced pair. Do you think that's what this is? Is it not really important to get the waves in phase if its a spaced pair?

    I probably will end up using a single mic, but I just want to practice stereo micing so that I will be able to do it will when the need arises.
  7. ClarkJaman

    ClarkJaman Active Member

    Boswell, I have a phase inverter, but would that help? The waves aren't in reverse phase, they are all over the place and out of sync. This is something that could probably be solved with that new waves Phase Shift plugin, but not just inversion.

    Here is a better resolution image of that screenshot:
    Imageshack - screenshot20120809at115h.png
  8. bouldersound

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

    Is it important? I don't know. How does it sound? If mono matters to you (think poor FM reception, AM, distributed speaker systems, laptops with mono playback, mp3 encoding and any other playback system that reduces separation) then how does it sound summed to mono?

    Using two mics may make them a spaced pair in a generic sense but that doesn't make them stereo. Mostly it just puts them at odds with the 3:1 rule of thumb. A coincident pair would solve that problem but I don't know if the result would be all that much better than a single mic. Although I'm fond of stereo mic techniques I don't bother unless I want to capture a source with significant horizontal dimension.
  9. bouldersound

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

    You can't invert phase on a complex signal like music. You can only shift it. There is simple phase shift resulting from differences in arrival time and there is more complex phase shift like what filters do. (The cello is a big filter altering the phase of the string's signal.)

    Any button that simply flips the waveform is inverting polarity. Swapping pins 2 and 3 of an XLR does the same thing. Although polarity inversion is often used to treat phase problems they are not the same thing.

    Phase is measured on the horizontal (time) axis of the waveform and is expressed as °@f (degrees at a given frequency). Delay is also measured on the time axis and expressed in seconds, ms etc. Phase and time values can be anything from 0 to ∞ including all intermediate values. Polarity is wholly on the vertical (amplitude) axis and can only be in or out, + or - with no intermediate values*.

    *I'm deliberately ignoring an exception that will just confuse things.
  10. Boswell

    Boswell Moderator Distinguished Member

    Now that you have told us this was a spaced pair and not a coincident pair, all bets are off. Spaced pairing (A-B) is usually set up using omni mics and not cardioids.

    To check on the phasing, put the two pencil mics together side-by-side (parallel) and record any localised sound source that has a distinct pitch - low C on an ocarina is a good example. Now expand the waveforms on your DAW until you can see individual cycles of the note, and it will be obvious whether the two different tracks have the same or opposite polarity. If they are opposite, re-wire the cable that is connected wrongly.
  11. ClarkJaman

    ClarkJaman Active Member

    The 180s come with screw on omni pattern tips that I was using.

    So basically you guys are saying the wave forms for spaced pairs don't have to be in sync like they do for coincident techniques. That's what I was missing. I guess I should have mentioned that it was a spaced pair earlier. :/
  12. bouldersound

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

    You first post had enough information to infer a spaced pair.

    Actually, a spaced pair should result in waveforms that are largely but not completely in phase for most sources due to differences in arrival times of sound approaching from different angles. But what you are getting is the sound radiating from the cello at different angles. The phase differences are built right into the sound as it leaves the instrument. Adjusting mic distances will never fix that.

    You have two options if you want to use two mics. Either use a coincident pair or observe the 3:1 rule and move the mics farther from each other and closer to the source.
  13. Boswell

    Boswell Moderator Distinguished Member

    No, it's not a question of being "in sync" or not. A spaced-omni configuration principally uses the time difference in the two path lengths from source to the spaced mics which the listener's brain interprets as spatial position, resulting in a stereo image. The waveforms at the two mics will not necessarily appear to line up or match.

    By contrast, a sound arriving at a co-incident pair of mics will generate very similar waveshapes in the two mics, and the principal mechanism for creating the stereo image is different amplitudes in the two channels rather than different time delays.
  14. ClarkJaman

    ClarkJaman Active Member

    Awesome. Thanks you two. Super helpful.

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