BeckyT, what MattD said is true--a line that remains vertical would indicate that the signal in the two channels is the same, i.e. that you have a mono (or near-mono) recording playing in the two channels at once.
As far as how the display "should" look, it depends on your specific requirements. Some people care a lot about mono compatibility (the ability to mix the two channels into one without losing any of the direct sound to cancellation). If you're recording for FM broadcast, some of your listeners will be listening in mono and others in a kind of "blended" (reduced-separation) stereo mode which some automotive and portable radios use to reduce "picket fence" noise; you care about mono compatibility a lot in that case.
Used to be, recording for LP release also demanded this concern, particularly at low frequencies since the difference between channels becomes the vertical modulation of a record, and large amounts of it at low frequencies can cause skipping (not that the person listening will start skipping around the room, which might be fun, but that the record would skip, which is not fun). For that reason, stereo FM and LP are often best served by coincident and near-coincident stereo recording techniques (which in turn require appropriate room acoustics and placement of performers so that those techniques can deliver their best results).
Such techniques tend to create Brillo-pad shapes on a Lissajous viewer like you've apparently got in your software, but not random ones; the overall shape of the trace will fall into a diagonal area that slants from the lower left toward the upper right, rather than the opposite way (more / than \, so to speak).
Many other people--probably most people nowadays--don't care much about mono compatibility, as long as the result sounds and feels good to them in stereo. If you throw all caution to the winds as far as mono compatibility is concerned, you can have some real fun with spaciousness that you can't have otherwise. If you know that your recordings will always be heard in stereo, "why not?".
The extreme of this approach is widely-spaced omnidirectional microphones, a peculiarly American approach which produce a swimmy, unstable stereo image with a "hole in the middle." More moderate approaches include narrower spacings (so-called "small A/B"), omni mikes with an acoustically opaque baffle between them (sphere stereo, Jecklin plate, etc.), and my own favorite of the lot, a good pair of so-called "wide cardioids" with moderate spacing and some appropriate angle between them--giving the best of ORTF and small A/B rolled into one.
The low-frequency pickup available from these methods is superior to anything you can get with conventional coincident stereo miking, both in quantity and in quality, while (room acoustics permitting, of course), the ease of listening and sense of spaciousness and envelopment are about as good as it gets with two-channel stereo. And if wide cardioids are used rather than omnis, the stereo image can be quite precise without giving up the spaciousness and low-frequency goodies.
You'd probably find that an X/Y scope display of such a recording may seem rather random much of the time, except when someone sitting near the center of the sound stage is playing solo. And those are the kinds of recordings that were frowned upon back in the vinyl era, because cheap players couldn't keep the needle in the groove very well when the going got intense (especially with low-frequency content that wasn't at the center of the stereo image). But there is much less reason nowadays to be concerned about maintaining high positive correlation between channels.
--best regards
Topic: Phase meter strangeness (Read 1617 times)