Inverting a side of your ambient pair

Discussion in 'Location Recording' started by David French, Jun 14, 2005.

  1. David French

    David French Well-Known Member

    I've been assisting another engineer on a harp CD project and he's been using an interesting trick that is new to me. The trick is inverting the polarity on one side of a stereo ambient pair in order to move the reverb away from the center. In this particular setup, he was using a coincident pair of cardioids with a 180 degree included angle. The mics were at least 40' from the close pair and the two were not time aligned. I had him do some AB-ing for me, turning the polarity shitch on and off, and it was very cool to hear the reverb move from a mass in the center to a huge, beyond the speakers inveloping effect. I understand the idea of antiphase and how all this works, but what I was wondering is this: Have you heard of or done this, and is it a good idea?
  2. DavidSpearritt

    DavidSpearritt Well-Known Member

    Wow, I would never do this. The main pair is the foundation image, with the most gain in the mix. If this is phase modified then it will sound really very silly indeed.

    Sorry, just re-read, its not the main pair. Still, I would be sceptical. Phase consistency between all the pairs and spots is very important. I would not do it. Even though reverb is uncorrellated, you have to be careful about how much of this pair you include in the mix. If there is enough gain and it contains enough direct sound, it will definitely upset your main image and sound.
  3. Cucco

    Cucco Distinguished Member

    I think the concept is a little different than you think Dave. He's not suggesting flipping the phase of the main pair, rather the hall mics.

    However, I still wouldn't do this - for different reasons.

    When you space 2 mics in 2 completely different areas of the hall (which ambient mics are usually spaced out a fair deal) the information that is reaching each mic is so uncorrelated that flipping phase doesn't really have the effect that it would on a close pair. The only thing you're essentially doing is changing the positioning of one of the mics only using a phase switch instead of moving the stand. I would think (and have experimented with such) that, with proper placement of ambience mics, the phase should be left alone.

    Just my $.02

    J. :)
  4. DavidSpearritt

    DavidSpearritt Well-Known Member

    Yeah I saw that and edited my post, probably after yours, sorry about that. But one still has to be careful about any direct sound in this pair and the level of gain used. Presumably there is also another "main" pair and it might still have the more reverb in the centre.
  5. David French

    David French Well-Known Member

    I'll have to let you guys hear it soon. Thanks for weighing in.
  6. pr0gr4m

    pr0gr4m Well-Known Member

    The ambient mics are 40 Feet away and the two mics are not in completely different areas. They are a coincident pair. So the Phase reversal is really only affecting the ambience, not the direct signal.

    I have done things with phase on stereo effects, but strictly as an effect when I was looking for a weird type of thing.

    I've never tried it using mics. I don't have a room big enough to try something like this.

    It may not be appropriate, but I'll be it sounds pretty cool.
  7. Cucco

    Cucco Distinguished Member

    Why on earth would your ambience mics be coincident pairs?
  8. David French

    David French Well-Known Member

    Jeremy, I think he chose a coincident pair (Schoeps CMXY 4V) specifically so that he could use this technique in order to get a surrounding very that extends beyond the listening triangle.

    Here's what I don't get: why is everybody so down on this technique? So far I haven't heard one technical reason why this isn't a good idea. I've heard it and I thought is sounded very nice. Can anyone provide proof that this is a technical mistake? I would very much like to learn the truth, but for now, I must trust my ears, and I thought it sounded nice. In a hall, you heard sound from a wider angle than 60 degrees. In surround recording, you would have signal coming from all around you. This sems like having a bit of surround in your stereo.

    I will be working with him again tomorrow morning. Is there anything any of you would have me ask?
  9. ghellquist

    ghellquist Member

    Just remember to check the sound in mono as well. If it sounds good it is good has always been my motto.

  10. DavidSpearritt

    DavidSpearritt Well-Known Member

    Good question. But there is no need for a technical mistake, necessarily. The mistake is one of incoherance and a stereo image that doesn't make sense. If reverb is skewed to one side then its not a faithful acoustic recording, in other words its not an accurate representation of the sound field in the hall.

    If you want to make this change and its part of the artistic vision for the final result, then its fine.
  11. Cucco

    Cucco Distinguished Member

    I agree with Dave S here.

    First of all, when we capture a recording of an ensemble and our intent is to place ambience mics, the intent is to capture the overall sound of the hall. By placing a coincident pair 40 feet back from the main array, your simply capturing what is essentially, a time delayed version of the original signal (with of course many early and late reflections mixed in). So in essence, you're capturing the hall sound from that one spot. Then, to go and phase invert it, you're simply trying to fool a person with psycho-acoustics into believing that the ambience is wider than simply a few inches.

    The problem with any phasing issues is, they only work to some degree on some people in some seating positions. So, if you get up and move around, this phasing thing can just sound wierd as the center of the image is no longer defined.

    My point - why use psycho-acoustics to accomplish something partly when by using proper technique, you can accomplish it in reality? If the answer is a simple one, such as, I only have 1 spot in the hall where I can place ambience mics - then I would give the following advice -
    Don't use ambience mics. Or, if you do, use 1 and use it to help you build a realistic reverb in a program such as altiverb or other ir plugs.

  12. David French

    David French Well-Known Member

    I think I see now. It seems that in doing this, concessions have to be made. It does sound cool when youre in the sweet spot in stereo, but I hope this never makes it to mono.
  13. Cucco

    Cucco Distinguished Member

    You know, I used to think that exact way. As a matter of fact, I got into an off-line discussion with a very knowledgable gentleman from France (who I don't see around here lately) concerning the merits of mono-compatibility. His statements to me at first were that, in Europe many radio stations broadcast in mono and even if they don't many people listen over mono radios.

    I felt this to be a piss-poor argument and I said so. (Yeah, I know, I'm usually SO reserved... :roll: ) But then he asked me how I check for phasing issues and listen for an objective idea of how the stereo image remains stable. My response was that, if my phase scope doesn't vary that much, then that was objective enough for me.

    Well, then I tried it. I began mono-ing out pairs of mics (a practice which I, up to that point, found assanine) to see how their mono image appeared. I soon discovered a few things. First, my mic placement must not have been as flawless as I would like to have thought, as small time adjustments made on one mic would bring the image into sync. Second, many of the width problems that I was battling in the past had much more to do with phasing than panning.

    Now, when I set up on location, I always solo my main pair of mics and mono them out. I'll then make any minor tweaks (including phase inversion) necessary to bring them into a nice coherent center as much as possible. Then I'll readjust panning til it sounds beautifully natural.

    I'll then check outriggers and spots. Of course, bear in mind, there are very few possible phasing issues with mics that are that far apart. Often, my outriggers are as much as 24 feet from eachother making phasing issues either blatantly obvious or non-existent. The same usually applies to spot microphones as well. Of course, for spots, I usually do most time aligning and phase manipulation in post production as it takes me all of 30 seconds and is much easier to determine over a proper monitoring system vs. a Mackie mixer with AKG cans.

    Of course, I understand that I'll never release an album in mono (voluntarily that is), but if it can't stand up under mono scrutiny, educated listeners will complain (or worse, buy someone elses products).

  14. Zilla

    Zilla Active Member

    Having stereo mixes which still sound reasonable monaurally helps insure playback consistency over a wide range of systems. Anytime you start inverting polarity for effect, rather than for correction, you open yourself up to the unexpected.
  15. David French

    David French Well-Known Member

    Well, here's an example of how it sounds.

    The close pair was KM184s in NOS about 4 feet from the instrument on the player's left side, aiming about 45 degrees forward of perpendicular.

    I must say that i'm not all that crazy about the tone here, but I do like the reverb.

  16. Cucco

    Cucco Distinguished Member

    It's not that it sounds bad - don't get me wrong. It actually does sound decent. However, I would like to hear the same thing with well placed omnis as your reverb mics without inverting the phase. I definitely don't think it would be center-focused as the two mics pick up so much different stuff.

    I think the concept is an interesting theory on paper and obviously one that works to some degree, but I strongly agree with Zilla's statement and add that, if you invert phase, you must do it to solve a problem otherwise, you've got to question your motives and technique. Otherwise, you're playing with psychoacoustics which don't always translate well.

    j. :)
  17. David French

    David French Well-Known Member

    Did you try it in mono? All the low verb disappears as expected.

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