Absolutely right, Jörn. The difficulty with Soundfield type microphones is
that the 1st order B format signals are generated from differences between
capsules and these become very small as the frequency goes down which means
more and more make-up gain is needed which means, in turn, that any
differences between capsules become exaggerated. Capsules need to be
matched as accurately as possible over frequency and need to stay that way.
The MkIII workshop manual has (at least) five pages of instructions on the
alignment of the microphone to B format process and tt needs access to a
good anechoic chamber. Calrec used a smallish anechoic chamber for the
mid/high region and a plane wave tube for the low frequencies, if I
remember right. That was all done in the analog domain with preset trimmer
pots - I'm sure there are people (Richard? Fons? Eric?) on this list who
can comment better than me on the more advanced techniques available (and
used) in this digital age.
Dave
On 11 September 2015 at 23:21, Jörn Nettingsmeier <
[email protected]> wrote:
> On 09/11/2015 01:21 PM, James Anthony Enda Bates wrote:
>
>> Hi everyone,
>> I have a quick question about calibrating a Soundfield MKV. At the
>> moment
>> the test tone oscillators are coming in with the W channel about 3.5 dB
>> lower than X,Y, or Z (these three channels are within about 0.5 dB of each
>> other).
>>
>
> It's late and I've had a long day, but isn't that the expected W gain for
> classical Ambisonics?
>
> So obviously the best thing would be to return the unit to get it
>> calibrated as it's probably due for one; and this has been noted before on
>> this list;
>>
>> http://sursound.music.vt.narkive.com/UtLGYHeC/sursound-level-alignment-of-soundfield-microphones
>>
>> However, my question is, if we know what the gain mismatches are based on
>> the test tone output, is it sufficient to just apply gain adjustments to
>> the b-format recordings based upon these Test Tone signal levels? Or is
>> there a possibility that the test tone channel levels, and microphone
>> channel levels could differ?
>>
>
> The problem is not the B-format output. B-format is very robust wrt small
> gain mismatches - say your Y is 3 dB down, all it does is make the image a
> little less wide.
>
> IIUC, the problem is in the individual capsule gains - if one capsule is
> off by 3 dB (which would be a catastrophic mismatch), _all_ output
> components will be totally warped.
>
> The capsule EQs might have similarly severe consequences, leading to weird
> direction-dependant coloration.
>
>
>
>
> --
> Jörn Nettingsmeier
> Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
>
> Meister für Veranstaltungstechnik (Bühne/Studio)
> Tonmeister VDT
>
> http://stackingdwarves.net
>
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--
As of 1st October 2012, I have retired from the University.
These are my own views and may or may not be shared by the University
Dave Malham
Honorary Fellow, Department of Music
The University of York
York YO10 5DD
UK
'Ambisonics - Component Imaging for Audio'
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