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7. Use of coincident microphones in practice

7.1 Representation of sound sources on the stereo basis

The directions of sound incidence and distances of sound sources are picked up by the coincident microphone in a polar plane, which means that only sound sources located on a circle around the microphone will be represented during playback along a straight line (basis line) between the two loudspeakers, whereas sound sources that are arranged along a straight line in front of the coincident microphone will emerge from the basis line:

Sound sources on the stereo basis
Fig. 14


7.2 Orientation of the coincident microphone

As we have already seen, coincident microphones consist of two single microphones, (mounted on a common axis), that can be turned individully. Their directional characteristic is variable. The fixed (lower) single microphone or capsule delivers the X- or M-signal; for a recording in the XY-system it is turned to left, in the MS-system to the center (or most important part) of the sound source. The revolving (upper) capsule delivers the Y- or S-signal; for a recording in the XY-system it is turned right, in the MS-system to the left at an angle of 90 to the axis of the M-signal.

If the coincident microphone is suspended upside down, the M-signal remains unchanged, whereas after sum-difference conversion the L and R signals would be laterally inverted if the S-capsule is not turned by 180. For an upside down XY-arrangement, the fixed (now upper) capsule has to be turned to the left and the revolving (now lower) capsule has to be turned to the right.

In the XY-system, the angle of displacement [delta] has to be selected according to the desired recording range (see paragraph 6). The capsules are symmetrical to the axis that faces to the center of the sound source.

In the MS-system, the M-capsule faces towards the center of the sound source and the S-capsule (always set to bi-directional) at a right angle (90) to the M-capsule so that its positive side (in phase with M) is facing left.

Delay time differences between the two systems of a coincident microphone and the therefore resulting phase differences between the stereo signals falsify the sound picture: with the MS-recording system the stereophonic sound picture and at the XY-recording system the stereophonic and monophonic ones. This can only be avoided if sound hits the common axis of rotation of the two microphones exactly vertically:

Slanting sound incidence
Fig. 15

At a slanting sound incidence, there is a distance difference deltas, or a delay time and phase difference, which causes a frequency dependent levelreduction of the mono signal. If the deviation is about 15 (angle alpha in fig. 15), and the distance between the middle points of the capsules is about 40 mm, frequencies around 15.000 Hz are cancelled.

With stereophonic replay, delay time differences between the two XY-capsules will have a negative effect only in extreme situations; they will reduce the accuracy of directional perception. A phase difference between M and S of less than 90 will cause, after conversion into L and R, a reduction of cross talk attenuation, which means a displacement of sound sources to the middle. At 90 phase shift a total cross talk arises, which means reproduction occurs only in the middle (mono); at 180, L and R are reversed. This can lead to the effect that lateral sound sources that are arranged vertically will be represented horizontally during playback: e.g.the sound of footsteps beneath a person's voice.


7.3 Practical limitations of XY/MS equivalencies

In theory, the MS and XY microphone systems are equivalent, and their signals can be converted into each other by the sum-difference-transformation, but practical experience has shown some restrictions. The main reason for these restrictions is the deviation of the real polar diagrams from the mathematically defined ideal forms.

Fig. 16 shows the frequency dependency of the directional patterns of a coincident capacitor microphone with variable directional patterns:

Frequency dependency of directional patterns
Fig. 16

Between 1 and 4 kHz, the real directional patterns correspond very well to the ideal form. At lower frequencies, the cardioid mike approaches the omnidirectional pattern; at higher frequencies the omnidirectional mike becomes less sensitive at its sides and thus approaches the hyper cardioid characteristic; the figure-of-8 characteristic is the least frequency dependent one.

In principle, the deviations of the real patterns from the ideal ones in both the MS and XY systems will cause a frequency dependent displacement of sound sources during replay. With the help of fig. 13 these influences can easily be explained. For instance: at lower frequencies the MS combination according to fig. 13,1c changes into the combination of 1b, which means an increase of the recording range; at higher frequencies the MS combination according to fig. 13,1b changes into the combination of 1d, which means a decrease in recording range.

Due to the frequency dependency of the microphone signals, all MS combinations reproduce the higher frequencies better in the range of symmetry axis, all XY combinations better from the sides.

The equivalent mono-signal (fig. 13) also delivers information about how diffuse sound (reverberation, noise of the audience) will be recorded during a stereophonic recording.

Despite the theoretical equivalence of XY and MS systems, in practice the MS system offers some advantages:

 

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BackNextUp Stereo recording techniques