Jag svarar på utestående frågor från det tidigare inlägget först.
Jag det lugnt i denna storm, och hoppas det inte blir för höga vågor pågrund av min oträning i att debattera skriftligt och verbalt.
Det blev ingen torrent det blev en ftp server
ftp://faktiskt:bosse@flunk.mine.nu
där finns 2 filer inspelade i Söderledskyrka med 2 stormembranskondensatormikrofoner inställda i karaktäristiken för 8 och uppsatta enligt "Blumlein".
Inspelat i 44.1KHz 24 bitar nedsamplat till 16bitar.
Ladda gärna ned och säg vad ni tycker, lite för mycket "atmosfär" men det var denna ljudbild som fanns där jag satt.
Både kören och den lilla orkestern och kören rymdes inom de 90 graderna mellan de 2 mickarna. Orkestern fanns inom de första 20-30 graderna från ena sidan medan kören var utspridd över resterande 60-70 graderna.
Jag har lyssnat och tycker att jag har en stabil stereo bild utan hål i mitten från min normala lyssningsposition med 345 mellan högtalarna och ett lyssningsavstånd på 280 cm och ända fram till +/- 60 grader.
Jag erkänner att begränsningen för min högtalarplacering beror mest på möblering och ej på inlyssnat väl ljud.
Jag råkar bara ha snöat in på ambisonics som inte är det "erkända" systemet för surround återgivning, och har läst diskussioner i den mailing listan under ett antal år.
Jag är en Amatör i vardande och har bara gjort 2 in spelningar så redan här kan jag ju klassas som en som bara har missförstått teorier och inte har någon praktisk erfarenhet.
IngOehman skrev:Vad jag tror att han menar är att man med ambiosonictekniken ju bara lagrar en helt coincident men (amplitudiell+)vektoriell information (man lagrar tre dimensioners vektorer), som kan plockas isär i valfritt antal kanaler, vid behov (eller lust).
Fördelen är att man har större frihet att välja antal kanaler i efterhand, och att man med lätthet kan hantera (och skapa) information på detta vis i en dator.
Nackdelen är att man redan i valet av systemet har omöjliggjort kodning av tidsskillnad.
Man kan säga att normal "kanal-för-kanal-lagring" är ett lite primitivare, men ett oerhört mycket friare informationslagingssätt - givet att man redan i förväg har bestämt hur högtalaruppställningen skall se ut.
Vh, iö
En mycket tydlig och grundlig beskrivning av ambisonics som alltid när iö förklarar något.
Det som jag vill invända mot är betoningen av att det inte ingår någon tidsskillnad i den lagrade signalen. Är det verkligen nödvändigt med en inspelad tidsskillnad om vi inte skall lyssna med hörlurar och prestera konsthuvudstereo.
Som jag har förstått ambisonics så lagrar man informationen helt coincident med xyzw där xyz är de 3 vektor riktningarna och w är amplituden, detta format kallas B format och måste avkodas för att sändas till högtalare.
Vid avkodningen tas hänsyn till tidskillnader genom att avståndet och vinklarna från den teoretiska coincident punkten tas med vid genereringen av högtalarsignalen.
Den tidsskillnad som behövs för att lokalisera ljudkällor anser/tror jag att återinförs i hela ljudkedjan från mikrofon till hjärna genom att jag sätter in mitt eget huvud i det ljudfält som skapats genom att man sänder signaler från högtalarna mot den återskapade coincidenta punkten.
Om man distribuerar färdigt avkodade ambisonic högtalarsignaler så kallas det g-format. För att det skall bli bra krävs det att högtalarna står nära de ställen där man förutsatte vid avkodningen.
Ett exempel på g-format signal kodad som dts finns här:
http://www.ambisonic.net/decodes.html
Ambisonics skall kunna spelas in med 3 stycken 8 mickar och en omni som man sätter coincident (så nära som möjligt) det var nog mest det
+ detta inlägg som gjorde att jag valde att börja min blygsamma inspelnings karriär som "Blumleinare" efter att ha inhandlat mina 2 första mikrofoner.
-- BAMBAM/Bo-Erik
-----Original Message-----
From:
sursound-bounces@music.vt.edu
[mailto:sursound-bounces@music.vt.edu] On Behalf Of
ebenj@pacbell.net
Sent: den 11 december 2005 07:42
To:
jouzts@mcgalumni.com; Surround Sound discussion group
Subject: MICROPHONE Re: [Sursound] Blumlein (was Waterlily, Mahler 5)
John,
I'm sorry if my post wasn't clear. I tried! And I'll try again, if
you'd like.
Meanwhile I'll attempt to address your specific questions from this
email:
> ribbon mics, they were
> marketed as, and generally considered to be, velocity mics.
A sound wave has both pressure and velocity (or 'particle velocity')
components. This is all just jargon until you understand what it means.
If you put up a pressure microphone (which has omnidirectional polar
patterns) it only senses the pressure of the sound waves. As the
wavefront passes the microphone the pressure will go up and down, and
the signal from the microphone will go up and down in a corresponding
way. But the microphone doesn't 'know' what direction that wave came
from.
Think of the old expanding ripples in a pond analogy. If a rock is
dropped into the pond the ripples expand outwards in every direction,
until they hit the edges of the pond - but we will imagine that the pond
is infinite. If we observe a leaf floating in the pond as the wave goes
by, it just goes up and down. From observing the leaf we can't tell
anything about the direction of the wave. But as observers from above,
we can see that the wave does have direction to it. From the position
of the leaf we can point back to where the wave started - where the rock
was dropped in.
The motion of the leaf is like the output of the omnidirectional
pressure microphone. It has no direction; only up and down.
Now imagine a hair suspended vertically in the pond. As the wave goes
by, the hair will be deflected. And it will be deflected In The
Direction of The Wave! It gives an indication of the direction of the
wave propagation.
Note that that direction depends on the position of the observer. Its
an "it came from over there" sort of thing, and depending on where you
happen to be sitting, the direction could be different.
I used the analogy of the hair deflected by the wave because that's how
insect's ears work. Ours are pressure sensors. We can sense direction
primarily because we have two ears. (even though persons who hear with
only one ear can still manage to sense direction via other means.) We,
or rather our hearing, look at the differences in the pressure signals
at our two ears, and from that determine the direction of arrival.
The method of the vibrating hair is like the ribbon microphone. The
reason that the hair moves, and the reason that the ribbon moves, are
the same.
When the wave reaches the ribbon, and flows around it, it creates a
small zone of increased pressure on the near side and a sort of shadow
of decreased pressure on the far side. It is the difference in pressure
between the two sides of the ribbon that makes it move, and only when
the ribbon moves can the fact that there was sound be sensed.
But wait! I thought that the ribbon was supposed to be a velocity
sensor.
And now we've decided that it's pressure that makes it move. That's
true.
It's the difference in pressure between front and back, mathematically
described as a pressure gradient, that makes the ribbon move.
To codify our terminology:
a pressure microphone senses pressure, and as a result it has a
fundamentally omnidirectional polar pattern. It can't tell what
direction the sound came from.
a velocity microphone senses the pressure gradient, which is
proportional to the acoustic particle velocity. The microphone has an
output which is a cosine function (figure-of-eight) of the direction of
the sound. Positive for sounds from in front, zero for sounds from the
sides, and negative for sounds from the rear.
Two of these placed at right angles to each other give a full
description of the acoustic particle velocity in the plane of the
microphones. Three of them at right angles to each other give a full
description of the particle velocity in three-dimensional space. Add an
omnidirectional pressure microphone to the mix and you have a complete
physical description of the sound field at a point in space.
> What is this all about with regard to the velocity/pressure ratio? How
> are these things measured?
Now we know about acoustic pressure and particle velocity. And we know
that our ears aren't directly sensitive to particle velocity, but we can
still sense direction by measuring the pressure gradient between the two
ears.
And this is a big part of how we sense the direction of a sound.
So to get the reproduced sound right, ideally we would want to get both
the pressure and the particle velocity right. We would like for the
pressure and the particle velocity at the listener's position to be the
same as the pressure and the particle velocity were in the recording
venue. This is what Ambisonics can do, and what 2-channel stereo can't
do. But obviously 2-channel stereo works pretty well. Otherwise we
wouldn't spend so much time listening to it.
In acoustics the pressure and particle velocity are wrapped up in a
combination called acoustic intensity. There are devices made by Bruel
and Kjaer and by one or two other companies that are called intensity
probes, and that is what they measure. In effect an intensity probe and
a Soundfield microphone are really the same thing. It's just that the
first is a laboratory device intended for measuring things, and the
second is a recording microphone.
> I assume that the figure-eight
> components of the B format remain pure velocity mics. Are there real
> world pure velocity mics that could be used to record B format
> directly (along with an omni pressure mic for W)?
I'm not sure about the word "pure" in your question. The cardioid or
subcardioid capsules in the Soundfield microphone have a mixture of
pressure and pressure gradient sensitivity. When the capsule outputs
are matrixed together, what is recovered are the pressure and three
pressure gradient components.
In that sense the output of the Soundfield microphone is exactly like
the output of an omnidirectional (pressure) and three figure-of-eight
(pressure
gradient) microphones. But better, because the Soundfield microphone
has output signals that behave as though all four of those microphones
were exactly coincident in space.
> Are there real
> world pure velocity mics that could be used to record B format
> directly (along with an omni pressure mic for W)?
Why sure! You can use your stereo ribbon microphone to recover the
horizontal velocity components of B-format and add an omni to get the W
component.
Thomas Chen and I wrote an AES paper (AES preprint 6621, The Native
B-Format Microphone; Part I) on this subject, and boiled down the
thrust of that paper is that you can record B-format in the way that you
suggest. We were trying to encourage people to do just that. In some
ways this technique is inferior to a Soundfield microphone for two
reasons. One of them is that you can't make the discrete microphones be
truly coincident. The other one is that the polar patterns of
real-world omnidirectional microphones aren't all that omnidirectional!
In some ways the "Native B-format microphone" is better. You get to use
the microphones that you prefer.
Perhaps most important is that it may make the difference between having
a B-format recording, or not having a B-format recording. That argument
supercedes a lot of the theoretical BS surrounding what is best.
For those who are interested in the subject of recording B-format with
discrete microphones, Thomas and I plan a second paper detailing the
results of our listening tests. That paper is planned to be presented
at the 120th AES in Paris this spring. Wish us luck. We have a lot of
work to do.
Eric Benjamin
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Sursound@music.vt.edu
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) hur många är det som kommit fram till ungefär de avstånden som jag angav på förra sidan, alltså:
Jag har lekt med den mikrofonen lite och kommit fram till jag föredrar A-B eller modifierad ORTF (lite mer avstånd mellan mikrofonerna). Men jag ska absolut plocka fram min QM12 igen och testa lite mer.
! - blir det inte ett "hål" i mitten?
