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Old 20th December 2014, 03:21 AM   #1
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Default MidSide decoding

Suppose you have L and R signals. MidSide encoding produces signals M=L+R and S=L-R before transmitting.

On the receiver side, decoding is operated as L = (M+S)/2 and R = (M-S)/2. See Joint (audio engineering) - Wikipedia, the free encyclopedia

Question: I have a differential transmission with XLR connectors, so getting M on pin 2 and -M on pin 3 on channel 1; also S on pin 2 and -S on pin 3 on channel 2.

Can I obtain a simple passive decoder by just wiring together the above pins as follows without any further components? Any drawback?

IN:

Channel 1
pin I2: M
pin I3: -M

Channel 2
pin J2: S
pin J3: -S

OUT

Channel 1
pin 2: I2 and J2 wired together (gives 2L)
pin 3: I3 and J3 wired together (gives -2L)

Channel 2:
pin 2: I2 and J3 wired together (gives 2R)
pin 3: I3 and J2 wired together (gives 2R)

Thanks
-Roberto

Edit: I think I actually obtain L, -L, R, -R in the output pins, as each input pin is used twice, so its signal is halved.

Motivation: the M and S signals are produced in the digital domain and it seems to be better to decode them as L and R in the analog domain after that M and S have been converted through a DAC: https://translate.google.com/transla...-text=&act=url

Last edited by robertopisa; 20th December 2014 at 03:34 AM. Reason: added comment
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Old 20th December 2014, 03:20 PM   #2
DF96 is offline DF96  England
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Join Date: May 2007
If you wire an output directly to another output then the join can only have one voltage. This is still true if you add more outputs to the mix.

I1 and J2 are joined, so they have the same voltage. You then join I2 and J3 so they too have the same voltage - but J2 and J3 have opposite signals. Repeat as necessary. The result is no signal anywhere.

As a minimum you need to add some resistors so the outputs can be mixed without forcing equality. To find suitable resistor values just do a forum search or use Google. Look for 'mono mixing' or 'stereo to mono' or suchlike. Your requirement is slightly different, but the technique is the same.
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Old 20th December 2014, 08:38 PM   #3
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M-S is relevant only to the original recording side of the chain, where a side-facing bipolar mic is matrixed with a front-facing, matched-response(the tricky part) cardioid/unidirectional mic. Unless you have two tracks where each track is just the direct signal from each such mic, M-S decoding will produce a completely erroneous result. If you do have such tracks, just do a web search on "passive m-s matrix".
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Old 20th December 2014, 08:55 PM   #4
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@DF96: it makes sense now to me, you are right, no signal anywhere...

@stephensank: I want to experiment, so searching for a passive m-s matrix, I found this info http://www.uneeda-audio.com/ms/ms-mat.htm

Thanks to both
-R
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