[Sursound] question about simple equation

[Giuseppe Silvi]

Hi I'm Giuseppe from Rome,
i apologize if my question could be trivial, but I have this problem in my mind 
to be solved.

If cardioid pattern is descrbed by equation:

cardioid = 1/2 ( 1 + cos Theta)

and W equation is:

W = 1 / sqrt2

replacing numbers with letters the equation should be this:

cardioid = 1/2 ( sqrt2 W + X )

but for Mid-Side decoding from B-Format, where M is the same cardioid 
component, I often find this equation:

M = sqrt2 W + X

WHY? [this is a question, not a strange ambisonic configuration :]

Help me please!
Thank you very much.

Giuseppe Silvi

On 11/mar/2012, at 17:00, sursound-requ...@music.vt.edu wrote:

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> Today's Topics:
> 
>   1. Re: blindly identifying matrix encodings (Aaron Heller)
> 
> 
> --
> 
> Message: 1
> Date: Sat, 10 Mar 2012 14:23:01 -0800
> From: Aaron Heller 
> Subject: Re: [Sursound] blindly identifying matrix encodings
> To: Surround Sound discussion group 
> Message-ID:
>   
> Content-Type: text/plain; charset="iso-8859-1"
> 
> Following up, I made a UHJ version of "Eight Directions".  Grab it and the
> MATLAB code that made it from
> 
>  http://ambisonics.dreamhosters.com/  (at the bottom of the page)
> 
> As you can see from the code, is is easy to plug in any coefficients needed.
> 
> 
> On Fri, Mar 9, 2012 at 2:07 PM, Aaron Heller  wrote:
> 
>> Perhaps a useful first step would be to take some B-format recordings from
>> Ambisonia and encode them into the various systems, so we could listen to
>> the differences in some controlled manner.   That would also comprise a
>> reference data set to test automated algorithms.
>> 
>> Sorry if this has been answered before, but is there a concise summary of
>> the encoding and decoding equations for the formats?  With that in hand, it
>> should be quick MATLAB/Octave exercise to encode some B-format recordings.
>> 
>> 
>> On Fri, Mar 9, 2012 at 10:22 AM, Eero Aro  wrote:
>> 
>>> Richard Lee:
>>> 
>>> I'm actually listening to the "stereo" presentation of the reverb
 rather than individual sources.
 
>>> 
>>> My respect!
>>> But I know what you are talking about. However, your skills need a lot of
>>> listening experience.
>>> 
>>> 
>>> You'll probably find a stereo fig-8 @ 90 decode from the B-format
 gives sharper images than "UHJ encoding listened in stereo" of the
 same B-format.
 
>>> 
>>> Naturally.
>>> 
>>> Eero
>>> 
>>> __**_
>>> Sursound mailing list
>>> Sursound@music.vt.edu
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>> 
>> 
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Re: [Sursound] question about simple equation

[Fons Adriaensen]

On Thu, Mar 15, 2012 at 03:16:05PM +0100, Giuseppe Silvi wrote:
 
> If cardioid pattern is descrbed by equation:
> 
> cardioid = 1/2 ( 1 + cos Theta)
> 
> and W equation is:
> 
> W = 1 / sqrt2
> 
> replacing numbers with letters the equation should be this:
> 
> cardioid = 1/2 ( sqrt2 W + X )
> 
> but for Mid-Side decoding from B-Format, where M is the same cardioid 
> component, I often find this equation:
> 
> M = sqrt2 W + X

Which is the same, just 6 dB more gain. When using MS you will adjust the 
relative gains of M and S anyway to obtain the desired width. You need to
do that also when using 'real' microphones for MS - in general they won't
have the same sensitivity. 

Ciao,

-- 
FA

Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.

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Re: [Sursound] question about simple equation

[Jörn Nettingsmeier]

On 03/15/2012 03:16 PM, Giuseppe Silvi wrote:

Hi I'm Giuseppe from Rome,
i apologize if my question could be trivial, but I have this problem in my mind 
to be solved.

If cardioid pattern is descrbed by equation:

cardioid = 1/2 ( 1 + cos Theta)


this is a fact.


and W equation is:

W = 1 / sqrt2


this is just a convention, which comes from the days of magnetic media. 
i guess the observation was that in normal soundfield recordings, W is 
usually a bit louder than X,Y, and Z, so in order to ensure optimal tape 
saturation with maximal signal-to-noise ratio, W was reduced by 3dB 
(that's the 1/sqrt2) before being recorded to tape. this way, you could 
get roughly the same recording level for all 4 channels.


the -3dB convention is technically not necessary any more (which is why 
the new proposed ambisonic standard does away with it). but it makes a 
lot of sense to keep the old WXYZ format as it is, in order not to break 
all existing recordings.





--
Jörn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487

Meister für Veranstaltungstechnik (Bühne/Studio)
Tonmeister VDT

http://stackingdwarves.net

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Re: [Sursound] question about simple equation

[Giuseppe Silvi]

Thanks Fons and Jorn.

writing the equation inside max6 I had several clipping problems with the
decoding of a Soundfield ST450 signal, that has four matched line under single 
knob.
With the “correct” equation I have a signal that falls within the correct 
dynamics
like decoding into Blumlein fig-8 pair.

I think the W component of ST450 comes out scaled by sqrt2.

I record the b-format in a single quadro file.

the code:

// max6 CodeBox - B-Format to MS to LR decoding

W = in1;
X = in2;
Y = in3;
Z = in4;

// M = (sqrt2 * W) + X; // several clipping problem
M = .5 * ((sqrt2 * W) + X); // without clipping
S = Y;

L = (M + S) / sqrt2;
R = (M - S) / sqrt2;

out1 = M;
out2 = S;
out3 = L;
out4 = R;

// fine

Thanks.
Bye.

> --
> 
> Message: 2
> Date: Thu, 15 Mar 2012 14:51:49 +
> From: Fons Adriaensen 
> Subject: Re: [Sursound] question about simple equation
> To: sursound@music.vt.edu
> Message-ID: <20120315145147.ga22...@linuxaudio.org>
> Content-Type: text/plain; charset=us-ascii
> 
> On Thu, Mar 15, 2012 at 03:16:05PM +0100, Giuseppe Silvi wrote:
> 
>> If cardioid pattern is descrbed by equation:
>> 
>> cardioid = 1/2 ( 1 + cos Theta)
>> 
>> and W equation is:
>> 
>> W = 1 / sqrt2
>> 
>> replacing numbers with letters the equation should be this:
>> 
>> cardioid = 1/2 ( sqrt2 W + X )
>> 
>> but for Mid-Side decoding from B-Format, where M is the same cardioid 
>> component, I often find this equation:
>> 
>> M = sqrt2 W + X
> 
> Which is the same, just 6 dB more gain. When using MS you will adjust the 
> relative gains of M and S anyway to obtain the desired width. You need to
> do that also when using 'real' microphones for MS - in general they won't
> have the same sensitivity. 
> 
> Ciao,
> 
> -- 
> FA
> 
> Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.
> 
> 
> 
> --
> 
> Message: 3
> Date: Thu, 15 Mar 2012 16:18:14 +0100
> From: J?rn Nettingsmeier  
> Subject: Re: [Sursound] question about simple equation
> To: sursound@music.vt.edu
> Message-ID: <4f620836.3020...@stackingdwarves.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> On 03/15/2012 03:16 PM, Giuseppe Silvi wrote:
>> Hi I'm Giuseppe from Rome,
>> i apologize if my question could be trivial, but I have this problem in my 
>> mind to be solved.
>> 
>> If cardioid pattern is descrbed by equation:
>> 
>> cardioid = 1/2 ( 1 + cos Theta)
> 
> this is a fact.
> 
>> and W equation is:
>> 
>> W = 1 / sqrt2
> 
> this is just a convention, which comes from the days of magnetic media. 
> i guess the observation was that in normal soundfield recordings, W is 
> usually a bit louder than X,Y, and Z, so in order to ensure optimal tape 
> saturation with maximal signal-to-noise ratio, W was reduced by 3dB 
> (that's the 1/sqrt2) before being recorded to tape. this way, you could 
> get roughly the same recording level for all 4 channels.
> 
> the -3dB convention is technically not necessary any more (which is why 
> the new proposed ambisonic standard does away with it). but it makes a 
> lot of sense to keep the old WXYZ format as it is, in order not to break 
> all existing recordings.
> 
> 
> 
> 
> -- 
> J?rn Nettingsmeier
> Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
> 
> Meister f?r Veranstaltungstechnik (B?hne/Studio)
> Tonmeister VDT
> 
> http://stackingdwarves.net
> 
> 
> 
> --
> 
> ___
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> End of Sursound Digest, Vol 44, Issue 11
> 

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