11/01/15

PRINCIPLES OF THE XY STEREO TECHNIQUE

This configuration consist of two microphones (often first order cardioids) positioned at the same point (coincident) and angled to create the stereo image.

Keywords: XY, stereo, recording, coincident, coincidence, microphone angling, angle, cardioid, first-order-cardioid 

XY stereo set-up is a coincidence stereo technique applying two microphones arranged to position the capsules at the same point. The most commonly used XY-set-up consist of two first-order cardioid microphones angled 90° to produce the stereo image.

Theoretically, the two microphone capsules need to be at the exactly same point to avoid any phase problems due to the distance between the capsules. As this is not physically possible, the best approximation to placing two microphones at the same point is to put one microphone on top of the other with the diaphragms vertically aligned. In this way, sound sources in the horizontal plane are picked coincidently.

The off-axis attenuation performed by each of the cardioid microphones creates the stereo image. While AB stereo is a difference-in-time stereo, the XY stereo is a difference-in-level stereo.

However, as the off-axis attenuation of a first order cardioid microphone is only 6 dB at the 90° angle, the channel separation is limited and wide stereo images are not possible with this recording method. Therefore, XY stereo is often used where high mono-compatibility is needed – for example, in broadcasting situations where many listeners still receive the audio from monophonic reproducers.
 
Opening angles (angles between microphone capsules) of 120° to 135° or even up to 180° may also apply, which changes the recording angle and stereo spread. Bidirectional (figure-of-eight) microphones and supercardioids may also apply for XY-stereo.

Since the sound sources are mainly picked up off-axis when using the XY stereo setup, high demands are placed on the off-axis response of the microphones used. If the off-axis response of the microphones is poor, the center of a reproduced stereo image is not very precise.

The use of directional microphones at large distances may reduce the amount of low-frequency information in the recording, due to the proximity effect exhibited by the directional microphones. Always be aware of the frequency response of directional microphones. Some are designed to be used at short distances (i.e. vocal microphones). These microphones have a reduced low-frequency output when used at a distance. Other microphones are designed to provide a full frequency response at larger distances. Specifications must state the distance at which the frequency response is valid.

The XY configuration is often a choice for close-miking applications.

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