Stereo recording techniques

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In case of intensity stereophony the stereo signals L and R have different levels; delay and phase differences are unwanted recording or transmission faults, especially in respect to the demands of compatibility.

The single microphone technique uses several, mainly directive, mono microphones. In the control room (with the help of basis and direction control) the respective signals are composed to the final stereophonic sound impression. Microphone I for example is exclusively assigned to the left channel, microphone III to the right channel, whereas the signal of microphone II as Middle signal is equally distributed to both channels. A basic condition for the single microphone technique to function perfectly is a high degree of acoustical separation of the different microphones. Two single microphones can be considered to be sufficiently separated if their cross talk attenuation is bigger than 15 dB. This separation can be improved with the help of sound absorbing barriers or bunks, or by skillfully placed sound sources.

The single microphone technique can also be used together with other microphone techniques. It is especially favorable

1. if the desired arrangement of the sound sources in the stereophonic impression does not correspond to the actual arrangement in the recording room.
2. if the different sound sources differ widely in their natural loudness.
3. if the signals of various sound sources have to be differently manipulated by the recording engineer (use of different filters, equalizers, delays, reverberation).

The use of many microphones is also called polymicrophony.

Fig. 10

The XY-microphone technique uses a stereo coincident microphone, whose two microphone systems are set to the same directional characteristic, either cardioid or bi-directional. The main sensitivity axes of the two systems are symmetrical to the fictitious central axis, which is directed to the center of the sound source (orchestra). The respective main sensitivity axes form together with the central axis the angle of displacement []. The angle of displacement depends on the necessary recording or acceptance angle [2 ]. The recording or acceptance angle is the angle within which effective, definite stereophonic resolution is possible. It thus includes the recording range, which during playback is represented between the two loudspeakers.

The angle of displacement influences the recording angle (recording range):

The recording range increases with decreasing angle of displacement.
The sum M of the stereo signals L (left channel) and R (right channel) will be a perfect mono signal.

Like the XY-microphone technique, the MS-microphone technique uses a coincident microphone. In this connection the one system (S system) has to be set to the bi-directional characteristic and to an displacement angle of 90°: the other system (M system) can be set to the omnidirectional, bi-directional or cardioid characteristic, but the angle of displacement has to be 0°.
The two systems do not immediately deliver the signals L (left channel) and R (right channel), but the signals M and S (Middle component and Side component signal, or sum and difference signal), which after reconversion deliver the L and R signals.

Fig. 11

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Stereo recording techniques