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