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Mid/Side a Primer: encoding and decoding
Old 28th March 2010
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Mid/Side a Primer: encoding and decoding

MID/SIDE ENCODING AND DECODING in brief.

There has been a great deal of information and misinformation on the forums about Mid Side recording and how it pertains to stereo recording. In order not to derail any of the previous threads I thought I would start a thread starting with some simple math.

There has been a great deal of good information posted from very knowledgeable individuals on this form over the past years, but it seems to be scattered about. For example, in a four year old post Kiwiburger gives a most stellar explaination of the process that turned the light bulb on for me, and also discloses his personal bias against MS as a recording technique that not everyone who is knowledgeable shares-- read his posts carefully. https://www.gearslutz.com/board/so-mu...ing-works.html

I am hoping that you all will contribute to this thread in order to build a foundation of understanding for people that want an introduction to the Mid Side Process and how it relates to stereo recording. All questions, answers, theories, relevant anecdotes are heartily welcome. I am hoping that people who have a personal displeasure or axe to grind against the MS approach will take their medication before reading and contributing.

But first, what is "TRUE STEREO?" This reminds me of the Supreme Court argument regarding art and pornography. ...... Jacobellis v. Ohio, 378 U.S. 184 (1964)...."I know it when I see it." For many of us it is "I know it when I hear it.

The Mid Side process was useful early on for broadcasting stereo, because the two signals (left and right) would separately waver resulting in the sound wavering from side to side. The information was encoded to MS and transmitted and then decoded, if there was separate wavering of the two M and S signals the listener would only hear a widening or shrinking of the stereo image, which didn't seem to bother listeners. The encoding and decoding does not change the final stereo image unless the M or the S signal is changed in some way.

MID-SIDE or SUM DIFFERENCE TECHNIQUE
ENCODING: The left and right channels of a stereo recording are encoded to Mid and Side.
M = L + R
S = L - R

DECODING
LEFT = M + S = (L + R) + (L - R) = 2L
RIGHT = M - S = (L + R) - (L + R) = 2R

The summing process has resulted in 2L and 2R, so the gain must be dropped to get back to unity.

Given: A stereo image can be encoded as above.
Given: Arithmetic holds true for acoustics as well as other things in our universe, i.e. reversing the process of addition and subtraction gets you back where you started mathematically. daaa!
Given: A stereo field can be captured directly using Mid microphone facing the sound source and a perfect figure of 8 microphone at 90 degrees.
Given: manipulating the gain of M or S will change the perceived stereo width.

Last edited by midnightsun; 28th March 2010 at 08:17 PM.. Reason: typos
Old 2nd April 2010
  #2
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Saddle's Avatar
 

Thanks for posting this. Very informative.

Saddle.
Old 3rd April 2010
  #3
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rjacobsen's Avatar
 

Yes, thanks for the informative thread.

rjacobsen
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