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Author Topic: The World's Finest Blumlein Array  (Read 2800 times)

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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #60 on: August 10, 2020, 12:09:15 PM »
Yes, you're correct, if that's what you're seeking. I neglected polarity.
« Last Edit: August 10, 2020, 12:12:11 PM by Len Moskowitz (Core Sound) »
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Offline kuba e

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Re: The World's Finest Blumlein Array
« Reply #61 on: August 19, 2020, 12:34:18 PM »
I read more about ambisonic microphones. I'll be happy if someone corrects me. Maybe it will also be useful for someone else.

The easiest way to understand the ambisonic principle is a 1st order microphone (eg TetraMic). If someone is starting to read about ambisonic, it is best to first thoroughly read how Mid/Side of a coincident stereo pair works. When we understand Mid/Side, then understanding  1st order ambisonic is easy.

The 1st order microphone is made of four coincident directional capsules evenly angled in space. From their signals, it is possible to get any pattern of the 1st order (omni, fig.8 or their combination  - cards, hypercards, ...) and we can angle this pattern arbitrarily in space. Everything is built on the basis of summing and subtracting coincident directional microphones (eg cardiode):
-when we sum the signals from two coincident directional microphones rotated 180 °, we get omni.
-when we subtract these signals from each other, we get fig. 8.
-when we sum two coincident directional microphones that have an angle of less than 180 °, the result is a less directional microphone in their axis.
(explanation - each directional pattern contains a certain ratio of the components omni and fig. 8., it's just composing fig.8s and summing the omnis.)

By summing and subtracting the signals of the four coincident cardiodes evenly angled in space, we obtain one omni and three figs. 8 in the perpendicular directions X, Y, Z. This is the 1st order B-format. And to get the final pattern from the B-format, we use the following manipulation. The sum of the signals of three fig. 8 (X, Y, Z) in the corresponding ratio is resulting to fig.8 in the desired spatial direction. And when we add the omni signal in a certain ratio to the resulting fig.8, we get the resulting desired directional pattern rotated in the desired direction.

The principle of higher order ambisonic microphones (eg OctoMic) is different. There are multiple microphone capsules that are distributed on a spherical surface (imaginary or rigid) and a mathematical model is used to evaluate their signals. The mathematical model is based on the fact that the values ​​of acoustic pressure on a spherical surface can be expressed using the sum of the infinite number of spherical harmonics. And these spherical harmonics correspond to our polar patterns.
The input to the mathematical model are signals from individual capsules. A matrix operations are performed with these signals and the output are spherical harmonics. 1st order spherical harmonics and their combinations correspond to the generic microphones patterns. Higher orders and their combinations are ambisonic specialties. The more microphone capsules we have, the higher spherical harmonics we can derive.

The principle of the higher order microphone (capsules on a spherical surface/space + mathematical model) is different from the principle of the 1st order microphone (combination of 4 coincident cardiodes). It would be interesting to hear their comparisons, eg TetraMic vs OctoMic with 1st order patterns.
« Last Edit: August 19, 2020, 01:52:19 PM by kuba e »

Offline kuba e

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Re: The World's Finest Blumlein Array
« Reply #62 on: August 19, 2020, 12:35:55 PM »
Thank you Len, you are right. I read that omni capsules are also used in higher order microphones. The mathematical model is very similar. But the physical design of the microphone is different. It is necessary to separate the capsules from each other to obtain signal differences from the omni capsules at low frequencies. But this in turn brings limitations for high frequencies. It is probably only used in a microphone, which consists of a large number of capsules.
« Last Edit: August 19, 2020, 12:44:29 PM by kuba e »

Offline kuba e

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Re: The World's Finest Blumlein Array
« Reply #63 on: August 19, 2020, 12:38:18 PM »
Ambisonic microphones have their advantages and disadvantages. Unfortunately, I don't understand why the OctoMic should have a better polar pattern than generic microphones. The input to the OctoMic mathematical model are signals from cardioid capsules. These signals are transformed into spherical harmonics (polar patterns). And the quality of these spherical harmonics corresponds to the quality of signals of cardioid capsules - generic microphones. How can the OctoMic be better than a generic microphone?
(I don't consider equalizing low frequencies as an improvement. DSatz has already explained to us that we can equalize any microphone, this is not an OctoMic advantage.)

It is also not clear to me how can be the proximity effect removed from the B-format?

I am left with these ambiguities. But those are the details.
« Last Edit: August 19, 2020, 01:33:19 PM by kuba e »

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #64 on: August 19, 2020, 02:01:22 PM »
Unfortunately, I don't understand why the OctoMic should have a better polar pattern than generic microphones.

It's truly so. Please have a look at OctoMic's measured patterns, and compare them to the best of the world's mono mics of all directivities.

It has to do with the resolution of OctoMic's spatial sampling and the precision of its calibration. That's all I'll say for now.

Quote
It is also not clear to me how can be the proximity effect removed from the B-format?

It's not removed. It's never there from the start.

---

Also, please note that OctoMic is not the kind of spherical microphone you described.
« Last Edit: August 19, 2020, 04:05:45 PM by Len Moskowitz (Core Sound) »
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Offline kuba e

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Re: The World's Finest Blumlein Array
« Reply #65 on: August 19, 2020, 05:31:46 PM »
Thank you Len for your patience.

I have seen charts of fig 8.

Yes,  I probably understand now. The question is what do you mean by better polar pattern.
I understand this that pattern is not changing with frequency. Quality generic microphones meet this. OctoMic also meets this up to 10 kHz.
But you probably mean that OctoMic is better because it has a little different shape of polar patterns than generic microphones. Is this difference in shape really beneficial? This should be judged by listening.
(Haha, I got idea how to do it. From the OctoMic, we can take the direct signal of two cardiods that have a larger angle with each other. This gives us a recording with generic microphones in xy. And also we can create the second recording by ambisonic virtual patterns identical as the two cardioids in xy. Then it is easy to compare it.)

The proximity effect is difficult. I believe you. I only asked because I wanted to understand how ambisonic works. It is not easy -  the B-format, which is based on cardiod signals, has no proximity effect. And when we compose a virtual pattern from the B-format, the proximity effect is created there. Not everything may be clear. I will need time to understand.

Thank you again for all your help.
« Last Edit: August 19, 2020, 06:24:23 PM by kuba e »

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #66 on: August 19, 2020, 11:51:48 PM »
The question is what do you mean by better polar pattern.
I understand this that pattern is not changing with frequency. Quality generic microphones meet this. OctoMic also meets this up to 10 kHz.

Have a look at all of the directivity patterns. You'll see that OctoMic has more stable patterns for all directivities.

Quote
But you probably mean that OctoMic is better because it has a little different shape of polar patterns than generic microphones.

No, that's wrong. The ideal pattern shapes are identical across both ambisonic and mono mics. You may be mistaking the linear scale plots for log plots.

---

We've strayed very far from the subject of the original post. I'm going to lock the thread.
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Re: The World's Finest Blumlein Array
« Reply #67 on: August 20, 2020, 11:22:51 AM »
We've strayed very far from the subject of the original post. I'm going to lock the thread.

Is this not a discussion forum?
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Offline Gutbucket

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Re: The World's Finest Blumlein Array
« Reply #68 on: August 20, 2020, 03:22:01 PM »
Just started a new thread dedicated to discussion of ambisonics in general-
Ambisonics (general discussion, as related to recording)
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #69 on: August 20, 2020, 03:32:44 PM »
Quote
Just started a new thread dedicated to discussion of ambisonics in general-
Ambisonics (general discussion, as related to recording)

Thank you!

Quote
Is this not a discussion forum?

This post was transferred here by the moderator from Retail Space.
« Last Edit: August 20, 2020, 03:34:52 PM by Len Moskowitz (Core Sound) »
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #70 on: November 10, 2020, 06:04:32 PM »
Very nice samples, Len.  I'm a choir director, and this music could not be more in my wheelhouse.  I also love Stile Antico - own several of their albums.

The most convincing for me was the straight-up Blumlein of the Anglica Women's Choir, as it captured the acoustic the best while still giving very good balance to the choir.  That's really impressive - the best Blumlein recordings I have heard usually use single-diaphragm fig8s like MKH 30 or MK 8, and there are some that it's verboten to do it any other way (I'm not one of them).

Thought you might enjoy another track from the same concert: Magnificat

https://www.core-sound.com/temp/Magnificat.wav
Len Moskowitz
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