A friend bought one, so I've been doing some reverse engineering.
In the 96kHz stereo file, the samples are interleaved
L: 1313131313
R: 2424242424
litepc2aformat reformats this to a 48kHz, 4-channel file
1: 11111
2: 22222
3: 33333
4: 44444
I also measured the impulse responses of Twirling720 Studio.app when
converting from 4-channel A-format to 4-channel B-format FuMa. It is just a
4x4 matrix, no coincidence correction or any other filters. (view this in
a fixed-width font)
1 2 3 4
FRU BRD FLD BLU
W 0.0354 0.0354 0.0354 0.0354
X 0.0107 -0.1220 0.1220 -0.0107
Y -0.1220 -0.0107 0.0107 0.1220
Z 0.0866 -0.0866 -0.0866 0.0866
If you invert that matrix and convert the velocity components to polar,
they assume the mics are pointed at,
az
-85.0000
-175.0000
5.0000
95.0000
el
35.2644
-35.2644
-35.2644
35.2644
so the whole mic array is rotated 40 degrees clockwise. The elevations are
correct for a tetrahedral array.
Also the euclidean length of the X, Y, Z components equals W*sqrt(2), so
the gains are for perfect cardioid capsules.
Looking at some sample recordings that were made well into the diffuse
field of a fairly reverberant hall, W looks about 3 dB too low, so I'm
guessing the capsules are more directional that cardioids, something
between a cardioid and supercardoid (which would make W 4.6 dB too low).
Without coincidence correction filters, the diffuse field response will be
wrong.
Aaron Heller (hel...@ai.sri.com)
Menlo Park, CA US
On Thu, Mar 8, 2018 at 4:27 PM, Marc Lavallée <m...@hacklava.net> wrote:
>
> Hi Steven, and thanks for the useful info.
>
> The fact that the Twirling720 presents itself as a stereo 96Khz
> device and the possibility that the 4 channels are "encoded" in 2
> channels is very interesting, because it could potentially be used with
> any computer. I still don't use mine because the Android app is asking
> for too many permissions on my phone. I'm waiting for the official SDK,
> hoping I can program simple and safe custom apps. I also plan to build
> a mount because holding it only from its USB plug is very risky; a
> modified phone shell could be a good start.
>
> Le Thu, 8 Mar 2018 18:40:20 +0000
> Steven Boardman <boardroomout...@gmail.com> a écrit:
>
> > I have one too. Been using it successfully for a while with my android
> > phone.
> > It does seem to present itself to other audio apps as 2 channel 96khz
> > device.
> > When using the apk it records either stereo, or 4 channel A and B
> > format at 48khz. I think theres some sort if matrixing going on.
> > I haven't done any vertical tests, but the horizontal works well for
> > the price.
> > It is also pretty easy to rotate the capsule spindle, so not sure how
> > accurate the positioning is. Mine is also not quite perpendicular!
> > The manual is useless.
> > They are quick to fix bugs, and implement suggestions thoigh. (the
> > A-Fornat one was mine.)
> > Its way better than a h2n in my opinion, and really easy to carry, as
> > i always have my phone anyway.
> > Because if this i use it a lot, as i carry it at all times.
> > I just have to make a mount for use with my Samsung gear 360.
> >
> > Best
> >
> > Steve
> >
> > On 8 Mar 2018 16:05, "John Leonard Main" <j...@johnleonard.uk> wrote:
> >
> > > Mine (pre-ordered for some small amount) arrived a couple of days
> > > ago and I’ve got it hooked up to my MacBook via a suitable USB
> > > adapter and an old Apple keyboard extension cable. At first, I
> > > couldn't get a sensible signal out of it, but then discovered that
> > > it needs to be connected via USB3, or it won’t work. Then I took a
> > > look at the capsule orientation, which, although it is indeed a
> > > tetrahedral array, seems to be skewed by 45º off centre, but as the
> > > output is encoded in some way into two channels, this may not be a
> > > problem. By using their 720 Studio app, I can get a sort-of
> > > surround signal out of it, although it appears to have no vertical
> > > information. The skimpy on-line manual is pretty useless for Mac
> > > users, so I wonder in anyone else has had better or more consistent
> > > results?
> > >
> > > Bruce - I could send it to you for chamber analysis, if you’re
> > > interested.
> > >
> > > All the best,
> > >
> > > John
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