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  4. Mic Distance and Interaural Delay

  • Mic Distance and Interaural Delay

    R
    R

    Hi everyone,

    I know this has been addressed but I want to make sure I'm understanding things correctly.  FWIW, I love MIR Pro so far, as well as the spaces I've purchased, but overall I get the sensation that instruments placed off-angle to center aren't "in the space".  If I've read things correctly, the spacing between mic capsules is not modeled in MIR, and there is, therefore, no interaural delay created for off-angle placements.  Is this correct?  I'm referring to the direct signal between the instrument and the microphones (not reflections).  Even when I place a meter between two omni microphone capsules, I get a pure mono signal at the output.  I just want to confirm this before I add another layer of channel plugins on every performer position.  I hope this isn't the case, it seems like a very basic thing that MIR should do.  Thanks!

    ~Bob Currie
  • DietzD
    VSL Team
    DietzD

    Welcome Bob,

    thanks for your interest in Vienna MIR Pro.

    You are absolutely right: The whole MIR-concept relies completely on Ambisonics, a meta-format which relies on the idea of an coincident microphone array. By definition, there's no runtime delay between the individual channels of an coincident setup (opposed to a spaced multichannel microphone array). - You can think of it as some kind of three-dimensional MS.

    I tried to cover the basics in an addendum to MIR Pro's Manual called "Think MIR!". It's available as a download from your MyVSL area: -> http://eu.vsl.co.at/downloader.aspx?FileID=7629 (... please see p.9 ff)

    _Any_ signal that runs through MIR is encoded to Ambisonics. It is the only possibility to match the virtual position of the direct signal component _exactly_ with the position if the impulse responses recorded in a hall for the use with MIR Pro, while allowing for seamless interpolation of any in-between positions not covered by an actual recording.

    Another great thing about coincident multichannel formats is that they don't introduce any acoustic "smear", thus the positioning that can be achieved is second to none. Consequently any downmix to "narrower" audio formats (e.g. stereo to mono) is free of artifacts which will typically occur with spaced microphone setups (phasing etc.).

    And finally, Ambisonics allows for the free modelling of virtual microphone capsules. This is a unique feature and very flexible, as long as you keep in mind that the underlying "real" microphones have been set up as a coincident array. This means: Two (or more) coincident Omni-capsules in a coincident setup will always lead to a mono signal, because there is no runtime delay between them. MIR is able to introduce some "virtual" spacing for the reverb tails (for even more acoustic "enveloping"), but there is no delay between the individual channels in case of the (readily positioned) direct signal and the early reflection part of the IRs.

    HTH,

    /Dietz - Vienna Symphonic Library
  • R
    R

    Thanks Dietz (sorry for the late reply).

    That makes sense.  I can see that the number of permutiations of room positions and inter-mic distances would have been impossible to deliver.  I've gotten to a point where I'm quite satisfied by using MIR as a wet-only processor, and then using the Matrix Mixer to introduce spatial-delay between the L/R channels on the direct/dry signals.  I calculate the delay and attenuation in an Excel spreadsheet and copy the values over to VEP for each performer position.   It definitely took some time, but once saved, I can recall different ensemble setups quickly, and the results are fabulous.  Thanks again for a great product!

    ~Bob Currie
  • DietzD
    VSL Team
    DietzD

    Hi Bob,

    it's always great to hear how different approaches work for different people! :-)

    Like I wrote in other threads before: The MIR development engineers and I have tried several ways to implement non-coincident microphone setups during the years. In fact there is a very recent prototype which allows us to toy around with some Decca Tree-based IR recordings. While the reverb tail itself sounds quite nice (although not significantly better than the results derived from our existing Ambisonic setups), the all-important positioning cues inherent to the IRs are blurred significantly. 

    But the real problem is the postioning of the direct signals: Sometimes it's enough to reduce the width of an artificially introduced runtime delay stereo effect to add some strange, nasal quality to the signal, and subsequently to MIR's output. But you will _definitely_ run into all kind of phasing artifacts and tinny-sounding direct signal components as soon as you downmix to mono. ... something to watch out for! :-)

    Kind regards,

    /Dietz - Vienna Symphonic Library