Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Sampo Syreeni]

On 2017-06-25, Fons Adriaensen wrote:

Could it be that you're just talking about different perceptual 
weightings? I mean, if we talk about noise, there we shouldn't ever 
go with A-weighting, or even C-weighting, but the ITU 468 curve.


Electrical noise from active mics and mic preamps is normally white 
and Gaussian, except at low frequencies where you will typically find 
some 1/f noise. If that is not the case there is something seriously 
wrong (but see remark about A/B conversion below).


Correct, but then soundfield type mics are compound beasts with things 
like differencing and capsule matching going on as well. Those can lead 
to issues foreign to the natural noise characteristic of any monolithic 
mic, like a highpass component on the noise floor of the first stage 
amplifier, and (guessing on theoretical grounds only since I'm no 
practitioner like you) perhaps most crucially high frequency ripple in 
the noise floor (too), because of capsule spacing and phasing 
imperfections.


The reason I brought up the ITU curve is that simply by reading from the 
response curve, such anomalies would be belittled by A-weighting with 
its rapid HF rolloff, while being accentuated by the brutal peak of the 
ITU curve, smack in the center of the speech intelligibility band and 
reaching up from it to the 6kHz range where matching issues first start 
to show. At least with lower end soundfield kinda designs, with smaller 
diaphragms and more, detailed, physical geometry between the capsules, 
instead of the more symmetrical, large diaphragm design of the likes of 
the original SoundFields.


(I'm grasping a bit here, but wasn't it so that the MkIV and MkV only 
started to exhibit mismatch above 10kHz, where the ITU curve already 
rapidly rolls off?)


Plus of course the ITU curve was designed with a noise measurement goal 
in mind, unlike the A-weighting and C-weighting ones (their only 
difference being in the reference SPL, both being linear weightings 
derived from equal loudness contours of the Fletcher-Munson and later 
Robinson-Dadson kind). I believe it really is Better wrt to what Enda 
started with, on top of this thread.


But of course its derivation or at least its most common intended 
application also leaves a lot to be desired. ITU-R BS.468-4 as it stands 
doesn't really tell much about what is being measured or how, except 
that it's noise at electrical signal levels, and that we measure it 
using a particular analogue-implementable circuit consisting of a 
passive filter plus a quasi-peak detector. I.e. 1) the filter definition 
itself is a remnant of analogue era gone by, and perhaps most saliently, 
2a) the reasoning behind the precise processing engendered by the filter 
and 2b) its proper ambit of application are *thoroughly* obscured. Most 
importantly we don't know whether the standard was meant to quantify 
noise in an idle channel, by itself, or noise relative to a utility 
signal present at the same time -- a huge difference even when you apply 
the idea to analogue noise reduction, as the


it's been mostly 
applied to environmental noise pollution, where the noise is the utility 
signal under study, but also to the quantification of background noise 
in the presence of a utility signal, such as in noise reduction. Those 
two scenarios aren't interchangeable by a long shot, even if the curve 
itself seems to perform rather better than its primitive elders in both 
its roles; the extra nonlinear processing applied to the signal in the 
base standard,



If the noise is white then it doesn't matter much which weighting 
filter is used, as long as it is specified.


Doesn't it though, when you want to translate a purely objective 
measurement into perceptual noisiness? I thought that was the very 
essence of why all of the weighting curves were conceived in general, in 
the first place, and the ITU one in particular. (Notwithstanding the 
rest of the processing which goes along with the curve.) I mean, 
essentially any weighting curve is a frequency-wise decision of what 
matters and what does not; where the curve peaks, you'll have the 
frequencies which most contribute to the aggregate noise figure, and 
where you have the most attenuation, you're essentially downgrading the 
importance of noise over that band.


Given my argument from compoundness of spatial mic designs above, I'd 
argue a weighting which peaks around the frequencies which lead to 
soundfield typical aberrations is more sensitive to the perceptual 
shortcomings of this class of mics.


Granted, I can't really argue that such a weighting would be better 
except by half-coincidence: while the ITU-468 curve really was designed 
for the measurement of confounding noise in the presence (implicitly?) 
of a utility signal, instead of audibility of signals as such like the 
A, C and whatnot weightings, and so is probably better in the role we're 
currently discussing in any case...on the other hand the perfect 

Re: [Sursound] Re Re: Ambisonic Mic Comparison

[David Pickett]

At 01:34 25/06/2017, Sampo Syreeni wrote:

It's then amazingly difficult to get a rig amenable to the job. At 
my rather low price point, it's almost impossible to get any numbers 
on how your tentative loudspeakers behave. Pretty much no speaker 
manufacturer wants to publish even such basic measures as impedance 
curves at contact, driver thermal constants/dynamic compression time 
constants, polar response plots, waterfall plots, crossover 
frequencies, phase plots, and the thing.


In recent years, I have only seen such plots (and not all of them) in 
reviews by Stereophile magazine, after seeing which, and listening to 
in the shop, I bought four B DM603 S3 speakers for about $600 each 
about 10 years ago. I am still very happy with them though, had I the 
money and the space, I would be even happier wih their studio 
monitors! Lower quality speakers that that I wouldnt expect to give 
decent 4.0 results. But what price range are you looking at?


Undoubtedly it's more complicated on the speaker side. But the 
difficulty is manifest on the mic side as well: all of the 
measurables dual to those of a speaker can in fact sometimes affect 
the performance of a mic, and then even there they don't just tell 
you outright what those measurables *are*.


No microphone known to me, at any price, has sufficiently defined or 
credible specifications.  You just have to use your ears, I am afraid!


David




 Then because of the unknowns, you might well end up paying several 
thousands of euros extra, for nothing at all. Even at the high end, 
which people here talk about... :/


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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[John Leonard]

> On 24 Jun 2017, at 23:09, Fons Adriaensen  wrote:
> 
> If it does that means the amplifier has a problem.

And that, in my experience anyway, is indeed often the problem. When I started 
using the Tetramic, it was with low-cost portable recorders  and at the gain 
levels that I thought sensible for recording, the pre-amps were indeed adding 
noise: swapping to the Metric Halo ULN-8 made a big difference, as anyone who’s 
heard the recordings I’ve made with that combination will attest.

I’m not a MOTU user, so I can’t comment on how it behaves, but I’m very happy 
with the combination that I use.

Regards,

John


Please note new email address & direct line phone number
email: j...@johnleonard.uk
phone +44 (0)20 3286 5942



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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Sampo Syreeni]

On 2017-06-24, Fons Adriaensen wrote:


... In that scenario, the TetraMic recording was definitely
noisier, purely due to the additional gain required.


That doesn't make much sense.


I think as much as well.


Noise level (relative to signal) shouldn't increase with gain.


At least when noise level is well defined...

The Tetramic capsules have an acoustic noise level of 19 dB(A), and 
sensitivity is 7 mV/Pa. [...]


Enda didn't quantify what s/he meant by "noise" too well. You talk about 
A-weighted measurables, then, while Enda probably talked about program 
level overall noise and distortion, or something like that.


Could it be that you're just talking about different perceptual 
weightings? I mean, if we talk about noise, there we shouldn't ever go 
with A-weighting, or even C-weighting, but the ITU 468 curve. The one 
which peaks as fuck between 2-6kHz, and explains how things like Dolby B 
and C sliding band companders work so well; the one which also fails to 
explain the loudness of impulsive, nonstationary, nonlinear noise, yet.


I mean, the perceptual cognates of lower quality in this test appeared 
to be in precisely that frequency range.


That means the electrical noise level is -116 dBm(A). If the EIN of 
the preamp is say 6 dB or more better then most of the noise comes 
from the mic. So with an EIN of -122 dBm(A) you should be safe, and I 
wouldn't call that 'high end'.


I'd call that just "sane engineering for a sane gain structure".

The specs for the Motu 8m don't even mention EIN (which isn't a good 
sign). The 'dynamic range' figure of 112 dB can mean all sorts of 
things and is pretty useless.


Precisely.

Funnily enough, I'm about to buy meself four speakers right about now. 
For the first time. So that I could finally, eventually, at least do 
some pantophonics for myself before I *die*. Compose for at least a 
simple setup of four identical floor standing speakers, and whatnot; for 
the very minimum of proper spatial reproductive rigs.


It's then amazingly difficult to get a rig amenable to the job. At my 
rather low price point, it's almost impossible to get any numbers on how 
your tentative loudspeakers behave. Pretty much no speaker manufacturer 
wants to publish even such basic measures as impedance curves at 
contact, driver thermal constants/dynamic compression time constants, 
polar response plots, waterfall plots, crossover frequencies, phase 
plots, and the thing.


Undoubtedly it's more complicated on the speaker side. But the 
difficulty is manifest on the mic side as well: all of the measurables 
dual to those of a speaker can in fact sometimes affect the performance 
of a mic, and then even there they don't just tell you outright what 
those measurables *are*. Then because of the unknowns, you might well 
end up paying several thousands of euros extra, for nothing at all. Even 
at the high end, which people here talk about... :/

--
Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
+358-40-3255353, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Fons Adriaensen]

On Sat, Jun 24, 2017 at 05:07:15PM +, Enda Bates wrote:

> ... In that scenario, the TetraMic recording was definitely
> noisier, purely due to the additional gain required.

That doesn't make much sense.

Noise level (relative to signal) shouldn't increase with gain.
If it does that means the amplifier has a problem.
A well designed mic preamp will in fact have better EIN at
high gain. 

> I've heard from quite a few people that given a high end
> preamp with sufficient clean gain, that's not so much of
> an issue.

The Tetramic capsules have an acoustic noise level of 19 dB(A),
and sensitivity is 7 mV/Pa. That means the electrical noise
level is -116 dBm(A). If the EIN of the preamp is say 6 dB
or more better then most of the noise comes from the mic.  
So with an EIN of -122 dBm(A) you should be safe, and I
wouldn't call that 'high end'. 

The specs for the Motu 8m don't even mention EIN (which
isn't a good sign). The 'dynamic range' figure of 112 dB
can mean all sorts of things and is pretty useless.

Ciao,


-- 
FA

A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Gerard Lardner]

Following on from Steve's comment, if it is of any interest to you Enda, I can 
offer an early Brahma (serial no 008), about 18 months old, and an old Oktava 
MK-012 D4 (maybe around 20 years old). I'm in Bray, so only about 11 miles from 
TCD.
I had some problems with the calibration files for the Brahma, but I had it 
recalibrated and now I am very satisfied with the result.
The Oktava was calibrated by Fons Adriaensen. It produces a very good sound on 
orchestral and large-scale choral recordings (the only ones I have used it in 
conjunction with conventional microphone techniques, where I could make a 
comparison), but the very large array size - 48-49 mm radius - means that the 
directionality is noticeably quite poor. The current Oktava MK-012 D4 mount is 
different, more stylish; but I have no idea if the array size is any smaller. 
Gerard

Sent from my Samsung Galaxy smartphone.
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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Steven Boardman]

Hi Enda

Thanks for the update.

That all seems to make sense. I just wanted to confirm.

I haven’t been using the new Ambeo conversion plugin, so my 2 cents were based 
on the previous version. This may of skewed my perception! It’s good news 
anyway, as the top end needed eq’ing for my ears.
I have heard differences with different Tetramics, so there maybe discrepancies 
from one to the other. Especially when used out in the field, as mine sometimes 
suffers with humidity. 
I mentioned the cable as there are quite a few different flavours, and I have 
noticed RF problems in some scenarios. It seems you have used the same as me, 
so it is comparable.
The Ambeo didn’t come with the extension cable originally, so that’s also why I 
asked. It doesn’t seem to suffer from RF so much anyway. It’s presumably lower 
noise because it is balanced at the mic.

I was pleasantly surprised at the accuracy of the H2n, and TBH for the price 
the sound isn’t to bad either! Shame you didn’t have any of the Brahma mics for 
test..

Best

Steve


> On 24 Jun 2017, at 18:07, Enda Bates  wrote:
> 
> Hey Steve,
> 
>   well actually I think our results match your impression to some extent. In 
> our second listening test, the TetraMic was preferred to the Ambeo, 
> particularly in terms of high frequency timbre. In terms of directional 
> accuracy, our study did find the Ambeo to be slightly more accurate, but with 
> the difference in capsule spacing that was expected.
> 
> 
> Both the Ambeo and TetraMic were recorded with a MOTU 8m, with the stock 
> Ambeo cables, and the PPAc cabling for the Tetra over a very short cable run, 
> and yeah, the specific calibration was for sure used for the Tetra. In that 
> scenario, the TetraMic recording was definitely noisier, purely due to the 
> additional gain required. I've heard from quite a few people that given a 
> high end preamp with sufficient clean gain, that's not so much of an issue.
> 
> 
> Have you tried the latest version of the Ambeo A-to-B-format conversion 
> plugin? The new filter sounds a lot better to my ears.
> 
> e
> 
> 
> Steve
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Re: [Sursound] Re Re: Ambisonic Mic Comparison

[Sampo Syreeni]

On 2017-06-24, Enda Bates wrote:

In terms of directional accuracy, our study did find the Ambeo to be 
slightly more accurate, but with the difference in capsule spacing 
that was expected.


I glanced at the post, but don't seem to remember. Sorry about that. But 
I just wanted to make sure: was the test double blind? You talk a *lot* 
about what was to be expected, so that eliminating observer bias might 
be doubly important.


Both the Ambeo and TetraMic were recorded with a MOTU 8m, with the 
stock Ambeo cables, and the PPAc cabling for the Tetra over a very 
short cable run, and yeah, the specific calibration was for sure used 
for the Tetra.


In general, on a priori reasons, mics wouldn't be expected to be much 
affected by cable considerations. Pretty much all professional mics of 
today tend to be extremely low current devices, which means they aren't 
too sensitive to resistance, and because of that, their proper cabling 
is also rather thin, leading to low capacitance due to low effective 
cross section between the cables, and low inductance because of close 
lead spacing. Of course all that modulo shielding, but still.


Thus I think if the cabling needs to be mentioned, especially with high 
end, broad diaphragm capsules, someone, somewhere did something Nasty. 
It should be a given that with mics costing thousands of euros a piece 
the cabling at least can be assumed to be beyond audible reproach.


In that scenario, the TetraMic recording was definitely noisier, 
purely due to the additional gain required.


Is that because of smaller capsules, lower line levels, noise gain, or 
what, you think?


I've heard from quite a few people that given a high end preamp with 
sufficient clean gain, that's not so much of an issue.


How does that happen, precisely? I mean, analogue circuit wise speaking? 
If the noise signal is there, no amount of even cleaner gain is going to 
take it away. Input impedance issues might *generate* noise, true, but 
then again what *is* the precise issue, here?


Inquiring minds want to know. :)
--
Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
+358-40-3255353, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
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[Sursound] Re Re: Ambisonic Mic Comparison

[Enda Bates]

Hey Steve,

   well actually I think our results match your impression to some extent. In 
our second listening test, the TetraMic was preferred to the Ambeo, 
particularly in terms of high frequency timbre. In terms of directional 
accuracy, our study did find the Ambeo to be slightly more accurate, but with 
the difference in capsule spacing that was expected.


Both the Ambeo and TetraMic were recorded with a MOTU 8m, with the stock Ambeo 
cables, and the PPAc cabling for the Tetra over a very short cable run, and 
yeah, the specific calibration was for sure used for the Tetra. In that 
scenario, the TetraMic recording was definitely noisier, purely due to the 
additional gain required. I've heard from quite a few people that given a high 
end preamp with sufficient clean gain, that's not so much of an issue.


Have you tried the latest version of the Ambeo A-to-B-format conversion plugin? 
The new filter sounds a lot better to my ears.

e


Steve
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