Mid / side stereophony

The mid / side stereophony (English for "middle / side", also MS stereophony ) is a stereophonic signal coding method. The stereo channels are not separated into the left L and right R channels, but into a center channel M and a side channel S.

principle

MS signals can be obtained by forming the sum (middle channel M) and forming the difference (side channel S) of the left and right channels.

{\ displaystyle {\ begin {aligned} M ^ {\ prime} & = L + R \\ S ^ {\ prime} & = LR \\\ end {aligned}}}

For correct scaling, an attenuation must be introduced, which however has no functional significance in the matrixing: as well as . ${\ displaystyle M = {\ frac {1} {2}} \, M ^ {\ prime}}$ ${\ displaystyle S = {\ frac {1} {2}} \, S ^ {\ prime}}$

Conversely, MS channels can be converted to LR channels so that they can be played on common stereo speaker systems:

{\ displaystyle {\ begin {aligned} L & = M + S \\ R & = MS \ end {aligned}}}

From a mathematical point of view, the R components of M and S cancel each other out during the addition , which corresponds to a cancellation in the signal processing. When subtracting it is corresponding to the L part . This matrix circuit can be implemented electrically with transformers , electronically with differential amplifiers or as digital signal processing .

Since the complete information is retained, the conversion chain from LR – MS – LR is in principle lossless. In practice, of course, non-ideal properties (especially in analog signal processing) with the typical losses in signal quality (e.g. noise, distortion, rounding errors) occur.

MS coding has technical advantages in some applications. A popular example is the Joint Stereo setting when encoding MP3 files. Mono signals (such as speech) automatically mean that the side channel does not carry a signal and therefore does not have to be coded.

Another field is the implementation of stereophonic transmission in a manner compatible with monophonic transmission. This means that the center signal as was previously the mono signal is transmitted in such a way so that mono receivers only receive the center signal and thus a - if the output signal monokompatibel is - adequate mono signal reflect. This is the case, for example, with FM stereo broadcasting (see below) and with certain variants of analogue stereo television . With the flank writing on stereo records, the side writing portion can be interpreted as the center signal and the depth writing portion as the side signal. A mono record player (which only reproduces page writing) therefore only "sees" the center signal.

An original application of MS stereophony is the MS stereo microphone recording shown below.

MS miking

As with all stereo miking methods, two microphones are required to pick up a mid-side signal . With intensity stereophony , which includes the MS method, the stereo effect is created solely by level differences, not by time differences between the channels. Therefore, the microphones are placed as close together as possible. The MS stereophony wins the middle and side signals by combining two different directional characteristics :

A predominantly omnidirectional to directional microphone with a spherical to hypercardioid characteristic for the center signal
A pure pressure gradient microphone with the directional characteristic of an eight for the side signal.

The axis angle of the microphones is 90 °, the positive polarity of the figure eight microphone points to the left in accordance with the standard. Instead of a combination of two microphones, a coincidence microphone can also be used. The channels “left” and “right” can be obtained by forming sums and differences with a matrix circuit.

If no matrix circuit is available, the MS stereo signal can also be decoded with a mixer ( direction mixer ) (see picture): The symmetrical S signal is voltage-corrected to the left channel and at the same time voltage-inverted (reverse polarity, colloquially: "phase rotated") switched to the right channel. The M signal is mixed into both channels with the correct voltage. The voltage inversion is easiest to implement if the mixer has a switch for "phase rotation" (ϕ, phase) in the channel strip. Alternatively, the signal voltage can be inverted by swapping the symmetrical connections pin 2 (hot) and pin 3 (cold) on the XLR connector .

The connection between the polarity of the eighth microphone, mixer and stereo impression is best understood in the example: If an acoustic signal comes from the left side, it is reproduced with the correct voltage by the eighth microphone, since its positive membrane side points to the left. The omnidirectional microphone always picks up signals with the correct voltage, as it is a pressure receiver . In the illustrated mixer channel setting the mid signal is added to the voltage S correct signal on the left channel . On the right channel via the pole reversal adds spannungsinvertierte S signal with the M signal: . The signals are canceled out: The acoustic signal coming from the left is reproduced louder on the L channel than on the R channel. If an acoustic signal comes from the right side, it is picked up by the figure eight microphone with an inverted voltage, as the rear side of the membrane points to the right. It will cancel each other out with the voltage-correct middle signal on the left channel and add up on the right. ${\ displaystyle L = M + S}$ ${\ displaystyle R = M + (- S)}$

MS stereo microphones have the following advantages:

As with all intensity stereo methods, there are no runtime differences and thus no comb filter effects due to phase cancellation ( interference ). The signal is not discolored when played in mono.
By changing the M level to the side channel level (+ S signal and –S signal), the basic width of the stereo signal can be changed later. The level ratio M to S corresponds approximately to the axis angle (opening angle) of an XY stereo system .

In practice, the middle-side channels are recorded in order to be able to vary the basic width of the stereo recording in the recording studio. Matrix conversion to left-right channels must be available during recording in order to control the microphone perspective.

Significance in stereo radio

MS coding is fundamental for the transmission of FM stereo programs. Here the center signal is transmitted on the main VHF band and the S signal on a secondary band. This has the following reasons:

The signal can be played back without any problems with mono VHF receivers, they ignore the S signal and have a perfect mono signal. Mono compatibility , so backward compatibility of the signal is a precondition for this.
If reception is not good enough for a stereo receiver, i.e. if the transmission range is restricted, it can always access the more stable M signal.

A comparable principle can be found in analog color television , for example the PAL method. Here is also u. a. for reasons of compatibility, the sum signal of all colors (black and white signal) and two difference signals for the colors are transmitted.

More stereo forms

literature

Klaus Bertram: About dealing with stereo coincidence microphones. (PDF) In: Telefunken-Zeitung, issue 3/4. Telefunken AG, 1965, pp. 338–347 , accessed on August 5, 2013 .
W. Hoek, K. Wagner: Stereophony recording technology . Berlin 1970.