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How to read a Lissajous readout of phase....?

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  • How to read a Lissajous readout of phase....?

    I'm looking at a music file through a vectorscope set to Lissajous mode. shown here is one "frozen" moment. Lissajous is when you get this spidery pattern that dances all around inside a box.


    What on earth does it mean? What am I looking at, exactly? is it wrong/right? good/bad? The software's helpfiles do not explain it; they assume you know what it means already!



    Thanks, ras
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    <font color="blue"><b><font color="olive"><font color="sienna"><font color="purple">Every paint-stroke takes you farther and farther away from your initial concept. And you have to be thankful for that. </font> </font></font><font color="olive">Wayne Thiebaud</font></b></font><br />
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  • #2
    This is a Lissajous pattern that's been rotated 45 degrees so that the horizontal and vertical axes aren't horizontal and vertical. I think they do that so that the width of the pattern will be more obvious. You can get a better idea of what it's displaying if you see how the line changes directions as you mute the channels, one at a time. The wider and fuzzier the pattern, the more "stereo" you have.

    Mono should be a straight line along the 1-2 axis for one channel and along the 3-4 axis for the other channel. When the 2-channel pattern is oriented so that it's more horizontal than vertical, this indicates that there's material that's out of phase between the two channels. So, whatever is playing to produce this pattern is a little stereo and the channels are in phase.
    --
    "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
    Drop by http://mikeriversaudio.wordpress.com now and then

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    • #3
      See the labels? R+ and R-? If you just fed in a R signal, you'd see a line that goes from -R to +R. Likewise, if you only fed an L, you'd see a line that goes from -L to +L. If you feed a mono signal (same thing to both sides) you'd see a vertical line. If the signal was completely 180 degrees out of phase (inverted) you'd see a horizontal line.

      So, the closer to a vertical line, the less difference between R and L.

      There are two ways I've used these patterns. The main one is to check a stereo mic setup. If you have a point source of sound and record it directly in front of the stereo mics, with no reflections (haha), you'd see a vertical line, because the stereo setup would be recording essentially a mono signal. If you don't get pretty much a vertical line, something's wrong: fix it.

      The other way is to get a sense of how wide the stereo image is, on a mix. The wider the pattern, the wider the stereo image. Also, for a balanced stereo image, the width of the blob should be the same on the R axis as the L axis -- that is, the blob should be fairly symmetrical about the vertical center line. It's interesting, but once your ears are trained they're considerably better at this job. Regardless, it's useful to have an objective measurement. Just be aware that the measurement doesn't take psychoacoustics into account and therefore can be misleading for a mix where panning is done by phase more than amplitude. (It is sensitive to both phase and amplitude, but not exactly the same way your brain is: your brain thinks a 0.7 ms delay is panned fully to one side. The Lissajous pattern has no such bias.)

      To get a better idea of what it means, take a mono track and split to stereo. Feed that stereo track to the LJ display plugin (or whatever). Pan the signal left and right and see what happens. (The line will tilt left and right, but will stay a line: it'll be thin in the center.)

      Now stick a delay in front of the LJ display. Delay just one side a small amount (say, 0.1 ms). Listening in headphones, vary the delay gradually up to about .7 ms and compare what it does to the sound in your ears vs what it does to the display. Then keep increasing the delay: the LJ will get wider, but you'll lose the panning effect in your ears -- I believe you'll get a more complex stereo image due to different phase relationships for different frequencies. As you increase the delay, the middle will get fatter. If you feed a one-frequency signal and do this, you'll see it go from a line to a circle to a horizontal line. The horizontal line is when the delay is half the wavelength.

      The LJ display shows only amplitude and phase relationships, whereas your brain responds to amplitude and delay relationships. The two are related but not identical. That can make the LJ display seem ambiguous.

      As Mike says, this snapshot looks like a nice healthy stereo image, without serious phase problems.
      learjeff.net

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      • #4
        I am so-o-o deeply indebted to you guys and your amazing erudition. I'm humbled and appreciative. I like to hope that my questions here are dumb, but not stupid. Y'know.
        <div class="signaturecontainer"> <br />
        <font color="blue"><b><font color="olive"><font color="sienna"><font color="purple">Every paint-stroke takes you farther and farther away from your initial concept. And you have to be thankful for that. </font> </font></font><font color="olive">Wayne Thiebaud</font></b></font><br />
        <br />
        <br />
        <b><font color="#808000"><font color="blue"><a href="http://www.facebook.com/#!/rasputin1963/info" target="_blank">Friend me on FACEBOOK!</a> </font></font></b><br />
        <font color="#808000"> <br />
        <br />
        </font></div>

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        • #5
          No such thing as a stupid question!










          It's the person that's stupid, not the question :lol:

          (Neither, in this case!)
          learjeff.net

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          • #6
            If you have Native Instrument's Reaktor 5, there's a software instrument in the library called Skrewell that has a readout based on lissajous pattern and gives you the ability to manipulate it and listen to the results - as the video shows, lotsa wild racket is usually the result....handy for certain kinds of electronic music obviously:



            nat whilk ii

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            • #7


              nat whilk ii



              This would've made Joe Meek see God.
              <div class="signaturecontainer"> <br />
              <font color="blue"><b><font color="olive"><font color="sienna"><font color="purple">Every paint-stroke takes you farther and farther away from your initial concept. And you have to be thankful for that. </font> </font></font><font color="olive">Wayne Thiebaud</font></b></font><br />
              <br />
              <br />
              <b><font color="#808000"><font color="blue"><a href="http://www.facebook.com/#!/rasputin1963/info" target="_blank">Friend me on FACEBOOK!</a> </font></font></b><br />
              <font color="#808000"> <br />
              <br />
              </font></div>

              Comment


              • #8
                when the readout show this, then it means that you sit to much in your studio and

                you must get your ass immediately to the next jewlery shop, or she is gone next

                time you go to the kitchen, and you make your coffee & breakfast yourself


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                • #9
                  when the readout show this, then it means that you sit to much in your studio and

                  you must get your ass immediately to the next jewlery shop, or she is gone next

                  time you go to the kitchen...........


                  Oh no!!! Seriously.......you gotta be kidding me. I thought this guy was gone for good? It's been so nice and unpolluted around here. Just me but this sucks.

                  Comment


                  • #10
                    He's been playing nice, so give him a break. What's wrong with that post? It's not attacking anyone, just a bit of humor. We all have different senses of humor, so not everyone will find it funny.
                    learjeff.net

                    Comment


                    • #11
                      Well, I did notice his posts about the vocal sound, studio size, BG singers, effects etc was "normal" and helpful to read in terms of learning something. The other stuff.......count me out. I don't get how it goes from well written in his better tech posts to sounding half able to write/speak English or some sort of stream of bull**************** consciousness in the so called funny posts.

                      But I will take your words in the spirit meant.

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