The technique of handling stereo M-S microphones was started in the late fifties during a stereophonic transmission of Danish radio. From the moment of its appearance, this technique quickly gained followers thanks to its flexibility and monaural compatibility. But it was only widely known when the broadcasting of television signals with stereo sound appeared in the United States.
The principles
The fundamental principle of the stereo M-S system is the matrix that combines a forward-directed microphone (M, Mid or monophonic signal) with a bidirectional microphone (S, Side, side, or stereo signal) oriented laterally. Often, these microphones are stereophonic with M-channel remote control patterns that improve the flexibility of the capture device.
The signal collected by the M channel provides the central sound image and constitutes the single monophonic signal. The S signal provides the stereophonic directional information. Considering that the two signals respond in different ways to the environment or reverberation information present, this is the basis for the ability of the M-S technique to reduce the ambient sound component for those who listen to a monophonic signal and increase it for the audience receiving the stereo signal.
This principle is based on the natural condition of human beings and their ability to listen. Thanks to the fact that we have two ears, right and left, the most intuitive way to encode sound information in stereo is following the pattern of reception of our auditory sense.
The quality of stereophonic sound production with monaural compatibility could be achieved easily and quickly under the new stereo M-S technique, which in turn made it possible to achieve a sound image with sufficient versatility and degree of quality that maintained at the same time a fully monophonic individual signal.
The essential object of this invention is to provide a system of sound recording, reproduction and, or, broadcasting, which carries a realistic impression about the information being transmitted. This information travels through two different acoustic paths, in the same way that the listener experiences his auditory sense on a daily basis. Also the idea is to communicate a true impression in the case of sound association with images. In reality, it seeks to improve the illusion that the sound is on the way and that it only comes from the artist, the actor or a sound source that is present before the eyes.
Stereophonic intensity, addition and subtraction
The M-S stereo or mid-side technique is based, like the X-Y technique, on what is known as intensity stereo, that is, the stereo in which the spatial location is determined by the differences in the intensity of a sound wave as it reaches the pair of matching microphones in phase. The stereo intensity depends entirely on the directional characteristics (polar patterns) of the pair of microphones used to produce this effect, considering that there are only differences in intensity and not phase between the channels for any individual sound source captured by the matching microphonic pair.
Stereo X-Y mode, unlike M-S, is created when a pair of polar-pattern directional microphones are placed in perfect angular symmetry from the center line (axis of symmetry) of the sound position.
For its part, the M-S technique also uses two matching microphones. One is the microphone for the Mid signal, of any polar characteristic, and its shape can be one of the omnidirectional, cardiodes and bidirectional models. This microphone is oriented directly to the center line (axis of symmetry) of the sound source. The second microphone is the side (S, side) of bidirectional pattern (cos 0) oriented to the right, forming an angle horizontally with the center line or axis of the microphone M. The conversion of these two signals (Mid-side or sum and difference) into a conventional stereophonic signal is achieved by means of the matrix network of sum and difference in which the left signal is the sum (L = S + M) and the right signal is the difference (R = M - S).
Reverb and compatibility
The M-S technique offers many advantages over other microphone use techniques (miking or mic'in). For that it provides an individual monaural capture on axis, along with a very versatile and potentially accurate stereophonic image. Monaural compatibility is, by definition, absolute, because when the two channels are added together, only the signal coming from the microphone M is maintained (M+S) + (M-S) = 2 M. Therefore, the added monaural signal is the individual output of the M microphone.
Proper selection of a polarity pattern and the location of this microphone will result in a monaural quality that cannot be matched by any other stereophonic sound pickup technique. Carefully hearing the signal during the positioning of the M microphone allows monaural compatibility to be optimized. Another additional benefit is that with this microphone oriented directly to the center line of the sound source on its respective axis, the center section of the sound source is not subject to the ambiguity caused in the coloration of the sound image off the axis, common to many other stereo sound techniques, assuming, of course, that there are good frequencies and polarity characteristics of the M microphone.
Similarly, as long as the reverberant sound field is essentially inside, on the axis of the S microphone, the coloration of this field will also be reduced. In terms of the compatibility of monaural/stereo registers, the result offered by the M-S technique for small reverberation components in the added monaural signal is different from that derived from the left-right stereophonic signal. Because the ear is less tolerant of reverberation in a monaural signal than in a stereo signal, this reduction of the reverberant sound field when added to the monaural signal benefits radio broadcasting in stereo, television signals and sound production for cinematography, where a large segment of the audience listens to monophonic signals.
Another great advantage of the M-S system is that each component (M and S) is available to be treated separately before mastering. This allows to advance a corrective selection of some of the problems found in locations in which the sound was captured, such as noises outside the phase of low frequency coming, in general, from the environment. Here, the S component, which contains most of the information, can be sent through a high-pass filter to reduce the unwanted low-frequency component and this can be achieved without creating any alteration of the M component, until the left-right matrix image remains acceptable.
The flexibility of this technique is a great advantage for the sound engineer/producer, both during and after the recording session. In real-time situations, using a microphone with remote pattern selection (such as single-point single-point microphones available in the AKG, Milab, Neumann brands, among others), the pattern of the M component can be varied to improve capture.
It is also possible to perform a process of manipulation in the perspective of the sound image during the post-production process if the M and S signals are recorded directly as M and S instead of through the matrix as left-right. In this way, the stereo image can be chosen or varied to better fit the requirements of the moment, such as the visual perspective of cinema or television. Stereo/mono compatibility remains completely predictable because the individual monaural signal is available directly.
In short
The M-S technique, when used with a stereo microphone with remote pattern control capability and an adjustable matrix network, offers great flexibility and versatility in stereophonic sound production and has the additional advantage of superior monaural compatibility. This selection of perspective is one of the best virtues of this technique and becomes a great advantage for the work of sound capture in real time and in post-production processes.
Although each technique has its own set of commitments, its use as a recording technique for record production, FM broadcasting and multichannel film productions, requires fewer commitments and extends more useful functions than another stereo technique for the correct balance of the team formed by engineer and producer.
Sources
This article was written based on the following materials:
M-S Stereo: A powerful Technique for Working in Stereo. Wesley L. Dooley and Ronald D. Streicher. Audio Engineering Associates.
New M/S Stereo Technology. Audio Engineering Associates.
Understanding Stereo Microphone Techniques and M/S Stereo. Buzz Turner. www.paia.com
How MS Stereo Works. John Simonton. www.paia.com
