Phase Correlation Meter?

Forums Forums Qu Forums Qu feature suggestions Phase Correlation Meter?

This topic contains 11 replies, has 4 voices, and was last updated by Profile photo of Dick Rees Dick Rees 6 years, 4 months ago.

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  • #43555
    Profile photo of DocDocDocDoc
    DocDocDocDoc
    Participant

    Hi there,

    I’m currently mixing down a recording I did with the Qu-Drive Multitrack. To stay with a concrete example: The recording features a cajon, which is recorded using a boundary mic inside and a regular condenser from the front. (Btw, sound guys, this is the way to make a cajon sound real big! Use a Sennheiser E901 inside.) As always when having multiple mics for one source, I check the phase correlation. In any DAW software, this can be easily done using something like Voxengo SPAN (freeware). It turned out that I’d rather phase-invert the boundary mic’s channel.

    Now I have the same setup live with the Qu-16. Of course I can phase-invert the channel – but how would I find out about it? So this is my question: Would it be possible to have a correlation meter as a new feature?

    Or is there a way of getting a display of the phase correlation that I’m not aware of?

    Best
    Doc4

    #43556
    Profile photo of MarkPAman
    MarkPAman
    Participant

    “Now I have the same setup live with the Qu-16. Of course I can phase-invert the channel”

    Oooooo….. Did that get added in an upgrade? I missed that one, and don’t have the desk with me. How do I do it?

    If I’ve been running backwards and forwards with little adapters for no reason, I’m going to feel a bit silly!

    #43557
    Profile photo of DocDocDocDoc
    DocDocDocDoc
    Participant

    MarkPAMan, it’s called “Polarity” in the manual. You can find it in the preamp section of the “Processing” screen, the little on-screen button labeled “Ø”.

    Best
    Doc4

    #43558
    Profile photo of MarkPAman
    MarkPAman
    Participant

    OK, as I said, I’m feeling a bit silly now!

    <Opens manual>

    It’s not where I expected it to be on the screen, but it’s not exactly hidden is it. How can I not have seen that?????

    Thanks.

    I think I’d previously searched the manual for “Phase” and not found what I was looking for. But “polarity” is what I wanted – phase is actually something different as it is frequency & time dependent.

    #43559
    Profile photo of DocDocDocDoc
    DocDocDocDoc
    Participant

    Well, phase vs polarity: Depends on the view. Wikipedia claims: “A phase inversion is neither a time shift nor a phase shift, but simply a swap of plus and minus.” – anyways, “polarity” seems to be a precise word for it, that disregards different views on phase inversion.

    So, back to the start: using the Qu, how to detect that I need to switch polarity? (except from by ear)

    #43561
    Profile photo of cornelius78
    cornelius78
    Participant

    Inverting the polarity of a signal effectively rotates the phase by 180 degrees, so I think that’s where the confusion comes in. As phase is technically a function of time, I think “Polarity Inversion” is a more accurate term for what the “Ø” button does on most mixers.

    So, back to the start: using the Qu, how to detect that I need to switch polarity? (except from by ear)

    You could watch the RTA. If the RTA is following PAFL and you PFL two signals together (“Additive” mode needs to be switched on for this) where they may be some phase cancellation going on, if there is some cancellation you should see a corresponding dip in the RTA. If you then invert the polarity on one of the signals, that dip should return to normal. That said, the RTA is only 31-band, and may not have enough resolution in the frequencies that are being cancelled: your ears are probably more useful.

    As for where you need it, it can be useful if you notice phase cancellation when signals from two or more mics in close proximity are electrically summed (eg choir mics that are too close together, snare top and snare bottom etc.) A case can also be made for inverting the polarity on outputs too. Dave Rat has a video explaining how inverting the polarity on a drummer’s wedge can help get a few more dB of GBF before the wedge bleeds into the drum mics. This can be accomplished in software on the GLD on a per-output basis, but I don’t think it’s available on the Qu: physical adapters may be required.

    #43562
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Ear > Gear

    This is one of my mottoes. I prefer using my ears for such tasks. Meters are fine, but they don’t tell you what it SOUNDS like. That said…

    You might take a look at the RTA following the PFL. Run the two channels you wish to evaluate into a mix, then PFL the mix while flipping the polarity on one or the other of the two channels. Perhaps there will be a visible difference.

    Try it and see…so to speak

    #43563
    Profile photo of DocDocDocDoc
    DocDocDocDoc
    Participant

    Mh, a polarity switch on the mix outs would be another feature request for the Qu series! After all, it is a very cheap operation in terms of CPU time (n*-1, could perhaps even be done with some bit flipping/bit shifting? (It’s been a while that I failed the assembler programming class.)

    Concerning my ears: They work well for tonal signals in detecting phase issues, but I find it hard to do so especially on drums.

    Of course, one always has to check how it sounds. This is especially true for small venues, when you’ve got a lot of drums spilling from stage into the room. Depending on rock’n’rolliness the drummer, of course.

    #43564
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Why would you want to flip the polarity on a mix out? For that kind of specialized application I have in-line barrels with pins 2&3 reversed. That’s a fail-safe way of setting up.

    Did you understand what I was getting at in using the RTA as your visual representation of the effects of flipping polarity? Short of running SMAART that’s the first thing that comes to mind.

    #43565
    Profile photo of cornelius78
    cornelius78
    Participant

    Why would you want to flip the polarity on a mix out?

    For the reasons mentioned in the Dav Rat video. It would also be useful if you have a cable/amp/speaker that’s wired backwards and you don’t have any available in-line barrels with the the pins reversed.

    #43570
    Profile photo of DocDocDocDoc
    DocDocDocDoc
    Participant

    Dick, yes I got your point with the RTA. I’m gonna try that. Btw, Voxengo SPAN (freeware, mentioned earlier) has a very simple correlation meter that does the job for most cases. It’s just that I prefer not to drag around extra stuff like laptops or polarity XLR adapters and stuff. I’m constantly trying to own and drag around less stuff. 🙂

    #43580
    Profile photo of Dick Rees
    Dick Rees
    Participant

    I do not find it a burden to have a pouch of specialty devices. It easily fits inside the Qu case thanks to the “J” shape of the desk itself…along with headphones, WAP/cables, LED flahlight, tool kit and battery tester.

    It’s very handy to have such things when someone else needs them. Bail someone out once and like the mouse who pulled the thorn from the lions paw, you’ve got a friend for life.

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