04/03/16

PRINCIPLES OF THE MS STEREO TECHNIQUE

MS Stereo combines two different microphone characteristics and a special matrix to create the stereo image and localization. M is an abbreviation for Mid, and S for Side.
In the basic setup, a cardioid microphone capsule pointing forward provides the M signal. A bi-directional microphone (figure-of-eight) positioned at the same point but angled 90° provides the Side-signal. The positive lobe of the bi-directional microphone is pointing left.


It is not possible to monitor the stereo signal correctly without applying an MS-matrix.

To obtain the left / right stereo image, the M- and the S-signals are processed by simple addition and subtraction:

L = M + S
R = M – S

There are various ways to carry out this calculation:


The illustration shows two ways to reach the LR-format: Using a mixer or using a transformer.

If using a mixer, feed the M-signal (the cardioid microphone) to one input and set the pan-pot to the center-position (providing an equal amount of the M-signal on the left and right channel). The S-signal from the bi-directional microphone is split equally into two parts. One part is fed into a channel and panned fully left. The other part is fed into another channel – phase inverted – and panned fully right. If this channel does not contain a phase-reversing button, use an external phase reversing cable (swap of pin 2/3 at one end).

Notice that most DAWs can perform the process directly, perhaps by applying plug-ins.

The interesting thing about MS recording is the fact that the stereo width can be adjusted, during – or even after – the recording takes place. It is all about the balance between M and S. Gaining M and reducing S makes a more narrow recording. In the extreme, full M and zero S provides clean mono. Gaining S and reducing M provides over width.

This principle works due to the convention that the in-phase part of the bi-directional lobe is pointing to the left. Providing the left channel (M+S) most left-side acoustic information adds up, whereas as right-side information more or less cancel out. Obtaining the right side channel by subtracting the signals cancels out left-side information and add up right-side information. (Remember: minus minus, equals plus.)
Changing the width after the signals have been processed to the LR-format is possible because our simple “processing” equation also works the other way around:
 
M = L+R 
S = L-R
 
The procedure is: 1) Convert LR to MS; 2) Change the balance; 3) Convert back to LR.
We must, however, be aware that we have to correct the levels if we apply continuous conversion like MS→LR→MS→LR.

The correction is a subtraction of 3 dB from the level of both channels after each processing. Subtracting 3 dB is the same as dividing by √2.

The equations then look like this:




The MS technique was invented by Danish Engineer Holger Lauridsen in 1954 as a part of  his research on radio transmission and reproduction of spatial audio. Holger Lauridsen was the head of the R&D department of the Danish State Radio.

Ref: Heegaard, Fr.: The Reproduction of Sound in Auditory Perspective and a Compatible System of Sterephony. JAES Volume 40 Issue 10 pp. 802-808; October 1992.


Selecting microphones

DPA does not manufacture bi-directional microphones. However, if you want to try the MS principle, replace the bidirectional microphone with two first order cardioid microphones: one microphone pointing left and one pointing in the exact opposite direction, right. Arrange the microphones with aligned diaphragms (on top of each other). Reverse the signal of the right-pointing microphone. Then add the two signals, and you have now substituted a bidirectional microphone.

 

Suggested microphones

If you want to try this setup with DPA mics, we suggest:

d:dicate™ 4011A Cardioid microphone
d:dicate™ 4011C Cardioid microphone, Compact
ST2011A Stereo Pair with d:dicate™ 2011A Cardioids
ST4011A Stereo Pair with d:dicate™ 4011A Cardioids
SB0400 Stereo Boom, Modular
UA0837 Stereo Boom
DUA0019 Spacer for Stereo Boom, 19 mm / 0.75 in

 

Related content

In Microphone University we have loads of content that could be relevant to you. Learn more about multichannel techniques by reading the articles below.
 

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