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Phase issues with ORTF technique

Discussion in 'Microphones' started by jarjarbinks, Jul 18, 2013.

  1. jarjarbinks

    jarjarbinks Misa want to learn! Active Member

    Hello experts.

    Misa got a question. How far are the mics placed from the soundsource when using the ORTF technique?

    I tested it and noticed some phase issues.

    This is the setup I used:

    Recording acoustic guitar
    2 pencil mics angled 110 degrees and distanced 17cm (6.6 inches) apart from each other
    The mics were positioned 30 cm away from the guitar (12 inches)

    According to the 3:1 rule these technique should generate phase problems. ┬┐Am I right?

    In other words, if the mic heads are 12 inches appart, then they must be placed at a maximum distance of 4 inches from the sounsource. Otherwise, the 3:1 rule is not met.

    This is in direct conflict with the idea of capturing the stereo image by placing the mics 12 inches or more from the soundsource.

    What am I missing?
     
  2. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    the 3:1 'rule' is not talking about using a pair of mics in stereo. it is referring to the use of two mics on the same source as two discrete mono sources. for instance when you close mic a guitar cab, then use another mic further away, to grab more ambience. So if your first mic is 1" away, your next mic should be no closer than 3", (3 times the distance of the first. or if your first mic is 1 foot away, your next one should be at least 3 feet away, according to this guideline. this will help you get in the ballpark, of a phase coherent miking situation.


    The ORTF has nothing to due with the 3:1 rule. It's an old french radio broadcasting standard. It's specifications are simply 7" apart (okay 6.6"), and the 110 degree angle. The distance from the source is purely up to taste. While the phase coherency is generally a little less than an XY, it's generally pretty good phase wise.

    I'd just move the mics around a bit, or just flip the phase on one of the channels to see if you get a fuller sound. and check the mono button to see if things disappear.
     
  3. Boswell

    Boswell Moderator Distinguished Member

    I have to say I never use ORTF for close-miking a solo instrument like a guitar. In my view, you do not get a coherent sound in that configuration from working at distances of about a foot, probably as a combination of phase and amplitude factors.

    I suggest you try recording the same guitar piece twice, with the only difference between the takes being the mic configuration: ORTF and X-Y. Compare the result when played on different set-ups in different environments: studio monitors, headphones, domestic hi-fi etc. My guess is that you will find the X-Y better overall than ORTF. This does not say that X-Y is the best for those mics and that instrument under those conditions, but what it does say is that you can do better than ORTF. The challenge is then to find what you regard is the best configuration for you.

    By way of comparison, if you have omni capsules for your mics, doing a take in A-B configuration can often be an eye-opener in suggesting things that are not right about the recording environment (multiple floor reflections, slap-back echo from a rear wall, low-frequency HVAC noise etc). I have found that cleaning up what can be heard using the extended response and directivity of omnis results in a tighter sound when reverting to X-Y or other cardioid configurations for the actual recording.
     
  4. hueseph

    hueseph Well-Known Member

    Gotta agree with Boswell here. ORTF is better suited to recording orchestral or choir arrangements. X-Y is probably the way to go. Personally, 30cm is too close for micing a guitar. I would back up and give the sound some room to develop. Of course, it helps if you have a nice sounding room.
     
  5. bouldersound

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

    Yep, ORTF is for distance capture of a stereo field in a was similar to human hearing. You wouldn't listen to an acoustic guitar with your nose 15cm away from the body.

    The 3:1 rule means other sound sources should be 3 times the distance from the mic compared to the intended source and other mics should be 3 times the distance from the source compared to the mic on the source. The two sources could be different parts of one guitar such as using a body mic and a neck mic. Of course rules are made to be broken so if it sounds good it is good.

    3:1 doesn't apply to having a close and distant mic on one source, in which case you either live with the phase effects or time align the two signals later.
     
  6. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    my reference for this textbook stuff is the recording engineers handbook. i thought it based the 3:1 proportions on single mics from a source, gotta double check that. my thought was that it helped keep proper distancing when using multiple, non-coiincidental mic placement on a single source.

    i'm still curious as to what issues phase wise the OP was hearing?

    cheers all.
     
  7. bouldersound

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

    It applies to either:

    A) one mic and two sources, a source you want in the mic and a source you don't want in the mic

    or

    B) one source and two mics. a mic you want to pick up the source and a mic you don't want to pick up the source.

    Often you'll have two or more mics and sources and have to consider 3:1 from a variety of perspectives at once, such as on a drum kit. It doesn't apply to two mics intentionally capturing one source (your guitar amp example), one mic intentionally capturing multiple sources (i.e. overheads), or stereo techniques. The 3:1 rule helps isolate mics and sources with relative distance.
     
  8. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    hey cool man, thanks for clearing that up for me! i totally misunderstood that concept. thankfully i haven't tried to use it on a recording!
     

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